Tuesday, February 25, 2020

We Have Propagation

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.

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:
  • 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.
For me the factors that ultimately convinced me to go for the gamma match were driven element robustness and (I thought) relatively simplicity of construction and tuning. Gamma matches are peculiar creatures that defy intuition and easy answers. I had more difficulty than expected getting them to work. Let's dig a little deeper and learn why.

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.
As Cebik's theory makes clear there are many wrong design algorithms and right ones are difficult to come by. In his survey the Tolles-Nelson-Leeson algorithm seemed to be best. For my designs I relied on the gammaMW4 software that came with my copy of the ARRL Antenna Book. My experience with it was less than satisfactory, but I had it and it at least got me pointed in the right direction.

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.
With the lower yagis of the stacks on the tower I want to know how they tolerate several months of cold, ice and wind before putting up the top yagis of the stack. The top yagis are more difficult to lower and fix if faults arise.

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.
A side effect of the close wire spacing is that the model fails the average gain test. However, as explained in the EZNEC manual that if it is known that there are no true losses in the model it is safe to add the lost power to the far field calculations of the pattern.

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 careful manipulation of the model the match was achieved. The resulting SWR curve is virtually identical to the original beta match and measurement of the real antenna.

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:
  1. Move the strap until Ro is approximately 50 Ω.
  2. Adjust the capacitor to bring Xo near 0 Ω.
  3. 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.
Of course it isn't quite that simple. For example, you must have the antenna high enough or, alternatively, pointing upward in the proper fashion for the impedance measurement to hold when the yagi is placed on the tower. Small yagis with the driven element accessible from the tower and for VHF/UHF yagis that can be tuned near the ground you can do this job more easily that for my 20 meter and 15 meter long boom yagis.

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
Adjusting the driven element in this fashion has no impact on the yagi's pattern. It would take a far larger change to do that, enough to significantly affect the mutual impedance with the parasitic elements. Choose whatever combination of strap position and element length that works best for the mechanical design of the element and gamma match of your yagi.

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.

Magic reel of LDF5-50A
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.

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.