Duty Cycle

With the return of sporadic E season on 6 meters my thoughts turn to transmitter duty cycle. That might seem odd unless you know that I almost exclusively operate digital modes on 6 meters. I have to monitor my use of the amplifiers more carefully than I do on CW and SSB. Most hams know that it is recommended to operate transmitters and amplifiers at lower power on digital modes, yet the understanding of why is perhaps lacking. Failure to reduce power has not gone well for many.

Despite so much written on the subject it seems worth another article on the subject. Will it help? I don't really know. Repetition has its own benefits and perhaps putting it all in one place will help a few readers. Or not. I'll try anyway.

Here are a couple of samples. On top is audio from a recorded wav file from one of my contest messages. There is no compression in the recording so the duty cycle is low; compression is added during transmission. On the bottom is a sample of CW keying. The duty cycle of the transmitted signal is slightly lower due to shaping: gradual rise and fall times to prevent key clicks.

Duty cycle for both CW and SSB can be exquisitely calculated, if you wish. That isn't necessary since we don't need that degree of precision. CW is approximately 50% when you average over dots and dashes, inter-character and inter-word spaces, and of course ordinary brief pauses. SSB can be less than 20% though few of us operate that way. Instead we equalize and compress the audio so that the duty cycle of SSB is also approximately 50%. In contrast, digital modes like FT8 and RTTY are 100%.

That is (literally) only half the story. We interleave receiving and transmitting. Assuming typical communication, conversation or contesting, we do both approximately equally. Therefore the transmit duty cycle is closer to 25% for CW and SSB and 50% for digital. FT8 transmit cycles are really only about 42% and if you CQ a lot without replies, the duty cycle for CW and SSB may be closer to 35%. Again, we can mostly ignore these nuances for the purpose of this discussion.

One problem with the basic definition of duty cycle is that it is a moving average that changes from moment to moment. For example, although a single phone QSO may have a 25% duty cycle, when averaged over, say, 1 hour of time in front of the radio, the duty cycle can be much less since you spend more time listening than in QSO.

Digital is similar. An FT8 transmission interval is 15 seconds, where you receive for 15 seconds and transmit for 15 seconds (actually only about 12.6 seconds). Over the course of a QSO the duty cycle is 50%, double that for CW and SSB. If you behave like a robot the 1 hour average may be the same, and therefore far higher than for CW and SSB outside of contests. That is one difference between digital and modes like CW and SSB where robots are, at least so far, absent. 

That 100% duty cycle is only when averaged within the bounds of a single transmission interval. Said another way, the instantaneous duty cycle during an FT8 transmission is 100%.

Up to this point I very much doubt that I've said anything that the majority of readers don't already know. From here I want to combine the idea of a moving average with transmitter and amplifier operating parameters. The connection between them is heat, more specifically heat transfer.

This is me operating on 6 meters FT8 with an Acom 1200S solid state amplifier. Notice the power level. I rarely go above 650 watts on FT8 since the temperature soon rises towards 70° C, the nominal limit. When the shack is warm the power must be kept below 500 watts. The reason is that input air is warmer and is less able to remove heat from the amplifier. On CW and SSB I regularly operate at a full kilowatt, where the lower duty cycle keeps the amp within its temperature limit.

In this stylized diagram we can see how the temperature rises during alternating transmit and receive cycles, light gray for FT* and dark for CW and SSB. Average power output is the same for both. Although the reality is a more complex, the diagram communicates the important ideas. Line width shows the variation based on ambient temperature: the hotter the air going into the amp the less effective the cooling system.

At first the temperature rise is sharp, with the slope gradually declining as the amp heats up. The slope declines (the lines really ought to be curves!) because of the increasing temperature difference between the incoming coolant fluid (ambient air) and the surface of the heat sink.

The red line is the temperature at which the amp protection trips. Operating with lower average power is required on FT8 to keep the temperature within the acceptable range. However, as you can see, it is possible to operate at higher power for a short period. Robot operators and contesters need to be more mindful of the long term average, and therefore the duty cycle.

Cooling effectiveness is a function of coolant heat capacity, ambient temperature, airflow volume, and the surface area of the material to be cooled. As is the case for any heat pump, the ability of the coolant to draw heat from the material increases with the temperature differential. Cool air cools better, and the hotter the material the more heat can be drawn off by the coolant.

At right is a picture I took of my Acom 1500 output air vent showing the integrated heat sink (cooling fins) of the 4CX1000A, the chimney directing air through the cooling fins and the exhaust air temperature sensor (top centre). 

In contrast, consider the picture that I pulled from the internet of an LDMOS device being bonded to an amplifier heat sink. We have more options on where to place the temperature sensor. It can be a thermocouple in the device, on the heat sink or a sensor placed in the airflow as is done for tube amps.

With respect to heat transfer, tubes like this have the advantage. Bonding of device elements to the heat sink is entirely integrated. The designer's job is to ensure that the air flow is properly routed and of sufficient volume to meet the cooling requirements. 

Glass envelope tubes are more challenging since there is a combination of conducted heat (bottom pins and top anode) and radiated heat from the interior metal components, especially the anode. Typically the air flow is from the bottom, to cool the pins and their glass seals, then around the glass envelope. Air flow is directed using vents and chimneys. The temperate sensor should always be at the exhaust.

Solid state devices are acutely sensitive to good thermal bonding since there is so much heat concentrated in a small volume and with limited surface area to conduct the heat away. For example, for 1000 watts of RF at 60% efficiency, the heat produced is 670 watts. That's a lot to transport over a few square centimeters of heat sink contact area! Once you get the heat over that barrier, cooling the heat sink is relatively easy.  

It is no surprise that LDMOS longevity is highly dependent on transferring that thermal load to the heat sink. Thermal protection must trigger quickly and reliably to protect the devices. The MTTF chart is from an old Freescale presentation on LDMOS. The device mentioned is likely obsolete now, however newer devices, like any semiconductor, will have similar thermal characteristics.

This brings us to the question of what we're actually measuring and where we're measuring the temperature.

All methods of thermal protection are by proxy. That is, we're measuring temperature at some remove from the locations where the heat is generated and the points of greatest criticality. Indeed, most amplifiers have more than one measurement system to detect thermal problems. For example, in a tetrode like the 4CX1000A the control grid has almost zero ability to dissipate heat. Exhaust air temperature won't detect that. It is necessary to monitor grid current and quickly shut down the amp when current indicates excess power dissipation (by P = I²R). 

Despite the sensitivity of the grids, overall tube temperature can be very high. It is common for me to measure a temperature of 90° C when continuously running ~1000 watts of FT8 on 6 meters during warm July days when the house air conditioning is off. That's enough to brew tea yet the thermal protection trigger is even higher.

LDMOS devices can quickly fail if the semiconductor junction temperatures exceed their limits. Unless there is a thermocouple built into the device we are limited to proxy measurements outside the device. 

Ideally it should be on the metal body or, if that isn't possible, on the heat sink near the LDMOS. Exhaust air measurement may be too far removed from where the heat is generated. Unlike a tube, a brief internal temperature spike may be unrecoverable. Remember that when you absolutely must work that new one.

Proxy measurements are dependent on good design and construction practices so that the thermal transfer from the interior to the heat sink is predictive of the junction temperatures. Unfortunately that is not always the case and the devices don't last for long. The screenshot is from a video by W8JI demonstrating poor thermal bonding of a FET in a late model Ameritron amplifier.

To quote the aforementioned Freescale presentation: LDMOS device thermal resistance benefits from having a backside source that is thermally and electrically bonded to the package flange, which in turn is directly mounted to the heat sink. Metal to metal contact is best. I've built power supplies and other projects where the power transistor requires a thin insulator between it and the heat sink, with thin coatings of toxic conductive paste to minimize thermal resistance. There is also the hazard of capacitance between the transistor case and the heat sink in RF applications.

It should be clear by now that thermal protection circuits on amplifiers, tube or solid state, require a physically removed sensor that measures by proxy, which demands excellent construction so that the proxy measurements are predictive of temperatures at the critical points. Otherwise we should expect regular and expensive repairs. 

In this context, duty cycle is really only one factor among many, and not necessarily the most important. Indeed, it can be expensively misleading. 

Increasing airflow (bigger and noisier fans) can only be effective if thermal transfer from device to heat sink meets the design specification. In this respect, solid state devices are more difficult to reliably cool than tubes. 

Although competent amplifier designers take these factors into account and incorporate thermal protection, failures can still occur. A little common sense on our part can pay dividends:

• Buy from companies with a reputation for good build quality, that stand behind their products and don't practice blame shifting when failures occur
• Device ratings and duty cycle matter less, often far less, than amplifier design and construction 
• Install equipment so that airflow is unconstrained even if the fans are annoying; many modern solid state amps can be operated remotely if it's a problem 
• More protection circuitry is better than less, despite the annoyance of false alarms 
• Understand that nothing is forever: failures will occur, even in expensive equipment 

I intend to follow my own advice when I buy my next amplifier, probably this summer. 

Modern Spotting

DX spotting clusters have existed for decades. Originally they were isolated systems accessible via packet radio on VHF or UHF, and therefore limited to the local community. Unless you already had the equipment it required time and money to get connected. The value was dependent on the size of the local DX community and their willingness to connect and spot what they heard.

We built an AX.25 VHF cluster in Ottawa almost 40 years ago. It didn't go well. There were too few of us, the node was difficult to reach (too far out of town) and spots were infrequent, mostly just evenings and weekends. After about a year the node was decommissioned and we donated the equipment to other local VHF projects that had nothing to do with DXing.

Today it's easy. PCs, smart phones and internet connectivity are universal. Pick an app or an internet DX node and you're ready. You can spot at the click of a mouse or do it automatically. You can receive spots from hams across the globe or just your region. There's so much traffic that it must be filtered to avoid being overwhelmed. Then there are the perpetual incompetent spotters and also the limited utility in knowing what DX stations across the ocean might be hearing and working.

Do you want to work the latest rare DXpedition? Chances are that they have a live stream where they post each station worked, including mode and frequency. That's the ultimate in self spotting behaviour. You don't even need to check for spots of these stations. Filling band slots and reaching DXCC Challenge endorsement levels has never been easier. Some think it demeans the value of DX awards, but most hams love it.

Nobody listens anymore. It's time to revisit the topic of DX spotting.

CW

When I tuned to 20 meter CW one afternoon this week there were no spots on the band map. Yet there were many signals visible on the transceiver's waterfall display. I enabled skimmer spots and within 10 minutes there were dozens of spots, only two of which weren't from a skimmer. I am careful to filter for skimmers located in northeast NA to reduce spots of stations I'm unlikely to hear.

CW skimmers work so well that most operators no longer bother with spotting. Whether that's good or bad is a matter of opinion. Skimmers make mistakes, including decoding errors and phantom signals. They may also have antennas too poor or too good to mimic a typical station. 

An increasing number of nodes are installing filters to weed out the problematic spots, whether human or skimmer generated. They're becoming quite good at it, often better than proficient CW ops -- typos are common. Skimmer spots don't include splits (for rare DX), and human split comments are frequently unreliable.

Even on 160 meters where I have excellent Beverage receive antennas there is little need to tune the band. There are skimmers with dedicated receive antennas that perform admirably. It is also useful for any weak station (e.g. QRP) since skimmers pay more attention to the weak ones than humans do. Once your weak signal is picked up by the skimmer you will attract callers, be it for POTA, in a contest, or other operating activity.

The technology has progressed to the point that, like many others, I rarely spot stations on CW, and I can do it without feeling guilty. One exception is rare DX that may rarely identify or not follow standard CQ pattern that might not be picked up by skimmers.

SSB

There are no phone skimmers, at least not yet. They'll come eventually but until then we are reliant on human spots. For those that rely on spots to find stations, rare DX or not, be aware that most stations do not spot the phone stations they hear or work. If you dislike spots and prefer to find stations yourself, this may be seen as unimportant or perhaps a positive. That is not a common view.

There is a way to increase phone spots. It remains largely a feature of contest logging applications such as N1MM Logger+. It is a simple configuration change with which every station you work by S & P (they run and you find them) is spotted, if you are connected to a DX cluster. 

While not mandatory it is recommended by many contesters. The objective is to increase the number of phone stations that are spotted to a level comparable to that achieved by CW skimmers. 

This is the best we can do until we have software that can reliably recover call signs and other critical spoken QSO data, whether in English or other languages. It's a challenging technical problem. My prediction is no less than 10 years until it is broadly available and sufficiently accurate. I could be wrong and we'll get there sooner, or much later! There is no financial incentive so we are dependent on technology developed in fields outside of amateur radio and, of course, dedicated and capable volunteers.

Digital

Digital modes are particularly amenable to automated spotting. It has been available for years through use of WSJT-X and similar apps in concert with services like PSK Reporter. One only needs to click a check box to report all that the software hears. The data can be retrieved and analyzed via online services, whether PSK Reporter itself or downstream services that utilize the data feed.

You can manually inspect what other reporting stations are hearing or use services that graphically present the data. Again, I am no expert on those services so I'll let you discover those on your own.

Even if you don't send your data to PSK Reporter, you will be found since most of the stations you work on digital modes upload their reception data. 

Contests

Spotting in contests is different. The major difference is that, for a big station like mine, you run most of the time. When you run you have nothing to spot. Not only that, the spots that do appear are less appealing. Certainly you want to chase multipliers and other contacts, but if you're in an assisted class you click on spots and so you have little to contribute to the community. That is, you are not spinning the VFO and spotting stations that are not already spotted.

If you operate unassisted you do not spot. It is true that some contests allow you to spot others but not see others' spots. However that's rarely done even when permitted by the rules. Operators just don't connect to DX clusters when they're not in an assisted class.

For an increasing number of contests it may be that most of your spots will be for yourself: self spotting, where the rules permit it. When you're running there's little else for you to spot. This benefits you and nobody else, but that's the nature of the game: you want to be found and others want to find you.

In CW contests the skimmers are so successful that I often turn skimmer spots off. The reason is that the flood of data is overwhelming and not necessarily useful. Early in a contest everyone is a new contact so the band map isn't needed. Later after many stations have been worked, I turn skimmer spots back on.

For the casual operator in a contest, you are more likely to be a consumer of spots rather than a producer. Considering the high activity level in contests that really isn't a problem -- you probably aren't adding anything that others, including skimmers, aren't already producing.

Tools

There are many tools to analyze spots in real time or near real time. Some are downloadable apps while others are web based. You can track DXpeditions, where and on which bands there are openings, get alerts for grids, countries and specific stations. Many are free. The data is also used for research, mostly amateur but some professional, to correlate with solar and geomagnetic data and refine prediction algorithms. With so many spot sources there is a lot of data available.

I've used a few of these tools though far less than many others. Since I am not in a position to make recommendations, I won't. You can find them and their champions through an internet search. I'll leave it to others to guide you if you want to pursue the topic.

Where we are

Like it or not, spotting is the way a large majority of hams find stations to work. Older hams may reminisce about the old days when we spent many evenings spinning the VFO looking for stations to work, or that elusive rare DX with an unpredictable operating pattern. Those days are gone and they are not going to return.

The truth is that all that VFO spinning was tedious and frequently fruitless. I recall the days when I left my 6 meter rig parked on 50.125 MHz during sporadic E season. The hiss of the receiver filled the house since squelch wouldn't trigger on weak signals. Don't try this if you're married or living with others! The discovery potential of FT8 and other digital modes is one reason it has large replaced CW and SSB as the mode of choice for the serious 6 meter DXer.

Skimmers do much the same for CW, discovering stations and rare openings that would otherwise be missed. Although there is less need to spin the dial, it is helpful to CQ into the aether from time to time for the skimmers to have something to copy. If nobody transmits the band is dead to humans and also to our automated listening devices. That at least hasn't changed.

6 Meters is Heating Up

It's said that sporadic E does better when solar activity is low, thus favouring the years surrounding the cycle minimum. With the solar flux falling maybe we'll get some fireworks this summer.

I've worked the first DX of the 2026 season. Nothing too exciting and nothing new, but that's to be expected with 150 DXCC entities worked on 6 meters; every new country is harder to work than the last. Of those, 140 are confirmed on LOTW, my only QSL route.

So far this May I've worked S0 and EA8. I've heard other countries including several in Europe, D2 and in the Caribbean and South America. Around the continent, I've worked stations in VE7 and W7 along with several stations closer in. I could work many more if I was interested in short haul contacts, which I'm not.

DX signals are weak and fleeting this early in the season. From PSK Reporter data I know that I've been heard in Europe. I have no contacts with Europe yet this year. The summer solstice, the usual peak of the sporadic E season, is still 6 weeks in the future. It will get better.

Despite the negativity often seen with regard to digital modes, 6 meter DXing is one place where FT8 shines. It is perfect for discovering and exploiting the brief intensification of ionization that transport signals between continents at VHF frequencies. You can't chat over FT8, and that's fine with me. I don't chat much over CW or SSB either since my interests strongly lean towards contests and DX chasing. I see no reason to have a conversation when 6 meter opens; I have other priorities.

To enhance my enjoyment this year I configured my station so that I can operate HF and 6 meters at the same time. My hope with this change is to catch openings earlier than I otherwise would. 

My usual SO2R contest setup is ideal for this. It's quite easy to do by moving the 6 meter antenna to the second radio (Icom 7600 & Acom 1200S) and using it with WSJT-X. The main radio (Icom 7610 & Acom 1500) is dedicated to HF. Both radios connect to the same PC, one using N1MM and the other using WSJT-X. 

To limit interference when I'm transmitting on either radio I switch in the contest band BPF on the 7610. I have no BPF for 6 meters. That will remain a low priority despite its potential benefits. I know fervent contesters that dedicate a radio and amp to 6 meters so that they can work sporadic E openings during contests. FT8 is easy to operate in parallel since it is slow and only requires the occasional click of the mouse. There are no audible distractions to the CW or SSB contest activity on the HF radio.

Although less powerful on high duty cycle digital, the solid state amp is near instant on versus the 3 minute warm up period of the tube amp. With such fleeting openings the rapid readiness of solid state is more important than another 1 to 2 db. I prefer not to keep the tube amp idling all day long, especially in summer when the shack is already too warm. When I buy another solid state amp later this year it will increase my flexibility, and power, in this activity and in contests.

Returning with the propagation are the 6 meter robots. They are growing in number. Unfortunately, WSJT-X-improved supports a blacklist of only up to 12 call signs. I wish it were larger. I find myself editing the list as robots appear to strike out the ones bothering me at that moment. It would be easier to add each of them once.

I don't really mind the existence of robots, I simply ignore them. The problem is that they fill the decode screen with their endless CQs that push the wanted DX signals off the messages received pane of WSJT-X. I have to scroll the screen to see what I might have missed. It is easier when I can just glance at the screen from across the room. So I filter them into invisibility. I find the behaviour of robot operators perplexing but it isn't my concern.

As always, don't be surprised or offended if I don't reply to you when I call CQ DX and you are not DX. It's nothing personal: you're not my intended audience. My primary interest on 6 meters is DX. I don't hunt grid squares, states or special call signs. One of the joys of amateur radio is that we can pursue our individual interests, together or separately, while sharing the same spectrum.

Enjoy the season. Soon the DX will be rolling in. If I get lucky a few new countries will fill my log this year. Working DX on 6 is enjoyable to me even if I work no new ones. I hope that you have success on 6 this season no matter your objectives or station size. If you've never given 6 meters a try, you should. Sporadic E season is the ideal time of year to take the plunge.

Another Rotatable Side Mount - 15 Meters

One lack in my station is a sufficiently strong signal on 15 meters into Asia, especially east Asia. While we cannot run Asian stations like our friends further west there is a rich vein of multipliers to be mined during major contests. I was reminded of this during the last All Asia SSB while running Japanese stations by how incredibly weak most were, barely a whisper above the noise. And this is from a quiet rural QTH.

The 5-element stack for 15 meters has a rotatable upper yagi at 43 meters and the lower yagi is fixed on Europe at 32 meters. Individually and together they are very effective antennas. In daily operating the stack can easily break pile ups with just 100 watts. To achieve stacking gain the lower yagi must become rotatable. That is never easy for side mounted antennas.

There are two possibilities: a swing gate or ring rotator for close to full 360° rotation, or a fixed side mount for about 130° of rotation. The first is not really necessary in my station since I have other yagis that can fill the gaps. It is also the more mechanically challenging of the alternatives. 

The latter method can work well if the 130° of rotation includes both Europe and east Asia. That I can do. The plot shows the approximate coverage for the 15 meter yagi's rotatable side mount.

Unlike the 40 meter Moxon, the 5-element yagi can't be reversed to double the azimuth coverage. That's okay for my situation on 15 meters so I followed the same design as for the first rotatable side mount. Deviations from that design are minor, to accommodate differences on the mounting location and the coverage objective. 

Last fall I picked up a used Ham-IV rotator and controller. It looked clean on the outside and proved to be in good condition on the inside. That's more than enough rotator for the weight and wind area of the 15 meter yagi. A swing gate would require a far heftier rotator to deal with the high torque of an offset load.

Since the weight of the yagi, mast and rotator bears on the lower strut, I used 4" × ¼" steel angle. It was scrap and rusted but perfect for the application. I cut a rectangle of ¼" steel plate with conveniently located holes that lined up with slots in the angle stock. This made a sturdy platform that put the rotator far enough from the tower to allow rotation from about 60° (through north) to 280°, just as I wanted. The slots permit a small adjustment range to align the rotator and mast bushing

The strut is strong enough to support my weight and even jump up and down on it, which I did to test it after installation -- I should point out that I don't weight much so be very careful about trying this yourself! Unlike the aluminum strut that I used on the other rotatable side mount, there is no need for a support cable to keep it from deflecting under load. If it becomes a problem later I can easily add it.

The LR20 guyed tower is popular in Canadian big stations while rare elsewhere. Mechanical details will therefore be of little interest for most readers. Nevertheless, the design and construction process can be applied to other towers. I keep a section of LR20 (10' high, 120 lb) in my workshop as a jig for jobs like this.

The first challenge is that I had to use a different tower girt for this side mount. There are two guy/support girts on each section. Due to the yagi's position the girt for the lower strut has to fit between the guy yokes and section splice bolts. Pictures will help so here are a few.

On the left you can see the two cutouts for the splice bolts. I cut the angle stock with an abrasion wheel on a circular saw. Since it's out of sight high on the tower it doesn't have to be pretty! I regularized the slots as well as I could and removed all sharp edges and points. The strut was descaled and painted.

The strut couldn't be centred on the girt due to the guy yokes. A short length of steel angle stock seats the strut on top of the girt. 3" ×⅜" aluminum angles are used as fore and aft clamps to the girt to keep the strut stable under the stress of the weight of the yagi, rotator and mast on the far end of the strut. The clamps are cut with peculiar angles to fit the insides of the tower legs. 

The rotator plate will be shown in a later picture. It is outboard of the strut to maximize the rotation range. They are joined by ⅝" grade 5 hardware.

The upper strut only has to deal with lateral forces. The 1.9" O.D. 6061-T6 pipe mast runs through a 2-⅜"O.D. schedule 80 pipe section, which is used as a bushing. This is identical to the other rotatable side mount that is currently used for the 40 meter Moxon. I considered a polymer bushing, which I have in stock, but that is more difficult to clamp to the strut without crushing the polymer. I took the easy path rather than fuss with a more elaborate design to accommodate the polymer cylinder. 

The upper strut pivots on the rear bolt to the girt. The outer end is adjustable with a threaded rod that runs through the strut and a bracket bolted to the back girt. While not very visible in these pictures the design is almost identical to the one for the other rotatable side mount (I provided a link to that article above). By swinging the upper strut and sliding the rotator plate on the lower strut slots the mast can be vertically aligned and rotated without binding. 

On the right you can see the boom of the yagi still supported on the original plate that is attached to the tower with pinch clamps. A chain holds the boom to the tower. That becomes the pivot for swinging the boom over to the mast. A pulley and rope support the boom on the rotator side of the tower (centre photo). With the chain and rope lightly supporting the boom, the boom truss and the saddle clamps of the existing fixed side mount are removed. 

With the yagi's weight fully supported by the swing the mast is lifted out of the rotator with one hand and the boom is pushed to the outside of the mast with the other hand. Then the mast is dropped back into the rotator, as seen in the left photo. Although a second pair of hands would have come in handy it wasn't difficult to do.

While not too heavy, it was still awkward to lift and level the yagi to permit attachment of the boom to the mast clamp. The front rope was pulled to raise the boom up to the clamp and tied off to the tower. A second rope was used to lift the boom within the confines of the chain. There is no risk of the yagi escaping during the operation.

Unfortunately the saddle clamps that I selected didn't fit the 3" boom. That was quite a surprise! Since it was time for lunch I left the yagi to bounce in the wind and climbed down. On examining the clamps on the ground I discovered my mistake: I had selected clamps to fit 2-⅞" O.D. pipe. The u-bolts fit but not the saddles.

Since I was out by a small amount I tried to fit the u-bolts through the 3" saddles taken from the old fixed mounting system. To my relief they fit. After lunch I took the mix of u-bolts and saddles up the tower and completed the installation of the yagi to the mast. I was lucky that I didn't need to machine a new mast clamp to accommodate the 3" saddle clamps.

With the yagi attached to the mast it was time to quit for the day. The rain clouds were getting very close. I kept the antenna pointed at Europe and reattached the coax. The clamps were not too tight so the yagi swung on the mast a bit in the high winds that accompanied the rainstorm. It was almost a week until the cold, wind and rain subsided and I could resume work. The remaining tasks included: boom truss, protective rubber bumpers on the tower, coax rotation loop, and alignment with the upper yagi for maximum stacking gain.

Wiring of the rotator was completed last fall when the struts were installed -- this and several other projects were supposed to have been completed before winter struck us early and more severely than expected. The motor phasing capacitor is encased in UV-resistant plastic and a barrier strip matches the 8 wires required for Hy-Gain rotators. 

Wire #1 for ground/common (on the left) is connected to the tower, thus eliminating one of the 8 wires. The phasing capacitor is connected to #4 and #8 to eliminate the need for those wires. Cat5 cable is used for the direction pot wires #3 and #7. The high current wires for the brake (#2) and the motor windings (#5 and #6) are 14/2 electrical cable. 

This is my preferred wiring method for Hy-Gain rotators with long cable runs, well over 100 meters in this case. It gets the job done for the least cost. Copper isn't cheap. Extra cables to support future projects such as this one were thrown into the trench when it was dug during the early days of COVID.

I am still experimenting with the rotation loop, as I will explain shortly. The photos show its current configuration. When the phasing lines from the stack switch were made I included a few feet of slack to ensure adequate length for routing around obstacles and other unforeseen issues. For the lower 15 meter yagi there is approximately 3' (1 m) extra, with most of that coiled at the stack switch.

To make a sufficiently long rotation loop the full length of the LMR400 phasing line was pulled down through the cable ties. Since this is semi-rigid coax the amount of flex during rotation must be limited. The present configuration accomplishes this. It took a few tries to get it tracking properly and ensuring that there was very little stress on the cable.. 

Note: In these and a few other photos you'll see a few seemingly out of place wires and ropes. Those connect and support the two halves of the boom truss during the project. It's no fun having to lean far out from the tower to retrieve a dangling cable and turnbuckle that can easily escape from one's hand. For most of my yagis I tie a short length of rope between the two turnbuckles in case of turnbuckle or cable failure. It's cheap insurance.

The yagi in the previous pictures is pointed at a bearing of approximately 45°. The yagi further down the tower is the lower yagi of the 20 meter stack pointing to Europe at a fixed bearing of about 50°. Due to the offset of the rotator the clockwise stop for the 15 meter yagi is approximately 55°. In the photo immediately above, the yagi is at its counter-clockwise stop, pointing almost west at about 275°. This is the rear of the yagi (facing east) with first director adjacent to the tower. The rubber bumper protects the tower and boom when the operator over-rotates the yagi. The Ham-IV has limited torque and so is easily stopped when it hits a solid obstacle without the risk of damage to the yagi or tower.

Yagi de-tuning with the director so close to the tower is minimal. Coupling interaction is minimum at element centre and maximum when near the element tips. There is in fact more interaction as the yagi is rotated towards north. The measured SWR barely budges. Gain is difficult to measure or model due to the structure of the lattice tower. Modelling that I've done in the past of similar tight coupling is encouraging so I choose not to worry about it.

Of greater concern is stacking gain over the rotation range. Previously, the stacking gain was optimized with the lower yagi fixed towards Europe. Since the rotator is offset from the tower centre, where the upper yagi is centred, the yagis can only be in phase in one direction. In all other directions the feed points are not vertically aligned. For the rotatable side mount the phases are also aligned when both yagi point to Europe. The question is then how much the stacking gain is reduced in other directions due to the phase error.

The top-down diagram demonstrates how the analysis was done. The red dot in the triangular tower's centre shows the position of the upper yagis' mast and rotation centre. The blue dot is the lower yagi's rotation centre. The red dot adjacent to the tower is the 0° phase point that is in alignment (main lobes in phase) with the upper yagi when both yagis point to Europe (northeast).

The circle shows how the phase centre of the lower yagi moves in relation to the tower and upper yagi as the rotator is turned. The phase offset is quite small when the lower yagi points north. Modelling shows that the gain reduction is less than 0.1 db. That's negligible. 

The misalignment becomes quite large when the lower yagi points west. The offset is approximately 5' (150 cm) or roughly 38° at 21 MHz; 150 cm is 0.105λ for the 14.3 meter wavelength; the phase calculation is approximate, not exact.

The elevation plots are overlaid to show the effect. Since the resolution is limited, making it difficult to see small details, I'll mention that main lobe's gain is reduced by 0.5 db. That's not bad. The greater impact is that the nulls between the multiple elevation lobes are shallower, pretty well disappearing at high elevation angles. In a contest that can be helpful since fewer signals will be attenuated when the arrival angle falls into one of those nulls. It is inconvenient to fiddle with the stack switch (BIP, lower and upper by mouse control) for every contact. 

While not shown, the lateral offset of the booms has negligible impact. This is unsurprising since the vertical alignment of the boom (when pointed in the same direction) has little to no impact on the formation of the far field pattern. I took a few minutes to confirm it in the model.

The lower yagi could be slid backward to improve phase alignment to the west. However that would unbalance the wind load, require adjustment of the boom truss, and require greater extension of the coax rotation loop. The last will be difficult. For my style of operating the improvement is not worth the effort. It is rare that I'd benefit from stacking gain to the west. When I do I can live with the 0.5 db deficit. 

Another potential benefit of sliding the boom back is that the first director would be farther from tower when the yagi point west (see above). On the other hand, the front of the yagi (third director) would come closer to the upper guy that is due west, as shown in the adjacent photo. The top guys descend at a sharp angle and this is a large yagi. That interaction could be worse than the one between the first director and tower.

As I finish writing this article there is still some tidying up to do. The most important is the forward boom truss for which the cable isn't long enough to reach the mast. Pieces of the turnbuckle unscrewed and fell down over the winter months and I haven't found them all. 

I am also contemplating replacing the aluminum mast with steel. While not serious, it abrades a little in the rotator mast clamp and bushing. I'll check it again before winter and decide whether to replace it.

Other than these minor items the rotatable side mount is done and I've already tested it to Asia on air. The results are encouraging. As with any combination of antennas like this, especially so they're so high relative to wavelength, there are times when the lower yagi is superior. The reason is that with many elevation lobes there is an increased risk of a signal's arrival angle falling into a pattern null. The more expansive stack arrangement gives me greater ability to deal with those signals.

Multi-band Antennas & Inter-station Inteference

When I first built this station I put up a few multi-band yagis: TH6, TH7, Explorer 14. When operating SO2R in contests I could not share these yagis since I don't use triplexers but I had enough antennas to get by. Over time these antennas have been displaced by mono-band yagis. The last of them, the TH6, is slated for removal later this year.

Trap yagis are inefficient but they can be very convenient for most hams since it is rare to have more than one tower, or one robust enough to support a "Christmas tree" of mono-band yagis for 20, 15 and 10 meters. Let's set that aside in this article to talk about interference between radios -- SO2R and multi-op contesting -- when using multi-band vs mono-band antennas.

Above are multi-band yagis from Optibeam, DX Engineering and Hy-Gain (clockwise from top left). Only the last has traps. However, non-loaded multi-band yagis suffer various gain and bandwidth deficits in comparison to mono-band, full-size yagis. Coupling among close spaced elements, even if each element is resonant on a single band, create many anomalies. The same is true for mono-band yagis placed too close together. 

In general, multi-band antennas are often high-Q, with narrow operating bandwidth, due to traps and other devices to enable resonance on more than a single band. Simple antennas without those devices can also be multi-band by exploiting harmonic behaviour. These include full-wave loops (resonant on all harmonics), dipoles (resonant on odd harmonics), and EFHW (end fed half wave) multi-band antennas that are popular with many hams that lack towers. 

Multi-element mono-band directive antennas like yagis may resonate on harmonics but they will not be yagis at those frequencies. Therefore they are potential interaction sources while not being useful at those higher bands. For example, a 40 meter yagi on 15 meters can behave as a resonant long dipole with a complex but largely non-directional pattern. It can seriously degrade the pattern of a 15 meter yagi quite far away, depending on their positioning; reciprocally, the 40 meter yagi will induce significant current from a 15 meter yagi and send it down the transmission line to the receiver.

This brings us to BPF (band pass filters). They are the almost universal choice for managing interference between radios in a contest or DXpedition operation with two or more stations. For the present I'll focus on the most common situation of one station per band and not on the more difficult one where there are in-band stations.

BPF are not a universal solution since there are things they can't do. Our primary concern is reception, where we want the BPF in the transmission and reception paths to minimize interference and performance degradation. It is important for the station designer to understand how BPF function to determine how they can be best deployed. For stations with high power amplifiers the interference can be far worse. In this case the placement of BPF between the rig and amp or between the amp and antennas can have substantially different outcomes.

Other station elements impact inter-station interference, either by enhancing the effects of BPF or rendering them ineffective. For example, poor port isolation in an N×M antenna switch. BPF are critical but are not by themselves sufficient to eliminate interference.

It must also be noted that BPF are not always necessary. I operated SO2R very successfully at QRP power levels. When the antennas don't strongly couple modern receivers typically won't overload from the 5 watt fundamental on a different band. You just need to select operating frequencies to avoid harmonics from the other transmitter. If the antennas are far apart you can even do SO2R with 100 watts, and I have done so. In both cases you must also avoid transceivers that generate unacceptably high broadband noise; there are a few notorious examples that I won't mention here.

For the rest of this discussion let's assume a station that requires BPF for acceptable performance. As noted, BPF are just one element of station design. Now consider the following case of a tri-band yagi.

In this example the receiving radio is on 10 meters while the other station transmits on 20 meters on a different antenna. Even if the transmitter BPF cuts the 2nd harmonic by 70 db it can't be entirely eliminated. It can still be an overwhelmingly strong signal on 28.024 MHz that may require the 10 meter operator to QSY. If the 20 meter amplifier has poor harmonic suppression and the BPF is between the rig and amp (low power BPF), the interference can be worse.

On the other hand, with appropriate protections it is possible to share a tri-band yagi, and to do so without destruction degradation of receiver performance. There are several manufacturers of triplexers and associated high power BPF that make this possible, but it can be very expensive. Yet it may be cheaper than another tower and yagi suitably placed and filtered. 

The diagram is from VA6AM's web site where he presents numerous options for antenna sharing and what can be expected with respect to performance. I know sharing can work since I've operated at stations with this setup. One cautionary note: don't try this with trapped tri-banders and high power! The traps are unlikely to survive 2 to 5 kW of combined power from up to 3 transmitters.

Returning to the previous example, the 10 meter station's antenna contributes to the problem. The tri-band yagi is resonant on 10 meters and will take that harmonic and deliver it to the amplifier. It will pass through the amp unattenuated (on receive) and be attenuated by the BPF. It would seem that disaster has been averted, but has it? There is more to the story.

The annotated Google satellite view of my station shows the south pointing TH6 in relation to the high band stacks, the 40 meter antennas and the Skyhawk tri-band yagi. Let's say the TH6 is used by the 10 meter station in our example and the other station uses the 20 meter stack pointing to Europe (northeast).

As already shown, the 20 meter station's 10 meter harmonic is loud but is otherwise not a serious problem. It is usually sufficient for the 10 meter op to QSY 5 to 10 kHz on CW -- more separation is needed on SSB due to the wider signal (~6 kHz for a clean signal on the 2nd harmonic). Despite the actions of the two stations' BPF, during last year's CQ WW SSB our team discovered a serious interaction that I had not anticipated or previously experienced.

The 10 meter amplifier tripped and went offline. The problem wasn't the harmonic, it was the fundamental 20 meter signal that did it. The full kilowatt on 20 meters on the stack pointing at the TH6 was unattenuated by the yagi since it is resonant on 20 meters. The 10 meter station's BPF, which in my case are low power BPF between the rig and amplifier, protect the receiver but not the amplifier.

Still surprised? Here is what happened. Quite a lot of power appeared at the 10 meter station amp's antenna port. It is clearly visible on the amp's power meter which, like many modern amps, measures power whether the amp is in transmit or receive (bypass). There were several 10s of watts displayed on the amp's power meter. 

In this instance the amp was an Acom 1200S. The amp's protection circuits saw this as unexpected power output during receive. It was interpretted as an amp fault, such as oscillation while in receive (standby). The protection circuit flags this when there is no corresponding RF power on the input (transceiver) port. It's a sensible precaution despite the aggravation it caused us.

Had there been a high power BPF on the antenna side of the amplifier the problem would not have occurred since it would block the 20 meter fundamental signal picked up by the tri-band yagi. Our options were to switch to a 10 meter mono-band antenna -- which passively rejects 20 meter energy -- or turn the Skyhawk tri-bander south and hope that it was far enough out of the 20 meter stack's main beam not to pose a problem for the amp. Fortunately both options worked, but it was inconvenient for the operator to deal with.

I've previously discussed why I chose low power as opposed to high power BPF for my station, so I won't repeat that here. With that as a given, my task is to minimize interactions using some combination of BPF, stubs and antennas. Stubs can greatly attenuate harmonics produced by amplifiers if that is a problem; low power BPF filter the transmitter's harmonics. 

Solid state amps are more likely to generate harmonics than tube amps. Hams tend to run amps hard and that can markedly reduce linearity. Tube amps are better at harmonic suppression when that happens since the output impedance matching network (usually Π or T) also serves as a low pass filters. The output transformers typically used in a solid state amp don't have that feature.

Since my objective is to have solid state amps on both operating positions in my station -- for contesting agility and guest operator convenience -- it is worthwhile to invest in amps that have enough headroom with respect to our legal limit so that they are less likely to be operated beyond their linear range. The 1200S may have to be replaced.

There is more that I can do with antennas to reduce interaction. As already noted, even a dipole is resonant on odd multiples. That also applies to mono-band yagis that are made with dipole elements. They will pick up substantial harmonic energy from other antennas even though they are not exactly resonant on the harmonics.

One good example in my station is the 3-element 40 meter yagi. Through extensive modelling I discovered that the pattern of the 15 meter stack could be severely distorted when point at the 40 meter yagi, which is on the popular European path. I used small capacitance hats to move the 3rd harmonic well outside the 15 meter band to solve that problem. A side effect is that the 15 meter fundamental signal from the stack is greatly attenuated in the direction of the 40 meter receiver when using the big yagi.

The reversible Moxon on the same tower is naturally non-resonant on 15 meters since the topology of Moxon elements incorporates capacitance hats. It is also true to a lesser degree for the more conventional Moxon rectangle. The XM240 and similar inductively shortened 40 meter yagis are the same. The only difference in this respect is that inductive loading lowers the 3rd harmonic resonant frequency while capacitive loading raises it. 

The XM240 works pretty well on 17 meters and the 3-element 40 meter yagi works well on 12 meters. Since I have no resonant antennas for 17 and 12 meters the designs of those loaded 40 meter yagis had an accidental benefit for operating on those bands.

Even the 30 meter delta loop shown at left has interesting interactions that prevent its resonance on 15 and 10 meters, which are close to the 2nd and 3rd harmonics. In the EZNEC plot above, the currents are for the delta loop excited at 28.5 MHz. There are two effects that are worth attention.

First, the location of currents can be quite complex at harmonic frequencies. In this case there is very strong coupling to the tower; it is less at 21 MHz but still quite large. For the interaction case we are more interested in the feed point impedance than the pattern. The SWR is very high on both of these higher bands. At 21 MHZ the impedance is around 210-170j Ω and 2+20j Ω at 28.5 MHz. 

Part of the effect is due to tower coupling but also the 75 Ω ¼λ transformer (cut for the 30 meter band). The transformer is approximately ½λ at 21 MHz and ¾λ at 28.5 MHz. Although in the last case it is approximately a ¼λ transformer, the loop's feed point resistance can be quite different on it harmonics. This is an excellent example of how a matching network can beneficially filter unwanted energy from other antennas.

This is only bad news if your intent is to use this antenna on other bands. I am not. It's potentially good news for antennas with "interesting" matching networks. Matching networks, other than transformers, are narrow band, typically working on just one band or a portion of a band. 

Antennas that are resonant on harmonic bands typically will strongly reject the fundamental signal since the feed point impedance, due to the antenna being quite short on the fundamental frequency, and the feed point matching network (gamma, beta, etc.) presents a large mismatch to the signal on a different band. For example, a 10 meter yagi with a beta match presents a high SWR to a received 20 meter signal. Checking the impedance on other bands of a few mono-band yagi models in my files confirms this.

While inconvenient, you can test what you have by making and testing a model of the antennas on other bands. Or you can climb the tower and measure the feed point impedance on those bands. A high SWR in this case is desirable because the antenna is unusable on those bands. This is exactly what you want for contesting. The time spent can be especially beneficial before an antenna is bought or built.

To conclude this long article, I'll give my thoughts with respect to my own station:

• No triplexers or high power BPF: It's expensive, but so is another tower and antenna. While not ideal, I will make do with more modest objectives for contesting using low power BPF. The TH6 is being replaced since, due to its placement, there is no good way to effectively decouple it from the 15 meter stack. Its replacement will be 20 meter mono-band yagi, and possibly 15 and 10 meter mono-band yagis on a different tower. Harmonic pickup will thus be strongly attenuated. The Skyhawk is far enough out of the stacks' lines of fire (west of the big towers) that it doesn't suffer the same ills.
• Stubs: I've been planning to experiment with coax stubs to better deal with amplifier harmonics (I use low power BPF) but haven't gotten around to it yet. That may be enough to solve the few remaining interaction issues I experience, especially on CW where you often find yourself on the other station's harmonic. These must be switched and installed at both stations' amplifiers. 
• Transceivers with high blocking and mixing dynamic range: Direct sampling receivers like on my Icom 7610 are susceptible to overload that can be somewhat ameliorated with its internal filtering option. Superhet designs are still typically more resilient than direct sampling receivers. The advantage is gradually diminishing as SDR technology evolves.
• Situational awareness: Understand the potential for interference from the other station and each station's antenna choice, then avoid those situations. You might think this is easier when operating SO2R than in multi-ops since I know the station well and control all choices, but it's really easy to make mistakes in the heat of high rates and frequent band and antenna changes. There's a lot to keep track of. 
• Polarity: Antennas with opposite polarity interact far less than those with the same polarity. It is for this reason that multi-ops will use verticals for their in-band stations. I haven't done this yet in my station but I may yet do so if I get more serious about hosting multi-op teams. Keep in mind that the often quoted 20 to 30 db of polarization attenuation is an exaggeration. Relative positioning, induction in various structures and squint angle can reduce attenuation to 10 db or less. 

Decades ago I did many multi-op contests using kilowatt class amps without BPF or stubs and we somehow survived. The receivers didn't always survive so we kept spares on hand. Repairs weren't difficult with those simpler rigs, often no more than replacing a neon lamp: the so-called lamp fuse. Mostly we just argued about who should QSY when inter-station interference became a problem.

No matter your contesting objectives, hopefully there are a few useful ideas in this article. There is a wealth of data out there if you want to do a deep dive into the topic.

Springtime Lull

The bad spring weather continues. This has been the wettest early spring I can remember for perhaps 10 years. At least it has warmed up, although we still have regular cold weather. That isn't so bad except that it comes with high winds and snow. It is always amusing to see the tulip shoots pushing up through the snow cover. This weekend it was a summery 25° C on Saturday and snowing on Sunday.

About the only productive work I've done outdoors was several hours clearing brush along the Beverage lines in preparation for summer growth, and to take down several large dead maples that threatened the towers or buildings. Felling trees isn't so hard but cutting them up and disposing of the debris is a lot of work. I also spent time putting connectors on and testing Heliax runs that I hope to bury in a trench to one of the big towers either this spring or in the fall.

I did manage one tower climb to resume work on one of my projects. That was great until I fired up the station for the Ontario QSO Party and found out I had trouble with both prop pitch motor rotators. One wouldn't turn at all and the other had an intermittent direction pot.

Remember this uncomfortable truth with large stations: you can be as thorough as you like yet there are so many things that will go wrong. Maintenance is never ending, no matter how well you build it.

I've had time to operate yet admit to doing little. General chatting doesn't often interest me and there has been little happening on-air to attract me. I've been passively monitoring far more than I've operated.

• There are no interesting rare DXpeditions at present. 3Y0K brought excitement for a while and then there were S21ZD and XX9W, plus a few others that drew my attention. The latter were difficult due to being shots over the north pole. They seemed to have more luck with Europe and with FT8 so that's what they did. 
• Despite that difficulty, with daylight shining on the Arctic there have been regular openings most evenings to Asia over the pole, at least on 20 meters, and some on 17. The several days long dive of the solar flux below 100 bodes ill for 15 and 10 meters. The years long slide down to the solar minimum is well underway.
• 
  6 meters briefly showed signs of life around the spring equinox, as expected. I heard South American stations almost daily for nearly 3 weeks. That dried up in early April. In any case there were no new ones to work. There was a CE0Y station active but not on 6. It will be several weeks yet until we see the first glimmerings of the summer sporadic E season.
• 160 meters has been quite poor. Most evenings the Europeans are very weak. Even the FT8 activity is subdued. There has been little DX at our sunrise openings, just the occasional VK and KL7, with just one JA heard on FT8. I expect better with the quiet geomagnetic conditions at present. As previously mentioned I heard but did not work 3Y0K on top band. I could have tried harder but it was inconvenient. Soon I'll be rolling up the radials for the duration of the farming season.
• There are no major contests. The next is WPX CW in late May, which I may enter for practice since it's not one that I particularly like. The only QSO parties that interest me are the Ontario QSO Party, where I want to raises the activity level, and perhaps the Florida QSO Party since it is one with lots of activity. Handing out a mult was fun in OQP but the contest was otherwise not terribly exciting.

While the rain pours and the winds howl, keeping me off the towers, I am dabbling with various indoor projects. 

• 
  I made significant strides on the software for the next version of my antenna selection software. Although it won't be ready for a while yet I am getting close to connecting the new UI (user interface) to the Arduino-based antenna switching system. The design process has been interesting since the revised UI requires a different relationship between clients and server. I may write about it when the project is done.
• To support the antennas slated for construction this year and next I am preparing Heliax runs to the tower that hosts the 20 and 15 meter stacks. That tower also has a 30 meter delta loop and the tower itself is shunt-fed for 160 meters. A new trench will be required, running parallel to the one I dug 6 years ago. I'll toss a Cat5 cable into the trench for control line expansion.
• Most of the material for the new 3-element 20 meter yagi has been assembled. I'm thankful that the aluminum is on hand because, have you see the price of aluminum recently? Antenna prices are going up, commercial or home brew. It ain't easy being a big gun!

This article took me so long to finish that I finally did spend a few hours on the towers. Many more will follow as the spring weather stabilizes. I'm beginning to realize how much work there is to do this year. I'm getting exhausted thinking about it. The springtime lull won't last much longer. Then, if I feel lazy, I'll have different excuses to avoid work on the station, such as ticks and the growing hay.

Stray Capacitance in Interaction Models

Measuring stray reactance -- L and C -- can be difficult because the values are typically quite small. Yet that is a requirement I've dealt with when modelling antenna interactions. Test fixtures can only be built with some difficulty and then measured with suitable instruments, and I've never done that before. The literature addresses some of the cases I run into but not all. Then there's the matter of whether the quoted figures are reliable since the measurements methodology may not be described.

There are two cases in particular that I want to discuss since they are prevalent in my station and I've harboured doubts about my methods and calculations:

• Coupling of guy cable segments at insulators. There is coupling due to field interactions (naturally dealt with in NEC engines) and the series capacitance of the overlapping guy grips.
• Coax common mode leakage across transformers with galvanically isolated windings. I use lots of these in my Beverage systems, and I use more in long runs of RG6 lying on the ground that may be parallel to Beverage antennas for some distance. 

When I first developed interaction models in EZNEC for my guyed towers I modelled a wire for each non-resonant guy segment and overlapped by a length and separation approximately that of what was built. That was cumbersome and not really very accurate, but probably good enough. For the small Beverage transformers I represented the coax with connected wires and placed series capacitive loads on the wire to model the series capacitance between the transformer windings. I used values gleaned from ON4UN's Low-Band DXing and other sources.

The latter method of modelling stray capacitance is easier than the former, so I've been using it almost exclusively for the past several years. However, I don't know how accurate either method is or can be.

It struck me as an ideal small project to tackle during the extraordinary and persistently bad weather we're going through this April: rain, snow, wind, cold. There are always jobs to do in the shack and the workshop when tower work is too uncomfortable. I set out to measure the series capacitance in the cases listed above with the hope of designing better interaction models in the future.

I have a 1:1 Beverage transformer on a BN73-202 binocular ferrite core with 3 turns of insulated #26 wire for both windings. It was left over from a project and is identical to what I use for isolating coax segments in my long RG6 runs to the Beverage antennas. For the other case, I quickly constructed a "dummy" guy segment termination from a 504 insulator and two 5/16" guy grips. This is what I use on my big towers supporting stacked yagis and large 40 meter antennas.

There are 3 instruments that I have available for the measurements:

• RigExpert AA54 antenna impedance analyzer (single port)
• VNWA3 2-port VNA
• 35 year old LCR meter (made in Taiwan, and the brand is defunct)

I first did the measurements with the AA54. The results, at first, seemed sensible, but did not pass scrutiny. This shouldn't have been a surprise since these analyzers, even one of this quality, have increasingly poor accuracy as you move farther from its 50 Ω reference point. A pure capacitance in series with an infinite resistance (open circuit) has a very high SWR indeed. It is a poor LCR meter in these circumstances.

You may have to click on the image to improve the resolution. On the right are two measurements of the transformer, one open and one with the transformer in circuit. The ends of each winding are joined for the measurement of inter-winding capacitance. I compared the two calculated capacitance values with the expectation that their difference would be close to the actual value. In this instance, the difference fell between 2 and 3 pf: measurement precision is no better than 1 pf.

Notice that the R value is 0 Ω for all of the measurements. That casts suspicion on the results. The open circuit is so out of bounds from what this single port device can accurately measure that some deviation was to be expected. The measurement of the guy cable series capacitance is about the same, further casting doubt on the suitability of the instrument for this application.

I tried the measurement from 1 to 50 MHz in the hope that there would be an island of stability, if not accuracy, at least at low frequencies. At low frequencies the calculated capacitance was far too high, only stabilizing above about 5 MHz. So I chose 50 MHz and got the same result at a few other randomly selected frequencies. The poor accuracy really isn't the fault of the antenna analyzer. I asked it to do a job it was not designed for. 

I moved on to my old and trusty LCR meter. Like most of these instruments it does its measurements at a low frequency, although that is not documented. But I have had a lot of success with it over the many years I've owned it, measuring fixed and variable capacitors from a few pf up to a large fraction of a μF. A major downside of the meter is that it eats through 9 volt batteries very quickly.

The LCR measurements were quickly done with the aid of the short alligator clip leads. The stray capacitance of the instrument plus leads is slightly below 6 pf, so we subtract that from the measurements.

The results are far more in line with my expectations: 5.6 pf for the transformer and 11.3 pf for the guy termination. The Xc at low HF frequencies is close to what others have measured for a transformer of this design. That gives me confidence that these values improve my interaction models. Greater measurement accuracy may be desirable for capacitors used in antennas and matching networks.

I did not do measurements with the VNWA 3 at this time. With the many recent PC upgrades I made this winter it would take some time to set up the software to use it. The single port (S11) measurement would likely be better than with the AA54 but probably not by a lot. More accuracy requires a 2-port (S21) measurement of the series reactance, using a fixture that includes the outer coax conductor. Maybe when I have nothing better to do at a future date I'll make the measurements if only to satisfy my curiosity.

In practice there are many parameters that determine guy wire interactions, with the series capacitance just one of them. As you go higher in frequency the Xc falls enough that there is significant conductance through those guy terminations. For example, Xc of an 11 pf capacitor 30 MHz is just 480 Ω. It's 10 higher on 80 meters. Those non-resonant guy sections are not as isolated as they appear! Don't be surprised if they model differently than you expect when you include the stray capacitance between supposedly non-resonant guy segment.

There are also induced currents from the antenna itself and among the guy segments despite each segment being non-resonant in isolation. Perfection isn't possible: we can only do the best we can. I intend to use the LCR meter measurements in my EZNEC interaction models until I have better.

Is it worth the effort? I believe so. I've put enough years and sweat into my "big gun" station that it is only sensible to get the most from it. That doesn't mean I'll make major changes when an interaction is worse than I'd like, but I'll know what to expect. That's valuable information even if it isn't good news.

Ignorance is not bliss.