I often hear hams expressing impatience with the current state of propagation. There are few or no sunspots and the cycle minimum drags on and on. At least it seems that way when you're in the midst of it. Some go so far as to speculate (based on nothing at all) that the poor conditions will never end.
A big DX contest changes that. The bands are lit up with megawatts of RF from all corners of the globe. If there is a path some of that energy will push through. The following brief notes are my experience of DX propagation in the previous weekend's ARRL DX CW contest in which I made 2.5k DX contacts on 160 through 10 meters. If you were active you'll know what I mean.
Over the pole
As we creep towards spring the hours of daylight are rapidly increasing. Northern paths benefit as the ionization in the far north spikes. There were lengthy openings to Japan, UA0 and parts of the Far East on 20 meters. It was a joy to hear so many strong signals and the running was good.
On 40 meters there were intense though brief opening after sunrise on the short path. Japan, China and others sounded like they were next door. VR2XAM in Hong Kong was at least S9+10. I missed JT though others logged them. Since I was unassisted I couldn't find everyone. An even shorter though weaker opening to the Far East on 80 meters added multipliers.
The long way around
Long path was quite good during the contest. While running I was called by a number of VK and ZL stations on both 40 and 20 meters. Signals were surprisingly strong. It was about 2 hours before sunset and 2 hours after their sunrise. These openings are common if unnoticed much of the time. For us it's the most reliable path to VK.
In the evening there was long path over the south pole on 40 meters to Japan and elsewhere in the Far East. I missed most of it since I was busy on 20 working many of them short path.
QRP makes the grade
Harking back to the several years I operated only QRP, including successes in the major contests, I was pleased to log many QRP stations from around the world. In the ARRL DX contests the DX sends their power in the exchange making it easy to identify who was running QRP.
While running Japan on 20 meters I was called by 2 or 3 running 5 watts. They were easily copied. That's how good the openings were. On 40 meters there were QRP stations calling me from across Europe and well into Russia, all in the afternoon sunshine. Some were weak but most were copied well. I had to use the Beverage to pull some of them out of the noise. Many more QRPers were worked on 20 meters and a handful make the grade on 80 meters.
Dancing with the MUF
This far north the first direction to open when the MUF rises is usually south. Although there are not a lot of stations to work down there the multipliers are worth gold. You have to monitor since the openings can be quite brief and limited in scope. When 15 meters opens south you need to work the closer DX in the Caribbean fast since they fade out early.
As the band goes long South America comes through. There was a short opening on 10 meters which I unfortunately almost entirely missed. I used the sub-receiver to check on 10 periodically while running Europe on 20 meters. This required switching to my south tri-bander and listening with the split headphone audio of the FTdx5000 (similar to SO2R).
I called many as the band was fading but only logged Brazil. Most others did better, especially the multi-ops, those in assisted categories and SO2R operators who could monitor more effectively than I. Few hams notice those southern openings during daily operating yet they're there.
On 15 meters the openings on the northerly paths were marginal, just in the vicinity of the MUF. Many European countries could be worked if you paid attention. As patches of ionization grew and faded one station would get loud and another would dip into the noise. A few minutes later the situation would reverse. These conditions are there most days but go unremarked since most hams believe the band is dead.
Low band pile ups
The long winter nights, low atmospheric noise and a dearth of sunspots justifiably drives a great deal of low band activity. A little propagation enhancement during a major contest can turn a graveyard shift into the highlight of the weekend.
The 160 meter band is the least adversely affected by few sunspots and geomagnetic activity. It marches to a different beat and can offer surprises any night. Saturday was one such night. For hours signals from Europe were strong and numerous. With a kilowatt and full size antenna I was able to run DX stations for several hours. My low noise receive antenna helped me to hear the weakest callers.
Working over 60 countries on 160 meters over the course of a weekend is an excellent result. I did far better on 80 meters, adding the Far East, Africa and Pacific stations to my log. Smaller stations were also having good luck. While openings to many rarer locales were not on tap from my area stations further south and west had successes.
On both 160 and 80 meters signals continued to pour in from Europe well after their sunrise. Propagation this good doesn't happen all the time but more than many realize. But if you're not listening or calling CQ you won't know.
For the DXer
Big guns routinely exceeded DXCC on 20 and 40 meters. In a few cases they did so on 80 meters as well. While this is difficult for the contester with a modest station you can get close. Indeed many small stations and non-contesters in the assisted categories harnessed the skimmers and spotting networks in their DX pursuit.
One of the benefits of big antennas and power is that very often the DX finds you. Run on a band with good propagation and you will be amazed by who calls you. This weekend I was called by NH2, TZ, EX and other uncommon if not very rare stations. Last fall in CQ WW SSB I was called by VP6D (Ducie I.) on 80 meters, and I was running just 100 watts.
On 160 and 80 meters I was pleasantly surprised by calls from the Middle East, Asia and South America, and occasionally from the Caribbean and Central American contest operations that I was hunting for multipliers. If you have a decent station call CQ even if you are not a contester and you will sate your DX appetite. My best country count during the contest was 95 on 20 meters.
Get active
Solar cycle 25 is coming and there will be sunspots. As to how good,
mediocre or bad it'll be will remain the subject of conversation for
some time. Instead let's focus on the now. Yes, there is propagation,
often excellent propagation and not just the low bands. Good low band
propagation is dismissed by many hams since it is difficult to put up an
effective DX antenna on 80 meters and 160 meters. High band openings
are shorter and more fleeting than at other points during the solar
cycle, and can pass in the blink of an eye on 15 meters and above.
I
became a ham in the early 1970s when we were fast approaching a solar
minimum. Older hams mused on the great propagation of the cycles that were. Those stories meant little to me and my friends. I was thrilled to be
working any DX at all, and I was. The pickings were paltry but I didn't know that. I did dream a little about the peak conditions the old timers talked about.
My
first contest experiences were in the major domestic events such as
Sweepstakes where we were not so dependent on sunspots. Working several hundred contacts in DX contests was
thrilling. Then the sunspot number began its long climb to 1979 and the
propagation became eye popping. As chance would have it that was the time
when I was busy finishing my post-graduate degree and moving from the
propagation black hole of VE4 to VE3. I had no station at all for 5 years.
It
is a good idea to appreciate what nature throws our way. HF propagation
is quite good during the minimum though not spectacular. One has only to get on the
air in the right place and the right time and the DX is there. Many open
bands lie fallow because pessimism keeps hams away from the shack. Don't be one of those hams.
Tuesday, February 25, 2020
Wednesday, February 19, 2020
Methodology for Adjusting HF Yagi Gamma Matches
There are many feed systems used in yagis over the years. Gamma matches are not as common as they once were. More typical are beta matches and T matches to convert the low impedance of a yagi to 50 Ω. Moxon (rectangle) yagis and OWA (coupled resonator) designs are becoming more popular for their direct match to 50 Ω coax.
For my home brew stacked 20 meter and 15 meter yagis I originally planned for beta matches. I bought the fibreglass tubes for the split driven element. Then I changed my mind and switched to gamma matches. Although in disfavour for their unbalanced and asymmetric topology the impact on yagi performance is negligible.
For me the deciding factor was a more economical and robust driven element. That is not to say the beta match is deficient however the construction for the home brewer is perhaps more challenging. I am a little less convinced now that I've built, tuned and installed the lower yagis of the stacks.
Gamma matches are finicky creatures. For commercial yagis the challenge for the ham is modest since all the design work has been done. All you need to do is set it up according to the manual and tweak it as necessary to achieve the desired SWR curve. When you design and build from scratch the challenge is of a higher order.
Which matching system is best?
Before delving deeper into gamma matches let's compare it to alternatives. There are both objective and subjective aspects depending on materials, preferred construction method and other factors unique to every builder. This is not about commercial products where the design and construction are outside of your control other than choosing which antenna to purchase.
Just about every common matching network is equivalent -- gamma, beta, T, transmission line transformer, etc. -- in that they are essentially L-networks. There is a C and an L, one parallel (shunt) and one series, sometimes split in half for a balanced feed. The gamma is a little more complex due to the step-up function of the gamma rod and an L in series with the a couple of C (capacitor and shortened element). You could in fact feed the yagi with an L-network although that is rarely done with rotatable yagis.
With respect to achieving the best SWR curve they are essentially equivalent. Any simple matching network is constrained by the range of R and X across the band of an inherently high Q antenna like a yagi. When I first modelled the 5-element 15 meter yagis I built I utilized a beta match for its simplicity and compatibility with EZNEC's stepped diameter correction (SDC) algorithm. Compare that SWR curve with the one measured on the completed yagi with its gamma match and you will see they are the same.
Construction of any of the matching systems is straight-forward. However all of them require unique mechanisms:
What could possibly go wrong? Theory vs. practice
I am not an expert on the theory of gamma matches. I will leave that to others. There are ample discussions to be found in various places and software tools to design gamma matches. Some of them might even be correct. But as I said above, gamma matches are peculiar creatures.
Perhaps the clearest description of the gamma match that I've found is by W8II. A survey of theory and design algorithms was done by W4RNL (SK). Going back further there is a good description of the gamma match by W3PG in a 1969 QST article. Predictable gamma match designs are stymied by sensitivities of aspects of their construction:
With an approximate impedance from the EZNEC model of the 5-element 15 meter yagi I used the software to design the gamma match. Unfortunately it didn't come close when I put it to the test in the field. Trying the software again, after achieving a match in the field, with a range of deviations from the original impedance still did not get close to the experimentally found dimensions. Keep that in mind when you use gamma match design software. The sensitivities mentioned above are significant.
Choosing a design
I bought 1/16" thick mild aluminum alloy 1" wide to make the straps. I formed the strap by manually folding the strap around tubes of the same diameter. The ends were then bent and drilled for stainless fasteners. It took a little experimentation to get the folds and bends in the proper positions to have correct element to rod spacing and good grip to the tubes.
Alternatively you can use a stainless u-bolt and a straight strap. For best long term contact between the strap and tubes a fold at the top and bottom of the strap protects against the weather and increases the contact surface.
A short section of PVC pipe is used for the inner spacer. The ½" gamma rod pierces the pipe. An aluminum strap or a set of UV-resistant cable ties secures the pipe to the element.
There is a coax connector on an aluminum L hanging from the element-to-boom clamp (closeup here). It is sealed on the inside to prevent moisture penetration through it and into the coax. RG213 with the outer jacket and braid removed is the inner "plate" of the gamma capacitor. Measured capacitance is a little over 2 pf per inch (25 to 27 pf per foot) for a ½" OD gamma rod.
There are many component parts to fatigue or fall prey to the weather so you want it to be robust. After matching I took the following steps:
Gamma match model
With some difficulty it is possible to model a gamma match using NEC2. I did so using EZNEC and had some success. There are a few rules to follow:
When the -2.0 db of the average gain (shown when calculating a 3D plot) is added back the gain and other pattern measurements were essentially identical to the original yagi model. Pattern symmetry is maintained (as shown by Cebik) despite the asymmetric construction of the driven element. I'll come back to this shortly.
Below is a closeup of the model and compared to the real gamma match. The letter labels were chosen to be self-explanatory.
Notice the green dots (hard to see) that show the segment edges. The blue squares are wire connections. The red circle is the source and the red square is the capacitor. Let's list the active components of the gamma match that affect the tuning:
With the model thus constructed I was able to explore how various adjustments affect the match and compare it to physical measurements. There was close agreement, telling me that the model is effective. The objective is to develop a deterministic and rapid process to reach a match. Without this insight my first attempts at adjusting real antennas became a confusing and lengthy chore.
One aspect is the myth that the gamma match causes an asymmetry in the yagi's pattern. You can see in the plot above that this is not so, or more accurately that the amount of asymmetry is negligible. You'll end up with far more asymmetry due to interactions with guys and other antennas. The model provides a clue.
Driven element centre is at the source (wire #14). The full half element to the right is #12 and the portion of the half element to the left, beyond the gamma match, is #11. The current in #10 is ~2.2 A with the source I chose. The current in the gamma rod #16 is ~1.0 A and 170° out of phase. This would be almost complete cancellation if the currents were equal (as we'd expect in a transmission line section) so we are left with ~ 1.2 A.
The current of #12 at element centre is ~1.25 A, almost identical to the net current and in phase with #10 (10° difference). That argues for excellent symmetry especially when viewed from the perspective of mutual impedance with the parasitic elements. Indeed, the EZNEC calculated currents in all the parasitic elements are symmetric to at least 3 decimals which is symmetric considering the numerical precision we can expect from the calculating engine.
Current in #17, the dangling end of the gamma rod, is very close to 0 so it really doesn't do much of anything. That would appear to confirm the conclusion of Cebik and others, therefore we can ignore its length during adjustment of the match. If it were an effective open stub its current should be a sizable fraction of that in #11.
The small mismatches of current and phase mentioned may be artifacts of the model since NEC2 is stressed by tightly coupled wires. That is, the actual situation may be even better but without NEC4 and deeper study I don't really know. For now I'll accept these figures since they appear to correspond well to the real world
Of course you must use a common mode choke to ensure the best pattern integrity. It is possible to model the outer coax sheath as long wires connected to the element centre, and I have done this in the past. But with the boom also connected and the unpredictable influence of the coax running along the tower and adjacent to other cables a model is impractical. Install a choke and forget about it, whether the ultimate or one that's good enough.
With the benefit of what I learned the first time I attempted gamma match tuning and with the this model the adjustment of the permanent gamma matches proceeded quickly. That's the subject of the next section. A few more insights from the model will be sprinkled in the following discussion as appropriate. I plan to play with model a little longer to see what more it can teach me.
Getting a match: to be methodical you need a method
On the first tuning of the gamma matches I mounted a 150 pf variable capacitor to the gamma rod. There were only two adjustments: strap position and capacitor value. Potential confounding factors mentioned above were thus eliminated. Even so I quickly ran into trouble. It was my own fault since I trusted the gamma match software and believed I could adjust the gamma match to perfection with a little bit of tweaking. I was very wrong.
The first problem was the coax. Some is needed since my system for lifting the antenna on the tram line to get it high enough for measurement requires coax down to the ground and the antenna analyzer. The chart below is from one of my first tuning attempts. All that I'm doing is moving the shorting strap in one inch increments with the capacitor value fixed. Zo is the impedance at the feed point and Zm is the impedance measured by the analyzer at the end of the coax.
It is commonly recommended that you use a length of coax that is an exact electrical multiple of λ/2. For example, LMR400 with a VF (velocity factor) of 0.85 should be 19.8' (6.04 m) long or a multiple thereof. If you do this the impedance at the far end is the same as at the antenna. Otherwise you get something else, which will confuse you if you aren't aware of the issue.
There are infinite combinations of R and X value for every SWR greater than 1. Indeed, if you try to tune the gamma match by only measuring SWR you could be there for a very long time! I tried trial-and-error at first because I mistakenly thought I could adjust the match with a few tweaks from the initial software calculated values.
With the 29' length of LMR400 the Rm and Xm values seemed utterly random. Tweaking only resulted in more nonsense measurements. Rm and Xm bear no resemblance to Ro and Xo even though the SWRs are identical. I used TLW to convert Zm to Zo. Only then could I begin to methodically adjust the gamma match.
This is an example of an impedance "rotation" from one of my test values. Since the length of coax is close to an odd multiple of /4 this is a worst case situation. The R value is approximately the reciprocal of the measured value with respect to 50 Ω while X "flips" sign with a smaller or larger magnitude. The Zm = 27 - j31 Ω value is actually quite good since we have only to cancel the series reactance (Xo).
Look again at the chart. Notice that Ro increases as the shorting strap moves outward. As discussed earlier, the gamma match and its cousins are essentially odd looking L-networks. Change C or L and both R and X change. However, the rate of change is not the same. I talked about the utility of this behaviour for the L-network in my 80 meter vertical yagi. We can put the same principle to use here.
The basic gamma match tuning method I use is as follows:
When I used software to determine the gamma match dimensions the position of the shorting strap was quite different on the real yagi. It had to be moved outward by about 1' (30 cm). For my 15 meter yagi this meant the strap had to reformed to fit the ¾" tube instead of the 1" tube.
There is another way to change the R value without moving the strap: change the resonant frequency of the driven element. Sliding the element tips in and out changes the series capacitance of the element and therefore Zo. I used the very same technique on the 80 meter yagi to adjust the L-network with a relay to accommodate the different impedances of its yagi and omni-directional modes. The only difference here is that the capacitor is in series (like in a beta match) and not a shunt.
To avoid having to make the gamma rod longer I changed the length of the driven element to increase Ro. You can deduce the general procedure from the following 3 measurements I made while leaving the strap position and gamma capacitor unchanged:
Now we need to talk about the gamma capacitor. Its adjustment can be straight-forward or confusing depending on your approach. Sliding the gamma rod to adjust the capacitor affects the "stray" lengths at both ends of the transmission line formed by the element and gamma rod. Refer to the annotated model shown above.
I would like to tell you that I now understand how sliding the gamma rod is affected by those end effects but I cannot. At first I thought I had found something when, very close to a match, sliding the rod had the opposite effect on the capacitive reactance. That was interesting but wrong. It turns out that the length of LMR400 was really 30', not the 29' I first measured. However, this is almost exactly λ/2 on 20 meters so tuning the 20 meter yagi was quicker: no need to convert Zm to Zo.
When I fed the measurements into TLW the effect disappeared. My conclusion, in agreement with the experts and my model, is not to worry about it since the effect either doesn't exist or is too small to matter. I must also caution that the accuracy of the antenna analyzer is paramount otherwise you will to your dismay discover you are dealing with a fictional reactances. My weapon of choice is the RigExpert AA54 which I find to be very good for my antenna projects. There are other equally good products and there are many that are far worse.
We now need to discuss what the capacitor does and how to adjust it. Unlike inductance an increase in capacitor value reduces the reactance (for a constant frequency). Recall that the formula for capacitive reactance is:
Adjusting the capacitor to exactly cancel the residual inductive reactance can be difficult since a small change has a large effect. Let's say we have Z = 48 + j25 Ω when the capacitor value is 40 pf -- this is approximately the case for my 15 meter yagi. We also know (as discussed earlier) that the capacitor I am using changes at a rate of ~2.1 pf / inch (~0.85 pf / cm).
By how much should we change the 40 pf capacitance to cancel the 25 Ω of inductive reactance? This is easy to explore with spreadsheet software, and I wrote one years ago for antenna projects. For C = 40 pf at 21.1 MHz we have X = 189 Ω. To increase X to 214 Ω [25 - (214 - 189)] we need to decrease C. If we shorten the gamma capacitor by 1" so that C becomes 38 pf the revised X is 199 Ω. That's a 10 Ω increase for a 2 pf (1") decrease.
You can estimate the correct answer with a linear extrapolation to 35 pf and that is indeed just about right. I did the same in the EZNEC model I developed and it agreed exactly. The antenna itself behaved just as the calculation and model predict. With that I was done.
One final note about the capacitor. It is a good idea to make the stripped length of RG213 longer than you expect. If you discover you need more capacitance you will have to make a longer one; save the short one in case you later build a higher frequency yagi.
On the other hand if it is too long the gamma rod will have to be overextended and that can cause mechanical instability. What I do is cut short lengths from the RG213 and keep the gamma rod where it is. If you do this don't get overconfident since you may have to increase the capacitance again in the tuning process. Cut off less than you need to until you are very close to a final match.
Parting words
That is my full gamma match tuning procedure. Trust me, speaking from experience, that the task goes far quicker by being methodical. You may get lucky with trial-and-error but that is a low probability outcome. If you prefer a different method go for it, but do use one.
I now have two properly working large HF yagis and gamma matches, and a methodology to attack the many more yagis I plan to build. Considering how many yagis with gamma matches I've used over the decades it's a little surprising that I learned so little about them. Of course those were almost all commercial yagis with recommended presets from their field tests and only required small tweaks to get them tuned up.
Had I run into serious difficulty I would have reverted to the original plan to use beta matches. Those are far easier to model and are more predictable. However they have their own challenges as I summarized at the start of this article. I am pleased with my choice of the gamma match. For me it was a bonus that I got to learn something new.
Update Feb.21: I have made several minor corrections to units and quantities that slipped in. I don't usually edit to correct typos and grammar but I do want to keep the technical matter accurate.
For my home brew stacked 20 meter and 15 meter yagis I originally planned for beta matches. I bought the fibreglass tubes for the split driven element. Then I changed my mind and switched to gamma matches. Although in disfavour for their unbalanced and asymmetric topology the impact on yagi performance is negligible.
For me the deciding factor was a more economical and robust driven element. That is not to say the beta match is deficient however the construction for the home brewer is perhaps more challenging. I am a little less convinced now that I've built, tuned and installed the lower yagis of the stacks.
Gamma matches are finicky creatures. For commercial yagis the challenge for the ham is modest since all the design work has been done. All you need to do is set it up according to the manual and tweak it as necessary to achieve the desired SWR curve. When you design and build from scratch the challenge is of a higher order.
Which matching system is best?
Before delving deeper into gamma matches let's compare it to alternatives. There are both objective and subjective aspects depending on materials, preferred construction method and other factors unique to every builder. This is not about commercial products where the design and construction are outside of your control other than choosing which antenna to purchase.
Just about every common matching network is equivalent -- gamma, beta, T, transmission line transformer, etc. -- in that they are essentially L-networks. There is a C and an L, one parallel (shunt) and one series, sometimes split in half for a balanced feed. The gamma is a little more complex due to the step-up function of the gamma rod and an L in series with the a couple of C (capacitor and shortened element). You could in fact feed the yagi with an L-network although that is rarely done with rotatable yagis.
With respect to achieving the best SWR curve they are essentially equivalent. Any simple matching network is constrained by the range of R and X across the band of an inherently high Q antenna like a yagi. When I first modelled the 5-element 15 meter yagis I built I utilized a beta match for its simplicity and compatibility with EZNEC's stepped diameter correction (SDC) algorithm. Compare that SWR curve with the one measured on the completed yagi with its gamma match and you will see they are the same.
Construction of any of the matching systems is straight-forward. However all of them require unique mechanisms:
- Beta: Series C comes from a driven element shorter than its resonant length and which is adjustable. The driven element must be split and made mechanically robust. Parallel L is with a shorted stub or a coil, with the former easier to adjust with a custom made slider, and is more robust.
- Gamma: Series C should be a variable capacitor, either a component or (as shown and popular) insulated wire inside the gamma rod. The amount of capacitive reactance in the driven element is about the same as for a beta match. The strap and insulated support must be fabricated. After adjustment the capacitor must be robust enough to withstand weather and maintain a stable value. The driven element is continuous and easy to make robust using "plumber's delight" construction.
- T: Similar to a gamma with symmetric halves to provide a balanced feed. It requires a step-down transformer.
- OWA: The driven element is split for a dipole feed, just like the beta match. Adjustment is by varying the lengths of and spacing between the driven element and coupled resonator. NEC2 models can get you close but inaccuracy is unavoidable so field adjustment is required and can take some time. The coupled resonator adds wind load (and cost!) but no gain or pattern improvement; it is not a director.
- Transmission line transformer: Split driven element. Reactance set to zero by adjusting driven element length. Coax of the required value can be difficult to find and so is usually made from parallel runs of 75 Ω coax to boost 25 Ω (typical for a mono-band yagi) to 50 Ω.
- Broadband transformer: Residual reactance must be zero and achieved by adjust the driven element length. A 2:1 ratio is often a good choice since the impedance of a typical full size mono-band yagi is near 25 Ω. If the yagi has a high Q the transformer ferrite core is at risk of overheating at frequencies where the SWR is high.
What could possibly go wrong? Theory vs. practice
I am not an expert on the theory of gamma matches. I will leave that to others. There are ample discussions to be found in various places and software tools to design gamma matches. Some of them might even be correct. But as I said above, gamma matches are peculiar creatures.
Perhaps the clearest description of the gamma match that I've found is by W8II. A survey of theory and design algorithms was done by W4RNL (SK). Going back further there is a good description of the gamma match by W3PG in a 1969 QST article. Predictable gamma match designs are stymied by sensitivities of aspects of their construction:
- Ratio of element to gamma rod diameters: The resulting step up ratio is sensitive to the diameter and spacing, as is the inductance of the shorted stub (L) they comprise.
- Feed impedance: Since it is difficult to measure the impedance of a continuous driven element we usually have to estimate it.
- Gamma capacitor: A long wire inside the gamma rod can display transmission line behaviour since it is a non-negligible fraction of a wavelength. W8JI discusses possible consequences.
With an approximate impedance from the EZNEC model of the 5-element 15 meter yagi I used the software to design the gamma match. Unfortunately it didn't come close when I put it to the test in the field. Trying the software again, after achieving a match in the field, with a range of deviations from the original impedance still did not get close to the experimentally found dimensions. Keep that in mind when you use gamma match design software. The sensitivities mentioned above are significant.
Choosing a design
I bought 1/16" thick mild aluminum alloy 1" wide to make the straps. I formed the strap by manually folding the strap around tubes of the same diameter. The ends were then bent and drilled for stainless fasteners. It took a little experimentation to get the folds and bends in the proper positions to have correct element to rod spacing and good grip to the tubes.
Alternatively you can use a stainless u-bolt and a straight strap. For best long term contact between the strap and tubes a fold at the top and bottom of the strap protects against the weather and increases the contact surface.
A short section of PVC pipe is used for the inner spacer. The ½" gamma rod pierces the pipe. An aluminum strap or a set of UV-resistant cable ties secures the pipe to the element.
There is a coax connector on an aluminum L hanging from the element-to-boom clamp (closeup here). It is sealed on the inside to prevent moisture penetration through it and into the coax. RG213 with the outer jacket and braid removed is the inner "plate" of the gamma capacitor. Measured capacitance is a little over 2 pf per inch (25 to 27 pf per foot) for a ½" OD gamma rod.
There are many component parts to fatigue or fall prey to the weather so you want it to be robust. After matching I took the following steps:
- Sealed the end of the RG213 capacitor to protect against water and high voltage. Arcing probability increases as environmental contaminants accumulate.
- Sealed and taped the inner end of the capacitor to protect against water and capacitance changes due to movement.
- Stranded wire from the coax connector to the RG213 was replaced with a solid wire. Although there is more risk of fatigue the stranded wire developed kinks from repeated stress.
Gamma match model
With some difficulty it is possible to model a gamma match using NEC2. I did so using EZNEC and had some success. There are a few rules to follow:
- Close spaced wires (element and gamma rod) must have their segments line up exactly. This must be redone as the lengths and connection positions are adjusted.
- The short wires for the source and straps should have one segment.
- Stepped diameter correction does not work with the gamma match present. I replaced it with the equivalent diameter wire (calculated by SDC without the gamma match present) beyond the shorting strap. The inner section of the driven element must be true diameter since it affects the step-up ratio.
- The gamma capacitor is a load placed at the inner end of the gamma rod. This simulates the effect of sliding the gamma rod outward but without compensation for transmission line effect of the length of RG213. The exposed section of RG213 is modelled to the actual diameter of the wire and polyethylene insulation.
When the -2.0 db of the average gain (shown when calculating a 3D plot) is added back the gain and other pattern measurements were essentially identical to the original yagi model. Pattern symmetry is maintained (as shown by Cebik) despite the asymmetric construction of the driven element. I'll come back to this shortly.
Below is a closeup of the model and compared to the real gamma match. The letter labels were chosen to be self-explanatory.
Notice the green dots (hard to see) that show the segment edges. The blue squares are wire connections. The red circle is the source and the red square is the capacitor. Let's list the active components of the gamma match that affect the tuning:
- Length of the shorted stub L is determined by E, R and S.
- There is a second and inner section the same transmission line between the source and the capacitor C.
- There is an open transmission line stub outboard of the strap between the end of the gamma rod and the element. It is designated as a small capacitance 'c'.
- Moving the strap affects L and c. Moving the rod affects C, c and, by a small amount, L.
With the model thus constructed I was able to explore how various adjustments affect the match and compare it to physical measurements. There was close agreement, telling me that the model is effective. The objective is to develop a deterministic and rapid process to reach a match. Without this insight my first attempts at adjusting real antennas became a confusing and lengthy chore.
One aspect is the myth that the gamma match causes an asymmetry in the yagi's pattern. You can see in the plot above that this is not so, or more accurately that the amount of asymmetry is negligible. You'll end up with far more asymmetry due to interactions with guys and other antennas. The model provides a clue.
Driven element centre is at the source (wire #14). The full half element to the right is #12 and the portion of the half element to the left, beyond the gamma match, is #11. The current in #10 is ~2.2 A with the source I chose. The current in the gamma rod #16 is ~1.0 A and 170° out of phase. This would be almost complete cancellation if the currents were equal (as we'd expect in a transmission line section) so we are left with ~ 1.2 A.
The current of #12 at element centre is ~1.25 A, almost identical to the net current and in phase with #10 (10° difference). That argues for excellent symmetry especially when viewed from the perspective of mutual impedance with the parasitic elements. Indeed, the EZNEC calculated currents in all the parasitic elements are symmetric to at least 3 decimals which is symmetric considering the numerical precision we can expect from the calculating engine.
Current in #17, the dangling end of the gamma rod, is very close to 0 so it really doesn't do much of anything. That would appear to confirm the conclusion of Cebik and others, therefore we can ignore its length during adjustment of the match. If it were an effective open stub its current should be a sizable fraction of that in #11.
The small mismatches of current and phase mentioned may be artifacts of the model since NEC2 is stressed by tightly coupled wires. That is, the actual situation may be even better but without NEC4 and deeper study I don't really know. For now I'll accept these figures since they appear to correspond well to the real world
Of course you must use a common mode choke to ensure the best pattern integrity. It is possible to model the outer coax sheath as long wires connected to the element centre, and I have done this in the past. But with the boom also connected and the unpredictable influence of the coax running along the tower and adjacent to other cables a model is impractical. Install a choke and forget about it, whether the ultimate or one that's good enough.
With the benefit of what I learned the first time I attempted gamma match tuning and with the this model the adjustment of the permanent gamma matches proceeded quickly. That's the subject of the next section. A few more insights from the model will be sprinkled in the following discussion as appropriate. I plan to play with model a little longer to see what more it can teach me.
Getting a match: to be methodical you need a method
On the first tuning of the gamma matches I mounted a 150 pf variable capacitor to the gamma rod. There were only two adjustments: strap position and capacitor value. Potential confounding factors mentioned above were thus eliminated. Even so I quickly ran into trouble. It was my own fault since I trusted the gamma match software and believed I could adjust the gamma match to perfection with a little bit of tweaking. I was very wrong.
The first problem was the coax. Some is needed since my system for lifting the antenna on the tram line to get it high enough for measurement requires coax down to the ground and the antenna analyzer. The chart below is from one of my first tuning attempts. All that I'm doing is moving the shorting strap in one inch increments with the capacitor value fixed. Zo is the impedance at the feed point and Zm is the impedance measured by the analyzer at the end of the coax.
It is commonly recommended that you use a length of coax that is an exact electrical multiple of λ/2. For example, LMR400 with a VF (velocity factor) of 0.85 should be 19.8' (6.04 m) long or a multiple thereof. If you do this the impedance at the far end is the same as at the antenna. Otherwise you get something else, which will confuse you if you aren't aware of the issue.
There are infinite combinations of R and X value for every SWR greater than 1. Indeed, if you try to tune the gamma match by only measuring SWR you could be there for a very long time! I tried trial-and-error at first because I mistakenly thought I could adjust the match with a few tweaks from the initial software calculated values.
With the 29' length of LMR400 the Rm and Xm values seemed utterly random. Tweaking only resulted in more nonsense measurements. Rm and Xm bear no resemblance to Ro and Xo even though the SWRs are identical. I used TLW to convert Zm to Zo. Only then could I begin to methodically adjust the gamma match.
This is an example of an impedance "rotation" from one of my test values. Since the length of coax is close to an odd multiple of /4 this is a worst case situation. The R value is approximately the reciprocal of the measured value with respect to 50 Ω while X "flips" sign with a smaller or larger magnitude. The Zm = 27 - j31 Ω value is actually quite good since we have only to cancel the series reactance (Xo).
Look again at the chart. Notice that Ro increases as the shorting strap moves outward. As discussed earlier, the gamma match and its cousins are essentially odd looking L-networks. Change C or L and both R and X change. However, the rate of change is not the same. I talked about the utility of this behaviour for the L-network in my 80 meter vertical yagi. We can put the same principle to use here.
The basic gamma match tuning method I use is as follows:
- Move the strap until Ro is approximately 50 Ω.
- Adjust the capacitor to bring Xo near 0 Ω.
- Since each adjustment for Ro and Xo affects the other repeat steps 1 and 2. A few iterations will bring you to 50 + j0 Ω.
When I used software to determine the gamma match dimensions the position of the shorting strap was quite different on the real yagi. It had to be moved outward by about 1' (30 cm). For my 15 meter yagi this meant the strap had to reformed to fit the ¾" tube instead of the 1" tube.
There is another way to change the R value without moving the strap: change the resonant frequency of the driven element. Sliding the element tips in and out changes the series capacitance of the element and therefore Zo. I used the very same technique on the 80 meter yagi to adjust the L-network with a relay to accommodate the different impedances of its yagi and omni-directional modes. The only difference here is that the capacitor is in series (like in a beta match) and not a shunt.
To avoid having to make the gamma rod longer I changed the length of the driven element to increase Ro. You can deduce the general procedure from the following 3 measurements I made while leaving the strap position and gamma capacitor unchanged:
- -1": 36 + j13.5 Ω
- +0": 47.3 + j17.5 Ω
- +½": 51 + j24.5 Ω
Now we need to talk about the gamma capacitor. Its adjustment can be straight-forward or confusing depending on your approach. Sliding the gamma rod to adjust the capacitor affects the "stray" lengths at both ends of the transmission line formed by the element and gamma rod. Refer to the annotated model shown above.
I would like to tell you that I now understand how sliding the gamma rod is affected by those end effects but I cannot. At first I thought I had found something when, very close to a match, sliding the rod had the opposite effect on the capacitive reactance. That was interesting but wrong. It turns out that the length of LMR400 was really 30', not the 29' I first measured. However, this is almost exactly λ/2 on 20 meters so tuning the 20 meter yagi was quicker: no need to convert Zm to Zo.
When I fed the measurements into TLW the effect disappeared. My conclusion, in agreement with the experts and my model, is not to worry about it since the effect either doesn't exist or is too small to matter. I must also caution that the accuracy of the antenna analyzer is paramount otherwise you will to your dismay discover you are dealing with a fictional reactances. My weapon of choice is the RigExpert AA54 which I find to be very good for my antenna projects. There are other equally good products and there are many that are far worse.
We now need to discuss what the capacitor does and how to adjust it. Unlike inductance an increase in capacitor value reduces the reactance (for a constant frequency). Recall that the formula for capacitive reactance is:
X = 1 / (f × π × C)For f in MHz and C in pF change the numerator to 1,000,000.
Adjusting the capacitor to exactly cancel the residual inductive reactance can be difficult since a small change has a large effect. Let's say we have Z = 48 + j25 Ω when the capacitor value is 40 pf -- this is approximately the case for my 15 meter yagi. We also know (as discussed earlier) that the capacitor I am using changes at a rate of ~2.1 pf / inch (~0.85 pf / cm).
By how much should we change the 40 pf capacitance to cancel the 25 Ω of inductive reactance? This is easy to explore with spreadsheet software, and I wrote one years ago for antenna projects. For C = 40 pf at 21.1 MHz we have X = 189 Ω. To increase X to 214 Ω [25 - (214 - 189)] we need to decrease C. If we shorten the gamma capacitor by 1" so that C becomes 38 pf the revised X is 199 Ω. That's a 10 Ω increase for a 2 pf (1") decrease.
You can estimate the correct answer with a linear extrapolation to 35 pf and that is indeed just about right. I did the same in the EZNEC model I developed and it agreed exactly. The antenna itself behaved just as the calculation and model predict. With that I was done.One final note about the capacitor. It is a good idea to make the stripped length of RG213 longer than you expect. If you discover you need more capacitance you will have to make a longer one; save the short one in case you later build a higher frequency yagi.
On the other hand if it is too long the gamma rod will have to be overextended and that can cause mechanical instability. What I do is cut short lengths from the RG213 and keep the gamma rod where it is. If you do this don't get overconfident since you may have to increase the capacitance again in the tuning process. Cut off less than you need to until you are very close to a final match.
Parting words
That is my full gamma match tuning procedure. Trust me, speaking from experience, that the task goes far quicker by being methodical. You may get lucky with trial-and-error but that is a low probability outcome. If you prefer a different method go for it, but do use one.
I now have two properly working large HF yagis and gamma matches, and a methodology to attack the many more yagis I plan to build. Considering how many yagis with gamma matches I've used over the decades it's a little surprising that I learned so little about them. Of course those were almost all commercial yagis with recommended presets from their field tests and only required small tweaks to get them tuned up.
Had I run into serious difficulty I would have reverted to the original plan to use beta matches. Those are far easier to model and are more predictable. However they have their own challenges as I summarized at the start of this article. I am pleased with my choice of the gamma match. For me it was a bonus that I got to learn something new.
Update Feb.21: I have made several minor corrections to units and quantities that slipped in. I don't usually edit to correct typos and grammar but I do want to keep the technical matter accurate.
Friday, February 7, 2020
Magic Reel
Since I began planning this station, long before I went property hunting, I opportunistically purchased large items I'd need. One of these items was low loss transmission line. By the time I moved in late 2016 there was 2000' (600 m) of Andrew Heliax in my hoard. Since then I have continued to buy and scavenge Heliax so that I have far more, deployed or in storage.
I have never bought Heliax for anywhere near full price. It's all NOS (new old stock) or used. The used stuff varies from gently used to very nearly dropped from towers.
The last case is an amusing example. A friend in the business did me a favour by having his crew save what they could of a long Heliax run on the tower they were decommissioning. They had to be quick to avoid annoying their client. After cutting out the kinked sections I had 150' of perfectly good LDF5.
The reel of LDF5-50A on the right is NOS. I don't know the original capacity of the reel but it would at least 1200'. When we agreed on a price per foot the seller and I had to decide how many feet were on the reel. This is not as easy as you might imagine.
With both of us working with a tape measure and calculator we used our fingers to probe down the layers to estimate the number of turns. The task is near impossible without unwinding the reel, and you can't do that without great effort and risk of damage.
Reasonable people will be reasonable. Other times the calculation will be more contentious: the seller wants a higher number and the buyer wants a lower number. We were both reasonable people and after our best effort we agreed on 400' and shook hands.
When the picture was taken I had already taken two runs from it: 125' and 250'. Do the arithmetic and glance again at the picture and you'll notice that something must be amiss. There cannot possibly be that much left after removing 375'.
With the reel now significantly depleted I was able to reach all the remaining layers and confidently estimate that there are 250' to 300' on the reel. Obviously the deal was in my favour. I call it my magic reel for that reason, seeming to grow more Heliax when I'm not looking!
The deal could easily have gone the other way and it has on other occasions. Even an open reel of used Heliax, such as those on the right, can be difficult to measure. The turns are usually tightly taped (as they must be to keep the springy stuff intact) and are of variable diameter. Errors of 10% or 15% are easy to make. The same can be true of smaller cable like LMR400 when it is tightly taped.
One of the people I've bought NOS and used Heliax from uses a simple mental calculation to estimate length. For cable on the reel it's 6' per turn and for used coils it is 12' per turn. This works well for LDF5 size cable since the typical diameter on the reel is 2' and 4' in an open coil.
C = πd and π is approximately 3 so we have 2' × 3 = 6' and 4' × 3 = 12'. For metric length skip the multiplication by π. Hence, 2 meters and 4 meters, respectively, for diameters measured in feet.
For a simple rule it works well enough to usually achieve an accuracy ±15% in my experience. The magic reel is an exceptional case.
If you only ever buy one reel of hard line you may be sensitive to errors. When you buy many it is less concerning since the errors tend to average out. On some deals you win and on others you lose. The key is to be reasonable. Yes, be happy when you eventually discover you've been lucky (as with my reel above) but you should not be angry when you find you bought less than you paid for.
After all, what is the correct price anyway? When you paid $1/ft that is a fraction of the new price. As you unroll the reel later and you find the true price was $0.80 or $1.25 per foot does it really matter? It's still a good deal if the cable is in good condition. What is important is to inspect the cable for kinks and jacket cuts. Those are far more important to the price than an exact length.
Often there is a connector on at least one cable end. That's alone can compensate for any shortfall. Scavenging for deals should be a fun part of the hobby, not a chore or a source of aggravation. Be vigilant but relaxed when money changes hands. It's better to assume the best in people rather than the worst.
 |
| Magic reel of LDF5-50A |
The reel of LDF5-50A on the right is NOS. I don't know the original capacity of the reel but it would at least 1200'. When we agreed on a price per foot the seller and I had to decide how many feet were on the reel. This is not as easy as you might imagine.
With both of us working with a tape measure and calculator we used our fingers to probe down the layers to estimate the number of turns. The task is near impossible without unwinding the reel, and you can't do that without great effort and risk of damage.
Reasonable people will be reasonable. Other times the calculation will be more contentious: the seller wants a higher number and the buyer wants a lower number. We were both reasonable people and after our best effort we agreed on 400' and shook hands.
When the picture was taken I had already taken two runs from it: 125' and 250'. Do the arithmetic and glance again at the picture and you'll notice that something must be amiss. There cannot possibly be that much left after removing 375'.
With the reel now significantly depleted I was able to reach all the remaining layers and confidently estimate that there are 250' to 300' on the reel. Obviously the deal was in my favour. I call it my magic reel for that reason, seeming to grow more Heliax when I'm not looking!
The deal could easily have gone the other way and it has on other occasions. Even an open reel of used Heliax, such as those on the right, can be difficult to measure. The turns are usually tightly taped (as they must be to keep the springy stuff intact) and are of variable diameter. Errors of 10% or 15% are easy to make. The same can be true of smaller cable like LMR400 when it is tightly taped.One of the people I've bought NOS and used Heliax from uses a simple mental calculation to estimate length. For cable on the reel it's 6' per turn and for used coils it is 12' per turn. This works well for LDF5 size cable since the typical diameter on the reel is 2' and 4' in an open coil.
C = πd and π is approximately 3 so we have 2' × 3 = 6' and 4' × 3 = 12'. For metric length skip the multiplication by π. Hence, 2 meters and 4 meters, respectively, for diameters measured in feet.
For a simple rule it works well enough to usually achieve an accuracy ±15% in my experience. The magic reel is an exceptional case.
If you only ever buy one reel of hard line you may be sensitive to errors. When you buy many it is less concerning since the errors tend to average out. On some deals you win and on others you lose. The key is to be reasonable. Yes, be happy when you eventually discover you've been lucky (as with my reel above) but you should not be angry when you find you bought less than you paid for.
After all, what is the correct price anyway? When you paid $1/ft that is a fraction of the new price. As you unroll the reel later and you find the true price was $0.80 or $1.25 per foot does it really matter? It's still a good deal if the cable is in good condition. What is important is to inspect the cable for kinks and jacket cuts. Those are far more important to the price than an exact length.
Often there is a connector on at least one cable end. That's alone can compensate for any shortfall. Scavenging for deals should be a fun part of the hobby, not a chore or a source of aggravation. Be vigilant but relaxed when money changes hands. It's better to assume the best in people rather than the worst.
Tuesday, February 4, 2020
Beverage Maintenance and Repair
For the past few weeks I found myself rationalizing the puzzling behaviour of my 175 meter long northeast Beverage (my only receive antenna at the moment). European stations were often no better copy on the Beverage than on the transmit vertical. My rationalization was the possible presence of skew path, a frequent phenomenon on 160 meters.
When I found myself making the same rationalization almost every night I decided that something must be up. I pulled out my antenna analyzer and swept the SWR from 1 to 5 MHz.
This is very bad. Nominally the SWR should be about 1.5 with a small to moderate undulation. The 1.5 SWR comes from measuring the nominal 75 Ω impedance of the RG6 transmission line and antenna matching transformer with a 50 Ω analyzer. The undulation is mostly due to the termination resistor not being exactly the surge impedance of the antenna.
This SWR is far worse than that and is quite different to what it used to be. Clearly something was amiss. Since it was time for annual maintenance of the antenna I grabbed a ladder, my snowshoes and tree pruning implements and headed into the bush. After uncovering the feed point from the tangle of hawthorn trees that had sprouted around it I checked the connections and all looked good.
The bush clearing took 2 hours of hard work, slogging through the snow and bush and climbing the ladder in snowshoes to cut the limbs that threatened the wire. On the termination box containing the resistor the nuts were not snug but not tight. Everything else looked good.
Returning indoors the SWR showed no improvement. The next day I headed straight to the termination with my tools and opened the box. The adjacent picture is what I discovered. (Note: I spread the halves slightly apart to make the damage more visible.)
Other than the long trudge back and forth through the bush troubleshooting is pretty easy. The only parts are resistor, transformer, long wire, ground rods and coax. It is no surprise I found the trouble so easily. I pocketed the box and trudged back to the shack. In a minute it was disassembled and the halves of the 470 Ω resistor inspected.
There are no char marks to indicate a lightning surge. This is any case highly unlikely during our winter. As W8JI has written that is a common failure mode. He recommends carbon composition resistors for their relatively good tolerance to millisecond surges from near strikes. If not for that any suitably valued resistor other than wire wound works well as 1.8 MHz.
I suspect a pre-existing stress fracture or a couple of years of thermal cycling between -35° C and 35° C did it in. That these resistors are many decades old may have played a role. I have a bunch of them so it is easy to replace.
Rather than a direct replacement I opted for a lower value resistor installed without the box. I used the same technique on my temporary west Beverage last winter. The 390 Ω resistor is in the ground rod clamp with a thin stranded wire and small wire nuts to connect the resistor and antenna wire.
Thin flexible wire is necessary to avoid fatigue failure of the resistor. Alternatively use a box or, as some do, a fuse holder or other fixture to take the mechanical stress.
This is all very temporary. I am considering "twinning" the northeast Beverage this winter to make it reversible between northeast and southwest. I'll use the box to house the termination transformer. There will be no termination resistor, just a ground connection from the transformer secondary winding.
Satisfied with my temporary repair I hauled everything back out of the bush and headed for the shack. I couldn't try it out immediately since it was still daylight.
Sometime between then and sunset I remembered that I ought to recheck the SWR. I pulled out the analyzer and had a look. The improvement should be obvious.
The undulations can be reduced with more fine tuning of the termination resistor. Some imperfection is allowable in the termination resistor without noticably affecting the antenna pattern. I am not going to bother.
After the sun set I got on 160 meters and quickly found a few European stations. This time there was no need to rationalize about skew path. Signal-to-noise (SNR) comparisons between the Beverage and vertical were no contest: the Beverage was by far the superior, just as it used to be.
The break of the resistor made the Beverage unterminated and therefore bidirectional. I did notice stations to the southwest came in pretty well on the Beverage during the CQ 160 contest yet I did not wonder too much about it at the time. I was simply happy that I could work Europe and the US on the Beverage and didn't worry about the fictional propagation to explain what I heard.
I am now clearing bush to run a reversible Beverage for north and south directions as part of my quest for full compass coverage. This will be my first receive antenna of this type so it should prove interesting. More on this when the antenna is complete.
It isn't a rush job so it may not be ready for use until late February or March. Even then it'll need a remote switching system for which I am in the process of gathering parts.
When I found myself making the same rationalization almost every night I decided that something must be up. I pulled out my antenna analyzer and swept the SWR from 1 to 5 MHz.
This is very bad. Nominally the SWR should be about 1.5 with a small to moderate undulation. The 1.5 SWR comes from measuring the nominal 75 Ω impedance of the RG6 transmission line and antenna matching transformer with a 50 Ω analyzer. The undulation is mostly due to the termination resistor not being exactly the surge impedance of the antenna.
The bush clearing took 2 hours of hard work, slogging through the snow and bush and climbing the ladder in snowshoes to cut the limbs that threatened the wire. On the termination box containing the resistor the nuts were not snug but not tight. Everything else looked good.
Returning indoors the SWR showed no improvement. The next day I headed straight to the termination with my tools and opened the box. The adjacent picture is what I discovered. (Note: I spread the halves slightly apart to make the damage more visible.)
Other than the long trudge back and forth through the bush troubleshooting is pretty easy. The only parts are resistor, transformer, long wire, ground rods and coax. It is no surprise I found the trouble so easily. I pocketed the box and trudged back to the shack. In a minute it was disassembled and the halves of the 470 Ω resistor inspected.There are no char marks to indicate a lightning surge. This is any case highly unlikely during our winter. As W8JI has written that is a common failure mode. He recommends carbon composition resistors for their relatively good tolerance to millisecond surges from near strikes. If not for that any suitably valued resistor other than wire wound works well as 1.8 MHz.
I suspect a pre-existing stress fracture or a couple of years of thermal cycling between -35° C and 35° C did it in. That these resistors are many decades old may have played a role. I have a bunch of them so it is easy to replace.
Rather than a direct replacement I opted for a lower value resistor installed without the box. I used the same technique on my temporary west Beverage last winter. The 390 Ω resistor is in the ground rod clamp with a thin stranded wire and small wire nuts to connect the resistor and antenna wire.Thin flexible wire is necessary to avoid fatigue failure of the resistor. Alternatively use a box or, as some do, a fuse holder or other fixture to take the mechanical stress.
This is all very temporary. I am considering "twinning" the northeast Beverage this winter to make it reversible between northeast and southwest. I'll use the box to house the termination transformer. There will be no termination resistor, just a ground connection from the transformer secondary winding.
Satisfied with my temporary repair I hauled everything back out of the bush and headed for the shack. I couldn't try it out immediately since it was still daylight.
Sometime between then and sunset I remembered that I ought to recheck the SWR. I pulled out the analyzer and had a look. The improvement should be obvious.
The undulations can be reduced with more fine tuning of the termination resistor. Some imperfection is allowable in the termination resistor without noticably affecting the antenna pattern. I am not going to bother.
After the sun set I got on 160 meters and quickly found a few European stations. This time there was no need to rationalize about skew path. Signal-to-noise (SNR) comparisons between the Beverage and vertical were no contest: the Beverage was by far the superior, just as it used to be.
The break of the resistor made the Beverage unterminated and therefore bidirectional. I did notice stations to the southwest came in pretty well on the Beverage during the CQ 160 contest yet I did not wonder too much about it at the time. I was simply happy that I could work Europe and the US on the Beverage and didn't worry about the fictional propagation to explain what I heard.
I am now clearing bush to run a reversible Beverage for north and south directions as part of my quest for full compass coverage. This will be my first receive antenna of this type so it should prove interesting. More on this when the antenna is complete.
It isn't a rush job so it may not be ready for use until late February or March. Even then it'll need a remote switching system for which I am in the process of gathering parts.
Friday, January 31, 2020
2020 - Looking Ahead and Looking Back
It's that time of year again: the end of one year and the start of the next. It's my cue to take a look back at the year that was and plan for the coming year. This is a ritual I've followed on this blog for several years. In our extreme climate the weather makes its own statement this time of year. Antenna and tower work stops or slows for several months.
Over the winter my radio activities are mostly confined to indoor activities, whether it be operating, equipment work or planning for the future. There is always a lull during the changeover as priorities reset and a human tendency to "hibernate". Soon enough the energy returns as the sun begins its long climb out of the winter solstice. This is an opportune time to look back and consider the path ahead as I do each year since starting this blog 7 years ago. Even so I have been active outdoors, as you'll discover.
This article is of the type that will interest few readers. I do it for myself, to honestly measure my progress and to plan for the future. What readers may get from it is the discipline of making a plan and being accountable, so unlike new year resolutions which are lightly made and soon forgotten. Anyone can check on how I'm doing by comparing my written plan at the start of the year and my year end review. Although I almost always fall short I still prefer to think big.
If this is not the type of article that is of interest to you, stop reading. For everyone else you are welcome to follow along. What's a little different this year is the combining of the 2019 review and 2020 plan in one article.
If you do continue reading keep in mind my primary interests in ham radio determine the projects I take on. These are contests, DXing and playing with antennas. Even if these are not your cup of tea hopefully there are ideas and thoughts that will be of interest.
I called 2019 the year of the yagi for the major activity in my station building plan. It didn't go as fast as planned. The new 140' (40 meter) tower had to be raised and the details of large HF yagi home brew construction ate up the calendar. Even so I can see the light at the end of the tunnel, it was just a little further away than estimated.
As matters stand 2020 will be the final year of the intensive phase of station building that began with my move to this QTH over 3 years ago. In 2021 I'll most likely be tying off loose ends and focussing on matters in the shack and operating. Things inevitably break so maintenance is always there to be done.
15 and 20 meters
The 15 meter and 20 meter stacked yagis are partially complete. The side mounted lower yagis are up and working. As of this writing only the 20 meter yagi is usable since the 15 meter yagi needs another 100' (30 m) of coax on the tower to reach the main run of Heliax to the shack. A future article will cover the side mount yagis, including some of the lifting and mounting details and initial performance insights.
All this work was done this month with the help of VE3DQN and VE3KAE who, like me, know that the best way to survive a Canadian winter is to go outside and do something!
The upper rotatable yagis are partly assembled and will wait for warmer weather to be lifted. You can see them in foreground. The nearly complete 15 meter yagi is on the left and the bare 20 meter yagi boom is on the right. The 20 meter upper yagi will be a monster weighing ~115 lb (52 kg) that requires careful thought to the lifting process. I redesigned the 20 meter boom to be more robust.
The prop pitch motor for the upper 15 and 20 meter yagis is partially disassembled in my workshop. I am performing preventive maintenance so that it is as reliable as possible. The mounting position for the motor will make it inconvenient to pull out for repair. More on this when the rotation system is complete and installed, hopefully this spring.
For the present the TH6 and TH7 on the other tower will remain as they are until I make progress on 40 meter yagis. The TH6 is fixed approximately south up 23 meters as a contest multiplier antenna to Caribbean, Central America and South America, at which it is very effective. It is also used to work the southeast and south-central US. The TH7 up 43 meters is used for long haul DX and the west coast. Eventually the TH7 will come down the tower to be stacked with the TH6.
When the stacks are complete I will have excellent instant compass coverage for DX contests. In everyday use I can pounce quickly on a DXpedition without having to rotate an antenna in the needed direction. Those prop pitch motors are powerful but can take up to 2 minutes to turn 360°.
Until the sunspots return I am content with the two tri-band yagis for 10 meters. I expect to design, build and install a 10 meter stack on the 43 meter tower as early as 2021. Along with the tri-band yagis this will position me well on 10 meters during the productive high sunspot years. That time is closer than you think!
40 meters
This band is a dilemma. My long term goal is to have a full size 3-element yagi on the 43 meter tower. I have been investigating electrical and mechanical designs and I've talked to a few hams with one of these monsters. A friend with a full size 3-element yagi sent me pictures of it under the weight of at least ¼" of ice. It looked awful yet it bounced right back when the ice melted. His antenna weighs substantially more than the one I'm planning.
No reasonably survivable element is less than 50 lb (23 kg). Add two more elements and a boom and the antenna is certainly going to weigh 250 lb (115 kg) or more and have a cylindrical wind surface of about 30 ft². I decided to proceed in steps. At some point I will decide I've gone far enough for my comfort with a smaller design or continue all the way. Later in the year I'll talk more about this subject.
Step one is to make a full size rotatable dipole and put it up 46 meters. I will see how it performs relative to my other 40 meter antennas and, importantly, how it survives at least one cycle of seasons. We get a lot of ice storms and that is the greatest danger. For most hams it is high winds.
Unfortunately I failed to get the dipole up this year. In preparation for it I lowered the XM240 from that 46 meter high perch and put it back on the Trylon tower where it is up 21 meters. I have no other 40 meter antenna. When the XM240 developed a problem during CQ WW CW my score suffered accordingly. That is the danger of getting only halfway through an annual plan.
I will get the dipole up sooner rather than later. Although I scavenged part of it to repair the ice storm damaged 80 meter vertical yagi it is readily replaced with aluminum on hand.
In addition to these two antennas I hope to put up a wire yagi on the same tower this fall. It will be at least 3 elements and pointed at Europe to boost my contest scores. Making it reversible remain a possibility but not a necessity with the XM240 for working the North American and further south.
80 meters
I am pretty well set up on this band so no major changes are on my priority list for 2020. Alternatives to improve the performance of the 80 meter vertical yagi were touched on in previous article and will be dealt with as time (and ticks) permit. Adding yagi operation for SSB is not a priority since my interest in SSB contests is not high. That may change.
The high inverted vee can remain for the present since it is low maintenance and gives me a fallback in case the yagi suffers a catastrophic failure. Other antenna ideas are shelved for reconsideration further into the future.
160 meters
My top priority is to add 160 meters to the 80 meter vertical yagi before haying season gets underway this spring. That will give me year round access to the band. It's a simple addition -- a coil -- since everything else is installed including a switch on the operating desk.
After haying is done and the time comes to reinstall the T-top vertical there will be changes. The first is to reconfigure the element to raise the radiation resistance and thereby improve efficiency. The set of 8 × 30 meter long radials will be at least doubled for added efficiency. Between these two changes I expect from 1 to 2 db of improvement.
Considering how marginal conditions usually are on top band the small improvement will pay dividends in contest scores and DXing. Gain and directionality with two or more elements is left to future years when I have more time to play with various designs.
Receiving antennas
This winter I have lower receive antenna capability on the low bands than last year. I did not reinstall the west Beverage so all I have is the northeast Beverage. It's a great antenna but not nearly enough. Especially now that I'm using the amplifier more often it would help me to work the weaker stations that call me.
I have admittedly given priority to transmit antenna. Since, as they say: you can't work 'em if they can't hear you. Okay, that's not really what they say but you get the idea. I can no longer defer work on receive antennas.
My dilemma is whether to stick with the tried and true Beverages or switch to vertical arrays. The vertical arrays require more work and deliver better RDF (receiving directivity factor). I have ample room on my 48 acres (20 ha) for both although the feed line lengths can extend many hundreds of meters. The antennas, feed lines and remote switching systems would be difficult to access for over 6 months out of the year.
My tentative decision is to proceed with Beverages. Additional RDF is helpful for general DXing but not for contesting; in contests some leakage that will let me hear callers from other directions is a feature not a bug.
I want to install at least one reversible Beverage this winter pointing north/south or east/west. The second will go in next fall after the first killing frost. I also hope to make the northeast Beverage reversible northeast/southwest. That will give me 6 directions on all the low bands.
The switching system can be kept reasonably accessible (out of the thick bush). The cabling and switching requirements are modest. I have trenches to dig this spring and that is my opportunity to lay the infrastructure for the system.
Operating equipment
There will be changes inside the shack as well as outside. The major planned changes are to replace the FT950 with a modern high-end transceiver. That and a 160 to 6 meter amplifier will put me in good stead for SO2R and multi-op contests and provide a degree of redundancy should something fail. I do not want to be in the position again where an amplifier fails during a contest and there is no backup. I delayed replacing the FT950 in 2019 since my SO2R and multi-op progress has lagged.
The operating desk will be replaced. Although I love my hand built desk (made in 1985 with the help of a woodworking friend) it is unsuitable for multi-op contests. I have developed a design to meet my needs and most of the lumber is on hand. It'll be cheap but functional. With a bit of paint it can be made presentable as a piece of furniture.
With the addition of a second prop pitch rotator and just the one power supply and control unit I am planning to home brew a control panel to allow operation of both. Not being able to turn both motors at the same time should be acceptable although I can always build another or larger power supply later. A new position indicator is being designed, possibly with software for at least the display function. I have candidate designs that will lead to implementation later this year.
Station automation and contesting equipment
For a complete two position contest station I need the following:
Future years will likely see more improvements to station automation and possibly remote operation capability.
VHF and other HF bands
I would like to return to more active VHF DXing. At present my only VHF antenna is the Cushcraft A50-6 for 6 meters. This antenna must come down for adjustment because the gamma match slipped when it was lifted two years ago. I want to correct the 1.7 SWR before I buy an amplifier. The rig's ATU is sufficient for the present. My immediate aim is to achieve 6 meter DXCC using FT8. My country count over the previous two sporadic E seasons is 71. It won't be easy!
Last year I replaced the old RG213 transmission line with LMR400. This is better but still not good enough. Now that I know I have enough Heliax to spare the LMR400 will be replaced with LDF5-50A. It will run directly into the shack instead of going through the 8 × 2 antenna switch where it is occupying a valuable port. Over the winter it is disconnected (except during the RAC Winter contest) to make room for new antennas on the antenna switch.
I have a 7-element 2 meter antenna in my junk pile that I may put underneath the 6 meter antenna. With a remote antenna switch to select between them I will have all the VHF antennas I want for now. Of course I will need a 2 meter rig since I sold my VHF and UHF equipment many years ago while I was out of the hobby.
I'll admit I've given little thought to other HF bands. The big gaps are 30 meters and 17 meters. The latter I can do reasonably well using the ATU into the XM240 40 meter yagi. For 30 meters all I have is the 80 meter inverted vee. It performs poorly since on the antenna's third harmonic there are many lobes and nulls in the azimuth pattern. On some stations it works well and on others it is awful.
Anything I do for 30 and 17 meters will be simple. It isn't worth the effort in 2020 considering the many important projects I have planned. The best I'll accomplish is software designs for antennas to be considered in 2021.
Wrap-up
With this annual ritual out of the way the blog with return to antenna and operating topics. I have quite a few projects in progress so there will be lots to write about. There are several draft articles in the pipeline which are begging for more of my time to finish them. Winter antenna work will continue, weather permitting.
You can comment on articles, but realize that because of all the spam I get these are moderated. Direct email is to my call sign at rac.ca. Readership is stable to rising and I often hear from hams on the air that they follow the blog. I hope you are enjoying it. Among the mix of topics there should be something for most everyone.
I am not an expert in any one technical or operating aspect of ham radio and I try to steer readers to reliable external resources where appropriate. I try my best to avoid the myths and guesswork that is all too common among hams.
Onward to a new decade. For my amateur radio pursuits it may be the best one yet. May your decade be the same. 73
Over the winter my radio activities are mostly confined to indoor activities, whether it be operating, equipment work or planning for the future. There is always a lull during the changeover as priorities reset and a human tendency to "hibernate". Soon enough the energy returns as the sun begins its long climb out of the winter solstice. This is an opportune time to look back and consider the path ahead as I do each year since starting this blog 7 years ago. Even so I have been active outdoors, as you'll discover.
This article is of the type that will interest few readers. I do it for myself, to honestly measure my progress and to plan for the future. What readers may get from it is the discipline of making a plan and being accountable, so unlike new year resolutions which are lightly made and soon forgotten. Anyone can check on how I'm doing by comparing my written plan at the start of the year and my year end review. Although I almost always fall short I still prefer to think big.
If this is not the type of article that is of interest to you, stop reading. For everyone else you are welcome to follow along. What's a little different this year is the combining of the 2019 review and 2020 plan in one article.If you do continue reading keep in mind my primary interests in ham radio determine the projects I take on. These are contests, DXing and playing with antennas. Even if these are not your cup of tea hopefully there are ideas and thoughts that will be of interest.
I called 2019 the year of the yagi for the major activity in my station building plan. It didn't go as fast as planned. The new 140' (40 meter) tower had to be raised and the details of large HF yagi home brew construction ate up the calendar. Even so I can see the light at the end of the tunnel, it was just a little further away than estimated.
As matters stand 2020 will be the final year of the intensive phase of station building that began with my move to this QTH over 3 years ago. In 2021 I'll most likely be tying off loose ends and focussing on matters in the shack and operating. Things inevitably break so maintenance is always there to be done.
15 and 20 meters
The 15 meter and 20 meter stacked yagis are partially complete. The side mounted lower yagis are up and working. As of this writing only the 20 meter yagi is usable since the 15 meter yagi needs another 100' (30 m) of coax on the tower to reach the main run of Heliax to the shack. A future article will cover the side mount yagis, including some of the lifting and mounting details and initial performance insights.
All this work was done this month with the help of VE3DQN and VE3KAE who, like me, know that the best way to survive a Canadian winter is to go outside and do something!
The upper rotatable yagis are partly assembled and will wait for warmer weather to be lifted. You can see them in foreground. The nearly complete 15 meter yagi is on the left and the bare 20 meter yagi boom is on the right. The 20 meter upper yagi will be a monster weighing ~115 lb (52 kg) that requires careful thought to the lifting process. I redesigned the 20 meter boom to be more robust.
The prop pitch motor for the upper 15 and 20 meter yagis is partially disassembled in my workshop. I am performing preventive maintenance so that it is as reliable as possible. The mounting position for the motor will make it inconvenient to pull out for repair. More on this when the rotation system is complete and installed, hopefully this spring.
For the present the TH6 and TH7 on the other tower will remain as they are until I make progress on 40 meter yagis. The TH6 is fixed approximately south up 23 meters as a contest multiplier antenna to Caribbean, Central America and South America, at which it is very effective. It is also used to work the southeast and south-central US. The TH7 up 43 meters is used for long haul DX and the west coast. Eventually the TH7 will come down the tower to be stacked with the TH6.
When the stacks are complete I will have excellent instant compass coverage for DX contests. In everyday use I can pounce quickly on a DXpedition without having to rotate an antenna in the needed direction. Those prop pitch motors are powerful but can take up to 2 minutes to turn 360°.
Until the sunspots return I am content with the two tri-band yagis for 10 meters. I expect to design, build and install a 10 meter stack on the 43 meter tower as early as 2021. Along with the tri-band yagis this will position me well on 10 meters during the productive high sunspot years. That time is closer than you think!
40 meters
This band is a dilemma. My long term goal is to have a full size 3-element yagi on the 43 meter tower. I have been investigating electrical and mechanical designs and I've talked to a few hams with one of these monsters. A friend with a full size 3-element yagi sent me pictures of it under the weight of at least ¼" of ice. It looked awful yet it bounced right back when the ice melted. His antenna weighs substantially more than the one I'm planning.
No reasonably survivable element is less than 50 lb (23 kg). Add two more elements and a boom and the antenna is certainly going to weigh 250 lb (115 kg) or more and have a cylindrical wind surface of about 30 ft². I decided to proceed in steps. At some point I will decide I've gone far enough for my comfort with a smaller design or continue all the way. Later in the year I'll talk more about this subject.
Step one is to make a full size rotatable dipole and put it up 46 meters. I will see how it performs relative to my other 40 meter antennas and, importantly, how it survives at least one cycle of seasons. We get a lot of ice storms and that is the greatest danger. For most hams it is high winds.
Unfortunately I failed to get the dipole up this year. In preparation for it I lowered the XM240 from that 46 meter high perch and put it back on the Trylon tower where it is up 21 meters. I have no other 40 meter antenna. When the XM240 developed a problem during CQ WW CW my score suffered accordingly. That is the danger of getting only halfway through an annual plan.
I will get the dipole up sooner rather than later. Although I scavenged part of it to repair the ice storm damaged 80 meter vertical yagi it is readily replaced with aluminum on hand.
In addition to these two antennas I hope to put up a wire yagi on the same tower this fall. It will be at least 3 elements and pointed at Europe to boost my contest scores. Making it reversible remain a possibility but not a necessity with the XM240 for working the North American and further south.
80 meters
I am pretty well set up on this band so no major changes are on my priority list for 2020. Alternatives to improve the performance of the 80 meter vertical yagi were touched on in previous article and will be dealt with as time (and ticks) permit. Adding yagi operation for SSB is not a priority since my interest in SSB contests is not high. That may change.
The high inverted vee can remain for the present since it is low maintenance and gives me a fallback in case the yagi suffers a catastrophic failure. Other antenna ideas are shelved for reconsideration further into the future.
160 meters
My top priority is to add 160 meters to the 80 meter vertical yagi before haying season gets underway this spring. That will give me year round access to the band. It's a simple addition -- a coil -- since everything else is installed including a switch on the operating desk.
After haying is done and the time comes to reinstall the T-top vertical there will be changes. The first is to reconfigure the element to raise the radiation resistance and thereby improve efficiency. The set of 8 × 30 meter long radials will be at least doubled for added efficiency. Between these two changes I expect from 1 to 2 db of improvement.
Considering how marginal conditions usually are on top band the small improvement will pay dividends in contest scores and DXing. Gain and directionality with two or more elements is left to future years when I have more time to play with various designs.
Receiving antennas
This winter I have lower receive antenna capability on the low bands than last year. I did not reinstall the west Beverage so all I have is the northeast Beverage. It's a great antenna but not nearly enough. Especially now that I'm using the amplifier more often it would help me to work the weaker stations that call me.
I have admittedly given priority to transmit antenna. Since, as they say: you can't work 'em if they can't hear you. Okay, that's not really what they say but you get the idea. I can no longer defer work on receive antennas.
 |
| With some imagination I see Beverage antennas |
My tentative decision is to proceed with Beverages. Additional RDF is helpful for general DXing but not for contesting; in contests some leakage that will let me hear callers from other directions is a feature not a bug.
I want to install at least one reversible Beverage this winter pointing north/south or east/west. The second will go in next fall after the first killing frost. I also hope to make the northeast Beverage reversible northeast/southwest. That will give me 6 directions on all the low bands.
The switching system can be kept reasonably accessible (out of the thick bush). The cabling and switching requirements are modest. I have trenches to dig this spring and that is my opportunity to lay the infrastructure for the system.
Operating equipment
There will be changes inside the shack as well as outside. The major planned changes are to replace the FT950 with a modern high-end transceiver. That and a 160 to 6 meter amplifier will put me in good stead for SO2R and multi-op contests and provide a degree of redundancy should something fail. I do not want to be in the position again where an amplifier fails during a contest and there is no backup. I delayed replacing the FT950 in 2019 since my SO2R and multi-op progress has lagged.
The operating desk will be replaced. Although I love my hand built desk (made in 1985 with the help of a woodworking friend) it is unsuitable for multi-op contests. I have developed a design to meet my needs and most of the lumber is on hand. It'll be cheap but functional. With a bit of paint it can be made presentable as a piece of furniture.
With the addition of a second prop pitch rotator and just the one power supply and control unit I am planning to home brew a control panel to allow operation of both. Not being able to turn both motors at the same time should be acceptable although I can always build another or larger power supply later. A new position indicator is being designed, possibly with software for at least the display function. I have candidate designs that will lead to implementation later this year.
Station automation and contesting equipment
For a complete two position contest station I need the following:
- Band pass filters for 160 through 10 meters. These will be placed between the rigs and amplifiers. They must be automated so that the filter follows the rig. Outside of contests the filters will be bypassed. These will be commercial products. Additional harmonic filter stubs are not planned this year and will only be added in future as experience dictates.
- Antenna switching: The manual switching system will be automated. I am leaning toward a home brew software-controlled system. It must follow the rigs and support multiple antennas per band, multi-band antennas and non-contest band antennas. I will likely retire the existing 8 × 2 switch, which has insufficient ports and ongoing performance problems, and opt for a home brew system. It is probable this project will be delayed to next winter.
- SO2R control system: I recently purchased and built the SO2R Mini kit. It will replace the WinKeyer and my manual headphone splitter. It has microphone switching for SSB contests which my home brew SO2R starter system doesn't have. It's the box on the right.
- A second networked PC will be needed for multi-op logging software. I use N1MM Logger+ and I plan to stick with it. The SO2R Mini work with Logger+ software.
Future years will likely see more improvements to station automation and possibly remote operation capability.
VHF and other HF bands
I would like to return to more active VHF DXing. At present my only VHF antenna is the Cushcraft A50-6 for 6 meters. This antenna must come down for adjustment because the gamma match slipped when it was lifted two years ago. I want to correct the 1.7 SWR before I buy an amplifier. The rig's ATU is sufficient for the present. My immediate aim is to achieve 6 meter DXCC using FT8. My country count over the previous two sporadic E seasons is 71. It won't be easy!
Last year I replaced the old RG213 transmission line with LMR400. This is better but still not good enough. Now that I know I have enough Heliax to spare the LMR400 will be replaced with LDF5-50A. It will run directly into the shack instead of going through the 8 × 2 antenna switch where it is occupying a valuable port. Over the winter it is disconnected (except during the RAC Winter contest) to make room for new antennas on the antenna switch.
I have a 7-element 2 meter antenna in my junk pile that I may put underneath the 6 meter antenna. With a remote antenna switch to select between them I will have all the VHF antennas I want for now. Of course I will need a 2 meter rig since I sold my VHF and UHF equipment many years ago while I was out of the hobby.
I'll admit I've given little thought to other HF bands. The big gaps are 30 meters and 17 meters. The latter I can do reasonably well using the ATU into the XM240 40 meter yagi. For 30 meters all I have is the 80 meter inverted vee. It performs poorly since on the antenna's third harmonic there are many lobes and nulls in the azimuth pattern. On some stations it works well and on others it is awful.
Anything I do for 30 and 17 meters will be simple. It isn't worth the effort in 2020 considering the many important projects I have planned. The best I'll accomplish is software designs for antennas to be considered in 2021.
Wrap-up
With this annual ritual out of the way the blog with return to antenna and operating topics. I have quite a few projects in progress so there will be lots to write about. There are several draft articles in the pipeline which are begging for more of my time to finish them. Winter antenna work will continue, weather permitting.
You can comment on articles, but realize that because of all the spam I get these are moderated. Direct email is to my call sign at rac.ca. Readership is stable to rising and I often hear from hams on the air that they follow the blog. I hope you are enjoying it. Among the mix of topics there should be something for most everyone.
I am not an expert in any one technical or operating aspect of ham radio and I try to steer readers to reliable external resources where appropriate. I try my best to avoid the myths and guesswork that is all too common among hams.
Onward to a new decade. For my amateur radio pursuits it may be the best one yet. May your decade be the same. 73
Wednesday, January 22, 2020
Air Core Coax Chokes: Good, Bad and Ugly
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| Ugly coax choke, but it worked |
Common mode chokes on antennas can be very beneficial. All kinds of ills result from RF currents flowing on the exterior of the coax, on both transmit and receive. However it is surprisingly easy not to recognize the problems since they are often blamed on other causes such as proximity to the antenna, a poor receiver and cheap consumer electronics, among others.
It is usually easy to install a common mode choke so there is no reason not to do so even if there are no obvious common mode ills present. The choke presents a high impedance on the coax outer shield surface to prevent conducted current on the coax. The impedance can be a resistance or a tuned circuit, with the former having the most predictable characteristics and being broad band and the latter being narrow band with relatively unpredictable characteristics.
I am not here to tell you how to choose or build a common mode choke. For that I'll refer you to the experts. Probably the best explanation, analysis and recommendations was written by K9YC. There you'll learn that the best chokes use large ferrite toroids, wound with the coax itself or even THHN electrical wire. Instead I'll speak to a commonly used and admittedly inferior solution: the air core coax choke -- in this article I'll often call it a "coax choke" for brevity.
I've used many over the years, sometimes because I was cheap and it's easy to make. It is nothing more complicated that the coax wound into a coil. The coil inductance itself has choking properties, as does any inductor. However most of the choking is courtesy of the distributed capacitance among the turns that in combination with the coil inductance forms a resonant circuit. If the circuit has a high impedance at the frequency of operation (resonant) it can work very well indeed.
Unfortunately it's easy to get it wrong or to have unreasonably founded expectations. Most hams don't have a two port VNA (vector network analyzer) or the expertise to use them properly. I don't have one either. The question is: can we rely on an unmeasured coax choke to be effective?
Before going further we have to review what we mean by effective. What effect? How much? Without a clearly defined problem to solve or an operating objective to be met there is the risk of doing too little or too much. At least with the latter you can be successful though perhaps at a price or time investment that isn't strictly necessary.
To study effectiveness let's briefly list the potential benefits of common mode chokes:
- Antenna impedance: Common mode on the coax will alter the antenna feed point impedance. This may be unnoticed since when we tune the antenna (e.g. beta or gamma match) that component of the impedance is accounted for. However that may be insufficient since the common mode impedance can vary widely with frequency, especially when a reactive choke such as a coax choke is employed.
- EMC (electromagnetic compatibility): RF travelling down the coax will radiate. In our dense neighbourhoods this places the "antenna" closer to us and our devices. There is an increased probability of EMI to those devices and received EMI from those devices. Expect the tower and other cables to join the fun since the common mode current will couple to those parallel conductors.
- Antenna pattern: Even a small amount of radiation from the coax can ruin otherwise good antenna directionality. When the F/B is 20 db or greater it takes very little undisciplined radiation from reducing that to 10 db or worse. Routing the coax perpendicular to the antenna reduces induced RF but does little to suppress conducted common mode RF.
- Modest choking is sufficient to avoid difficulties with the antenna impedance. Once the choke impedance is above ~500 Ω further improvements have negligible impact.
- EMC is not a large problem in my station since the towers and antennas are far from the house and my neighbours are far away. This isn't typical of most hams. I can get away with a little leakage.
- Very little choking is needed to protect antenna gain. Directionality requires better choking. For an omni-directional antenna this is of low importance. I am more interested in gain than directionality since my primary interest is contesting where it can be beneficial to attract callers from directions other than where the antenna is pointing.
- For low band receive antennas such as Beverages and vertical arrays the use of high impedance common mode chokes is mandatory. These require ferrite cores to cover both 160 and 80 meters. Besides which a coax choke at 1.8 MHz is quite large and difficult to build, and can actually be more expensive than a ferrite core choke.
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| "Scramble wound" coax choke that I made in ~1987 for a TH6: bad! |
The scramble wound choke pictured above replaced the original burned out BN86 balun on my TH6 ~30 years ago; the Hy-Gain voltage balun is in any case a poor choice for a common mode choke. Asking a coax choke to be effective across the 2:1 frequency range of a tri-band antenna is inordinately optimistic. I could say the choke "worked" in that I didn't have any obvious common mode problems. It is likely that it had a choking impedance of less than 500 Ω on one or more of the three bands. Depending on circumstances and expectations even that small an impedance can be considered effective.
There are impedance measurements for a variety of coax chokes available. Most hams would prefer to rely on those rather than do their own measurements, in the hope that they deliver the published performance. The important parameters are diameter, coax type (outer conductor OD) and winding style (solenoid, scramble wound, etc.). Scramble wound is the easiest (see picture above) but has unpredictable performance due to the unpredictable L and C values.
One resource I've turned to many times is the measurements of various air core and ferrite core coax chokes by G3TXQ (SK). I've extracted part of the table below since I cannot assume that the web site will be around forever now that he's passed on. This is the data by which I recently made chokes for my new 15 and 20 meter stacked yagis.
These are solenoid wound coax chokes. Always wind air core coax chokes in this manner and never use scramble winding. That's the only way to achieve predictability performance. Notice in the table how difficult it is to make a high impedance coax choke that covers more than one HF band. You can do reasonably well if, like me, your station and operating style can tolerate imperfection.
For the 15 meter yagis I used 5 turns and 6" diameter of LMR400UF. On the above chart you can see that I interpolated between two known designs to get one that has the diameter and turns count that I prefer for the chosen coax. The PVC strips and cable ties hold the turns in a solenoid form. Tape was used while winding the coax to keep the diameter consistent and to discipline the turns. A temporary form can be used if you have one of the desired diameter.
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| Solenoid wound single-band 15 meter coax choke: good! |
Pay close attention to the bending radius specifications of the coax before winding your choke. I prefer to use RG213 or the ultra flex version of LMR400 since they are more flexible. Greater care must be take with foam dielectric coax to prevent the centre conductor from pushing through the foam and shorting to the outer conductor or altering the impedance.
These danger of excess or repeated bending can take months or years to manifest so build carefully and don't rely on a one time measurement. Avoid bending the coax more than once, especially with LMR400 with its solid centre conductor, since the minimum bend radius is far higher for multiple bends. Study the mechanical properties specs and use accordingly.
Here's the same choke installed on a 15 meter yagi. One of the PVC clamps doubles as a boom clamp. Yes, I do antenna and tower work in winter! Shortly after the picture was taken the antenna was trammed to 100' (32 m). The yagi is side mounted and fixed northeast as the lower yagi in the stack.
Speaking of stacking, if you use common mode chokes of any variety in a stack it is important to use the same choke (or current balun) on all yagis in the stack. Otherwise there will be a phase shift. Unless you compensate for the phase shift, gain and lobe formation for the stack will suffer. For these coax chokes I measure the length and type of coax, data which I'll use to ensure the yagis are fed in phase.
As much as I love coax chokes I avoid them for multi-band antennas and receive antennas. Almost all my tri-band yagis and lower frequency antennas have commercial ferrite core chokes that have high impedance across the bands the antennas cover.My Beverage receiving antennas use binocular ferrite core 1:1 transformers at the feed points. It is also good practice to use them at the switching system ports and at intervals on the transmission line, especially if it parallels a Beverage.
Breaking up the system in this manner keeps common mode currents at bay thus protecting the high directionality of the antennas. You can also wind the coax on a suitable ferrite toroid. It is very difficult to achieve high choking impedance on the low bands with an air core coax choke.
In conclusion, go ahead and use a coax choke if it suits the application. Remember to wind them properly, use reliable specifications or measure them yourself with a two port VNA and try to limit their use to one band rather than two or more. But don't expect more from them than they can deliver.
Friday, January 17, 2020
Call History
For the first time I made use of a call history file in a contest this weekend. It was the North American QSO Party (NAQP) CW contest. To those unfamiliar with this feature of modern contest logger software a call history file contains fixed exchange information cross-referenced with call sign. For example, in NAQP when you enter a call sign and tab to the exchange it will be pre-filled with RON for the name and ON for the state/province/country.
I want to talk about the how and why of call history usage and philosophical objections. The mechanics of call history can be found in the manual for your favourite contest logging software. I will not provide a tutorial.
This screen capture shows what I get when I enter the call of my friend VE3JM -- a dedicated contester with a big antenna farm. After I enter the call sign and call him by pressing enter (ESM, or by manually tabbing to the exchange) the software pulls exchange data from the call history file. The exchange can be left as is, saving typing, or overwritten with what is copied by the operator.
Call history is not the only source for pre-filled exchange data. It is typical that for stations already worked the exchange data can be pre-filled from the current contest log. For these contacts the call history only assists with the first time a station is worked in the contest. Some exchange data requires neither since it can be often be derived from the call sign. Examples include CQ and ITU zones, Canadian provinces, etc.
An extract of the N1MM call history file for NAQP surrounding my own call looks like this:
It should be obvious that call history cannot help you with serial numbers and other unpredictable exchange fields. A wise contester will always verify that the pre-filled exchange data is the same as what you copy. Trust your ears. Override the pre-filled data as necessary. The call history saves typing but should never be relied upon as a primary source.
The exchange in some contests is so predictable that a call history file is unnecessary. A good example is CQ WW in which the zone number can almost always be uniquely derived from the call sign. In cases where it is wrong call history can help as can other data sources used by modern contest logging software.
Why I used call history
I practiced SO2R (single-op, 2 radios) in the RAC winter contest last month at a more intense level. That includes running on two bands at once. The RAC contest exchange does not benefit much from a call history file since the exchange is either unpredictable (serial number) or very predictable (province).
That went well enough that I decided to apply the lessons learned to NAQP CW this month. I used call history as an insurance policy in case I found myself getting confused or out of sync between radios that would cause a lapse of concentration when copying the exchange. It did indeed help by lowering my stress level. SO2R novice mistakes due to the stress of operating two radios simultaneously were reduced.
That said the benefit was not large. Once a station was worked the pre-fill from previously working them on another band took precedence over call history. That's a good thing since call history does not always predict what the other station sends.
What to watch for
Call history is not reliable. Give more credence to what your ears hear that what the call history pre-fill provides. There are several reasons:
Philosophical perspective
As a matter of operating ethics I remain leery of the call history files, whether those compiled by others or from my own historical contest logs. Pre-fills reduce operator involvement in the QSO by reducing the need to fully copy and enter the exchange. The mental effort of doing these tasks adds stress by increasing operator focus to ensure no mistakes are made with a consequent penalty during log checking by the contest sponsor.
It's a philosophical issue, one with adherents and proponents on both sides of the question. The matter appears to be far less controversial than some others such as excess power and remote receivers yet it does raise interesting questions of just what skill set exemplifies excellence in contesting. Until now I was of the opinion that I ought to copy the full exchange.
Call history is not the only exchange copying aid:
Does it matter? Until now I believed that it does, which is why I have not used call history. I also felt uncomfortable when I enabled SCP. Yet I've never had a qualm about pre-fills from the current contest log. Operating ethics is a slippery concept. I see no clear answer. Resorting to using the same aids as your competitors is understandable.
I don't expect to use call history in many contests and I may decide to stop using it altogether. Most contesters do not share my misgivings and perhaps they're right. It's an individual choice based on one's personal view. I will not judge others.
I want to talk about the how and why of call history usage and philosophical objections. The mechanics of call history can be found in the manual for your favourite contest logging software. I will not provide a tutorial.
This screen capture shows what I get when I enter the call of my friend VE3JM -- a dedicated contester with a big antenna farm. After I enter the call sign and call him by pressing enter (ESM, or by manually tabbing to the exchange) the software pulls exchange data from the call history file. The exchange can be left as is, saving typing, or overwritten with what is copied by the operator.
Call history is not the only source for pre-filled exchange data. It is typical that for stations already worked the exchange data can be pre-filled from the current contest log. For these contacts the call history only assists with the first time a station is worked in the contest. Some exchange data requires neither since it can be often be derived from the call sign. Examples include CQ and ITU zones, Canadian provinces, etc.
An extract of the N1MM call history file for NAQP surrounding my own call looks like this:
VE3VFN,VINCENT,ON,You can create a call history file from your own past logs. However it is usually better to use one like the one depicted (compiled by VE2FK) since it is current, cross-checked against logs from many people and will include entries for stations you do not have in your old logs. I used a different publicly available call history file in NAQP. I probably ought to have used Claude's since the one I used had problems, as I'll discuss later.
VE3VGI,JOHN,ON,
VE3VN,RON,ON,
VE3VRC,VARC,ON,
VE3VSM,DAVE,ON,
VE3VV,TED,ON,
VE3VY,AL,ON,
It should be obvious that call history cannot help you with serial numbers and other unpredictable exchange fields. A wise contester will always verify that the pre-filled exchange data is the same as what you copy. Trust your ears. Override the pre-filled data as necessary. The call history saves typing but should never be relied upon as a primary source.
The exchange in some contests is so predictable that a call history file is unnecessary. A good example is CQ WW in which the zone number can almost always be uniquely derived from the call sign. In cases where it is wrong call history can help as can other data sources used by modern contest logging software.
Why I used call history
I practiced SO2R (single-op, 2 radios) in the RAC winter contest last month at a more intense level. That includes running on two bands at once. The RAC contest exchange does not benefit much from a call history file since the exchange is either unpredictable (serial number) or very predictable (province).
That went well enough that I decided to apply the lessons learned to NAQP CW this month. I used call history as an insurance policy in case I found myself getting confused or out of sync between radios that would cause a lapse of concentration when copying the exchange. It did indeed help by lowering my stress level. SO2R novice mistakes due to the stress of operating two radios simultaneously were reduced.
That said the benefit was not large. Once a station was worked the pre-fill from previously working them on another band took precedence over call history. That's a good thing since call history does not always predict what the other station sends.
What to watch for
Call history is not reliable. Give more credence to what your ears hear that what the call history pre-fill provides. There are several reasons:
- The call history file includes errors. Logging errors from previous contests reappear when those logs are used to build the call history file. This is true whether it is built from your own logs or that of others. Typos and copying errors of name and state/province were not uncommon with the call history file I imported for use in NAQP.
- NAQP brings out the weirdness in some people. Very strange names may be used just on a whim or out of perverse pleasure. This is completely within the rules. Other contests have their own variation of this whimsy. One example is sending 000 as the power by KP4 and KP2 hams in a previous ARRL DX contest to draw attention to the failed power grid due to a hurricane.
- NAQP has become a means of paying tribute to recently passed on contesters and prominent hams. For example, many Florida participants used the name Walt to commemorate the recent passing of W7SE.
- Special multiplier stations in many regional contests send section/county/region other than what is usual, and may even be in a different format. I most recently ran into this one in the Worked All Germany (WAG) contest.
Philosophical perspective
As a matter of operating ethics I remain leery of the call history files, whether those compiled by others or from my own historical contest logs. Pre-fills reduce operator involvement in the QSO by reducing the need to fully copy and enter the exchange. The mental effort of doing these tasks adds stress by increasing operator focus to ensure no mistakes are made with a consequent penalty during log checking by the contest sponsor.
It's a philosophical issue, one with adherents and proponents on both sides of the question. The matter appears to be far less controversial than some others such as excess power and remote receivers yet it does raise interesting questions of just what skill set exemplifies excellence in contesting. Until now I was of the opinion that I ought to copy the full exchange.
Call history is not the only exchange copying aid:
- Current contest log: Fixed exchange data is pre-filled from previous contacts with the same station, usually on another band.
- Country file: Zone and country by prefix or individual call sign are pre-filled. This can be especially helpful when working Americans because their call signs do not correlate with a zone or country. For example, a KH6 in the continental US and vice versa.
- Super check partial: Master data base of call signs appearing in contest logs. It is used to correct call sign copying errors. Not really a pre-fill aid but it does reduce the importance of paying attention. I always confirm a call if I substitute an SCP recommended replacement.
Does it matter? Until now I believed that it does, which is why I have not used call history. I also felt uncomfortable when I enabled SCP. Yet I've never had a qualm about pre-fills from the current contest log. Operating ethics is a slippery concept. I see no clear answer. Resorting to using the same aids as your competitors is understandable.
I don't expect to use call history in many contests and I may decide to stop using it altogether. Most contesters do not share my misgivings and perhaps they're right. It's an individual choice based on one's personal view. I will not judge others.
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