Wednesday, January 22, 2020

Air Core Coax Chokes: Good, Bad and Ugly

Ugly coax choke, but it worked
To give you an idea of how effective a poorly made air core coax common mode choke can perform I'll refer you to one of the oldest articles in this blog. It's wound with spliced together lengths of RG58 I pulled out of a junk box. It's ugly yet it worked beautifully, in that it solved the common mode problem I had with an end-fed antenna.

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.
Coax chokes are very attractive since they are cheap, easy to construct and can work well in select situations. Let's consider them with respect to the above metrics.
  • 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.
A common impedance objective for common mode chokes is at least 5000 Ω, or 100× the nominal antenna feed point impedance. This is difficult to achieve with a coax choke. You can do it on a single band but may need to be optimized with the aid of a two-port VNA. They are narrow band since their resonance (LC tuned circuit) is sensitive to construction technique.

"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.

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:
VE3VFN,VINCENT,ON,
VE3VGI,JOHN,ON,
VE3VN,RON,ON,
VE3VRC,VARC,ON,
VE3VSM,DAVE,ON,
VE3VV,TED,ON,
VE3VY,AL,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.

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.
The lesson is to expect the unexpected. Call history is an operating aid not a crutch. Put too much of your weight on it and it will break, resulting is substantial penalties during log checking.

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.
With all these operating aids reducing the need for careful listening and typing is the use of call history a significant factor. Incrementalism can be insidious. Just like adding a fraction of a decibel at a time with station improvements you eventually have a very big signal the use of call history is one more incremental change reducing the required skill to be a top contester.

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.

Wednesday, January 8, 2020

L7 Amplifier New Filter/Rectifier Board

During the ARRL 160 meter contest my Drake L7 kilowatt amplifier failed. It happened while I was checking email during an off period. There was an almighty bang and the amp went dark. Out of the corner of my eye I saw a flash of light in the darkness under the operating desk where the power supply is located. I didn't leap too far out of my seat but it was startling.


The problem was easy to diagnose. The amplifier is ~40 years old and has the original filter capacitors in the high voltage power supply. Electrolytic capacitors have a finite lifetime, especially high voltage ones of an earlier generation. I made a note to replace them at some point. Of course I didn't.

A temporary repair to route around the failed capacitor was attempted so that I could run the amplifier at its lower B+ setting. It didn't work properly so I continued the contest without the amplifier.


There are two filter/rectifier boards in the power supply, one for each side of the full wave rectifier. The large cylindrical parts are 220 μF 450 VDC electrolytic capacitors wired in series to give 55 μF at 1800 VDC. They see half the 2800 VDC of the no-load plate voltage (B+).

You should have no difficulty identifying the failed part in the picture above. I cleaned the power supply and surface it was on of the solid mass and liquid electrolyte that escaped from the ruptured capacitor. The material is not dangerous to clean up if you are careful to wash your hands afterward.

Pricing of the individual parts is not high but inconvenient to order and would not easily fit on the original PCBs. For a modest premium I ordered the Harbach Electronics PM400 kit that includes all the parts and one PCB. Modern electrolytic capacitors of the same rating are much smaller so it all fits on one PCB.

The kit is excellent. I had heard good report of Harbach's amplifier kits and I was not disappointed. The parts and PCB look excellent and, perhaps most important, there are detailed instructions for installing the new board in vintage equipment like my L7. The instructions proved accurate, right down to the length and colour of wires in the power supply.

After I assembled the new filter/rectifier board it sat on my work bench for a couple of weeks. It was the holidays and my free time was spent on a variety of other projects. Then the ice storm hit. I was so relieved after repairing the 80 meter array that I dove in that very evening to install the board in the power supply.

Installation was quick and went smoothly. I carried it upstairs to the shack and plugged it in. It worked perfectly. My first QSO using it was ZC4UW on 160 meters, with whom I had been unable to complete a QSO running 200 watts.

Voltage and other operating parameters are the same as before. The power output is limited by the plate transformer not the power supply filter and rectifier so there is no increase in power output.

The pair of 3-500 tubes is capable of more than the L7 delivers with the relatively low anode voltage and current capacity. I have no intention of replacing the transformer.

I am now ready for the CQ WW 160 contest later this month. Eventually I will have a second and more modern amplifier added to my station for high power SO2R and multi-op contesting. It made good sense to start with an inexpensive vintage amplifier. Despite this understandable failure the L7 has not disappointed.

Friday, January 3, 2020

80 Meter Stinger Version 3.0

As I wrote several days ago an ice storm damaged my 80 meter vertical yagi. Weight of ice on the catenaries ropes supporting the parasitic wire elements was too much for the stinger at the top of the tower driven element. Unequal ice weight on the four wire elements was a factor, probably a result of partial tree cover on the southeast element and the variety of rope diameters being used.

This is the second stinger to fail. The first one failed due to expedience: I made a couple of poor material choices because I was in a rush at the time. The second stinger survived winds well over 100 kph last summer and I thought it was strong enough to last a while. However ice is often a greater hazard than wind, and around here ice is more common than high winds. Obviously I did not design the stinger well enough.

The stinger failed where the 1.5" OD tube joins to the 1.9" OD pipe below it. This is a high stress point. After tearing the antenna apart and lowering the broken version 2 stinger I inspected the break.

I thought I had used 0.095" wall tube for this section. Turns out it was no more than 0.065" wall. Thinking back to when I built it I remembered that the 1" PVC pipe above it was slightly too large at 1.315" OD to fit inside the 0.095" wall 1.5" tube so I substituted a thinner wall tube. Amateur forensic analysis of the bent tube leads me to believe that this surplus tube is not 6061-T6 unlike the standard pipe sizes I've accumulated over the past few years.

We just had two days of balmy 5° C weather that was perfect for tower work. I dropped other projects to focus on repairing the 80 meter yagi. As the sun set on the second (and last) mild day the antenna was back in service. I had to work fast and not make the same expedience driven error I made the last time.

The version 3 stinger is exactly the same length as version 2. This is important since now that the antenna is tuned and the matching networks are fixed changes would consume far too much time and be very uncomfortable in our winter weather.

I reused the lower 1.9" OD pipes since they were undamaged. The lower pipe was left on the tower. The broken 1.5" tube required effort to remove since it was distorted by the bend. It was replaced by a 1" schedule 40 6061-T6 pipe and reducer already fabricated for another antenna project.

I slipped it into the 1.9" pipe and drilled the new pipe through the outer pipe's existing splice holes. This was quick and allowed the stainless hardware to be reused. Beware cutting debris on stainless threads since that is guaranteed to seize and destroy the fasteners. I learned this the hard way some time ago. Brush the threads clean before tightening.

The 1" pipe is shorter than the old 1.5" tube. I cut the undamaged top end of the 1.5" tube to reused the PVC top support and make up the missing length. The top segment of catenary ropes could be left in place which saved a lot of time.

With the broken tube as a length guide the tube and pipe were drilled and screwed together. Now all I had to do was install the new stinger. This is the most difficult part of the repair job.

Despite a couple of fumbles the new stinger was installed in a couple of hours. The dangling ropes inevitably tangled and had to be carefully separated and then held apart while I went up and down the tower attaching and suspending the four parasitic elements.

With that done I pushed the stinger to its full height and tightened up the tower clamps. Some lateral adjustment was needed since the force of the ice pushed the clamps out of vertical alignment.

Back on the ground I walked back and forth among the four element anchors carefully pulling them to working tension. It is important to do it in steps so that the stinger is not put under bending stress. It's tedious but necessary.

Finally it was done as the sun set. I cleaned the site, put away my tools and went indoors to check it out. The SWR was perfect in all of its directional and omni-directional modes. Mission accomplished.

Losing this antenna worried me because it keeps me competitive on 80 meters. The high inverted vee performs poorly on paths longer than 2000 km.

Now I have to hope for the best with stinger version 3.0. It is stronger but still a concern since it is quite tall and under stress during adverse weather events. Ideally I would like to increase the tower height and use a short stinger. It's in my plan though perhaps not this year.

All the ice storm damage to my antennas has now been repaired. It could have been worse and for that I'm thankful.


Tuesday, December 31, 2019

Ice Storm

This is not the article I wanted to finish the year with. On December 30 we had an ice storm that deposited from ¼" to ½" ice on all my towers and antennas. Unfortunately there has been damage. With all the tree limbs and chunks of ice falling it's still too hazardous to do a full inspection, not to mention the treacherous surface ice.


Freezing rain requires a fine balance of atmospheric conditions. There was little of it 100 km north in Ottawa and 200 km to the west. It's very possible that I am the only ham with a large station that has been affected.

The worst was to my new 80 meter vertical yagi. The stinger at the top of the tower folded from the weight of the ice on the parasitic wire element support ropes. Yet the weaker PVC pipe at the very top, to which the catenary ropes are attached, survived.

Shortly before the failure I tried to shake the ice loose because the stinger had a pronounced bend. Ice tumbled off the smooth surface of the insulated wire elements but was hooked deep into the rope fibres. An hour later the upper 1.5" × 0.095" wall tube collapsed at the joint with the larger pipe below.


This is disappointing since I thought it would withstand ice as well as it has 100+ kph winds. In this region ice is a greater menace than wind. The weight of ice on those long ropes is substantial. The towers themselves are fine since the ice is a modest addition to the tower weight and 1000 lb guy tension. Self supporting towers are more at risk should the wind blow hard while ice is present.

Luckily the yagis held up to the abuse. The elements bent quite a lot under the weight of the ice but bounced back afterward. The tips on the Hy-Gain yagis worried me since they're thin and low tensile strength aluminum alloy. They will break off with severe ice loads.

The XM240 elements curled downward quite a lot then bounced back as the ice broke off. Notice the condition of the trees in the photograph below. The boom of the 6 meter yagi above it was also sagging. The foreground guys are twice their normal thickness.


With most of the ice now fallen or melted all antennas other than the 80 meter array test fine. The SWR of the 80 meter vertical is low enough at 1.7 to at least be usable in its omni-directional mode. Wire antennas have stretched from the high load and will have to be tightened. Going by the SWR the stretch is in the ropes and not the soft drawn copper wires.

If repairs to the 80 meter yagi have to wait until spring I can fall back to the high inverted vee. Hopefully there will be enough mild weather to permit repairs to be done. A thorough upgrade will have to be scheduled later in the years. Antenna repairs will inevitably slow the pace of work on new antennas.

It's a somewhat sombre Happy New Year at VE3VN. See you on the bands in 2020.

Tuesday, December 24, 2019

Tuning Big Yagis

Among the many projects simultaneously underway as 2019 draws to a close is the completion of my 15 meter and 20 meter stacked yagis. Design and construction of these home brew antennas took longer than expected so here I am working away into the coldest time of the year.

Progress was quite literally put on ice for over a month when winter arrived early and fierce. Although it's Christmastime I have been creative with my schedule to take advantage of a period of mild weather. I can even work outside without gloves, which is pretty good for our chilly climate.

Rough tuning of one each of the 20 meter and 15 meter yagis was done in unusually warm October weather with the help of friends. I rigged a temporary tram line and several ropes to manipulate the yagis to get them off the ground and relatively easy to access the feed points. Gamma matches were rough made to allow easy tuning using a variable capacitor.

For these monsters I found it easier to raise the yagis above ground in a horizontal orientation rather than attempt to point them vertically upward. This appears to be the preferred method of the friends I canvassed who have big antenna farms. You'll understand the challenge with these big antennas in the picture below taken in October when the weather was warm and pleasant.


This is my side mount 5-element 20 meter yagi with a 40' (12 meter) boom approximately 20' (6 meters) above ground. It takes four strong arms to haul this heavy antenna up the tram line for tuning. My ever dependable assistant Don VE3DQN (left) and Janek VA3XAR demonstrate how the ropes are used to swing the antenna. The feed point is reachable from the ladder when the boom is pulled downward. A short run of coax and an analyzer are attached.

Surrounded by guys and the tower the antenna must be carefully oriented for accurate impedance measurements. Best results were with the yagi pointed slightly upward and away from the guys, as shown above. Assembling the guys with non-resonant segments in any HF band is not enough to completely prevent deleterious interaction.

In this article I will discuss how high a horizontally oriented yagi needs to be raised for reliable impedance matching, and then how to adjust the physical antenna so that it performs according to the computer design. For this exercise I'll focus on the 5-element 15 meter yagis since this is the one I first ran through the full process to prepare it for use.

The side mount 5-element 20 meter yagi has been successfully rough tuned. It needs a permanent gamma match and further adjustment before being raised. I will gloss over the details of the gamma match designs and tuning process since it is a topic well worth its own article. Had I known what I was getting into I might very well have opted for a different feed system!

How high?

As we saw with pointing a yagi up there is little advantage going higher than the reflector being λ/4 above the ground. This works since field cancellation off the rear is typically high so that all we need is a modest reduction of mutual coupling with the non-resonant ground to achieve an impedance close to that in free space or high up a tower. A horizontal yagi is different since there is substantial radiation downward and therefore interaction with the ground reflection.

There is no general rule since yagis of unequal size and configuration have different elevation patterns. Luckily it turns out that you don't have to go too high for reliable impedance measurements. Performance metrics of gain and pattern need a little more height. The height of the 20 meter yagi shown above is sufficient for impedance matching.

Let's take the 15 meter 5-element yagi and model it at several heights. Comparison of the SWR curves is compelling. You can reference the linked article for further detail about the antenna design. The current model includes the actual tubing schedule. Although a beta match is used in the model there is negligible difference from the gamma network used in the physical antenna.


It is perhaps surprising that you need only go up 15' (4.5 meters) to have an impedance curve similar to that in free space. At 20' (6 meters) the difference is negligible. It is possible to rough tune the impedance even lower and do the fine tuning a little higher up if that is helpful. For the 20 meter antenna simply scale these heights by the wavelength ratio: ~1.5×.

I took measurements at both 15' and 20' with the gamma match adjusted close to its final setting. Pictures of the actual setups for the measurements are included.


There is a 9 meter length of new LMR400 hanging from near the boom centre. The AA54 analyzer is on an empty cable reel. The reels keep the antenna off the ground and protect the fragile gamma match. Ropes at both ends of the boom are used to orient the yagi.

Adjusting the SWR

As a general rule do not adjust a yagi for minimum SWR at the centre of the band or, on the low bands, the centre of the design frequency range. The R and X impedance components rarely change symmetrically on each side of centre: the SWR curve is not the perfect parabola often depicted.

My 15 meter yagi is an example of a wide band high performance design that exhibits two SWR minima. This is not unusual for optimized yagis with 5 or more elements. Adjusting the matching network for minimum SWR at band centre results in an inferior outcome.

Assuming the antenna matches the model (see next section) you should adjust for an SWR of 1 at the frequency where the model shows its lowest minimum. For my 15 meter antenna that frequency is 21.100 MHz. When adjusted that way and with the physical antenna matching the model the SWR curve across the band should match the model. For commercial antennas proceed as the manufacturer recommends.

Once you have the matching network at the sweet spot raise the antenna higher and confirm that the SWR curve across the band remains where it should be. Lower and adjust as necessary, then repeat. Make sure the components of the network cannot move around during and after adjustment. Yagis are finicky beasts and it takes very little motion of the network components and antenna elements to spoil perfection.

That said, getting to an SWR of precisely 1 is not necessary. The way antenna impedance typically varies with frequency you'll notice that although the minimum is a little high there is almost no impact on the SWR where is it normally higher. A few ohms of R or X make little difference where the deviation from 50 + j0 is greater.

More important is that the SWR across the band be below your chosen maximum, or what the design or manufacturer promises. Ideally it should be less than 1.5 everywhere, especially for a contester like me. Then you won't have to worry about tuners for your rigs and amplifiers as you change bands and frequency.

Interactions with guys, towers and other antennas will upset the SWR once you move it into position after it has been tuned. If you've planned well the change will be inconsequential. If the change is large there is no point in readjusting the impedance match since the problem lies elsewhere. Find that interaction and fix it. A deviation of the SWR often indicates that the pattern is being degraded by an interaction.

Confirming the design

For the typical amateur directly measuring and optimizing the pattern of an HF yagi is difficult and almost always avoided. I am no different. I rely on software models and careful construction for my home brew antennas. Even with NEC4 it is nigh impossible to get the physical antenna to exactly mirror the software model. With NEC2 and SDC (stepped diameter correction) the divergence can be worse when good modelling practice is not followed. NEC2 has numerous quirks.

I use EZNEC with the NEC2 engine and the supplied SDC algorithm. These antennas came surprising close to the model which was a great relief. But how do I know since I cannot do a field measurement of the pattern? There are ways to go about it so that one can be confident even without a direct measurement.

Impedance is easy to measure with accuracy using modern antenna analyzers. Fortunately impedance holds the key to an indirect though quite good method of confirming the model. Refer back to the SWR curves earlier in the article for the following discussion.

If the antenna impedance is adjusted as described earlier the SWR curve will closely match the modelled antenna for the antenna reasonably high and in the clear and a software matching network that follows the same procedure. In the EZNEC model I use a beta (hairpin) matching network since unlike a gamma match it can be reliably modelled, it closely mimics similar matching networks such as L-networks and gamma matches and doesn't preclude use of SDC on the driven element.

Although the curves appear to match there is an important difference. In the model the second dip is at 21.410 MHz and is around 21.450 MHz in the physical antenna. Assuming the yagi has been constructed per the design the divergence is most likely due to element self-resonance and not interactions and ground effects. A broader measurement spectrum is useful at this point so I raised the antenna higher and measured the SWR up to 21.600 MHz.


What we have is an impedance inflection point at 21.450 MHz. Above this frequency the radiation resistance drops sharply and the resulting SWR cannot be corrected with the matching network; no simple network can tame that slope while also matching the antenna within its design range. The software model exhibits the same behaviour.

The inflection point is a proxy for the true frequency range of the yagi. You'll find an inflection point like this in almost every yagi, perhaps two or three of them. Their presence at the correct frequency is strong evidence that the yagi is tuned for optimum gain and pattern. If not the antenna elements require adjustment.

Here we have the inflection point 0.15% higher in frequency. Considering all the small construction inaccuracies, reactance "bumps" from all the hardware, elements curving under their own weight the software did a remarkably good job predicting the yagi's real behaviour. In practice this small a difference can be ignored. I didn't ignore it.

Calculation suggests that the antenna elements should be lengthened by a little more than 1 cm (½") to bring it into agreement with the model. All the half elements tips were lengthened by ¼", except for the driven element: the DE length affects the impedance match not the gain and pattern. I adjusted the second untested 15 meter yagi at the same time so that I don't forget to do it later.

The yagi was lifted and measured. The improved match at 21.100 MHz is due to bumping the gamma match whose components at the time were not fully tightened. That was dumb luck.


I call this a tremendous success. Now I have confidence the antenna will perform as designed. Perfection like this isn't necessary but I do enjoy being presented with a measurement that so nicely mirrors the design. It makes me feel good after all the work that went into this project.

Next steps

For this tuning process the tram line was moved higher up the tower so that once the yagi is ready (choke, coax run, truss) no rigging change is needed to haul it up to the waiting side mount bracket. If the weather and my luck hold that should happen before the new year arrives.

There are two LDF5 Heliax run to the new tower ready for use. They are overground until the spring when a trench will be dug for burial, including control lines and rotator power. For now I will directly connect the side mount yagis to the transmission lines and add the stacking switches later.

I hope to have the 20 meter side mount yagi tuned and raised in January in time for late winter contests. The antenna is quite heavy and I'll need a couple of helpers, none of whom are (not surprisingly) unavailable this time of year.

Once all that is done I can tune the upper 15 meter yagi at my leisure and assemble the upper 20 meter yagi. A better and stronger boom for the upper 20 meter yagi is built and ready. As the weather allows the mast will be raised and then the upper yagis lifted. That may have to wait for warmer spring weather.

I will end here and prepare to wrap up the blog for 2019. Expect a year end review article before or shortly after January 1. Merry Christmas, Happy New Year and see you on the bands.

Wednesday, December 18, 2019

FT8 - The Universal Solvent

FT8 keeps eating away at the bands, one ham at a time. Like the mythical universal solvent it cannot be contained: FT8 dissolves every container traditionalists attempt to put it into. The digital wave inexorably marches onward.

Lately I've succumbed further. Until now I've restricted my use of FT8 to 6 meters. With the long winter nights of a deep solar cycle minimum there are only the low bands available most of the time. I enjoy the low bands yet it can get tedious outside of the excitement and intensity of contests and DXpeditions.

Every night there are same stations working each other on CW. Top band aficionados continue their vigil for propagation and welcome all comers. The regulars exchange signal reports and, this time of year, supplement that with seasons greetings, wishing MX and HNY to all. Activity briefly spikes to include a broader range of stations during sunrise and sunset enhancements.

It's all very cozy. I have good antennas for 80 and 160 so I can hold my own even without enhanced propagation, although I would benefit from more receive antennas (they're coming, eventually). With my amplifier out of service until parts arrive it is a little more difficult to work DX in the everyday challenging conditions.

Then there's FT8. I have taken to monitoring 1840 kHz many evenings just to keep an eye on top band propagation when I am busy doing other things and I'm uninterested in pursuing routine CW QSOs. Of course the inevitable happened: one day I hit the Enable button in WSJT-X. My log has begun filling up with top band FT8 QSOs.

The breadth and depth of activity is startling for anyone daring to venture beyond the traditional modes. In amongst the multitude of call signs never heard on CW there can be found familiar call signs of contesters and DXers. The DX to be found is itself quite surprising. Every night I hear UA0, Africa, South America and in the mornings there's the Pacific and Far East.

Try to find these distant stations on CW and you will be disappointed. It isn't that FT8 is so much better than CW (it isn't). You can only work what's there and what's there is on FT8. The clear implication is that many so-called difficult propagation paths on 160 meters aren't really difficult at all, there's just no one active on CW.

The transition to digital modes is less extreme than on 6 meters, so far. To escape from routine QSOs with the regulars it is necessary to spend some time on FT8. My top band FT8 log is filling up with DX QSOs and DXCC countries I rarely hear on CW outside of contests. In a way it's sad that the hobby is changing yet exciting in that digital modes are spurring activity from newcomers and old hands alike. That's a good thing.

Will CW survive? Perhaps until 2040 when most of the older generation will have passed on. There are not enough young people entering the hobby with an interest in CW although it may survive among a small minority. Obsolete technologies do attract some among the younger generations, whether it be vacuum tube appliances, vinyl records or mechanical clocks. CW will have its adherents as well for many years to come.

I will continue to spend a portion of my top band time operating FT8 although CW will remain my first choice. Two nights ago I heard A50BOC on 160 meters, barely audible and not workable and it was exciting to hear. CW signals from JA and HL are far more attractive to me than FT8 despite the difficulty of making the QSOs. However I will go where the activity is, just as I did on 6 meters.

Okay, that's enough philosophical rambling. During my short time on 160 meter FT8 I've been learning a few things. Operating there is not the same as 6 meters. Openings are longer, the atmospheric and man made QRN dominant, QSB slow and deep and the activity is far greater most of the time. The spectrogram shows a busy 1840 kHz on a weekday evening.


Reciprocity of station capability is less than on higher bands. Decoding a station does not mean they can decode you, and vice versa, when your power and antennas are comparable. This is as true for FT8 as it is for CW and SSB.
  • Local QRN can differ by 10 db or more. This varies by time of day, latitude, urban/rural locale and other factors well known to low band operators. Don't be surprised when some stations don't answer you.
  • Many top band hams are unintentional alligators since, apart from the above QRN factors, most do not have low noise (directional) receive antennas.
  • Too many stations call on the CQing station's transmit frequency, which often means none of them are successfully decoded. I don't know why this seems to happen more on 160 than 6 meters, or perhaps I am suffering from selective memory.
  • Clear frequencies don't last long in that 3 kHz FT8 window. It is commonplace to have someone start transmitting on another station's transmit frequency and time slot despite signal levels implying that they must be able to hear the other station. Everyone suffers as a result. 
  • You can see a couple of poorly adjust transmitters in the spectrogram above. It is often worse.
  • DX stations are regularly covered up by nearer stations that cannot hear them and think the frequency is clear. There is no QRL? equivalent on FT8. If the spectrogram looks clear (or not) away you go.
Some of the problem is poor operating though mostly it's just regular hams doing the best they can with what they have on a band with difficult operating conditions. It's all a part of the game so get used to it. Those with skill and superior antennas have an advantage as they do on any mode, on any band and whatever the prevalent propagation. Experience and practice make a difference.

While it's nice to try something new and work new stations I don't take FT8 operating on 160 meters too seriously. That may change if the migration from CW continues. If it does I may have to concentrate on 160 meter FT8 for real just like I now do on 6 meters. I intend to hold off on other bands for a while longer, hopefully a long while.

Change is good even when it makes us uncomfortable.

Thursday, December 12, 2019

Performance of the 80 Meter 3-element Vertical Yagi

The 3-element, 4-direction vertical yagi I recently completed is not a perfect antenna although it does perform very well. It has its pros and cons. I learned a great deal designing and building it, which was one of my main objectives apart from putting out a competitive signal on 80 meters. The antenna is a variation of the K3LR array described in ON4UN's Low Band DX'ing book.

No antenna stands on its own merits; every antenna must be compared to alternatives. For this discussion of the yagi's performance I will use the big gun antenna standard for 80 meters, the 4-square.

This is the sensible baseline since it is important how I do relative to other serious contesters and DXers. It makes little sense to compare the yagi to an inverted vee -- of course it's better but the comparison is of little value.

This article is not a mystery novel so I will put the answer right up front: the 4-square is superior on the majority of metrics. That said the details of the comparison can be subtle and enlightening for those with a passion for antennas. A truthful comparison helps direct my future plan for 80 meter antennas. That will be briefly addressed towards the end of this article.

Let's start with the basics before delving into details.

First up is a fundamental of physics: conservation of energy. For antennas of equal efficiency a corollary is as follows:
To achieve gain in one direction requires taking energy from other directions. Conservation of energy informs us that to achieve gain the antenna must be directive, and vice versa. The two are inextricably linked. The 4-square's better directionality largely explains its gain advantage over the 3-element yagi.

However it is not quite that simple. Dropping a secondary lobe from -10 db to -20 db (assuming for the present argument there is only one lobe other than the main lobe) the main lobe energy increases from 90% to 99% of the energy. This is an almost negligible gain improvement of 0.4 db. Reception improves but not transmission effectiveness.

Further gain improvement requires narrowing the beam width of the main lobe. For a non-rotatable antenna like a 4-square or wire yagi too narrow a beam width can be detrimental since there will compass points where gain is poor.

Although the 4-square is more directive than the yagi the gain improvement is not substantial. The better gain of the 4-square mostly comes from other differences between the two antenna types. Both have sufficiently modest gain/directionality that 4 direction switching covers 360°.

With that fundamental observation made let's look at how the antennas differ. There are several factors:
  • Element spacing: On a side the 4-square element spacing is 0.25λ, and the diagonal spacing is 0.35λ. For the yagi the element spacing is 0.125λ. The closer spacing of the yagi increases the mutual coupling. This is required in a yagi but not is a 4-square.
  • Element shape: It is typical to use straight elements in a 4-square although that isn't necessary. The yagi has a straight driven element and sloped T-top loaded parasitic wire elements. Again, that is a choice not a requirement. Element shape and diameter effects both antennas and we will have to normalize them to make a fair comparison.
  • Forcing: Yagis work by mutual coupling alone. The 4-square uses phasing lines and combiners to engineer phase and amplitude of antenna currents. However mutual coupling exists in a 4-square and is a significant factor in its design and engineering.
  • Ground dependency: The antennas behave differently for the same radial system (near field). Distant ground (far field) effects are the same for both.
These are the major electrical factors. There are also other factors, such as cost, that must also be considered.

Notes on modelling

All the software modelling is done with EZNEC. Medium ground (0.005, 13) is used throughout even though the ground conductivity in my rural locale is better than that. A fair comparison depend on a standard environment.

MININEC ground is used rather than "real" ground so that the radial system can be easily modelled as a resistance load at the base of each element. MININEC assumes a perfect ground with respect to the near field. The resistance accurately represents the equivalent series resistance (ESR) of the radial system and ground beneath. But you have to know the ESR of your radial system. The model departs from reality for a small number of radials since they affect the antenna resonance. These effects must be compensated for during antenna construction and testing.

There is loss in more than just the ground. Wire elements have non-negligible loss whereas tower and tubing vertical elements have negligible loss. Coil, capacitors, phasing lines and hybrid combiners each contribute loss. In particular the 4th port of the hybrid combiner used in most 4-square antennas goes to a 50 Ω dump load, which can be as lower gain by as much as -0.5 db at the band edges, though -0.1 to -0.2 db is more typically .

Since this is comparable to the approximate -0.15 to -0.2 db resistance loss in the wire yagis elements I will treat them as equal, and leave them out of the antenna comparison. The 4-square model is adapted from one packaged with EZNEC uses lossless phasing lines and no combiner or dump load.

The azimuth pattern comparison is typical. The difference in practice has many factors, as listed above. For my current radial system the 2 db difference of the inner plot is a fair representation. In other configurations the difference can be better or worse and in the ideal can approach equality. We'll come to that later in the article.

The elevation patterns are similar for both antennas. This is primarily determined by ground quality and topography outside of the antenna's local environs.

Turn a yagi on its side

Verticals arrays -- yagis and 4-squares -- have relatively poor side lobes in comparison to horizontal arrays. Many of you know why that is but let's review it anyway.

A dipole has low radiation off its ends. An array made of dipole elements is the same since adding nothing to nothing equals nothing. Therefore the typical horizontal yagi has deep side nulls. In free space the elevation pattern has quite a lot of radiation directly up and down. For a typical 3-element yagi in free space the blue plot is the elevation pattern and black is the azimuth pattern.

Over ground the way to remove the high angle radiation is to place the yagi at a height that is an odd multiple of λ/2 so that the ground reflection is out of phase with the direct wave resulting in cancellation. At intermediate heights there can be substantial high angle radiation, and that is rarely desirable.

Rotate the boom 90° and the elevation and azimuth patterns are swapped. That is in essence what you have with a vertical array: lots of radiation off the sides and very little at high elevation angles. In a conventional vertical yagi like my 3-element 80 meter antenna radiation to the sides is worse than shown in the adjacent plot.

For a driven array such as the 4-square it is possible to reduce the side lobes. Thus a 4-square can have better directionality than a 3-element yagi. Of course the 4-square has one extra element, which may seem an unfair comparison until you consider that the two antennas are of similar size.

Element spacing

Comparing element spacing of the two antennas can be confusing since although they occupy a similar area the yagi has a fifth element in the centre -- the driven element -- and two of the elements are inactive. Further, because two elements are inactive the 0.35λ spacing between adjacent parasitic elements is irrelevant. The element spacing is 0.125λ for the yagi and 0.25λ for the 4-square, a ratio of 2.

The significance is that the mutual coupling between yagi elements is higher than the 4-square. The elements can be more widely spaced to equalize the antenna footprints. This would increase the boom length to 0.35λ (0.175λ element spacing), a length that is near optimum for a 3-element yagi. That does indeed improve the yagi's performance, as we'll see.

In addition to achievable gain the increased spacing modestly improves F/B. Of perhaps greater importance is that the mutual coupling is reduced which increases radiation resistance, and that lowers antenna currents and I²R ground loss. Driven at 1000 watts the typical 4-square element current is ~2.5 A. Currents in the yagi elements cover a wide range, from as low as 1.5 A to as high as 9 A, with more typical values between 2 A and 7 A.

The gain improvement of 0.175λ yagi element spacing is ~0.6 db (perfect ground), which is marginally significant. Reduction in ground loss results in greater efficiency for the same radial system. Gain improvement is greater with a poor radial system and less with a better one.

Element shape

The sloping T-top wire parasitic elements are convenient since it uses the driven element as the support structure. It comes at a performance cost since the element shape is not optimal. There are two problems:
  • Radiation resistance: The acute angle on the lower side of the T causes field cancellation with the monopole part of the element. Field cancellation lowers radiation resistance and this increases loss in the radial system and to a lesser amount in the element wire.
  • F/S: There is a horizontal component to the azimuth pattern due to the T which lowers overall directionality by increasing radiation to the sides and rear.
The lower acute angle (close to 45°) requires NEC4 for accurate modelling. With the more commonly used NEC2 there is a significant discrepancy so the wire elements dimensions must be determined in the field. NEC4 isn't perfect but you will get close.

Modelling with EZNEC predicts an approximate 5 Ω reduction of radiation resistance from 31 Ω to 21.5 Ω. compared to a straight wire element. With my analyzer I measured 25 Ω including an estimated ground loss no worse than 5 Ω. For a 5 Ω radial system the loss is 16% versus 14% with straight wire elements. Although that's small the loss multiplies for poorer radial systems and in a yagi where the radiation resistance is lower and the current higher.

A comparison of straight wire elements versus the T-top wire elements was discussed in a previous article. Look there for the relevant charts since I won't reproduce them here. You will see that directionality and gain are better with straight elements, especially directionality . Unfortunately straight elements are not easy to make from wire due to the need for suitable supports. A coil loaded shorter straight element is feasible except that efficiency is far worse. If you go to the trouble of rigid parasitic elements I believe it is more sensible to build a 4-square rather than a yagi.

Forcing

Yagis rely on mutual coupling alone to achieve current amplitude and phase for desired behaviour. Current forcing is a feature of driven arrays. Since there is substantial mutual coupling in a 4-square it is not a purely driven array; the elements would have to be much farther apart for that.

Forcing is simple in its basic concept. The generator always sees a single impedance. By tying all the elements to the feed point the amplitude and phase is uniquely determined. Networks between the feed point and elements set the amplitude and phase to achieve the desired behaviour.

It is quite complicated since you want a 50 Ω load for the generator and accurate power splitting and phase across 4 elements with network that must sustain complex loads (high voltage and current) at high power and with direction switching. Elements must be made as identical as possible. Not many hams design and build their own 4-square control systems!

The EZNEC model used in this article is adapted from one provided by W7EL with the software. It uses fixed phase lossless transmission lines. This is impractical for real antennas due to the direction switching challenges and the frequency sensitivity of the phasing lines. Hybrid combiners are more suitable.

More than you could ever want to know about 4-square design and hybrid combiners can be found in ON4UN's Low Band DX'ing book. For the present discussion I will only mention a couple things. First, the phasing lines experience high SWR and have attendant losses, although those are low with good quality coax at 3.5 MHz.

Second, hybrid combiners are not lossless since frequency dependent imbalances among the 4 elements present at a 50 Ω port where a dummy load dissipates the power due to the imbalance. A failure in one element or icing can cause a large increase in the the dump power. Monitoring or protective circuitry is important.

Modern 4-square controllers usually offer an omni-directional mode in addition to the 4 directions, just like I built with my 3-element yagi.

Ground dependency

Ground ESR in series with the antenna impedance is the most important factor affecting the yagi in comparison to the 4-square. For the same radial system the ground loss for the yagi is higher, and it can be substantially higher. That is due to the low radiation resistance due to the aforementioned factors: element shape and mutual coupling. The better the radial system the closer the yagi's performance to that of a 4-square.

The yagi should have a radial system ESR of less than 5 Ω and lower is highly desirable. In my antenna I have twice the number of radials on the driven element as the parasitic elements since currents are highest in that element. Current in the yagi elements can be more than 3 times higher than in the 4-square. If you are limited in how many radials you can put down go with the 4-square.

Measuring the ESR of a radial system is difficult. My estimate for those in the yagi is based on the trend line of element self-impedance as radials are added. This is a common technique and usually the only practical one. The measurements suggest that the driven element radial system is in the range 2 Ω to 3 Ω, and that of the parasitic elements 4 Ω to 5 Ω. For modelling purposes I use the values at the high end of these ranges.

I will keep it simple and state a few modelled comparisons rather than draw up a bunch of charts. As a baseline with a perfect radial system of 0 Ω the 4-square has approximately 0.5 db more gain than my style of yagi, assuming the previously described internal loss typical of the 4-square and yagi. For a 5 Ω radial system the 4-square gain declines by 0.5 db and the yagi gain declines by 2 db. Therefore with a large but not extreme radial system the 4-square gain is better by 2 db. There is frequency sensitivity in these figures for the yagi so I took the average.

That's a substantial difference. With a smaller radial system the difference will be larger. You need a lot of radials to make the yagi perform well. As I said in an earlier article that although the directionality of the yagi is lacking it is of little consequence in contests since I can work stations off the back and sides with good success and that puts more QSOs in the log. Receive performance is compromised so it is occasionally helpful to use a high directionality receive antenna.

Pros and cons vs. the 4-square

This list is a set of subjective and objective observations of the yagi versus the 4-square. You may disagree with some points or weigh their importance differently.

Pros:
  • Low cost
  • 20% less land use
  • Flexibility of direction choice, more than 4 directions, and ability to add more directors
  • No dump load or phasing harnesses: all the power is radiated or lost in the ground
Cons:
  • Lower efficiency for the same radial system
  • Must home brew: there are no commercial control systems
  • Directionality and gain
I believe most hams would, and should choose the 4-square. It is possible to build it with wires to reduce the cost with external supports or with elements similar to that used in my yagi.

The decision is not quite so straight-forward since my yagi design is not the only one. The yagi can be improved in various ways.

Alternatives

The yagi will remain as it is for some time. There are too many antenna projects for the next year to worry too much about 1 or 2 db. What I can do, now that I am indoors more often due to the weather, is to explore alternatives. Alternatives range from the highly disruptive to modest.

Taller centre support to allow straight (unloaded) elements

Straight wire elements increase gain by ~0.5 db. Side lobe radiation is reduced almost to that of the 4-square. Directivity is improved so that that it comes close to that of a 4-square though frequency dependent. The taller central tower can serve as an efficient 160 meter antenna with suitable switching to retain its performance as an 80 meter omni-directional vertical and yagi driven element.

Increase boom length so that it covers the same area as the 4-square

Element spacing increases from 0.125λ to 0.175λ, for a boom length of 0.35λ. Gain increases ~0.6 db and directionality is improved. Of course the radials for the parasitic elements must be relocated, and that job takes several days. The increased spacing permits converting the array into a 4-square. The central tower can be used as a simple support, or continue as an omni-directional vertical (if the commercial 4-square controller doesn't have this feature) or as a 160 meter vertical. For the latter case modelling confirms that a central 160 meter vertical does not affect 4-square performance on 80 meters.

Bent elements

Removing the lower half of the sloped T-top loading section on the parasitic elements has several advantages. Parasitic element efficiency because the radiation resistance rises from 21.5 Ω to 31 Ω. Gain increases 0.6 db with the existing radial system. Directionality is within a few decibels of a 4-square. There are no mechanical changes. Parasitic elements must be tuned for different self-resonant frequencies and the L-network adjusted to compensate for the higher feed point impedance.

More radials

This is perhaps the simplest alternative. By doubling the radial count the ground loss is reduced. In addition to a gain increase of 0.6 db the directionality is modestly improved. Apart from laying the wire the L-networks must be adjusted for the lower feed point impedance and the parasitic elements retuned to the desired self resonant frequencies. Wire isn't always cheap and doubling the radials will require 1600 meters of wire and many days to install them.

Paths forward

The described alternatives can be combined for further performance improvement. For example, by doubling the radials, using bent elements and a 0.35λ boom length the gain comes within 0.5 db of a 4-square and directionality is similarly close.

One or more of the alternatives will be explored in depth in future articles. This one is already long enough and I don't have the time right now. Although I may eventually go for a 4-square I am not done exploring yagi designs. Modifying the existing yagi is far easier than rebuilding.

A little more gain and directionality would be beneficial, especially on receive. It would be advantageous during contests to keep receive antennas primarily for 160 meters to reduce contention between operating positions. In contests I often use the northeast Beverage while working Europe to improve copy of the weakest callers.

In conclusion there are many ways in which the yagi can be improved and experimented with. In that light this is an ideal antenna for me and my interest in antenna design. A 4-square with a commercial switching system is not so interesting to me. Others may have different objectives.

Wednesday, December 4, 2019

1,000,000 Point Coax Connector

The weather broke for long enough for lowering the XM240 40 meter yagi from my Trylon tower. I previously noted that I lost the antenna during the CQ WW CW contest, probably due to a loose connection at or inside the balun. It has happened once before. Losing this antenna cost me a lot in the contest since I had no backup 40 meter antenna due to ongoing antenna work.

The first day I rigged a winch on the ground to handle the lowering and lifting of the 75 lb antenna. My lawn tractor could not be used because its weight is too low for traction on snow. On the second day I climbed the tower with the cable and other ropes to rig the antenna. Before doing that I disconnected the coax at the rotation loop.

It's a good thing I did that first. I had a good look at the connection once I had the weatherproofing removed. At that point I could have dropped the cable and rope to the ground since I knew I wouldn't need any of it.

The problem was the connector. In a way that was fortunate since I really didn't want to lower and lift this yagi in the cold and snow, not to mention the difficulty of getting friends out in this weather and their increasing family commitments due to the approaching holidays. It's a busy time of year.

The UHF female barrel connector is old and not the best quality. Notice that the outer edge has only 4 indentations; the best have a continuous set of indentations. There are typically 2 or 4 matching tangs on the male PL259. When engaged they resist the connection from unscrewing from twisting and vibration. Failure to engage those tangs promises future problems, perhaps sooner than you think.

On one side you can see the orange discolouration and grime from past water damage. The dielectric should be white. Despite the appearance this is the side that is working properly. We have to turn it over to discover the proximate cause of the problem.

At first glance it looks good. A closer inspection reveals discolouration surrounding the centre conductor. By eye (obscured by the camera exposure) it is clearly visible. The same discolouration is on the centre pin of the mating PL259 from the rotation loop. It comes from vapourization of the metal plating.

Sliding the two connectors together and apart a few times tells the tale. There is almost no contact between them. The springiness of the female receptacle tabs is gone. This leaves a gap and hence the intermittent continuity, and arcing when running high power.

The barrel connector is old. How old I don't know, possibly decades. I pulled it out of a bin of new and used barrel connectors I keep handy I ought to have tested it before putting it to use. It is the same barrel connector that was on the yagi when it was atop the big tower, where it also was occasionally intermittent.

With the weatherproofing on the feed point side in good condition I never inspected it as I moved the antenna from tower to tower. The intermittency followed. Each time I managed to convinced myself that other known problems such as dirty relays were responsible. Assumptions are dangerous weapons.

I picked a new (and better quality) barrel connector from the bin, tested it and installed it between the rotation loop and the coax running along the boom to the balun. Done? No, not yet. Troubles never travel alone. They enjoy company.


After testing the new barrel connector -- it was good -- I proceeded to inspect the connector on the rotation loop. It was not good. In addition to water damage there was evidence of arcing. Worse yet the body of the connector rotated. Pulling back the outer shell I saw that there was more corrosion inside the PL259 solder holes and the solder had no hold on the coax shield.

Now thoroughly irritated with myself and the chill from working on the tower in cold weather I removed the rotation loop and descended. In my workshop I pulled it apart and quickly realized I had to discard the coax and connectors. I made a replacement with better connectors and, after testing it, ascended the tower. Although it was a chilly 0 C with snow flurries it was would the best weather for close to a week. So I kept at it hoping to get it done before sunset.

After successfully testing the rejoined sections of coax with an analyzer I did a careful job of weatherproofing with my best materials, then routed and secured the new rotation loop. I descended and cleaned the site in the deepening dusk.

With some trepidation I entered the shack and moved the rig and antenna switch to 40 meters. This time the antenna worked perfectly. Assuming I did it right this time the repair should last. An hour later I made my first QSO with 4U1UN for a new one, counting from when I returned to the hobby in 2013.

I call it my 1,000,000 point coax connector (connectors actually) because that it cost me at least that much in the recent CQ WW contest. My claimed score is ~3.5M. Had I done comparably well on 40 meters the estimated additional 700 contacts and 40 multipliers would have lifted my score to about 4.8M.

According to the raw scores that would have moved me from position #55 to #37 in the single op unassisted high power category. That's a big jump though still not a great score. The higher score certainly wouldn't win me a plaque, not even for Canada. It's annoying though not a disaster. I had no illusions about winning or placing high in the standings.

In a big station there are so many parts that some mistakes and unexpected faults are inevitable. When the mistake is up in the air at the end of a long boom it can be costly. An expedient decision at a critical moment is all that it takes. It isn't easy to force yourself to slow down and test every little thing yet it saves time and pain in the long run.

I'll try to do better. We all should.

Saturday, November 30, 2019

CQ WW CW: What I Learned

Despite being at the bottom of the sunspot cycle the bands came alive this past weekend during what is arguably the biggest contest of the year: CQ Worldwide. This was the CW weekend. The contest was an opportunity to test out my station. In pursuit of that objective I turned on the amplifier. Comparison with the best stations and operators is enlightening.

I had no illusion about doing well. My station is incomplete, both inside and out, and I have just the one amplifier and so SO2R is handicapped. Worse, during the contest I lost one of my antennas due to a known intermittent that I did not yet find the time to repair. Nevertheless I soldiered on, operating for 42 hours out of 48.

In the first part of this article I'll run through the bands to allow focus on antenna performance. In the second part I'll cover everything else from propagation to station equipment and the lessons to be learned therefrom. No matter how long I've been a ham and a contester there is always something new to learn and, ideally, use the knowledge to do even better next time.

What I won't bother with is my claimed score and placing. That would only be of interest to me and so would bore readers, not to mention that it is not impressive. Whether or not you find these lessons useful I hope you will enjoy following along.

160 meters

Lately I've taken to operating on 160 more often with the amplifier. Having a close to full size vertical makes me more competitive in the pile ups. When conditions are favourable it is not difficult to generate long QSO runs on top band almost any winter night. I expected to do well on 160 during the contest and I did. Had I been better configured for SO2R my QSO total would have been higher since it would have allowed me to do more running of US stations.

My country total of 68 is excellent despite the best stations in this part of the world exceeding that by 40% or so. Americans run 2 db more power and assisted stations benefit from spotting and skimmers. The best stations have antennas with gain, utilizing 2 or 3 elements yagis and a few have full size 4-squares. Those I'll never compete with. Unlike my first forays with my 160 meter antenna in contests I am pleased to confirm that the antenna is truly competitive.

The Beverage antenna to Europe continues to pay dividends. I did not put back up my short west Beverage and I have not yet had the time to put up other receive antennas. Perhaps this winter. Having good ears is critical to working the many stations with lesser antennas that can hear me but have difficulties putting out a powerful signal due to the inefficiency of the small antennas that are typical on top band. That conditions were excellent Saturday evening certainly helped but since it everyone has the same benefit it did nothing for my competitive placement.

To do better is difficult and expensive. While I do plan on certain improvements those are low priority and are unlikely to provide more than a few decibels of gain.

80 meters

One unexpected lesson is that when running high power the 30 meter high inverted vee is useless: I do very well with the vertical yagi alone. This is despite its superiority for working nearby US stations and the comparative advantage of horizontal polarization before and just after sunset.

The inverted vee has a maximum advantage of perhaps 10 db, which is compensated for by the 10 db of the amplifier. Since these signals are already quite strong the extra 10 db isn't needed and mulling over which antenna to use is a distraction. The inverted vee remains valuable for low power and QRP contest operation and for select DX paths, just not for high power contest operation.

The 3-element vertical yagi performed very well. Although I cannot say whether my results -- 800 contacts and 76 countries -- would have been substantially lower with high power alone. With rare exceptions if I heard it I worked it. In many of the cases where I couldn't work a station neither could many of the big guns I heard calling at the same time.

A good example is the Sunday morning opening to the far east. There were many weak Japanese and a couple of RT0 stations that were being heard here yet they could hear few of us in this region. A few more decibels are needed to compensate for the difference in band noise: low here post sunrise and high there post sunset.

I will have more to say about these elusive decibels when I write my article about performance of the yagi and how it compares to alternatives. Overall it performed very well. I could easily establish long runs to Europe and its modest directivity allowed Americans to hear me when I was pointed northeast. I'm happy.

40 meters

As noted above this was my disaster band. Had I been seriously competitive I might have quit or refocused on a single band effort. Since this contest was a learning experience I persevered with this severe handicap. It is not possible to do well in this contest without 40 meters. The estimated loss was at least 700 contacts and 40 multipliers.

Unfortunately I had no backup antenna. The 80/40 fan inverted vee was converted to 80 meters only when reinstalled and the new rotatable dipole for 40 meters is not complete.

This is entirely my fault. When I had the XM240 on the ground I checked the connections and all seemed good, despite knowing that the Cushcraft balun previously experienced internal loosening of connection studs. My inspection was cursory because I had discovered a faulty relay in the antenna switch in the port used for this antenna and assumed that was the problem.

The fault was rediscovered soon after the antenna was raised to its new location. I isolated the problem to the antennas itself and knew it would have to come down. When the intermittent went away I delayed the work to focus on other projects (80 meters, new 15 and 20 meter yagis, etc.).

Now the weather has turned foul. These words are being typed just after I called my friends to cancel the repair job because of ice on the tower and a howling north wind. It will get done. After all, I originally put this antenna up in January!

20, 15 and 10 meters

At present I have two tri-band yagis for these bands: TH7 up 43 meters and a TH6 fixed approximately south up 22 meters. Unfortunately this is a poor combination for SO2R with high power and my limited filtering so I had to stick to one high band at a time. Had the 40 meter yagi worked I could have put a second radio on 40 late in the afternoon when both 20 and 40 meters were productive.

One thing I noticed is that just like on 80 meters, despite the high directivity of the yagis, with high power an enormous number of stations could be worked off the back. I could run Europe and US at the same time without having to switch antennas. When I heard a multiplier I called and worked them no matter the antenna direction.

Again, that 10 db power boost makes this possible. While any power boost is beneficial in this regard the bump up to a kilowatt is the ultimate since that is everyone's maximum power level. That is why, with a few exceptions, you can work it if you hear it.

There is really little more to say. The antennas worked well for what they are. One surprise is that the low yagi was superior on 10 meters towards the Caribbean and South America during Saturday's opening. This is unusual. Perhaps the reason is that sporadic E provided the first skip and that is typically better at higher elevation angles.

I expect improved results and operating flexibility when the new stacks are operational.

With the band by band breakdown covered I'll now move on to more general topics.

Running

It is no surprise that with high power running is easy when propagation exists. Indeed it is mandatory. From a strategic standpoint the challenge is not so much whether to run but when to run and when to hunt for multipliers and other stations. For those in an assisted class you learn to interleave calling spotted stations into the ongoing run, or runs in the case of SO2R.

Regardless of your strategy do not be so enamoured of your audience that you forget to hunt others form time to time. You may forget due to the joy of having many new mults call you when you run with a big signal.

At this stage in my education I run only one band at a time. The second station is for S & P. So far the only exception has been Sweepstakes CW where my rate was low due to running QRP. During CQ WW I almost always abandoned the S & P station when I had multiple callers to my CQ. I need more practice and better SO2R equipment.

Running is hectic since a bigger signal draws more callers. Common difficulties includes several callers zero beating each other and those who continue to call when I respond to someone else. The bedlam is a challenge even though it isn't nearly as bad as what DXpedition operators endure. Spot clickers may not call twice in a row, opting to click another spot when they fail to work you. They come back later. I've done the same when I was in an assisted category.

Although running can be fun and productive it is also a chore. The faster you can service your "customers" the longer they'll stick around to work you and the better your rate. For example, if you copy a partial call -- multiple caller QRM or distracted by the other radio -- it is faster to respond to the partial call with a full exchange, copy it in full next over and confirm the correction in the solicitation for the next QSO. Soliciting repeats to get the full call before sending the exchange should be limited to cases when only one or two characters are heard. The solicitation also tends to incite others to try again, and you don't want that.

It is good practice to send your call at the end of every QSO: "TU VE3VN". Passersby hear it and stop. I may interrupt the message after "TU" for one two QSOs when I have a few callers in the queue to work them faster and encourage them to stick around a few more seconds. You may reduce the bedlam by not sending your call as often -- passersby pass by when they don't hear it -- at the price of missing some of the S & P crowd. Try it both ways and then use your discretion.

Power allows me to hold a frequency. Other big guns are wary of getting too close or trying to steal attractive real estate at the low end of the band. Conflicts do occur and must be dealt with. I continued to do a lot of running high in the band since many small stations like to avoid the noise and crowding.

Operating crutches

In unassisted class spotting networks and skimmers are not allowed. Others do use them. No matter what frequency you call CQ the assisted operators will quickly find you. Don't be discouraged when starting a run attempt that little happens for a minute or two.

The very same tools that deliver QSOs to your frequency can occasionally be the cause of unwanted problems. Skimmer and human spotter are not perfect. A mistake in your call will begin a string of dupes. Eventually they'll realize the error and skip over you. Until then be prepared for those dupes. In this contest there were a few times during which every second QSO was a dupe. Just work them since it's quicker than trying to explain the problem.

Another crutch is the master database of call signs known to operate contests. These are collected from submitted contest logs and distributed to the contest community. Hence the Super Check Partial database (SCP). In the past I avoided using SCP since it felt mildly unethical to have the computer present alternatives calls in case of copying errors or to confirm the potential validity of a call.

I have been using SCP for the past year. Although a crutch it does save some effort and that can stave off fatigue. Unfortunately when there are many similar call signs SCP can be more confusing than helpful. It is better to correctly copy a call sign and not lean too much on SCP. On the plus side it can trigger me to ask the other station to confirm their call when they are not in the data base.

In one instance this weekend the received call sign had a single close match in the database. Since it differed by just one dit from what I copied through the QRM I sent back the call sign suggested by SCP. The other operator energetically corrected me. The database was wrong and I was right. Perhaps the log that contained his erroneously copied call was not filtered out when the master database was built. Learn to trust your ears.

Many use call history files to pre-fill the exchange. These files can be built from your own logs of previous contests and there are publicly available history and country files. My current opinion is that this is a crutch too far. I don't use this feature. In any case it is not very useful in CQ WW since the exchange, other than 599, is the zone number. The zone can is in most cases uniquely derived from the call sign. That is not true for the US and a few other countries and regions. Again, learn to trust your ears and rely on that rather than blindly accepting the pre-filled information.

SO2R

My primitive SO2R setup is fine for getting started. That will change. It will include more equipment, station automation and practice, practice, practice. I am exploring options and expect to be in better shape by the end of the current contest season. I will continue using two keyboards.

I discovered early on that the second radio was not very useful. Since I have only one amplifier the second radio is 100 watts. That's fine if you're low power and not so fine otherwise. You cannot expect to get through on the first call or even the second or third. It gets tedious with a handicapped second station. It was also not possible to effectively operate on two high bands at the same time since with just two tri-band yagis, one of which is fixed south, with only select multipliers available on 15 and 10 meters due to propagation.

When the 40 meter yagi failed the possibility of operating on 20 and 40 meters at the same time vanished. By the time 80 meters opened there was little left to pursue on 20 meters. Operating on 80 and 160 at the same time seems attractive but not with 100 watts. Low power on the low bands results in a low rate and the high frustration. Although I love low power and QRP contesting it is a poor fit when the other radio is running a kilowatt.

When the running was fast on 20 meters I found it difficult to tune and listen to the second radio. I am not yet that skilled. In the end my SO2R operation was less than 10% of the time. It wasn't a significant score booster in this contest.

When you work an SO2R operator don't be surprised at the curious delay before their responses. The best operators run on two bands almost seamlessly except that many transmissions must be slightly delayed to prevent having two transmitters on at the same time. At first it may be mystifying since you don't hear the other QSO. Rather than fret about it be amazed that they have this advanced skill and can do it for hours on end. Although talent helps we can all do it if we have the drive and put in the work.

Amplifier

Operating the amplifier for 48 hours straight in a major contest was a risk. My primary concern was the T/R relay, which is original (over 40 years old), loud and aggressively repaired using sandpaper a few months ago. There was no point in putting off the inevitable so I took the risk. It performed flawlessly.


To speed band changes I put sticky notes on the load and tune controls and marked the positions for each antenna, band and select frequencies where it mattered (mostly the low bands). You'll get close but you won't hit the perfect spot doing it this way. Once settled after the band change I would often tweak the tuning to be sure the amplifier was operating at maximum efficiency.

Before the contest I ran through the bands to make the labels. You must do this at full power or the power you intend to operate since amplifier tuning is power sensitive. I fixed the transmitter power to 65 watts since coarse tuning at low power was never necessary and the fixed input power ensured that the markings could be relied upon. It is a good idea to remove the sticky notes after the contest so that the glue doesn't mar the front panel due to heating of the glue by the amplifier.

Fixed input power is helpful for rigs that do not have a front panel power control. Going into a menu to repeatedly change the power for amplifier tuning is a tremendous inconvenience. The FTdx5000 has a power control while the FT950, my second radio, does not. The lack of a power control is not unique to Yaesu. Consider that when shopping for a rig if you have an amplifier.

Another inconvenient fact about the FTdx5000 and many other radios is that there is no front panel control to transmit a carrier. I rigged a foot switch to a rear panel plug so that I could tune the amplifier. There is a keyboard feature in N1MM Logger that will do this if you have no hardware mechanism to generate a carrier.

Propagation

No matter how good your antennas there will always be stations you cannot work or that are the limit of intelligibility. However the better your antennas more stations are workable and fewer are unworkable. The weak ones will still tease you, but without better propagation there may be no hope. The only cure is to get on a plane and operate from the tropics.

XKCD
Eastern VE3 is better than some and worse than others with regard to HF propagation. Many paths north of an east-west line traverse the auroral zone so that we are at the mercy of geomagnetic activity. At all times and especially during a solar cycle minimum stations a short distance east and south can enjoy remarkably better propagation on those northerly paths. By short I mean as little as a few hundred kilometers. Many the time I could only listen as many W1/2/3/VE1 stations work what I cannot hear or hear weakly.

This contest is no different. Many stations I am familiar with, including low power ones, could either put hundreds more European QSOs in their 20 meter logs or work many more zone and country multipliers. The difference is reduced when the sunspot count climbs and we are more competitive. This is with some decent antennas and heights. Some VE3 stations with better antenna farms do better than me but still not as well as their better located peers.

Skew propagation is well known on the low bands although I noticed little of it during the contest. Instead there was skew path on the high bands. This may be less well known yet it is common during marginal conditions. Europeans on 20 meters in our afternoon were peaking towards the east. This is usually ionospheric scatter from areas with a high enough MUF and not a true skew. A similar phenomenon occurs at sunrise when the rule is to "shoot the sun" on the high bands since that's where ionization is densest. There is also back scatter to nearby stations when we all beam to Europe, North America or elsewhere. This may be the only way to work them on the high bands when skip is long.

Unfortunately back and forward scatter is attenuated relative to a direct path. You need high power to have good results and even then you will likely only work the biggest stations. But it's better than not working them at all. Auroral zone scatter is also common on arctic paths. This was responsible for the strong Scandinavian signals and a BY in zone 23 that I worked on 20 meters deep in their night times.

Those with knowledge of these probable though not certain propagation phenomena can boost to their multiplier count. You can do it too by paying attention during your everyday DXing and using that experience during the contest.

Taking breaks and comfort

With high power there's always something to work no matter the band or time even when propagation is poor. If you can operate for the full 48 hours your score will show it. In the extreme some forgo food and drink to avoid the inevitable. For the humans among us it is necessary to take an occasional break. To keep your butt in the chair (BIC) you need every comfort you can manage. I am not that fanatical although I do pay attention to aids that keeps me in the chair.

One recent change was a new headset. After some consultation I purchased the Yamaha CM500. Although I was most interested in a robust cord I found them so comfortable that I could keep them on my head for long periods. This was not true of my previous headset the Koss 45 which would make my head ache after a few hours from the pressure against my glasses and skull.

I need a new operating chair. The old wooden office chair I've used for many years was comfortable when I was younger but no more. I need one with better ergonomics. Speaking of ergonomics I suggest paying close attention to the desktop. You need the table top (or your chair) to be at the optimum height to avoid fatigue from typing, sending CW and operating the rig. The less you have to swivel your head or chair to reach something the longer you can remain comfortable.

When you really need a break take it. Do not torture yourself. A few minutes walking around the house and talking to family members will refresh you. There is no need to take an official 30 minute break; just accept that you'll miss a few QSOs. Pour yourself a glass of water and head back to the shack.

I am fortunate that I can get by with little sleep. Each nights I only slept 2 hours, approximately from 4 to 6 AM (09Z to 11Z) when there is little to work after the low bands close to Europe and before the morning grey line opening. Speaking of sleep be sure you have an alarm clock that reliably wakes you up. Contests have been lost this way.