Friday, December 29, 2023

Snap! Mechanical Failure at 130 Feet

One windy morning in early December I was out in the bush doing Beverage antenna maintenance. As I stepped back out into the hay field I happened to look up. I saw that something wasn't right. The upper yagis of the 15 and 20 meter stacks were not pointed where I had last pointed them. As I continued to watch the yagis slowly spun in the wind. The mast was turning freely. That's bad, very bad.

Despite the cold and windy weather I suited up an hour later and climbed the tower. I considered it an emergency because freely spinning yagis can do a lot of damage to the cables. Coax isn't expensive but the repairs would be lengthy and very uncomfortable due to the cold weather.

Without stopping to discover the cause, I installed the mast grip previously stashed at the top of the tower and secured it to a tower leg. That stopped the mast from turning. 

I inspected the coax rotation loops and I lucky to discover that I had caught the failure before damage was done. Later, from the shack, I confirmed that the impedance of both yagis was nominal.

After dealing with the emergency, I dropped a few feet to inspect the mechanical coupling between the prop pitch motor and the mast. The trouble was easy to spot. The 5/16" × 3-½" bolt that couples the prop pitch motor drive shaft to the 2-7/8" short coupling pipe had snapped. 

The original was a grade 8 bolt. After an earlier repair I carelessly substituted a grade 5 bolt. Shear forces snapped the head off the bolt. The headless bolt subsequently squirmed its way outward until the pipes were fully decoupled.

Rather than repeat myself, interested readers can refer to earlier articles with the description of the system and how I later improved it to prevent water damage to the upside down prop pitch motor. On a positive note, on this trip up the tower there was no sign of water getting into the motor or the return of motor trouble.

Returning to my emergency climb up the tower, my hands were going numb from the fierce wind chill. The mast now secure, I raced to the ground before I could develop hypothermia. The speed at which I worked meant that I had to leave the yagis pointing south, which is not a terribly useful direction for me. That's where the wind wanted them and I was in no condition to fight the frigid wind. The important thing was that I averted a worse outcome.

A few days later the weather became unseasonably warm. I spent 3 hours on the tower that day to jack up the mast, disassemble the rest of the drive system and park the yagis northeast. I neglected to take a picture at the time so I took the accompanying picture after the full job was done after Christmas. Here you can see the improvised jack at rest. The weight of the mast and antennas is once again supported by the lower bearing, 5' below the mid-mast bearing plate seen here. 

You can probably discern from the arrangement of the bolts how the jack functions. I used what I had available at the time, and I can't really recommend doing it this way. It was an improvisation when time was of the essence. The jack is only stable if the mast can't turn.

I tossed the parts of the motor drive and mast coupler onto the workbench for when I would have time to inspect them. I had to decide whether to repair or replace the system. It couldn't be done quickly since the cold weather had returned and the holidays were approaching.

I had long intended to redo the drive system because the design is awkward for maintenance and there is play in the system that allows the yagis to rock back and forth more than I'd like. Good intentions aren't good enough when I kept putting it off in favour of so many other items on my project list. This emergency moved it to the top of the list.

One thing regular readers will know is that I do not hide my mistakes. The design and its implementation are flawed. They are flaws that I believed would not cause problems or at least cause them far enough down the road that there would be ample time to plan and implement improvements. There was plenty of time, but I spent it elsewhere. It is easy to let things slip when they seem to work and so much else is waiting to be done. Let's take a closer look at the problems I uncovered.

I took the time to illustrate the design since it is critical to understanding it and its failings. I included a side view and a bottom view. I didn't have a blank sheet when I came up with the design since there were several constraints I had to accommodate:

  • The motor crown gear was welded to a 1-¼" steel pipe with an OD of 1.66".
  • A bearing had to take the weight of the mast and antennas since a prop pitch motor can't deal with that.
  • The 20' mast falls ~2' short of the bearing plate. It had to be extended to reach the bearing.
  • The available tower girts for bearing and motor plates resulted in a tight fit for the drive shaft between the motor and the bottom bearing.

Please note that the diagrams are not complete. I left out the many washers (flat and lock), where the pipes are tapped, and several other details.

I chose a 3.5" OD aluminum pipe from my stock of materials as the mast coupler. Two cross bolts (purple) connect them. There are 14 gauge (0.078") steel spacers to fill the gap between the coupler's 3.068" ID and 2.875" mast. Some play is acceptable but not too much.

The weight of the mast and antennas rests on the bottom 75 mm fully sealed industrial bearing that is designed for both axial and radial loads. A 2.875" aluminum (6061-T6) schedule 80 pipe serves as a drive coupler. It fits through the bearing bore and has one cross bolt to the mast coupler, and four 0.078" steel spacers. A short length of the drive coupler projects below the bearing plate. There is one cross bolt to connect the pipes. 

There is a large 0.33" gap between the drive shaft and the drive coupler. ⅜" nuts fill the gap, but not perfectly. They are difficult to install so a gap needed to be left. Two positioning bolts help to prevent movement of the drive shaft.

It worked for several years despite its flaws. There is too much play in the drive coupler due to the lack of spacers at the bottom of the mast coupler. The adjacent picture shows one of the dual spacers fitted to the top of the drive coupler. That and the use of only one cross bolt allowed the bottom of the drive coupler to sway side to side under torque. The creaking could sometimes be heard on the ground.

The bearing bore is ~2 mm wider than the 2.875" coupler. I didn't worry about it too much at the time because the crown gear on the motor can accommodate the play without binding.

However the most serious problem was the cross bolt to the drive shaft. There is a high shear stress on the bolt and pipe openings due to large gap between the pipes (red circles). The stress cycling is continuous due to the antennas rocking in the wind and when the mast is turned. That is why I selected a grade 8 bolt. In the earlier picture you can see the marks on that bolt, and on the grade 5 bolt that failed, where it passed through the pipes. There is less bending stress (orange circles) between the mast and drive coupler to the 3.5" mast coupler since the gap between them is small.

I could have had a larger pipe welded to the crown gear. I didn't because the tight fit between the motor and bearing plates required fitting the drive shaft at an angle through the tower face. While that may have been a mistake there was no opportunity at the time to design a better system. At least I couldn't come up with one that satisfied all of the previously listed constraints.

Another measure I could have taken was to install more cross bolts to reduce play. I tried to keep those to a minimum because I was concerned that more holes would increase the risk of stress cracks in the pipes. That may have been a mistake since the total load isn't higher and more bolts helps to distribute the load, thus lowering point stresses. Also, when the pipes twist out of alignment under load there are greater bending stresses on the pipes and bolts.

The big design questions must be left for the future. There was an immediate problem to be solved or I'd spend the first half of 2024 unable to rotate the upper 15 and 20 meter yagis. I inspected the components to determine whether they could be reused and what could be improved without full replacement.

Once I decided on a course of action I reascended the tower on a unseasonably warm Christmas day to retrieve the mast coupler. I had left it there as insurance 2 weeks earlier in case the jack failed. I adjusted the jack and pulled the bearing plate so that the coupler could be slid off the mast.

I was surprised that the aluminum (6061-T6) components were in good shape. The holes for the cross bolts were at most only slightly elongated by the repeated torque stress. Perhaps that was due to the thick walls and relatively large bolt diameters that distributed the stress over large surfaces. There was noticable elongation of the 5/16" holes in the steel drive shaft. There were also wear marks at the bottom of the mast coupler where movement abraded the aluminum. It was superficial so it, too, could be reused. 

I gave myself one day to rebuild and reinstall the system, in the interests of safety (I trusted the mast jack only so far) and to exploit the record breaking temperatures. It may have been a strange way to spend the holiday but you do what you have to do. I kept a couple of friends on standby in case I needed to interrupt their holiday to help with the tower work. Luckily there was no need to inconvenience them. I appreciated their willingness to come to my assistance if needed.

All the work was done in my basement workshop since my garage workshop isn't heated and I had more lighting available for evening work. The picture doesn't show the full setup and procedure where I used levels and other tools to ensure good alignment of the new holes. I didn't work blindly with a hand drill!

I made several improvements to the drive shaft and its coupler:

  • I widened the holes in both to accommodate a ⅜" grade 8 bolt as a coupling pin. I judged that the wider hole left more than enough material to avoid stress cracks. The widening conveniently removed the elongation of the drive shaft holes. The bolt is ½" longer than the one replaced so that the unthreaded shank is in contact with all 4 holes. For applications with shear stress you should avoid having bolt threads in contact with the pipes where the bolt diameter is thinner and therefore weaker. The threads also abrade the pipe.
  • Two more holes were drilled and tapped so that there are 4 positioning bolts. They stabilize the drive shaft within the coupler without the need for difficult to install spacers on the cross bolt.
  • The positioning bolts are now grade 8. I found that their use as set screws on a constantly wiggling drive shaft rounded the tips of the grade 5 bolt threads. They were difficult to remove and afterwards I had to chase the threads in the coupler.

I next turned my attention to the mast coupler. A second cross bolt was added. To avoid placing it too close to the bottom end, the second bolt is ⅜" rather than ½". Since there was no elongation of the original holes I saw no need to make the new cross bolt any larger. Both bolts are grade 5.

New spacers were shaped for the top of the drive coupler. I added a ring spacer below the cross bolts using the same 14 gauge steel.  The ring spacer is visible in the picture on the right.

There is significantly less play with this arrangement. A better solution would be to make the mast coupler from schedule 80 pipe where the ID is 2.9". That would leave a gap of 0.0125" between the couplers and eliminate the need for spacers. There was no time for that to be done.

The picture above shows the assembly from the bottom of the drive coupler. All the bolts are there, including the cross bolts through the mast coupler just visible through the pipe. A carabiner is already attached for lifting.

In addition to the enlarged holes, the water cap on the drive shaft had to be replaced. The glue on the aluminum disk seal didn't survive the trip to the ground. I cut a new one from the top of a food tin, glued and taped it to the pipe, and added caulking to prevent the tape from peeling.

Here you see the prop pitch motor attached with the new coupler system installed. There is almost zero play. For this design the difficult part of the installation is to wiggle the mast back and forth to align the pipes and drive the cross bolt through the 4 pipe holes. Luckily the wind was calm that day as I used a long wrench to rotate the mast back and forth until the bolt was driven home.

At right is the view looking down on the mast coupler and bearing. The only visible difference from the original is the added cross bolt a short distance above the bearing.

Since I didn't have an assistant, the final test had to wait until I climbed down (with a cumbersome load of steel). Back in the shack I confirmed that the rotator was working again. 

I would like to do another test with a friend operating the controller while I monitor the rotating couplers on the tower. That will have to wait for a warm winter day after the holidays are over.

In the spring I'll discuss the design with a machinist and see if we can come up with a better system. As already said, it must be easy to install, remove and service. It must also not involve jacking the mast a large distance or the removal of the prop pitch motor. There are several possibilities brewing in my mind so we'll see where this goes.

I am relieved to enter 2024 with a fully functioning rotation system for the 15 and 20 meter stacked yagis. There are months of contests and DXpeditions coming up before spring arrives. I can live without being able to rotate the yagis for a while during the summer lull, and that may be my best opportunity to address the system design. 

Although having a big station can be a lot of fun it is also a lot of work. Things break and maintenance is an unending job. The failure described in this article may be worse than most, but there are many smaller issues that regularly crop up. Despite few readers having towers and antennas of this size, my hope is that my description of the design, design flaws and the repair will prove to be helpful.

This is my final article for 2023. I wish all of you a happy new year.

Friday, December 15, 2023

QRP Lessons from the ARRL 10M Contest

Operating QRP is not typical behaviour for someone with a station in the so-called "big gun" class. I used to do all my operating as QRP and the pleasure of it has never abandonned me. It may seem odd to put 5 watts into big antennas, and yet it is a lot of fun. If you scan the results of any major contest you will notice that I am not the only one doing it.

But why QRP in the ARRL 10 Meter contest? My interest in the contest is limited and I needed a way to spice it up if I was to bother. Hams with small stations -- be it power or antennas -- know that the high bands make working stations easier due to the low atmospheric noise. Of course the MUF has to be high enough to make it possible. A solar maximum is the ideal time to have fun on 10 meters.

To answer my own question, I operated QRP in this contest for two reasons. One was that the pace is less hectic than with high power due to the pile ups a big signal attracts. The second is that it was an interesting test of how my antennas perform. With a solar flux of about 125, 10 meters was open but marginal over many paths. That means a few decibels can have a large impact on results. So I put the antennas (and myself) to the test.

To recap, I have 4 antennas on 10 meters:

It is handy to have all of these directional antennas on 10; I didn't need to rotate the rotatable antennas often. Just click the mouse to select an antenna and then call the station.

This was my first contest using the recently purchased Icom 7610 as my primary rig. After CQ WW SSB, I switched the transceivers so that the FTdx5000 is now the second radio, positioned on the operator's right. Getting used to the 7610 was a secondary objective for the 10 meter contest. It was a stretch for me to quickly learn how to use the 7610 for SO2V operation. To avoid the hassle I reverted to SO1R and used the second VFO/receiver to tune stations. I swapped the VFOs when I wanted to call a station during my occasional runs.

The 7610's power setting is measured in percent of full power (nominally 100 watts). That made me suspicious of the actual power output so I used a power meter to measure the power at various settings. The power percentage does not perfectly track the power output, although it is close. It varies by band and across the power range. For this contest I chose a setting that gave a power of slightly below 5 watts on the 10 meter band. At least this is more convenient that the 3 db attenuator I built to use the FTdx5000 on QRP due its 10 watt minimum power setting.

Conditions at the start of the contest on Friday evening were poor. It was well past sunset and few signals were to be found. Those that I did hear were weak and not really worth calling with QRP. I put off operating until Saturday morning. I got up early to catch the sunrise opening. Although many Europeans were heard before sunrise their signals were weak. I had to wait until signals became stronger. I began filling the log soon after 1230Z. 

Europe was coming in strong and it was easy to work stations with 5 watts and the 5-over-5 stack. Indeed, Europe was the source of about half my contest contacts and the bulk of the multipliers (65 countries overall). That is despite the brief openings. I was able to get as far as UA, UR, 4X and 5Z. By late morning the Europeans faded to only a trickle of stations in F, EA and CT. The path closed soon after their local sunset.

The rest of the day was spent hunting stations to the south, and later to the west. I could run Europeans with QRP but runs were painfully brief to the US and I frequently lost my run frequency to North American stations that couldn't hear me. However, running is mandatory, even with QRP. If you don't it is impossible to work the large number of casual contesters who only S & P. I kept my run attempts brief but frequent.

Propagation under the marginal conditions was intriguing. The most important aspect was the large size of the skip zone. It was both a curse and a blessing, as I'll describe. The map above is my attempt to illustrate my observations, and my successes and failures.

Stations at the edge of the skip zone would be S0 one minute and S9 the next. It was very evident with MN and MB to the west and W5 and W0 to the southwest. It was a lot like sporadic E openings on 6 meters, except that sporadic E, also a staple on 10 meters, did not appear to be evidence despite the nearness of the winter season peak.

As a QRP operator I had to check station signal levels often to catch those peaks to increment my small total of state and province multipliers. This is where entering the assisted class came in handy. I could rapidly check on stations with a click of the mouse. I used both human and skimmer spots.

Particularly galling was that I was shut out of most of W4, other than Florida. That is the reason my state and province total of 49 multipliers was so low. I missed GA, TN, AL and KY. I heard many of them but weakly on back scatter. Unlike W1/2/3/8/9 stations that I could work on back scatter (mostly just the big guns), the added distance to W4 made it too difficult for QRP. W5, W6 and W7 were easy to work most of the time. The same with the western provinces from VE4 to VE7. A few Maritime provinces were workable on back scatter but, again, I missed most of those multipliers.

That's the con. Now for the pro case. Everyone has a skip zone centred on their QTH. For the bulk of the US eastern seaboard and south, they were at a relative disadvantage working the Caribbean and, occasionally, Central America and the north coast of South America. There aren't many stations there but almost every one is a multiplier. There were many times when I could sail over their skip zone and work those stations, right through modest pile ups. That was only in the morning because in the afternoon I could not compete with those further to the west that were in a similar relative position to me. Skip went long early as the MUF fell.

Back scatter was a puzzle that I had to solve since signals are very weak in comparison to the direct propagation path. QRP is not conducive to effective communication via back scatter. Yet it is the only option to reach stations within the skip zone. Every attempt was a roll of the dice and I had to find a way to increase the probability of success. Sometimes it was simply of matter of trying every few minutes to hopefully catch a small upward variation of the scattered signal strength. 

Calling a big gun has the best chance of success on back scatter since they have the antenna gain to better hear my signal. Unless you recognize the call signs of those stations you can only go by what the S-meter tells you. Obviously, the stronger you hear them, the better.

Equally important is listening to who they're working. That tells you where their yagis are pointed. Your best chance of success on back scatter is to point in the same direction that they are. Sometimes that is northeast to Europe and other times it's south to the Caribbean or west to the Pacific. That's true no matter where the target station is located. Early in the day when the band was open to Europe, northeast was best. That suited me well since my 10 meter stack is most effective in that direction. 

With four antennas at my disposal and the ability to point in various directions, it was a simple matter of clicking through the options and seeing which antenna netted the strongest signal. Then I call and hopefully work them. When you have only one antenna, testing the back scatter path by constantly rotating the antenna is time consuming. The wealth of antennas was highly advantageous.

I decided early on Saturday to change my operating plan to better utilize the propagation that was available. I had intended to operate mixed mode (SSB and CW). Since signals levels were generally poor I decided to stick with CW. I certainly would have had a higher score operating both modes, although I suspect that I would have been less competitive. Due to the lack of daylight at this latitude, the openings were shorter. Stations further south had the advantage since they experienced a higher MUF. Their advantage is greater with SSB due to the lower SNR.

Late in the afternoon the band opens to the Pacific and east Asia. The pile ups on the few VK stations were intense. I had to wait for their rates to drop before I had a chance. With no other callers it was no problem to work them. The same can't be said for Japan and stations further west on mainland Asia. The low solar flux had the band closing soon after sunset, which is nearly coincident with our sunrise. Signals were weak and not workable with QRP, even with 5 elements 46 meters up. Very soon they were gone. I heard long path Asian stations in the morning that were also unworkable with QRP.

Was I competitive? Did I win? Reports on 3830 suggest that I may come out on top in CW QRP (unlimited/assisted). Not everyone posts on 3830 and scores will change after log checking. I didn't set out to win, just to have fun and test the station. It would be nice to win even though that was not my objective. Time will tell.

Thursday, December 7, 2023

Stubs for Attenuating Harmonics - an Experiment

I'll start with a funny story. There were 3 of us doing a M/2 (multi-op, two transmitter) operation in CQ WW CW at the end of November. We kept both seats occupied for almost the entire 48 hours. As you might guess it was a tiring effort when each of us had to put in 32 hours on average. We got into this situation when our fourth operator had to cancel at the last minute.

Sleep deprivation is an occupational hazard for dedicated contesters. We may be able to do it, but not always with good results. It explains what happened to me on Sunday morning when the band opened to Europe on 20 meters and the running began. The run was going quite well when I was perturbed to find that an imposter using my call had come on frequency and began to run stations. The interference was high and I was incensed that some joker was going to mess up a lot of logs.

After a string of expletives that surprised my operating partners, I QSY'd a few kilohertz and resumed running. I made a mental note about the letter I ought to write to the contest committee describing the incident.

Two days later, one of us (VE3KI) did a log analysis. He remembered the incident and pulled up the time interval in the log. I think I turned a little red when he shared what he found.

The data was right in front of me on the screen, but I didn't look. That's what sleep deprivation will do to you!

The incident had me thinking about harmonics and how much more I can do to reduce their intensity. I am using low power BPF on both stations, and nothing else to prevent inter-station interference. The BPF are good but they cannot suppress the harmonics generated by the amplifiers. That requires high power filters after the amplifier.

Low power BPF are not the ultimate solution for SO2R and multi-op contesting. I selected them well aware of the limitations. I understood that I might eventually require high power BPF or stubs, or both. I can avoid the harmonics very easily when operating SO2R, but it is not so easy to coordinate with others in a multi-op. That takes communication and vigilance, or perhaps software to flag the risk.

High power BPF are expensive and the insertion loss typically ranges from -0.2 to -0.6 db. For low power BPF you can compensate for insertion loss by increasing the amplifier drive by 5 to 10 watts. Not so with high power BPF. The dissipated power can be quite high and may require fans to vent the heat. Can stubs do an effective job of attenuating amplifier harmonics, and let the low power BPF take care of transmitter spurious emissions? I have tube amplifiers which generally suppress harmonics well, due to the tuned pi output network. When I switch to solid state amps, as planned, the amplifier-generated harmonics will almost certainly increase.

I decided that it was time to do an experiment. I wanted to see for myself how well a stub can suppress harmonics and (hopefully) not otherwise interfere with station performance. I have the necessary equipment so an experiment was easy to arrange.

Before delving into the experiment, it is worth taking a moment to consider what it is that we want to accomplish. The following are the concerns I identified at the start of my quest:

  • Which harmonics are potentially deleterious, and which are benign
  • Which harmonics can be effectively attenuated
  • Bandwidth and depth of the harmonic notches
  • Impact of the stub on the fundamental band of operation (insertion loss and SWR)
  • Effect of the load (antenna) impedance on stub performance

It should not be a surprise that questions about stubs overlap with those of filters in general because a stub is a filter. Fixed components (capacitors and inductors) have been replaced by a network with strongly correlated L and C components due to the structure of transmission lines. You trade flexibility of filter design in return for simplicity. We will see that, as always, there is no free lunch. 

For the purpose of the experiment I went into the basement and looked over my inventory of new and used coax. I chose an ancient length of Amphenol brand RG8, about 16' (5 meters) long that had N connectors on both ends. I have no recollection of where it came from, how old it is or what use I made of it in the distant past. It was therefore important to assess its quality before proceeding. I connected it between the ports of my recently calibrated VNWA3 and swept it from 160 to 6 meters.

It measured better than I expected. The impedance is good and the insertion loss is only a little worse than the specification for new RG8. For example, at the (randomly set) 33.5 MHz marker the loss is -0.24 db. That's only -0.055 db worse than the -0.185 db spec. However, its random length doesn't correspond to any amateur band. I only knew that, from its length, it would be effective within the HF spectrum. I declared the cable adequate for the experiment.

I inserted a BNC T between the VNA port leads and, with the help of adaptors, connected the coax to the T. The other end, with more adaptors, is terminated with a BNC short from a VNA calibration kit.

The length of the coax is 16'-8½", which is 16.7' or 5.1 m. The length is measured from the through-line centre of the T to the short cap at the far end. 

Assuming the nominal VF (velocity factor) of 0.66 for RG8, without compensation for the VF of the adaptors, the cable's electrical length is 1λ at 38.8 MHz.

The test setup was then swept from below the predicted fundamental frequency of 9.7 MHz (where the stub is ¼λ long) to beyond the fourth harmonic. The first predicted notch is at 19.4 MHz (second harmonic) where the stub length is ½λ; the second predicted notch at 38.8 MHz is where the stub length is 1λ. Markers were placed inside the notches and bracketing the fundamental frequency. [Note: the frequencies are not exact multiples due to the limited number of discrete points sampled by the VNA.]

Were this a proper stub the fundamental and the harmonic notch would be within amateur bands. However, this test is sufficient to the requirements of the experiment. It is straight forward to scale the measurements to nearby amateur bands.

As you can see, the predictions were accurate. In practice you should cut your stubs long and trim them until they resonate where you want, just like you would to adjust a wire dipole. Coax has manufacturing variations that can render an accurately cut stub too short for its design frequency. The only recourse is to toss it aside and try again.

I plotted a variety of S11 curves to illustrate how the notch behaves and works as a filter. Around the stub resonance the real component of the impedance approaches 0 Ω; that is, a dead short. This is an important factor for understanding how the stub work, that will be discussed further below.

The stub is not transparent away from its resonant notches. The presence of the stub adds reactance into the transmission line that can be highly disruptive across the spectrum of interest. It is only at the fundamental frequency that the reactances cancel and the stub effectively disappears. Markers 1 and 2 are placed at the SWR 1.1 points surrounding the fundamental frequency of 9.7 MHz since I consider those to be the boundaries of the stub's usable pass band.

Notice the depth of the notches: -27.7 db at 19.4 MHz (second harmonic) and -24.3 at 38.9 MHz (fourth harmonic). The notches can be deeper with better coax because loss reduces notch depth. That is also why the upper notch is a few decibels less. 

The reason coax loss matters is because for total cancellation of the incident and reflected wave the amplitudes must be equal and of opposite polarity. It is not enough for the phase difference to be 180°. To achieve a -20 db notch the amplitudes must match to within 1%. For a -30 db notch the amplitude match must be 0.1%. 

To illustrate the impact I tested a short length of LMR400 as a shorted stub. It was similarly equipped with N connectors and adaptors, and attached to the T connector between the VNA ports.

We've improved the notch a few decibels. Because the stub is short the harmonic notch is at 74.1 MHz. Since loss is (roughly) proportional to frequency and length, the results are applicable to HF stubs. 

It is possible to achieve deeper notches with hard line like Heliax. However, notch depth is not the only important parameter. 

The above plot is restricted to HF and the markers have been placed for a discussion about bandwidth. I want to know the widths of the pass band and stop band. Their definitions are not rigid so I chose my own bandwidth definitions: SWR better than 1.1 for the pass band into a 50 Ω load (matched antenna); and second harmonic suppression better than -20 db. You are free to choose your own definitions if you don't like mine.

The pass band width is 13% for both the 19.4 MHz RG8 stub and the 74.1 MHz LMR400 stub. That appears to be consistent with all or most 50 Ω coax. That is far wider than the HF band segments that are relevant to contesting; the worst case is 8.6% between 3.5 and 3.8 MHz. It is fair to conclude that a coaxial stub filter designed to suppress harmonics from operation on any HF band contest segment will not affect performance on that band. That's good news.

The bandwidth of the notches is not as good according to my criteria. The -20 db second harmonic stop band between 19.11 and 19.69 MHz is 3%. It is narrower yet for the fourth harmonic. For lower loss coax the stop band is wider, but not by much; it's also near 3% for the 74.1 MHz notch of the LMR400 stub. The stop band is wide enough to cover most but not all of the contesting segments of the HF bands from 7 MHz and up.

  • 3.5 to 3.6 MHz: 2.9%; 3.6 to 3.8 MHz: 5.6%; 3.5 to 3.8 MHz: 8%
  • 7.0 to 7.2 MHz: 2.9%; and 7.0 to 7.3 MHz: 4.3%
  • 14.0 to 14.35 MHz: 2.5%
  • 21.0 to 21.45: 2.1%
  • 28.0 to 29.0: 3.6%

The low band figures are not as daunting as they appear. First, 160 meters is not discussed since we are only interested in the pass band and not the stop band because it's the lowest of the 6 contest bands. Second, harmonic filters for 15 and 10 meter are not needed since there are no higher HF contest bands to interfere with. Third, phone contests are less plagued by harmonics because, due to the higher frequencies of those segments, it is usually adequate to tune the stubs for the CW and RTTY segments. For example, the harmonics of 3.7 MHz are 7.4 MHz, 14.8 MHz, 21.2 MHz, etc. That sixth harmonic on 15 meters is usually so weak that it requires no additional suppression. Harmonics from operation on 160 meters is similarly of limited concern for most contesters.

The third harmonic of 7 MHz can be notched with an open stub. However, the insertion loss and SWR on the fundamental are less than ideal because the optimum pass band for an open stub does not coincide with ⅓ the frequency of the available notches. For the length of RG8 measured above, when it is employed as an open stub, for the best correspondence of a notch and operating frequency, the insertion loss is -0.34 db and the SWR is 1.75. Those are not acceptable. [Note: VNA plots not included.]

That aside, it is arguably sufficient to have harmonic stubs for 80, 40 and 20 meters since the harmonics for 160, 15 and 10 meters are not a significant interference risk. The 80 meter stub deal with harmonics on 40 and 20 meters; the 40 meter stub for 20 and 10 meters; and the 20 meter stub for 10 meters. Since a shorted stub can only suppress even harmonics, we can't use one for the 40 meter third harmonic on 15 meters.

At this point it is worthwhile to step back and ask a rhetorical question: is a coax stub sufficient to attenuate harmonics to an acceptable level? I hope that you'll agree that the answer is no. Lowering the harmonic by -30 db is good but not great. It depends on the unattenuated strength of the harmonic. Since we're discussing this in the context of amplifiers, we'll set 1 kW as the baseline.

If the second harmonic specification of the amplifier is -50 dbC (-50 db relative to the fundamental carrier) the harmonic power is 10 mW. That's a lot, although context matters. A coax stub will further knock it down to -80 dbC. A well-designed BPF will do even better. Two coax stubs in series can perform as well as a BPF. It is equivalent to increasing the order of a traditional filter. 

A BPF attenuates all out of band energy and not just the even harmonics. Its pass band insertion is typically -0.2 to -0.6 db depending on the filter design and the frequency. High power BPF often need fans to exhaust the dissipated power.

Stubs are more commonly used in conjunction with low power BPF since the BPF can't suppress harmonics generated by the amplifier. Certainly they attenuate harmonics generated by the transmitter, so that the amplifier doesn't amplify them, but the amplifier has its own non-linearity that generates harmonics regardless of what appears at the input. Amplifiers with tuned inputs attenuate transmitter harmonics regardless of whether a BPF is placed between the transmitter and amplifier.

There is another place where harmonics can be attenuated: the antenna. Dipoles and yagis made of dipole elements present a high impedance to even numbered harmonics, assuming the elements are not loaded. If the elements are loaded, the third harmonic may also be attenuated. I used that to good effect when designing my 3-element 40 meter yagi to eliminate the resonance within the 15 meter band. Of course a multi-band yagi may not attenuate harmonics at all. For example, the second harmonic of 20 meters and a tri-band yagi.

The presence of a mismatch at the antenna may not be enough. Any harmonic energy that reaches the feed point has to go somewhere. If the antenna presents a poor match (high SWR) our best hope is that it is reflected. If the coax outer surface presents a more favourable impedance, the harmonic energy will "leak" around the feed point and become common mode current on the coax shield. It can then radiate and/or return to ground by some circuitous route. CMC (common mode chokes) help prevent harmonic leakage if designed to be effective at the frequencies of the harmonics. Often CMC are optimized for the fundamental frequency.

Assuming we have well-designed high power BPF and coax stubs, should we care what happens to harmonics at the antenna? Perhaps not, although there is a valid concern when the antenna is not a close match to 50 Ω. All filters, and that includes BPF and stubs, only work as designed for specific port impedances. When there is a mismatch at the antenna or a fault due to poor coax or switching equipment, the filter may not perform as intended.

For this reason I did a further set of tests. I created a mismatch on the load side of the stub and measured the effects with the VNA. These were simple tests to probe the parameter space rather than a comprehensive analysis. I am not an expert and there was a risk that I would not design the experiment to get a broadly applicable result.

On the left I placed a 50 Ω resistor in parallel with the transmission line between the stub and the load (right VNA port). On the right I substituted a 100 pf parallel capacitor. In both cases I am using the original RG8 stub that has its fundamental frequency at 9.7 MHz. I skipped trying this with series elements since they would have taken more time to set up. Let's look at these two cases in turn.

With the parallel resistor the notches didn't change. Not surprisingly the SWR at the 9.7 MHz rose to 2 and there is an insertion loss due to the use of a real resistor rather than radiating one as found in an antenna. This is a good result. Unfortunately it is unrealistic since it is rare for a mismatch to be purely resistive. Thus we come to the second case.

The notch position and performance are unchanged. What has changed is the behaviour within the pass band. The SWR at 9.7 MHz has increased to approximately 1.4. This is also a good result because, despite the imperfect load the coax stub continues to perform well. That might not be case with a much higher SWR. Although it is an interesting question, it is not one I'm too concerned about since I design my antennas to have a low SWR over the band segments of interest. Not always, it is true, but it is true enough for my purposes most of the time. I am not aiming for perfection.

One of the reasons why I did the mismatch test was that I have seen it reported that due to the varying R and X values along a mismatched transmission line, the placement of the stub affects its performance. I now doubt whether that is true. Further testing may be called for, but not now. I can explore it in greater depth another time if I feel so inclined. I'm supplementing my mismatch experiment with a simple diagram. [Too simple, but I didn't want to spend the time making it complete and complex; it is only intended as a guide while following the text.]

After the transmission starts, the wavefront splits at the T connector, half going each way (blue & red). It takes a fraction of a microsecond for the signal to travel down and up the coax stub where it interferes with the signal from the transmitter. In this case it is in phase at the fundamental frequency and out of phase at the even harmonics. 

The fundamental wavefront continues toward the antenna at full power (half + half). The even harmonics go no further because the signal from the transmitter and exiting the stub are equal and opposite and travelling the same direction. Other than a small residual energy that is not cancelled (orange), the antenna behaviour at the harmonic frequencies becomes irrelevant.

Harmonics exiting the stub also travel in the reverse direction, towards the generator (amplifier). It is not cancelled, and the reflection causes extreme standing waves between the stub and amplifier. That is seen by the amplifier as a high SWR at those frequencies. The power in the harmonics is very low so the operating parameters of the amp are unaffected.

The initial leading edge of harmonic energy, having gotten ahead of the signal transitting the stub, will in most cases reflect back from the antenna's high impedance at that frequency, as discussed earlier. It, too, encounters the stub and the same thing happens in the reverse direction as it did in the forward direction. It will eventually dissipate through attenuation and (some) radiation.

The fundamental and harmonic signals can be individually followed and analyzed. They do add in superposition but the harmonics are too weak to measurably affect the total system. From this brief analysis I suspect that K9YC is incorrect about the importance of stub placement on the transmission line.

Stubs are not the only way to attenuate amplifier harmonics, as already mentioned. Let's recap the alternatives:

  • Coax ½λ shorted stub (relative to the second harmonic): With low loss coax the second harmonic can be attenuated by at least -20 db across the higher band, often better than -25 db, with low insertion loss. The third harmonic cannot be attenuated. The fourth harmonic is attenuated only a few decibels less than the second. Filters can be ganged in series for greater harmonic attenuation.
  • High power BPF: All harmonics are attenuated. Depending on filter design, the second harmonic can be attenuated by -40 db or better, and higher order harmonics are often attenuated better than -50 db.
  • High power LPF: Low pass filters filter all harmonics. They are not as complex as BPF but neither can they work as well below fifth order for the second harmonic. The Elsie model below is an example of a fifth order Chebyshev 40 meter LPF. The marker is at 14.2 MHz. Insertion loss is better than a BPF for a similar or lower component count and can be cheaper if you build it yourself.

How much harmonic attenuation is enough? In the story at the start of this article the second harmonic was S9+, so it was loud but not louder than a great many signals on 20 meters that morning. That helps explain why I mistook it for another station. Let's see what Acom has to say about their amplifiers. I'll quote from their specs for the Acom 2000a:

"Classical Pi-L network, all-air coils (no ferrite), and carefully designed layout of the output tank offer typical harmonic emissions as low as - 55 dBc (second) and below - 70 dBc (third and above)."

And for Acom 1500 that I own:

  • 1.8-21.5 MHz - better than 50 dB below rated output;
  • 24-54 MHz - better than 66 dB below rated output;

That is about as good as it gets since tetrodes in a well-designed amplifier exhibit very linear performance. Grounded grid triodes do a little worse and solid state amp typically do worse yet. It is good practice to choose a solid state amp that has lots of headroom at your intended operating power to ensure maximum linearity. The rollout of DPD (digital pre-distortion) may play a role in removing the harmonics generated by solid state amplifiers by improving system linearity.

I can't do much better for harmonic levels with amplifiers other than the ones I own -- Acom 1500 and Drake L7 -- yet I have problems with harmonics. It won't get better because it is likely that I will eventually transition to solid state amps to allow rapid and reliable band and antenna switching. Based on my experience I will wither have to switch to high power BPF, as almost all the best contest stations have done, or use stubs. The wait for broad deployment of DPD in the shack could be a long one.

VA6AM makes excellent high power BPF that are used by many contesters of my acquaintance. There are other products of similar quality available. But high power BPF are expensive and in most stations will require heat management and custom switching.

The time has come to try coax stubs in my station. I will start small, and if it is successful I can expand their use. For my antenna switching architecture it is easiest to make a stub for either 80 or 40 meters. The reason is that there is no switching required. My antennas for those bands have auxiliary switches connected to single ports on the 2 × 8 antenna switch where the stub can be conveniently attached. Better, it will be outdoors and out of sight. 

I have an ample supply of short lengths of Heliax and connectors from which to make low loss coax stubs. They can finally be put to good use. Assuming all goes well, I can do the same for 160 meters. For 20 meters they will need to be switched, per station, since there are several antennas distributed across several switch ports. As previously discussed, stubs are superfluous on 15 and 10 meters.

I'll try to get the first one done in time for the ARRL DX contests later this winter if I can fit it into my schedule. I may have to make it easily removable between contests because I currently use the 40 and 80 meter antennas on 30, 17 and 12 meters (the WARC bands). The stubs are incompatible with that.

Monday, December 4, 2023

Post-contest: Silence of the Bands

Have you ever noticed how quiet the HF bands are after a major contest? It was true last week after CQ WW CW and it is true again this week after the less popular ARRL 160 meter contest. This is quite different from what I remember decades ago.

Contesters were a minority back then. There was a lot of conflict on the bands during major contests due to the abundance of conversations, DX hunting, nets and so forth. The HF bands are quieter now. There are several reasons for the change. Since I've discussed this before (as have many others!) I won't repeat them.

Despite the lower level of HF activity, why are the bands unusually quiet on the Monday morning after the contest? The reason appears to be that a large fraction of today's daily activity is by contesters. They (we) are resting.

Immediately following a contest that I have little interest in turning on the radio. There is fatigue, chores delayed and an aversion to returning to the air so soon. I am not alone. Unless there's an interesting DXpedition, a 6 meter opening or something else sufficiently attractive to draw me back, the radio stays off. About all I might do is to monitor DX spots for items of interest. 

On Sunday evening after the ARRL 160 meter contest, the lack of DX spotting activity was notable. I recall one period of 30 minutes when there were just a few spots from stations in the eastern part of North America (I usually filter spots from elsewhere). Flipping from band to band showed a blank band map. Tuning the CW segments uncovered nearly no signals at all. 

Activity will soon recover as the contesters rest and recharge. We don't stay away for long. There is no need to suspect an elevated geomagnetic activity level since that does not hurt conditions enough to keep the enthusiasts away. We really have become a hobby where radiosport is a top interest and motivator.

Of course there are many exceptions, but that does not detract from the point. Digital activity continues, though even that is at a lower level. Those normally absent from the HF bands, such as VHF aficionados and those strictly on FM, likely don't notice. Tinkerers and makers might similarly be unaware. Hams active in SOTA and POTA also seem not to notice, although they may enjoy the brief respite from QRM. Yet it is a phenomenon.

Is it a problem? Probably not, at least not yet. What it may suggest, however, is that unless radiosport is taken up in a big way by new and younger hams the HF bands will only become more silent in the future. This is already happening to a degree, but a full generational replacement is unlikely. 

While it is a sad development for many of us, the amateur radio hobby will continue to change in the direction set by trends in technology and culture. HF will likely play a lesser role in that future.

Wednesday, November 29, 2023

Improving (maybe) the 160 Meter Shunt-fed Tower

My shunt-fed 140' tower works very well on 160 meters. But it can be made better. With the winter top band season underway, the time has come to do so. While it may seem odd to put the effort into 160 meters when the high bands are consuming the attention of most hams, there are two strong reasons: contests and DXpeditions.

I would eventually like to achieve additive gain and directivity by shunt feeding both of my big towers, that is a project for another year. Making each of the verticals more effective are beneficial on their own and for when I proceed with the phasing project. It is time to make the existing vertical better. By better I mean the following:

  • Higher efficiency: lower ground loss
  • Match: broaden the SWR bandwidth
  • Permanence: replace the "temporary" construction with a more robust system
  • Arc elimination: lower the voltage across the gamma capacitor

Lowering ground loss is perhaps the easiest: add more radials. I increased the radial count from 8 to 16. SWR bandwidth and gamma capacitor voltage are both dealt with by increasing the gamma "rod" diameter. Permanence, well, we'll see how I've done. Let's look at each item in turn.

For convenience of construction and radial placement, I previously placed the radial hub at the base of the gamma rod/wire, positioned ~2 meters from the tower base. I have now moved it to the tower base. 

Radial hub

I wrapped the tower with a band of aluminum flashing and stainless ¼" studs to attach radial wires. First I had to move the heavy aircraft cable wrapped around the concrete pillar with a small one on the tower's pier pin base. I have these on both big towers to serve as anchors for antenna work.

The new radial hub is pretty flimsy, so permanence remains elusive. It is a cheap and easy solution until I see how well the new design works. Tripping on a radial (not uncommon!) bends it out of shape, but it hasn't broken, yet. 

Assuming it works out I'll replace it with a copper band next year. I could easily wrap the base with copper wire and solder the radials on, except that the radials must be removed each spring for the farming season. Mechanical connections makes that activity more convenient.

The new radial hub consists of a 2" (5 cm) wide strip of aluminum roof flashing (~0.015" thick). It's cheap and I have a lot of it, but it is not very strong. Tripping on a radial puts quite a kink in it. 

I am not too concerned by that weakness. As I said earlier, it's a temporary measure until I am satisfied with the overall design. It'll do for this winter.

There are a dozen ¼" stainless bolts with the heads on the inside of the flashing. A nut secures each to the flashing and a set of washers and another nut are for attaching the radials. In the picture you can see that I have two radials per bolt.

To support the radial hub, there are several short lengths of scrap ½" aluminum tubes screwed to it. That keeps it above grade but not so high that the radials are not too exposed to mishaps. A narrow trench has been dug around the concrete pillar that I plan to fill with stone. That's to discourage growth so that I can trim the hay around the tower base without damaging the radials. Yes, I have accidentally cut a few over the years.

Feed point

When I first built the antenna several years ago I used a margarine tub to hold the gamma capacitor. The intent was that it would be temporary. Of course it became permanent, as these things always do. As you can see the elements were not kind to it.

For the experiment with a cage gamma rod I used another margarine tub. Of course it'll also be temporary. Or so I hope. 

The new one has one added feature: a coax connector. It beats the wire nuts covered in plastic and tape that I used to connect the coax and jumpers to the radials and gamma wire. The picture below shows the inside, with the gamma capacitor I ended with after tuning (more on this later).

Seriously though, I do have a permanent feed point system half built that uses proper components. I can go ahead and finish it now that both CQ WW contests are behind me.

One curious effect on the feed point impedance is worth mentioning. When I had 8 radials the resonant frequency measured in the shack was about 15 kHz higher than that measured at the feed point. That's because outer conduction of the buried Heliax transmission line, when connected, becomes the ninth radial.

With 16 radials the frequency shift is negligible: what I measure at the feed point is very close to what I measure in the shack. The transmission line has less effect on resonance when there are more radials. That is not a surprise since it was what I expected.

Radial wire

Each bolt on the radial hub terminates two radials: one of the 8 original and one of the new 8. All are 30 meters long. The original radials are AWG 18 insulated solid copper purchased new. Of the new radials, two are the same and one is stranded. For most of the rest I used wire that I bought at flea markets at bargain prices. They range from AWG 20 hook up wire to AWG 16 electrical wire. When I ran out of wire I used AWG 17 aluminum electric fence wire left over from Beverage antenna construction. 

It's a hodge podge approach that's cheap. Cheap matters since the radials require 500 meters of wire. I was not concerned with wire gauge since the more radials you have the thinner they can be. The reason is that the antenna current is evenly divided among them. However, do not make that assumption for a low radial count such as 4 since they are susceptible to imbalance due to variations in the ground composition and therefore the velocity factor in each. Too thin should be avoided since the wire is easily damaged.

Power lost due to ohmic loss in the radials declines faster than the radial count increases. That is true due to the power equation P = I²R. When you double the radial count, as I have, the current in each is halved and the ohmic loss is ¼ what it was. Ohmic loss declines with the square of the radial count, all else being equal.

Of the 16 radials, 3 had to be bent near the end because they ran into the stone wall that surrounds my yard and house. For this many radials the impact on current balance and performance is small and is not nearly as important as having the radials.

Gamma rod

I perused ON4UN's book and I made an initial trial with two AWG 12 wires spaced 40 cm (16"). The model (see below) suggested that it would work well to broaden the 2:1 SWR bandwidth to 130 kHz. I went ahead and built it with wire scavenged from a 40 year old 40 meter delta loop. It was a great antenna but I have better ones now.

The upper cage support was made from junk box metal. The brackets that hold the aluminum tube are more than adequate to handle the dead load and the tension to keep the cage taut. I added a support rope as insurance. The pre-drilled galvanized angle is found in almost every hardware store. The multitude of holes ease experimentation with wire spacing.

Another bit of scrap tube and hose clamps keep the wires in position at the bottom. Rope and brick weights provide tension. Despite the high RF voltage at the bottom of the gamma rod there is no risk of arcing because both wires are at the same potential. This arrangement was to be temporary until tuning was completed but it works well enough that I'll keep it for at least this winter.

The top of the gamma rod is at 55', 5' lower than the previous 60'. According to ON4UN the tap point is slightly lower with a cage than with a single wire. The model confirmed that but the difference is so small that lowering it was unnecessary. During tuning (see below) I raised it back to 60'.


The tower is loaded with yagis which make the modelling process difficult. Since including them results in a large and unwieldy model, I substituted a single wire that is the electrical equivalent length of about 58 meters based on an earlier measurement of the tower monopole. The measurement was partly swamped by the ground ESR via the lightning ground rod (~75Ω) but gave a clear signal of resonance at about 1200 kHz. That is, the tower plus yagis is approximately an electrical ⅜λ on 160 meters. The physical height of the tower plus mast is 43 meters.

⅜λ is an excellent height for a vertical but it is difficult to match. The impedance is quite sensitive to the frequency. For that reason the SWR bandwidth is narrow. It was 70 kHz for the original gamma match. Further, the high inductive capacitive reactance requires a low value capacitor to cancel it and that results in a high voltage at the gamma match feed point. Increasing the bandwidth and taming that high voltage go hand in hand. 

Zooming into the feed point illustrates how the cage gamma rod is modelled. The top of the rod is the same but without the loads. MININEC ground is specified to simplify the model for the purpose of impedance matching; at this point I was less concerned with efficiency. The load in wire #1 is the estimated ESR (equivalent series resistance) of the soil and radial field. The load in wire #5 is the gamma capacitor. The source (feed point) is the circle in the bottom segment of the tower.

I adjusted the model parameters until I had a 50 Ω impedance. They were closely in line with the tables published by ON4UN. The gamma rod spacing to the tower was 0.7 meters (28"), with a 20 meter high tap point and the aforementioned 40 cm spacing between the cage wires.

The SWR bandwidth closely matches the 130 kHz predicted from the tables in ON4UN's book.

At 1000 watts the gamma capacitor voltage for the cage gamma is about half of what it was with a single wire. It is still high -- that's unavoidable in this situation -- but far safer and easier to construct with components found in my junk box.

When I reached this point I was sufficiently confident to proceed with building and installing the cage gamma rod. Before we turn to that let's first discuss an alternative model of the antenna.

In addition to the basic model, I also built one that includes additional detail. The 20 and 15 meter yagis at the top were modelled as single wires that are about 30% longer than the booms. That brings the tower resonance in accord with the measurement of about 1200 kHz. The lower yagis of the stack are not included since they typically contribute little to the total capacitive loading. I may add them later to confirm that considering that the lower 20 meter yagi is not far above the gamma rod tap point.

I constructed radial systems with 8 and 16 radials over EZNEC real medium ground. To keep them out and off the ground (not allowed with NEC2) they are positioned 10 cm high. The difference is slight in comparison to on-ground radials. It is a common workaround for modelling radials with NEC2.

The efficiency comparison is interesting because it is less than 0.1 db. However, this is likely not really true. NEC2 is really not up to the task of accurately modelling ground loss and I've run into this discrepancy many times before. 

The experiment measurements by N6LF and others for 160 meter radial systems suggest that the expected gain improvement is between 0.5 and 1.0 db. That may seem tiny but it can make a difference on marginal propagation that is routine on top band.

I am not equipped to measure field strength. Instead I measured the feed point impedance. In the initial configuration for the cage gamma rod, the impedance dropped from about 40 to 30 Ω. Although significant, the true improvement in ground ESR is not 10 Ω because this is the impedance as transformed through the gamma match. I did not reverse the calculation in an attempt to pin down the actual change.


At right is the tuning setup for the cage gamma. When the picture was taken the tap point was at 55' so the cage wires reach almost to the ground. I was using a large capacitor since, as discussed above, the model led me to expect the voltage to be about half that as before. That corresponds to a low capacitive reactance: Xc = 1/(2πfC)

After several iterations of adjusting the gamma rod spacing I achieved a 50 Ω match.

There are two possible reactions to this beautiful SWR curve:

  1. Wow! Mission accomplished.
  2. No way! The antenna physics don't permit this.

I was suspicious but enough of an optimist that I was leaning towards the first reaction. I constructed the prototype high voltage gamma capacitor in the margarine carton and hooked it up. That evening I gave it a try. None of the European stations I called could hear me or could not hear me well enough to copy my call. Yet they were not weak and I usually have no trouble working them. It was then that my reaction changed to the second one and I began searching for answers.

It didn't take long to discern the likely problem. The next morning I looked over the antenna and confirmed my suspicion. When I connected the radial hub to the ground rod I forgot to reconnect the wire from the tower to the ground rod. The only path for antenna currents to complete the circuit from the base of the tower to the radials and coax shield was via the soil. The loss was therefore excessive. In other words, the ground loss dominated the feed point resistance.

When I restored the missing connection the beautiful SWR curve vanished. To cancel the inductive reactance the capacitance had to be greatly lowered. The impedance was approximately 21 + j0 Ω. That's far lower than what's acceptable.

Unfortunately I could not raise the impedance to 50 Ω. The best that I achieved was in the vicinity of 30 Ω. It was after several fruitless trips up the tower to adjust the spacing between the tower and gamma rod that I restored the tap point to its original 60'. That, too, helped very little. Worse, the further outboard I placed the gamma cage the capacitor value declined and the SWR bandwidth narrowed. What I ended up with was little better than what I had before.


The impedance at 1850 kHz was 41 + j0 Ω and 2:1 SWR bandwidth of 80 kHz. With contests coming up and many other projects on my list I reluctantly stopped work on the antenna. It's good enough for this winter. The extra radials give my signal a boost and the SWR is easily dealt with by the amplifier. The rig's ATU is not needed across the DX segment of the band. It is needed when the I go outside that narrow range during contests when I don't use the amp.

I returned to the model to try and discover what might be happening. I can't say for certain despite learning a few new things about the antenna's behaviour. 

The first thing I learned is that a gamma match on a ⅜λ vertical does not react the same way as on a vertical that is closer to being ¼λ. The reason is that the impedance decreases as you move upward because the current node is ⅛λ above ground. That is about 20 meters in this case. With the tap point at that height, moving up or down increases the impedance. It is then transformed by the transmission line formed by the tower and gamma rod to what is measured at the tower base.

I confirmed the impedance behaviour with the simple model shown at right. The vertical wire is 57 meters long and is directly connected to MININEC ground. Peak R of more than 140 Ω occurs at 20 meters height. It is about 100 Ω at the base. I then performed a sensitivity analysis by varying the height in 1 meter steps. A 1 meter change in either direction changed R by ~10%. That's a lot!

Changing the antenna length by 1 meter in the model with the gamma match and cage caused large swings in both R and X components. Clearly the tuning is critical. It is possible there's a tap point further up the tower that will result in a 50 Ω match. Unfortunately that will bring the cage gamma rod close to the lower 20 meter yagi, and the possibility of the gamma rod threading between the elements. I'd like to avoid that if possible even though interaction ought to be minimal.

Clearly this antenna is more difficult to match with a gamma match than I expected. What I measured is not what I discovered in my cage gamma rod model or what I read in ON4UN's book.

That's all the time I have for the 160 meter antenna this season. I was already irritated that I missed the W8S Swains DXpedition appearances on 160 meters while I was in the midst of trying to resolve this mess. I likely would have been able to work them.

Planning for the next round

I am not so committed to the gamma match that I wouldn't throw it away and try something different. My options are limited since the tower is grounded. 

The only one that might work is an omega match. An extra capacitor is needed, and the best it can do is shift the impedance match from 40 Ω to 50 Ω. The bandwidth will not be improved and the voltage will remain high.

The alternative is to insert a switched L-network for the higher frequencies. I could then optimize the gamma match for 1810 to 1860 kHz (SWR better than 1.5). That addresses 90% of my needs. For contests where the activity runs up to 1900 kHz or higher, a switched L-network can lower the SWR up to at least 1900 kHz. The enclosure I plan to use is large enough to accommodate the gamma capacitor, an L-network and more.

To be completed

With the ARRL 160 meter contest coming up fast, I limited myself to an improved connection between the gamma rod and capacitor. It's cumbersome due to the 3 meter gamma rod spacing at the base (it's 2 meters at the top of the rod). 

Using an aluminum tube makes it easy to adjust the spacing without have to cut or add wire. The ABS pipe on the tower isolates the high gamma match voltage while allowing easy adjustment of the gamma rod spacing.

After the contest I'll take the antenna offline to move the gamma capacitor to its new enclosure. I will also take the opportunity to "rough in" the components for an L-network to lower the SWR higher in the band. Although I have no plan to add it this year, I want to make it easy to do next year.

I'll continue to contemplate and research the conundrum I ran into with gamma tap point and gamma rod spacing. I'd like to understand the problem better regardless of whether I install a switchable L-network.