Raising the 40 Meter Reversible Moxon

One month ago I discussed the construction of the 40 meter reversible Moxon. With the advent of NEC5 it became possible to develop accurate models of complicated antennas like this one. These include tapered elements, mid-span capacitance hats, and close spacing of elements. NEC2 and NEC4 cannot properly model this antenna without a great deal of effort with segmentation or to calibrate the incorrect calculations.

Well, that's the theory. Others have had success using NEC5 to develop models that closely match measurements of built antennas, and my initial experiments were promising. It is no trivial matter to build and test an antenna of this size and then use real-world measurements to revise and try again. Trial-and-error may be an acceptable strategy for small yagis and wire antennas but not for rotatable 40 meter yagis.

For those that prefer one article that encompasses the theory, construction, testing and performance of this unusual antenna, all I can say is "sorry". I am writing this up as I go and each stage from conception to on-air use is an interesting subject for a blog article. This blog has always been about the journey, not the destination. 

So, now that we've covered the conceptualization of the reversible Moxon, the model and most recently its construction. we proceed to the next step: raising it onto the tower. The unique attributes of the antenna complicate the process, especially for side mounting on a guyed tower. For those with a self-supporting tower and no obstructions within the footprint of the antenna (43' × 37'; 13 × 11 m), raising it is a simple matter: assemble it around the tower and pull it straight up. It is not so straight forward with a guyed tower.

Moving fast

As I stated in the construction article, time was of the essence. The yagi was sitting in a hay field and the hay had entered its rapid growth phase. One way or another I had to get it out of there. I intended to raise it earlier in the month but, as usual, higher priority jobs got in the way. Finally I the antenna was completed and ready to go on the tower. Then it became unseasonably cold, windy and rainy. 

I pointed a finger at the calendar on a day with no rain forecast and decide that would be -- must be -- the day. I contacted friends and soon had a crew ready to come over for the big event. They were Alan VE3KAE, Dave VE3KG and Vlad VE3JM. If you're a contester the last two calls may be familiar.

I rigged the tram line and other mechanisms on the tower and antennas one day in advance. I say "antennas" because the XM240 had to come down first.

Lowering the XM240

Luckily these antennas have short booms: 22' for the Cushcraft and 20' for the Moxon. There was a gap of just the right width between a tree and guy anchor (for the 140' tower off to the right in the picture below). There also was a tree perfectly centred in the gap to anchor the tram line winch. We chose manual power since we had enough people and gravity helped for taking down the XM240. It can be difficult to speedily communicate with a driver of a vehicle that can easily damage a fragile antenna that at some points must be moved inches, or less.

Lowering a yagi is not without drama. You are never quite sure whether you have the CoG (Centre of gravity) where you think it is, and with a 75 lb antenna you may not be able to easily make adjust the harness after it is unclamped from the mast. The XM240 is pretty well centred on the boom-to-mast clamp so I wasn't too concerned. However, the lever arm and rope I attached to the boom caused a slight tilt that we needed to closely monitor. The capacitance hats are fragile and it is far too easy for the long elements to snag a guy. Good communication between tower and ground is needed to accomplish the necessary choreography.

The lever arm is used to tilt the elements up as needed to clear the guys. Since I was on the tower near the antenna I had the best perspective to guide the crew handling the haul rope, the lever arm and the winch (tram line tension). With a series of rapid commands back and forth we were able to guide the yagi past the guys without any drama. It helped that one of my friend's set me up with a VOX-actuated HT clipped to my harness.

Rigging the Moxon

Unlike a typical yagi, the Moxon elements are joined at the tips. That imposed constraints on the rigging and operation of the tram line. There was also the challenge of guiding the antenna between the guys directly below the tower side mount and the TH6 above. The tri-bander was not so much higher that the upward tilt of the long 40 meter elements would not reach it. The many clamps and traps on the TH6 can and did snag the Moxon elements.

The major consideration for rigging the Moxon was whether to run the tram line over or under the rear capacitance hats. I decided to run it above the tips since that would help to keep the antenna oriented upward, which was needed to clear the guys. That is, the tips or the connecting fibreglass rod could rises no higher than the tram line. I used a ½" rope rather than steel cable for the tram line despite its greater elasticity since it was less likely to abrade the antenna. Some contact was inevitable.

The choice seemed reasonable and it worked as intended with respect to that one issue. Unfortunately it turned out to be a poor choice because there were greater concerns that I failed to account for in my rush to get the antenna out of the hay field. It was only after it was attached to the mast that I realized how seriously wrong my choice had been.

First, I'll note that it took 3 attempts to successfully pull the Moxon up the tram line. On the first try it failed to clear the guys. We added the lever and rope so that ground crew could steer the antenna. The second attempt failed because the weight of the rope on the lever tilted the antenna too far upward. Finally we got it more or less right. Well, right enough to guide the Moxon where we wanted it to go.

I took the picture at right when the forward capacitance hats reached the tower. You can see the tram line, haul rope and the rope dangling from the boom lever. After taking the picture I opened the tips and guided them around the tower. Pulling resumed after I reconnected the tips.

It was at this point that trouble arose. The angle of the antenna caused a collision of the forward hats and element tips with the TH6. There was no provision for tipping the boom more level to clear the elements of the higher antenna. The lever was still out of arm's reach. It is normal for the trammed yagi to increase its vertical tilt near the top of the tram line due to catenary physics: the proximity of the pulley carrying the weight increases the downward angle of the tram line at the tower anchor. I could have reduced this using steel instead of rope, however that entailed the risk that I described above. 

A further increase of tram line tension helped enough that I could jiggle the ropes to encourage the Moxon past each clamp and trap on the longest TH6 elements. Clearance was easily completed when my arm could finally reach the boom and lever.

We slacked the tram line and the Moxon's boom rested against the mast. The haul rope was tied off when the boom was centred on the mast clamp. The u-bolts and saddles were then installed. That's when the more serious problem was discovered. 

It was when I couldn't level the antenna with the lever arm no matter how hard I tried. I looked behind me and discovered the mistake. Have a good look at the picture above. Perhaps you will see the problem without me telling you, especially since I gave a strong hint earlier in the article. Now that you, hopefully, see it, I'll continue.

When the boom is rotated the rear tips strike the tram line. My attempt to rotate the boom pushed the inside ends of the capacitance hats downward. This occurred due to another mistake: I forgot to torque the nuts of the u-bolts on the element clamps. They were intentionally left snug but not fully tightened so that I could align the tips once it was cleared the ground. The antenna had been abandonned in the hay field for so long that I forgot. 

Although I couldn't fix that problem, it didn't overly worry me. The mechanical risk of failure is low and the impact on antenna performance would be negligible. In the former case the fibreglass and aluminum are flexible enough to survive. In the latter case, performance of an antenna is largely determined by where current is highest. The attitude of the tilted capacitance hats has the same loading effect and will still cancel the fields with each side of each hat and pretty well with the hats at the other end of the elements. The distance between hat tips is critical and that remained unchanged.

My big worry was removal of the tram line. I could either let it go from the top or pull the full 200' length up and over the capacitance hats. I judged that the risk of damage was far greater by releasing the top of the tram line since the rope's weight would whiplash the rope tip as it wrapped around the fibreglass at high velocity. I opted to pull the rope up and over instead. That worked well enough in that no more damage resulted.

A broken fibreglass rod is cheap to replace but expensive in time and effort. Since I expect to pull the antenna down in the fall to inspect and change the antenna according to how well it performs during summer storms, I may leave the curved hats as they are until then. However it's an interesting problem that had me considering a repair. I think I know how to do it. Perhaps it'll be the subject of a future article!

Finishing off

I decided to use the custom boom-to-mast clamp that came with the XM240 rather than I one I selected. It was more robust, already installed and it was aligned with the rotator direction indicator. I can make another for either the Moxon or XM240 when it comes down in the fall.

I kept my friends longer than planned so we quit for the day. Operating the winch and hauling up a 105 lb yagi is tiring work. All of us needed a break.

The next day I climbed the tower to test the antenna and connect the rotation loop. The coax to each element feed point are equal length should I ever be motivated to stack it with the 3-element yagi on top of the tower. Their lengths have no effect on Moxon performance. I made them longer than required so I coiled and taped the excess to the boom for the lift. 

Notice the very slight droop of the boom. That 20' long schedule 80 2-⅜" OD 6061-T6 pipe is strong. It was worth the weight penalty. I also happen to find it amusing that the elements are trussed but not the boom. 

All of the coax connectors were weatherproofed but more could be done for the feed point enclosures, relay keying wires and securing the cables to the boom.. Only one wire is needed to operate the reversing relays since the return path is via the boom and coax and tower. In my station I bond the grounds for DC, RF and ground rods.

Here you can see how I mounted the small box with the coax switch. I was careful to label all 3 boxes and the boom to indicate the driven element and reflector for normal orientation (relays using their NC contacts) to ensure the correct connections are made.

Before weatherproofing I measured the SWR by connecting the normal driven element to the analyzer. Compare it to the NEC5 model. That's pretty good agreement! Even though there are interactions with the guys and the TH6 above it, the wavelength is long and the TH6 coupling small since they are pointed the same direction. 

Perhaps more important than the low SWR was the frequency where it was lowest. That agrees with the NEC5 model within 10 or 15 kHz. That raises my confidence that the design frequency range was achieved in practice. That success is a testament to the accuracy of NEC5 modelling a antenna featuring stepped diameter tubes, capacitance hats and close coupling between their tips. Neither NEC2 nor NEC4 are up to the challenge. 

Performance verification will have to wait. During installation a wire must have been knocked loose so that the rotator turns in one direction but not the other. That left it locked west. I operated in the CQ WPX CW contest with it in that condition. It worked well but I don't yet know how well in comparison to the 3-element yagi. There was no difficulty transmitting with a kilowatt. The SWR barely changed with the Moxon pointed west and the TH6 pointed south. That was encouraging.

I had no time available to trace wires to patch a path back to the shack for the reversing feature. That will also require a software change to my custom antenna selection software. Due to enforced rest after routine surgery this week I likely won't get to it until at least mid-June.

Aftermath

The urgency to clear the hay field prevented completion of this antenna project. At least the antenna is on the tower and working, which is a great relief. Until it is complete and its performance properly assessed I am delaying the discussion of how I built and tested the switch boxes. An article covering performance and electrical construction will eventually appear, though I can't say when.

On a more practical note, I lost my 17 meter antenna. The XM240 is near resonance on that band so I've been using it that way since I don't have any WARC band antennas. The 18.1 MHz SWR on the Moxon is near infinite. After testing various antennas with the antenna analyzer I chose the 80 meter inverted vee as my 17 meter antenna. Its SWR is 1.5 across the 17 meter band. It works well enough that I've already logged one DXpedition. The pile up was small so its performance is difficult to assess at this time.

This is an opportunity to reflect on the lessons learned, from design through to raising of the reversible Moxon:

• NEC5 rocks! Although it can be slow due to the greater number of segments it typically requires, its ability to accurately model complicated antennas like this one is remarkable. It integrates nicely with EZNEC, which also supplements NEC5 with additional features such as insulated wires which NEC5 alone does not support. NEC5 is well worth the licensing fee.
• Double check all fasteners. While this is obvious it is easy to forget when one is in a hurry. 
• Ground crew get rightfully annoyed when you ask the seemingly impossible or at least improbable. It is easy to forget the strain they're under while holding a massive antenna in position while I fiddle with the mounting hardware. You should have heard the language when I asked them to raise the antenna exactly ½" so that I could drive in the last u-bolt.
• Label and record every control wire in the station, no matter how unimportant. I have countless runs of Cat5 and other cable for the dozens of control lines, including antenna switches, stack switches, antenna mode switches, rotators and more. I'm pretty good about labelling the cables and recording the details but there are gaps. Hence the need to do wire tracing. I'm now taking the opportunity to revise and expand my records.
• Running the tram line under the rear capacitance hats would have been the better option. I failed to think through the entire process from lifting the yagi, riding the tram line, fitting the antenna to its mount and then, critically, removing the rigging. Counting on luck is no excuse for taking short cuts. 

Despite the frustrations and mistakes the antenna works. I'll know more about how well it works after I repair the rotator and complete the reversing feature.

Photo Credits: Other than those I took on the tower, pictures in this article are by VE3KAE and VE3KG.

6 Meters Sputters to Life

I am disappointed to observe that 6 meters has a season again. While the solar flux marched higher in 2024 there could be openings on 6 at any time. Yes, there was still some seasonality affecting F-layer MUF but there could be excellent DX openings almost any day. And there were many! I lost several opportunities for long haul DX that may be gone for good in this solar cycle because the Sun in 2025 is no longer cooperative.

Which brings us to summer sporadic E. This year's may be better than it has been for the past 2 years since there appears to be a weak anti-correlation between solar activity and sporadic E. That is, you get great F-layer openings or great sporadic E openings but probably not both at the same time. Nevertheless, unusual long distance paths can be formed by linking sporadic E with tropical TEP. 

The summer E season has certainly begun. We had our first 6 meter openings to Europe, the west coast and the Caribbean over the past week. These will become longer and more frequent until the usual peak at the solstice in late June. Shorter, single hop E openings have become common here, in Europe and in Asia.

Regular readers will know that I have little interest in "local" 6 meter contacts. I will occasionally work single-hop contacts, however my focus is almost exclusively on DX. When I call "CQ DX" on 6 meters don't be surprised that I don't answer non-DX -- that's my expressed intent, not rudeness. I am not willing to spend my time trying to please everyone and not myself. 

That said, I may answer. When I'm CQing into a potential but not an active opening, it's to discover if I can raise flags on PSK Reporter in distant locales. Since any transmission will do for that, it doesn't matter whether I CQ or have a QSO. 

So far...

In May we had our first openings to Europe, Africa and the Pacific, along with the more reliable paths to the south. Perhaps it's sporadic E linking to TEP, but I really don't know.

The majority were stations I've worked before, including D2UY and 3D2AG. I tell my friends and keep listening for new ones. The frequency and strength of sporadic E openings will increase over the coming weeks. While that is certain, it may or may not deliver interesting DX.

Despite what I've heard, the spring season has been disappointing so far. At least from here; others have had more success. Unlike spring 2024 the MUF for F-layer propagation has been too low. I listened but mostly it was just the usual propagation to southern South American stations in CE, LU and CX, with the occasional PY and HC. I love working them, since they are long haul DX, but other than for grid hunters (which I am not) they are nothing new. 

Several of us were excited to hear the TX9A DXpedition on 50.313 MHz in early May. We had few opportunities when they were successfully working North American stations well to the west and south of us. I heard them on two consecutive days, May 5 and 6. The screenshot was taken on May 6 during the peak of the opening at my station.

As you can see it was very marginal, with several strong peaks and deep long fades. I tried and failed. Had they responded to me I wouldn't have known. The chase was enjoyable despite proving fruitless. 

Another that got away was PJ7EE. Several of my friends worked him for a new one. I missed out because I was out of the shack. By the time I returned the opening was fading and I couldn't get through the pile up in the few minutes I had before he was gone for good. This one is not so rare that it won't get into my log eventually.

The African openings have all been marginal. One day D2UY was very strong but otherwise signals have been exceedingly weak. There are of course many countries in Africa on my wanted list. At the same time D2UY was heard, others on the east coast were hearing or working TR8CA, 9J2FI and others. I heard a few calling ZS8W but I heard no one working them. Closer Africans such as EA8 and D4 have been heard recently so the easterly path to Africa has returned and should improve. 

I need good propagation and DXpeditions to work most of Africa. Several 7Q were worked last year thanks to the effort of 7Q6M to train and license several young Malawians. It's good to hear them with some regularity. It shows what is possible were there more activity in that vast continent.

Automatic operation

It should be no surprise that the presence of robot operators is increasing on the digital modes. 6 meters is not immune. I have heard rumblings that more effort will be made by awards issuing bodies that disqualify robot QSOs. It'll be interesting to see if this happens and how it can be policed. Suspected offenders include several notable DXpeditions. 

I use the filter feature in WSJT-X-i (improved) to remove suspected robot operators from my monitor screen. Although I can ignore them, the filter clears the screen of pages and pages of CQing robots. The filters help me to easily find the DX station messages without having to scroll through pages of robot muck. 

I don't care whether robot operations are legal, sanctioned or justified by arcane theories and opinions. They interfere with my operating pleasure so they are filtered or otherwise ignored. The filter list is frequently updated as robots come and go.

As more stations give 6 meters a try, it is expected that the same operating practices follow. Higher activity has its pros and cons. I make accommodations to deal with what I consider poor behaviour. Taking advantage of the available tools is more productive and satisfying than becoming angry or frustrated.

Prospects for F-layer propagation

Unless we get a double peak this solar cycle the prospects are not good. It's possible though I am in no position to make a prediction. If it does happen it won't be any later than this fall; solar cycle 25 will almost certainly be in decline by 2026. Fall is a great time of year for a high solar flux since seasonality enhances the MUF to increase the probability of 6 meter DX. Last fall's openings may be it for solar cycle 25. Southern Asia, for instance, is almost unworkable for us without strong F-layer propagation.

Even if F-layer can't do it alone at this high geomagnetic latitude we can look forward to occasional path linkage. Local sporadic E propagation can link to TEP and other F-layer modes and create elusive and brief worldwide DX openings. These can be hoped for but not relied upon. The linkage is more likely during the June-July sporadic E peak to the southern hemisphere due to the higher F-layer MUF at tropical latitudes. After 2025 we may lose that and must rely on links via TEP.

I really can't say any more. We can only monitor and hope for the best. Solar modelling and related predictions are not reliable enough to reliably inform us what will come. We have to listen. Beware of those who misinform with predictions that tell you what you want to hear. We should strive to do the best we can with the hand we're dealt.

The buddy system

We have a small group of local 6 meter DXers that shares news of openings via an email reflector. I believe that with the latest addition there are now 6 of us (an appropriate number). We benefit from helping each other, ensuring that none of us miss much; it's like growing several extra sets of ears. We've all worked new countries by receiving alerts from our buddies.

I've been both hot and cold on the value of the venerable ON4KST web-based 6 meter chat. Some years I don't sign in at all and other years I've been a regular participant. I won't get into the reasons here. If you don't have other 6 meter enthusiasts nearby with whom to form a group you should definitely look into using ON4KST chat. At its best you can really "hear" the pulse of the band and coordinate QSO attempts with others, sometimes in countries you'd love to work. 

Since sporadic E spotlight propagation is common on 6 meters, the members of your group should be nearby. Our group's members are in FN14, FN24 and FN25. Despite our proximity, quite often not all of us hear the same DX stations.

DXCC prospects

The poor likelihood of more F-layer propagation this solar cycle limits my ability to add to my country total. I now stand at 146 worked and 134 confirmed. I've worked just one new country so far in 2025. The reach of sporadic E is inadequate to make up for the loss of F-layer DX paths. I spoke about diminishing returns several years ago and it's only become worse. The new countries come more slowly now.

I often have to rely on DXpeditions to the countries that I need, and that are within the scope of possible propagation paths.When they happen I need a little bit of luck as well. My antenna system could always be improved, but it is already good enough to hear and work weak signals that others in this area cannot. A better 6 meter antenna system is not (yet) high enough on my priority list to incentivize me to make an effort beyond idle speculation about alternatives.

I'll be monitoring as often as possible over the coming months to take advantage of whatever propagation comes my way. If I can put 5 new ones in the log this year and thus surpass 150 countries worked I'll consider it a good year.

I wish the best of DXing luck for all of you who are as seriously infected with the 6 meter bug as I am. My infection has lasted 50 years and shows no sign of abating. 

2 + 3 = ?? : Stacking Dissimilar Yagis

You don't often see stacks made from dissimilar yagis. Using identical yagis simplifies their design and optimizes overall performance. That's how I built my stacks for 10, 15 and 20 meters, right down to the matching networks. The only difference is the booms, which have a negligible impact on antenna performance, instead being chosen for their mechanical properties.

On 40 meters it is rare to see stacks of other than 2-element yagis since 3+ element yagis are so large and expensive. Consider just the tower to support two of those behemoths. I have a 3-element 40 meter yagi and while I love its performance I will not build another. It is more usual to make 40 meter stacks from 2-element yagis. Indeed, my understanding is that W6NL designed the Moxon (XM240 conversion most often) as the foundation for an effective and efficient 40 meter yagi without incurring a large mechanical challenge.

I never considered stacking the XM240 and 3-element yagi even though they are on the same tower and are at heights (λ/2 and λ) and separation (λ/2) that make it feasible. They almost always point in different directions so the only application would have been to "spray" in two directions, such as Europe and the US. The XM240 can't be pointed to Europe since the side mount only allows rotation between southeast and west.

As I've mentioned on the blog, I am in the process of building a 2-element reversible Moxon for 40 meters. It is intended to replace the XM240 on the same rotatable side mount.  Its performance will be superior to the coil-loaded XM240 and since it is reversible it can point over 260° of the compass, including Europe and Asia.

To do so is not without its challenges, and its potential performance is less than with identical yagis. Let's review those challenges, great and small. These apply to stacking of any dissimilar yagis.

• Gain: Long yagis have greater gain than short yagis. The 2-element Moxon has less gain than its big brother and the deficit increases as you move higher in frequency. When you add a little to a lot the sum is less than one might expect. For example, if the antenna gains are 8 and 6 dbi and the main lobes are ideally combined, the net gain is 10 dbi. That's below the nominal 3 db stacking gain of 11 dbi when both have 8 dbi gain. Keep in mind that we're working with a logarithmic scale so you can't simply add and subtract decibels!
• Phase: Different yagis has different feed systems and the driven elements are not equidistant from the tower. Both contribute to phase differences. The relative phase must be determined and compensation built into the stack system. It may not be achievable across the band with a fixed compensation network (e.g. delay line).
• Impedance: Here we are concerned with power division and phase; that is, what SWR affects but not SWR itself. Dissimilar yagis are certain to have different impedances (R and X) across the band of interest. Networks to precisely compensate for those differences can be quite difficult to achieve in practice, and more difficult to have their phases track together.

Both of my yagis, the 3-element and the reversible Moxon, require NEC5 for accurate modelling so I'll be using it throughout. There are so many segments when the models are combined that run times can be long. 

To begin, let's review the gain and SWR of both antennas. I will do this using  EZNEC medium ground, the lower Moxon at 75' (22.8 m) and the upper 3-element yagi at 150' (45.5 m). Those are the approximate heights where they are (or will be) placed on my tower. Interactions with other antennas and guys are not included in the model, but are expected to have only minor effects in my station configuration.

Gain and impedance were first modelled in free space to confirm that ground has only a small effect on the individual yagis. As expected, ground has a negligible effect on the higher and longer 3-element yagi and only a small impact on the shorter Moxon.

Since the yagis are individually fed for this exercise, as they would be with a stack switch in a real deployment, gain, F/B and SWR are not equal to what they are modelled in isolation from other antennas. That is, there is mutual coupling. The λ/2 separated is enough to ensure the effects are modest, but should not be ignored. A single EZNEC model containing both antennas ensures that their interaction is included. 

λ/2 is usually considered to be the minimum separation recommended for best stack performance, however reality is more complex than can be represented by a simple heuristic. Other heights and separations can be enlightening for those who have yet to put up the towers and antennas. My station, already built, has constraints that I am staying within for this analysis.

This chart should not be a surprise. A Moxon isn't a magical antenna. Its gain is slightly lower than that of a conventional 2-element yagi but with better SWR and F/B -- please note that F/B is excluded from this analysis since our focus is stacking performance. 3-element yagis typically have gain that increases with frequency, until the radiation resistance dives and the SWR soars. The 3-element does poorly above 7.2 MHz, which matters in the Americas but not elsewhere. I rarely use the 3-element yagi above 7.2 MHz since it is primarily a DX antenna at its great height. That is by design.

The disparities between the yagis increase with frequency. That carries over when the antennas are stacked, as we'll discover in the present analysis. For now, note that the gains and are not so far apart and the matches excellent in the CW segment. The divergence becomes wide in the SSB segment, especially above the Americas segment between 7.2 and 7.3 MHz.

It is also notable that the elevation angle of the forward lobes are far apart. This is of course normal for a vertical stack though perhaps not to this degree. The difference is usually a benefit of stacking since it allows the filling of nulls in the elevation patterns of the individual yagis. Yet stacking gain -- often what we want most of all -- is impaired by a wide divergence of forward lobe elevation angles. Heights of λ/2 and λ with a λ/2 separation limits stacking benefits. 

Having noted all of the forgoing, the stacking prognosis is not great. Nevertheless let's proceed and see what we can do with it and where the deficiencies lie in the calculated performance. 

Phase matching

In this array there are several factors for achieving phase matching:

• Feed system
• Feed point impedance 
• Power splitting
• Coax phasing lines
• Elevation angle (antenna height)

Let's briefly look at each of these. The diagram will be an aid to the discussion -- I didn't bother with detailed annotations since they should be evident. While this article focusses on my unusual stacking scenario, I've done this type of analysis before for my existing stacks. The planning and design process is strongly recommended before ordering towers and materials and beginning construction. A major investment should not be made without solid evidence of desired performance.

The superposition of direct and reflected waves from the yagis determines the lobes and nulls in the stack pattern. Yagi coupling (mutual impedance) with each other and the ground have their effects as well, in accord with their separation and height (relative to wavelength). That is elementary. There may be surprises from what we expect since the 40 meter wavelength is quite long, which brings the yagis closer to ground and to each other.

The feed system for the 3-element yagi was originally modelled as a beta match since that is simple in EZNEC and delivers reliable results. It was changed to a gamma match since that can affect the feed phase angle. The EZNEC (NEC5) model I developed closely matches the dimensions and behaviour of the real gamma match on the yagi. 

The Moxon driven element is of course directly connected to the driven element. The reversing electronics are not included since that does not affect the model. That said, while the reversible Moxon is insensitive to the lengths of the coax from the central switch to each element, they should be the same length when the antenna is part of a stack.

When this measures are taken and the yagis are fed via a power splitter that provides a common current source, the feed points will be in phase, or close enough to ensure stack performance. It must be so since one side of the coax directly connects to the centre of the driven element of both. This was confirmed by inspection of the Currents table in EZNEC. Other feed point matching networks may not be so straight forward.

This desired outcome requires that the impedances are equal and that the phasing lines preserve phase (equal electrical length). That is more difficult than it might appear. Consider how power is split. We want the power splitter to split the power equally since only then is optimum stacking gain achieved in the cases where the yagi are identical. The case I'm evaluating is more challenging. 

In the model I used an ideal 2:1 transformer as a power splitter and impedance matching network. The 50 Ω source is on one side of the transformer and parallel 50 Ω feeds to the phasing lines are on the other side, which sum to 25 Ω only when the SWR is 1 to both yagis. An L-network can be used in place of the transformer, as I've successfully employed for my 20, 15 and 10 meter stacks. 

While a transformer can be broadband, an L-network is rarely suitable for more than one band. For those who choose to stack multi-band yagis a transformer is the right choice. For single band use, such as in the present case, an L-network is compact, easy to design and build, and can be more efficient.

In both cases, equal power splitting and impedance transformation are only achieved when the SWR at the matching device is close to 1. That is, an excellent 50 Ω match. For the dissimilar stack that criterion is only satisfied below 7.150 MHz. For identical yagis with identical SWR curves the power will split equally but efficiency will fall; that is, more power is converted to heat in the transformer or L-network.

The phasing lines from the splitter do more than just preserve phase for the dissimilar array. Even for that it is necessary to adjust for the different horizontal locations of the driven elements. For example, in the dissimilar yagi stack that additional distance is 8' (2.5 m) -- the DE is near the centre of the 3-element yagi and at the end of Moxon boom. 

Since the VF of coax is less than 1, the phasing line extension must be shorter than that. For RG213 it would be 5.3' (0.66 × 8). The extra length is a delay line that goes on the yagi (the Moxon in this case) with the driven element further away from the forward direction.

CMC (common mode chokes) should be identical or their differences accounted for in the model. For example, the lengths of coax wound on ferrite toroids become part of the phasing lines.

That simple delay line calculation assumes an elevation angle close to 0°, in the plane of the booms. That is close enough for my high band stacks since they are high enough (again, relative to wavelength) that the elevation angle of both yagis are quite low. This is not true for the 40 meter stack since they are lower, again relative to wavelength. This applies to both similar and dissimilar stacks.

Look again at the diagram above. The wavefront from the lower yagi has to travel farther for correct phasing. Thus the delay in its phasing line must be longer. By coincidence that works out to 8' or 9' of RG213, as determined experimentally with EZNEC. I put that value into the transmission line table.

Impedance mismatch is another confounding factor that affects more than equal power division. There is an infinite set of R and X pairs for any SWR other than 1. They form a circle on a Smith chart. The feed point impedance will not be the same as the impedance presented at the power splitter (transformer or L-network). That has to be calculated. 

The parallel complex sum of the impedances for significant disparities will cause unequal power splitting. The impedance transformation due to the phasing lines and deviation in the behaviour of the matching network (power splitter) are acutely sensitive to the magnitude of the SWR. The power split and feed point phase difference can be inspected with EZNEC. 

Open the Currents table and carefully compare the current magnitude and phase in the wires at the feed points. I say 'carefully' since the current directions (signs of the phase) may be opposite to each other. This is explained in the Currents section of the EZNEC manual. The phases in the two feed points above are close but not equal since some difference at the feed points is necessary to achieve phase alignment at the common wavefront at the stack main lobe's elevation angle. That was determined experimentally in the model, which is often the easiest way of doing it!

Any residual imbalance can be corrected with delay lines and compensation networks. If the values are fixed, as is typical, stack performance will be frequency sensitive, assuming (as is almost certainly the case with dissimilar yagis) the SWR/impedance difference of the yagis varies with frequency. This is evident in the earlier plot of each yagi's gain and SWR for the 40 meter dissimilar yagi stack I am evaluating.

The significant of the power and phase imbalance can be calculated in the model. Therefore let's do that to evaluate the performance. Then we can discuss the difficulty and value of mitigation measures. 

Performance

There is perhaps no better way to demonstrate the expected performance of the dissimilar stack than to compare the modelled patterns for the individual antennas (Moxon and 3-element yagi) and when combined.

The phasing lines in these patterns was optimized for 7.05 MHz, mainly for CW use. The result is modest stacking gain from 7.0 to 7.1 MHz. This is the range where the SWR is low for both the Moxon and the high 3-element yagi. The line to the Moxon is 9' (2.7 m) longer. It is certainly debatable whether a stacking gain of little over 1 db is beneficial. Well, it can be in competitive environments like contests and DXpedition pile ups, but perhaps not for other situations.

By the time the frequency rises to 7.150 MHz the performance of the stack is not good, by any standard. For SSB use there is little stacking benefit, or none at all. Recall from the first chart that at higher frequencies the gain of the two yagis diverges quite a lot and the SWR of the 3-element becomes extreme above 7.2 MHz.

That is the case for a fixed phasing system. In the adjacent 7.2 MHz plot one change has been made: a longer phasing line to the Moxon to optimize performance at that frequency. It has been increased from 9' to 21' (6.4 m). 

The pattern is certainly improved though perhaps not enough be of value. The gain is now equal for the stack and for the 3-element yagi alone. The only useful change is null filling, yet that can be accomplished by the lower Moxon on its own.

What was accomplished by restoring phase alignment disrupted by the high SWR of the 3-element yagi was to prevent the Moxon subtracting from the main lobe gain. That could not improve stack gain beyond that of the 3-element yagi alone because the Moxon gain that high in the band is 3 to 4 db below that of the 3-element yagi.

It is likely that the situation can be further improved with a network to lower the SWR of the 3-element higher in the band, say centred on 7.2 MHz. I've modelled networks of that nature and the low SWR bandwidth is small, often no better than 50 kHz for an SWR below 2. That makes the 3-element yagi more useful for SSB contests but, again, stacking gain really isn't there.

A switched delay line is needed to compensate for the phase shift introduced by the matching network. I don't see the worth of bothering since, as demonstrated, there is little to no benefit of stacking. The yagis are too dissimilar in the top half of the 40 meter band.

Conclusions

After all of this discussion and calculation, what will I do? The short answer is that I don't know. It isn't difficult to stack the yagis so I might, if only out of curiosity. All I need to do is measure the phasing lines and build a stack switch, and make a few changes to my antenna switching software. If it's only useful for CW, that's acceptable since that's my primary interest.

I was already considering an auxiliary switch on the tower for these yagis. That would free up one run of Heliax and open a port on the 40 meter auxiliary switch for the XM240. I am pondering whether to place the XM240 on the other tower fixed south as a multiplier antenna, similar to the TH6 for the high bands. Or I may sell it. The reversing feature of the Moxon might make the XM240 superfluous.

My immediate priority is to swap the Moxon for the XM240, make sure it is working, and only then consider how to get the most out of it. Stacking can be deferred for several months or more. 

Lifting the Side Mounted TH6

One of the smaller tasks I set out for myself this year was to increase the separation between the south pointing TH6 tri-bander and the 40 meter XM240 on the rotatable side mount. Their proximity isn't a problem most of the time, but when they are 90° to each other (XM240 pointed west) the TH6 degrades the performance of the XM240. That's because, from the side, the TH6 behaves as a short 40 meter dipole with large capacitance hats.

In the model I developed, the separation was increased from its previous 2.5 meters (8') to 5 meters (16'). That tamed the interaction to my satisfaction. Unfortunately that isn't achievable since there are guys and an 80 meter inverted vee above the yagis. The reflector element tips of the TH6 would get close to touch them and degrade the tri-bander's performance. I opted for a smaller step of about 11' (3.4 m) to preserve the performance of both antennas as well as the situation allowed. 

Even simple tasks like this involve complications, both mechanical and electrical. It took twice as long as expected when the unforeseen occurred. In this article I'll take you through the process, including what went wrong, and finally the results of the move.

I installed a pulley about 20' (6 m) above the TH6. The extra distance was needed to make it possible to lift the yagi and its tower bracket at the same time. The TH6 centre of gravity, like many Hy-Gain yagis, is at the boom-to-mast clamp so that's where I tied the rope. The bracket is much lighter than the antenna so it doesn't unbalance the load by much. To keep it level for the lift I cinched the rope to the upper end of the mast.

On the right you can see the problem. Convenient at the time, I installed the tower bracket for the TH6 at the desired height with the lower strut under the top strut of the rotatable side mount for the XM240. The TH6 had originally been there so it was easy to install the bracket that way and simply the yagi several feet. However, that made it a hassle for climbing since, pointed south, the boom and truss were near the one climbing face of the tower. I've had to squeeze past it many times! It would have been better, but quite a lot more difficult to mount the yagi on the other side of the face

For this lift the challenge was different: to swing the yagi and bracket outward so that the lower strut cleared the XM240 upper bracket. It was easy to do by placing the pulley high on the tower. I used the same technique for the more complicated repair of the XM240 last year.

After the tower was rigged I asked a friend to come over later in the week to help with the lift. In between we had high winds that may have contributed to a problem I discovered too late.

The lower pulley takes the haul rope horizontally to the winch. The pulley is extended out from the tower so that the rope clears the boom of the XM240 which is below the TH6. The hand winch is attached to a convenient guy anchor. The tarp is for tick protection.

The rigging is straight forward. It is more than capable of handling a 5' lift of a load less than 100 lb. Yet when my friend cranked the winch nothing moved. When I unbolted the bracket the rope held the load but it wouldn't budge.

After searching for obvious problems and seeing none, I climbed up to the high pulley and saw what I really didn't want to see. The ½" polypropylene rope had jumped the sheave and jammed between it and the pulley housing. I didn't take a picture at the time so I took one later in the workshop (below right). It is amazing to think that the rope could squeeze into that small gap. Yet it did.

I have several of these cheap pulleys and I really shouldn't use them. This wasn't the first time they've given me trouble. I picked it since it was first my glance fell upon when I was gathering material for the job. Since this wasn't going to be a heavy lift I didn't give it a second thought.

With a bit of jiggling I was able to lower the load slightly and reattach the bracket to the tower. I climbed up to disassemble the pulley to free the rope (left picture). After a critical appraisal of both rope and pulley I put it back together with the rope properly riding the sheave. The second lift attempt went smoothly. In less than 30 seconds the bracket was swung outward and upward and ready for bolting to the tower in its new location. 

The girts on the tower are approximately 5' apart so that was the minimum distance it could be moved. I lowered the TH6 on the stub mast to where I wanted it. The next day I took a few pictures to show how close the reflector is to wires and cables.

I took these pictures the next day after pointing the TH6 south. We are looking down on the TH6 and XM240. To the northwest (left) the rear reflector is several feet from a 105' level guy and one leg of the 80 meter inverted vee. To the northeast (right) the other leg of the inverted vee if not as close but the guy (nearly invisible in the picture) is just a few feet from the opposite reflector tip. The south guy (not shown) is centred on the boom and some distance from the TH6. From previous modelling of interaction scenarios I am confident the south guy has negligible impact on the TH6.

Despite the proximity, I was surprised to note that the SWR on all 3 bands (20, 15, 10) was affected only a little. SWR measurements on the tower were not accurate since I couldn't stand far enough away to avoid detuning the antenna -- the jumper to the analyzer was too short. Body interaction was particularly bad on 15 meters; it's happened before with this antenna. On 20 meters the SWR at the band edges was a little high at 2, however back in the shack it was about 1.7 to 1.8, probably due to some loss in the long run of LDF5 Heliax.

The 15 meter SWR was very good when measured in the shack. The 10 meter SWR was about the same as measured on the tower, with the SWR 2 bandwidth ranging from 28.0 to 28.8 MHz. 

That's good enough. The TH6 isn't used heavily since it is primarily a "multiplier antenna" used in contests to work the multiplier rich and QSO poor Caribbean, South and Central America while the other, larger antennas can stay pointed to Europe and elsewhere.

But what of the XM240? Did this manoeuver help? That's difficult to measure with any confidence. What I can say for certain is that the SWR deviation when the antennas are at right angles is significantly improved. 

The deviation is now small enough that there is little need to re-tune the amps when the XM240 is rotated. I consider that a win. If modelling is any guide, gain and F/B are also better preserved. That, however, is very difficult to measure and confirm.

With this job out of the way I can proceed to finish the 40 meter Moxon and swap it for the XM240. I hope to get that job done in the near future. 

One final and slightly amusing note. After measuring the distance from the TH6 feed point and the Heliax connector I realized it was a perfect fit for the 20 meter yagi delay line that was retired from service last year. It had been hanging on a hook ever since. Further reducing the clutter, the small box for the L-network is also being reused in the 40 meter Moxon project. 

There are a few benefits to being a pack rat who rarely throws anything away.