Exploring the W6NL 40 Meter Moxon with a Model

W6NL (some time ago) came up with the idea of making a 2-element rotatable yagi for 40 meters that uses no loading coils yet is no larger than typical commercial designs -- such as the Cushcraft XM240 -- and has better performance. It employs capacity hats that also achieve high element coupling to make it a Moxon rectangle. There is also a step-by-step set of instructions for converting the XM240 to this design for those who do not want to start from scratch.

In my continuing ruminations on gain antennas for 40 meters I have been intrigued by the W6NL design for some time. For now it is an intellectual exercise since I am unable to install such an antenna at my current QTH. Since this may change I have to plan ahead so I can move fast when the opportunity arises.

A 2-element Moxon yagi (or the Moxon rectangle) has no more gain than a conventional 2-element yagi. The W6NL is no different, where gain bandwidth is about what one would expect. Yet its performance improvements are notable:

• Broad SWR bandwidth: Covers 7.0 to 7.3 with an SWR below 1.5. This is an excellent operating convenience, and can help with removing the need for retuning kilowatt amplifiers when jumping around the band during contests.
• Improved F/B: A 2-element yagi has acceptable F/B over a narrow bandwidth. The Moxon design, by way of increased mutual coupling between elements, maintains a good SWR over a larger frequency range.
• No coil losses: The ESR (equivalent series resistance) of loading coils in commercial short 40 meter yagis varies from reasonably good to poor. It is not unlikely that power loss is as much as -2 db in some antennas. VE6WZ has a good discussion on this topic you ought to read.
• Direct match for 50 Ω coax: No need for a matching network at the feed (e.g. transformer or hairpin) as for a conventional yagi. A common-mode choke is still required.

There were a few things I wanted to know about this antenna so I built a model of it in EZNEC. Since there is more than one version of this antenna out there I used the XM240 modification as the template. If you are interested in the general performance and construction details you can find them in the various papers linked to above.

Model peculiarities

This is a difficult antenna to model with the NEC2 engine. Those who have tried have failed. This is no surprise since element tapering creates an anomalous reactance with NEC2. This is further exacerbated by non-linear elements, which is a feature of Moxon rectangles. The W6NL antenna has these challenges in spades. For further reading on this topic I suggest reading the analysis and modelling by W8WWV.

It is therefore no surprise the model I designed is resonant well below 7 MHz. This is expected in NEC2. To keep it simple I used the exact linear measures for the XM240 modification and substituted constant tubing diameters for the 4 wires that comprise each half-element. I placed the antenna in free space to eliminate pattern complications due to height. Performance is retained, as it is for most yagis, when it is moved from free space to at least λ/2 above real ground. In this case that would be 20 meters height.

I can get away with this shortcut since my objective is to test certain aspects of the antenna's behaviour, not to create a construction template. My model is therefore a reasonable proxy of the actual antenna that is suited to the analysis in this article. For the same reason I omitted the boom and a couple of other fine details. Should you ever build this antenna you must follow W6NL's instructions exactly, or go through the pain of a careful design using NEC4.

With the preamble out of the way let's continue on to the analysis.


Wind action

The elements are about 6.6 meters apart, as they must be when using the 22' boom of the XM240. The capacity hats do double duty as the inward turned segments to achieve the high coupling Moxon requires. The lateral distance between the tips is about 40 cm, plus an outboard offset. This is close. In a typical Moxon rectangle the distance is fixed, a critical feature since small changes can have a large impact on antenna behaviour.

The W6NL antenna must contend with the action of the wind. I wanted to know how the antenna would respond when the wind moves the element ends closer together and farther apart. My approach was to rotate one half of the reflector 5° inward and then 5° outward and check for changes in SWR and pattern. Although it is common in wind that that all of the 4 half-elements are away from the nominal positions, my modelling experiment is intended to provide a starting point for a more complete analysis. I have no immediate plans to do so.

Since the model has resonance shifted downward by ~300 kHz I tested the pattern at 6.8 MHz, which would be equivalent to 7.1 MHz for the actual antenna. I then overlaid the patterns (all azimuth at 0° elevation in free space) for the depicted bending of one reflector half-element.

The result is a modest change in the F/B, evidenced by a different shapes of the rear lobes. Gain is effectively unchanged. SWR (not shown) improves when the reflector bends inward, and is about the same when it bends outward.

This is a promising result. In winds that are below storm level there is little concern about poor behaviour. The same is not true of stronger winds. It is all too easy for the tips to touch, even if only momentarily. I modeled this by connecting the ends of wires #4 and #11 for the inward bending case. The pattern changes to one that is equivalent to a dipole: a loss of -4 to -5 db of gain and a total loss of F/B. While undesirable this intermittent performance loss is no a deal breaker.

The more serious problem is that if contact occurs during transmissions, especially high power, arcing and element damage are likely. Also concerning is that the SWR jumps to well over 3, which can place stress on amplifiers and can cause some transmitters to shut down or roll back the power.

I should note that I have never heard evidence of this occurring in practice. Yet it must happen. Ways to deal with this, if one wishes protection, include the following:

• Longer boom: Mutual coupling is reduced with some degradation of F/B. However even 50 cm could eliminate the problem. For example using the boom of a defunct TH6 (24') rather than that of the XM240.
• Tying the tips together: This is not easy since the capacity hats are not in a plane. However I suspect what with some modelling they could be mated with a solid dielectric (e.g. fibreglass rod) with only a small performance impact.
• Lateral guying: Rope or Phillistran guys can be used to hold the element steady in both the horizontal and vertical planes. The negatives are reduced visual appeal and increased difficultly raising and lowering the antenna.
• Insulation: Wrap or cage the last foot of the inward tips of one element with a high-quality insulator. An open cage is better, though more difficult to construct, so that the dielectric effect has little impact on antenna tuning.

Out-of-band resonance

It is rare for a 40 meter yagi to stand proud and alone atop a tower. At least one more yagi on the same mast and rotator is most common. Interactions must be found and dealt with if all the antennas are to perform to their potential.

Mutual coupling between antennas and their individual elements will at a minimum cause the appearance of an unwanted reactance that will first be noticable in a degraded F/B, then SWR and gain as coupling increases.

There are basically two types of interaction: non-resonant and resonant. Non-resonant coupling can be reduced and largely eliminated by increasing antenna separation. As we increase wavelength this becomes more difficult since a large separation (in wavelength units, which is what matters most) may be physically difficult. Any 40 meter yagi is susceptible to this effect since other yagis are typically less than 0.1λ above or below it.

On the positive side for our 40 meter yagi non-resonant coupling is greatest when the element tips are close to other metal. Since the 40 meter yagi is almost certainly the largest in the stack its ends are quite isolated. However the same is not true for higher band yagis. This may need to be addressed if, say, a 20 meter yagi is closer than 3 meters to the 40 meter yagi. Modelling can help.

Resonant interaction is a greater challenge, one that no reasonable stacking distance can entirely solve. We need to characterize the behaviour of the 40 meters yagi across the higher bands to discover potential problems.

It helps that the W6NL yagi is less than full length. As a general rule any loaded antenna is not resonant on its harmonics. Therefore we may be saved from potential destructive interaction with a 15 meter or tri-band yagi. However it is no guarantee so we must do the work. The first step is an impedance scan up to 30 MHz.

We are mostly in luck: the only out of band resonance is centred on 28.8 MHz. Be skeptical about this value since as we've already noted the NEC2 resonance calculations are suspect. That does not mean that the resonance is a mirage, only that it could be elsewhere in the 10 meter band.

As it turns out the 28.8 MHz resonance is an oddity. The element radiates broadside and transports energy to the hat which radiates end-fire. The short element tips contribute nothing. The reflector and its hat couple poorly and thus contribute little to the pattern. The radiation pattern at 28.8 MHz is a 4-leaved clover with no gain (pattern not shown).

With this analysis in hand we can move on to stack another yagi near the W6NL yagi and see what happens.

Tri-bander stacking

I simplified the interaction model by importing the small 3-element tri-band yagi I modelled in 2014 and placed it above the W6NL antenna. I tried two vertical spacings: 2 and 3 meters, which brackets the range most hams would most often attempt on their towers. Full-sized mono-banders and longer-boom tri-banders are also commonly stacked though I expect the interactions to be similar enough for 20, 15 and 10 meters that this model suffices for a first step.

Current plot at 28.8 MHz for the small tri-bander 3 meters above a W6NL Moxon

When spaced 3 meters the interactions were modest, even on 10 meters where there is a resonance on the 40 meter yagi. Gain on the tri-bander is typically reduced less than -0.1 db on 20 and 15. F/B does degrade though not by enough to be a concern. On 10 meters the gain reduction is closer to -0.2 db and F/B degrades more. Even so the performance is effectively unchanged.

When spaced 2 meters the interaction is more pronounced. SWR increases enough within some band segments to be a concern. Exactly characterizing the impedance change is difficult since the tri-bander model is itself imperfect (discussed in the referenced article above). Many hams choose 2 meter spacing since it is man-height, allowing installion by standing on the tower top plate. Getting a little higher is advisable when mounting above the W6NL or similar 40 meter yagi.

Stacking

Many hams choose to stack short 2-element 40 meter yagis rather than deal with the challenging undertaking of a full-sized 3-element yagi. That allows for equivalent gain at less expense and difficulty. It also gives greater operating flexibility with the ability to point the yagis in different directions.

The question is just how well it plays in practice for modest sized towers and stacking separations. A full analysis is not my objective at present so I chose one configuration to model. The intent is to gauge what to expect from a full analysis and an optimization. I placed one yagi at 20 meters height and a second at 40 meters height. That's a big tower yet stacking separation is a modest λ/2, about the minimum that can be expected to work well for this antenna.

To correctly model stacking the model must use real ground, not free space. In the model I first I measure the lower antenna alone as a baseline for comparison. Then I put the higher antenna into the model and measure performance for the lower, upper and both in phase (BIP). The overlaid elevation patterns of all four cases is at right.

The primary trace is the standalone yagi at 20 meters height. Its 10° gain is 5.7 dbi. This places it about -0.5 db below a full size 2-element yagi. This is quite good for a small yagi that has great F/B and SWR.

When the upper yagi is installed, but not fed, the gain drops -2 db. The upper yagi is coupling to the lower yagi and lowering its performance. This is about as one should expect. When the yagis are pointed in different directions the negative impact will often be reduced.

The upper yagi alone has two main lobes, with the lower one showing excellent low angle radiation. SWR curves for these three cases are similar and very good.

Where the array shines is when both yagis are fed in phase (BIP). Its gain is about 3 db better at low angles than the upper yagi alone, and 6 db better at 10° elevation than a single yagi up 20 meters. F/B is a bit worse at low angles. SWR is very low, even better than a single W6NL yagi up 20 meters. This assumes proper choice of phasing and power splitting systems. These are discussed in my introductory article on stacking.

Unlike my 40 meter stacking example in that article this 40 meter stack works well and can be recommended. The difference is that these smaller antennas have a broad main lobe that allows for better lobe "matching" when stacked at modest heights (in terms of wavelength). Long boom, multi-element yagis are more negatively affected by low height and separation due to their narrower main lobes.

Conclusions

Commercial short 40 meter yagis like the XM240 have comparatively lower bandwidth and greater loss (due to loading coils) along with poorer F/B. While it won't beat the gain of a full-sized 2-element yagi the W6NL Moxon does come close. That it plays well with a stacked tri-bander is an added bonus for those with one tower and want good contest performance on 40 through 10.

Since the W6NL 40 meter Moxon is difficult to model in NEC2 interaction testing and tuning with software is challenging. I have no intention of purchasing a NEC4 license. With the approximate NEC2 model I built I feel reasonably confident that this antenna can be effective and effectively stacked with one more antennas for the high bands. It is an antenna I will mark down for future consideration when I have the tower and space for it. However I do have some worries about the elements touching in a high wind. I would do something about that if I built one.

Unexpected Absence

When I wrote last month that my rate of blog posts might decline over the next few months I did not expect it would hit so hard so fast. The past few weeks are a textbook example of what happens when Real Life intervenes and firmly takes priority over our amateur radio activities. In this case a death in the family.

On the plus side I was able to attend a meeting of Radiosport Manitoba (RSM). While they did not disclose all their contesting secrets to this outsider (though former VE4) it was a great opportunity to see some new and old faces.

Unfortunately I didn't think to take a picture of these Manitoba contesters. Instead I'll mention who I met: Rob VE4GV (probably my oldest ham radio friend who I've known forever and an excellent contester), Dan VE4DRK, Leor VE4DXR, Cary VE4EA, Ed VE4EAR/VE4VT, Ed VE4YU and Adam VE4SN. Cary gave a brief talk of his family trip to Poland (SP) and his brief experience operating from there. This brought up the usual talk of how hard it is to work Europe from VE4 -- which I know myself all too well -- especially on the low band, and therefore why domestic contests like ARRL Sweepstakes are so popular there.

Be sure to give them a call and say hello when you hear them in upcoming contests. Better still, visit them on June 9 when they will host Glenn W0GJ who will visit Winnipeg. He played a central role in the recent Navassa Island (K1N) DXpedition. He will likely talk about this however I understand the main topic is "Is 3 db worth a divorce?" That's a worthy subject and one I'd already been thinking about for this blog.

In other news the CQWW CW 2014 results are out. As expected I took the #1 spot in the SOAB QRP class for North America. Unlike in SSB I had no chance to reign globally since there were some very attractive calls ahead of me. After all, would you rather strain to pull another weak VE3 signal out of the noise or, say, PZ5AV, GJ2A or 5H3EE? Even I was in there calling them, though sadly failed to work 5H3EE for the multiplier. They have a natural advantage in CQWW, just as VE4 has in Sweepstakes. VE3 has an advantage in no contest.

I am now back on the air and slowly getting back to this blog. Hopefully I`ll have a new post about antennas before too much longer.

Good Conditions

The past week has been cycling between very good and very poor propagation conditions on the HF bands. The high solar flux (~150) is a plus as is the season (just past the equinox). Except when we also get a mix of X-ray flares and geomagnetic disturbances. Here in the mid-northern latitudes we often lose good DX propagation since even minor geomagnetic disturbances cause high attenuation on paths in the quadrant between the northeast and northwest, which covers pretty much all of Asia.

Although propagation and solar prediction are inexact sciences we at least have a surfeit of data in comparison to past decades. That is, we may not know tomorrow's (or this evening's) conditions but we can explain current conditions in exquisite detail. This is the equivalent of discounting weather forecasts to having a look out the window.

Like looking out the window to determine the weather the best way to determine conditions is to turn on the radio and listen. I try to do this often even when I have no intention of making any contacts. It only takes a few minutes. Not only do I get a better feeling of what's going on I sometimes discover unexpectedly good conditions. This occurred on Monday evening this week. The temptation to get active was irresistible.

With the yagi pointing northeast I was looking for early morning activity from stations in Russia, the middle east and east Asia, with one ear out for a rarer south Asian midday opening. Good conditions may not be enough since few stations are likely to be active on a weekday morning where most hams are instead heading off to work.

Perfect for QRP

Although I now typically transmit with 100 watts I can recognize conditions that are ideal for QRP. After all, with only 5 watts the difference has to be made up with exceptional propagation, up to 10 or 20 db enhancement, or more. That evening on 20 meters I was surprised by strong signals from the other side of the world.

There were S9+ signals from a host of juicy DX tidbits: 4X, 9K, EK, A6, HZ, SV5, 3B9, among many others. These are always good catches though none are especially rare. The thing is this was the kind of night when, with QRP, I would have been in heaven. With 100 watts I easily worked every station I called.

While this one path on 20 meters was excellent the same was not true elsewhere. Signals on 30 and 40 were unexceptional and almost no DX was heard on 17 meters other than TX5P on Clipperton Island. Higher bands were closed. The polar path on 20 was heavily attenuated allowing only the strongest signals through. These included JT and a variety of Siberians. All this data told me that the auroral zone was active and that the MUF to the northeast was most likely only slightly above 14 MHz. European signals were also quite good, though there were few to be heard since for them it was the middle of the night.

You can never have it all. Appreciate what you get and work it when the propagation gods smile upon you. Great conditions rarely last. This is particularly worth noting if you run QRP or have poor antennas.

Making propagation

The next evening (Tuesday) the conditions were worse. A series of X-ray flares had ruined most paths on the higher bands. I went lower and found mediocre propagation on 30 on 40 and little in the way of exciting DX. On a whim I went down to 80 and tuned the CW segment. With my noise and poor antenna I didn't expect much, and that's just what I got: nothing.

On my final spin of the dial a strong and steady S9 signal calling CQ caught my attention. I expected it to from the US. Instead it was HA8RM, a call often heard during contests. I listened to his unanswered CQs for a few minutes while I scanned my email on the shack computer. It was more interesting to me to see who would go back to him than to call him myself; I've worked him many times, including with QRP on 80.

In the end I decided to answer his CQ just so he wouldn't think propagation was dead. I mentioned to him (Peter) that conditions seemed to be good. He told me he was running a kilowatt and a 2-element yagi. Yes, those are good conditions! When the ionosphere is less than cooperative an exceptional station often can make up the difference. If I was running QRP I am sure I would have worked him just as easily. His station is that good.

Which goes to show that with a sufficiency of time and money you can go a long way towards making your own good conditions. The rest of us have to make do with what bones nature tosses our way.

Spring Checkup

It took a while for spring to arrive in Ottawa. Yet it is now here and the snow is mostly gone. This gives me an opportunity to climb and inspect the towers and the antennas they support. It all looks quite good.  In the days or weeks following erection of a tower or antenna it is normal to be slightly anxious no matter how much care went into the task or even if over-engineered for the local conditions. After a few big winds and coatings of ice when nothing happens you gradually relax. I am now very relaxed.

I'll take this as a sign that I've perhaps done something right. Rather than repeat myself you can refer to the following articles (and their links) for the major structural components of my miniature suburban antenna farm. The only mid-winter work I did was to secure with tie wraps the foam bumpers on the coax up the mast to the inverted vee. The tape loosened in the cold air so the bumpers slid out of position and the coax resumed noisily slapping against the steel mast.

• House-bracketed tower
• DMX-52 guyed tower
• Tri-band yagi
• Multi-band inverted vee and suppporting mast

As I hinted in my year-end recap I have some thinking to do in 2015. I can either make the best of my present QTH or I can move somewhere more amenable to bigger and better antennas. However that is a discussion for the future once I've made some personal choices. For this article I want to cover the successes and failures over the past winter season, both in the antennas themselves and in operating.

Rotator

My now ancient Ham-M rotator performed well though not without some difficulties. Internally it appeared fine when I opened it up for inspection before discovering my cabling faults. That argued for leaving well enough alone, and putting it up to see how it fared. Although it showed no serious problems it did have some difficulty dealing with the brutally-cold temperatures we suffered. February, for example, was the coldest on record.

Starting around -20° C rotation was sluggish. Below -25° C it turned only very slowly, and only started turning after 5 seconds of power to the motor. Although the grease should be good enough for these temperatures I either need to completely clean the bearings and races or use something even better. Some care is needed since low-temperature grease is not always the best at the height of summer when the temperature inside the bell housing can climb far above the air temperature.

Another factor that is almost certainly contributing to slow rotation is the motor capacitor. The replacement I selected is not what is specified for this motor. This is a simple change in the shack when I purchase the appropriate part.

Of more concern was behaviour of the wedge brake at the lowest temperatures. I had a couple of instances below -25° C when the brake would not engage. That is, it would retract to allow rotation but not fully extend when power was removed. When this happened and it was windy the rotator freely turned. Even though it cannot turn past the end stops it was disconcerting and an inconvenience. After some minutes it would engage properly. This is likely a grease problem.

Rain, snow, wind and ice

There wasn't much rain during this cold winter since thaws were absent for a long time mid-winter. Build up of snow and ice did affect resonance and performance of all antennas. Ice also added mechanical load though never enough to be a concern. These are routine matters that did not really test my design of the masts and towers. Of course I'd rather not test them!

Bolts can loosen in extreme cold and metal becomes more brittle. A survivable wind in summer can prove disastrous in extreme cold. Luckily the strongest winds don't occur here in winter. It is critical to do regular inspections -- preferably twice annually, but at least once -- to ensure nothing is out of spec. Nothing untoward has been discovered during my spring inspection.

DXCC

The numbers are gradually mounting at VE3VN with very little effort at all. With a yagi and 100 watts it is almost too easy. There are enough DXpeditions and decent propagation to give everybody a chance. I worked over 200 countries with QRP (5 or 10 watts) and at least 100 countries on each of 40, 30, 20, 17, 15 and 10. Now that total stands at 243 countries. Included in that is 200 countries on 20 meters, just shy of 50 on 80 meters and 198 confirmed on LoTW alone. This is all CW, though I am well past 100 on SSB. I am only counting what I've worked since returning to the air in early 2013; my actual total is well over 300 mixed, though I can't be bothered to count them.

Perhaps it is my imagination that it seems so much easier to climb the DXCC ladder now than before my 20 year absence from the hobby. I suspect that it is partly due to demographics. Aging baby boomers have accumulated wealth that is spent on their stations and DXpeditions. The quantity of booming signals on 80 and contest super-stations amazes me in comparison to the 1980s.

This may be as good as it gets since younger generations are less involved in amateur radio so this spurt of frenetic activity will wane with the decline of my generation and those older. At least in the USA and Canada, and to a lesser extent in Europe.

When the next solar maximum comes around don't expect quite the same level of DXCC country-hunting opportunities. I think that's a good thing since it ought to be a challenge. If it's easy it quickly becomes boring. My preference is work hams that are native to those currently-rare countries, which is also necessary to the longer term global health of amateur radio.

Contests

My time as a QRP contester can be best described as: big fish in a small pond. I do well because those with bigger antennas and ambitions also run more power. I didn't go into QRP contesting with the intention of winning anything yet I do well regardless. For example, I achieved #1 world in SOAB QRP in last year's CW WW SSB contest.

Going up to 100 watts and the results are less impressive. Here the competition is already more fierce. My antennas are not equal to my ability so my results are relatively poor. This is true on both high bands and low bands. Still it is fun to work more stations, including frequently good runs, regardless of how I place. Jumping up to a kilowatt will hurt rather than help since in that category there are many impressive antenna farms. And my otherwise friendly neighbours would form a mob and lynch me.

I will continue to return to QRP with my KX3 in some of the larger contests, since with the high participation I can work many stations and place well. If nothing else it stokes my ego. For bigger scores I will do better to join a multi-op effort at a station with more substantial antennas.

Low bands

My results on 40 are good for a simple wire antenna though less good on 80. Even with no antenna for 160 I have had some surprising success. Unfortunately it won't get much better at this QTH. With a larger tower I could have a small yagi on 40 (wire or rotatable), and by finessing the placement of radials I could do better on 80 and even on 160.

My neighbours are reasonably accepting of my present towers and antennas. Putting up a taller tower (over 15 meters) requires crossing the regulatory threshold to consult with my neighbours and deal with push back from the city. If I were prepared to go that distance there is another less-correctable problem: noise.

Noise

The number of residential noise sources continues to grow. In tightly-packed suburbia it can get quite bad. In recent weeks I have been plagued by even worse noise than before. Starting a few days back after some strong winds S9+ power line noise appeared on all bands from 80 through 10. As I write this it has become intermittent and I can only hope it disappears before the Ontario QSO Party this weekend. Another recent noise source peaks to the north, often wiping out most signals on the path to Asia.

No matter the time of day or day of week there seems to be noise on at least some bands. Often the only noise free periods are in the middle of the night when everyone's lights are out and asleep. Since I too am asleep then it does little good. Even with 100 watts I prefer to stick with CW where I can narrow the filter bandwidth.

The only true solution is to get out of the city. Even then it is no longer a sure thing to escape the noise from our modern electronic appliances. Investing time and money into a big tower in the suburbs is often not worthwhile.

Going forward

With the warmer weather my operating activity will, as usual, decline. This is a good idea even for the greatest enthusiast since it replenishes the energy when we do return to the shack. I do have a few antenna experiments in mind for this year which I will undertake as time permits. My schedule includes numerous non-ham activities. However, as I said above, there is little reason for significant changes at the station this year. I am not terribly happy about that.

I will continue with antenna modelling and other planning activities with an eye firmly set on the future. Blog activity may temporarily decline since much of what I'll be doing will not be interesting to others. Reading, studying and planning are boring to watch though worthwhile for the one doing them.

Whenever something that may be of interest to others comes up I will write about it. As I've pointed out before my web site statistics clearly tell me that those of you who visit this blog want to hear about antennas more than anything else.

Improving the 2-element Connected-radial 80M Parasitic Vertical

In my earlier article on a 2-element parasitic vertical array for 80 meters I made no attempt to optimize the design. That was put aside in order to focus on radial topologies. With that out of the way I now want to take the best of those radial topologies -- connected radials -- and get more out of the 2-element antenna.

Since I will not be repeating myself you should use the link above to refer back to that article. For this one I have some objectives in mind when I use the word optimize or, if you prefer, improve the design:

• Simplicity: The more common 4-square used on 80 meters in many high-performance stations is in the judgment of many hams a good return on investment of time and money. But at its heart it is not simple, dependent as it is on some intricate phasing and power splitting electronics. My preference is for an antenna with similar performance that is less vulnerable to weather or component failure, and is far easier to tune and keep in tune.
• Gain and F/B: Gain is paramount to me though I'll take all the F/B I can get on 80 meters. With noise so prominent on the lowest bands gain has an outsize effect compared to higher bands since signals are often so near the noise level. Odds of working the rare DX improve and many more QSOs can be logged during contests. The same applies to F/B but only on receive, which can alternatively be addressed with a separate high-directivity receiving antenna. Thus for me gain trumps F/B.
• DXing and contesting from my QTH: For me the most productive directions are Europe and central USA, which happen to be in exactly opposite directions. The 4-quadrant switchable directivity of a 4-square is nice, in general, but when aligned to those two directions the other two are not so useful: namely the north Pacific and south Atlantic Oceans. One of those will at least garner some JA QSOs while the other is mostly useless, except perhaps for instances of skew-path propagation. I am willing to sacrifice performance in those directions to do better towards Europe and central USA. Other, simpler antennas can fill the pattern holes.

Your objectives will almost certainly differ from mine, even if only to a minor degree. Hopefully what you find here will permit sufficient knowledge, or at least data, to inform adjustment in accord with different objectives.

With that introduction out of the way let's proceed to improving (or optimizing) that 2-element parasitic vertical antenna.

Step 1: Optimize the element spacing

The 0.25λ element spacing I used in the earlier article was not selected for its superiority. Rather I chose it as a reasonable value that would allow radial coupling for overlapping radial systems, good though not necessarily optimal coupling between the monopoles, and lastly as an initial basis of comparison with 4-squares (typically arranged in a square with 0.25λ sides). Don't confuse this parasitic antenna with a dual-fed 2-element end-fire array which has a different optimal element spacing.

I ran the EZNEC model by adjusting element spacing in steps of 0.05λ while changing nothing else about the antenna. As before each standalone element is resonant at 3.6 MHz. I successively changed spacing in the increasing and decreasing directions until it was clear that the performance possibilities were exhausted. The model (which has close to 500 segments) requires some work at each step in order to properly maintain radial topology, so it is not a trivial process.

• Move one element to the required position. I nominally set λ = 84 meters (3.57 MHz), and I rounded the spacing to the nearest 0.5 meters to simplify radial calculations without significantly affecting the results.
• With the element moved it is necessary to extend or contract the radials that must be connected. These are radials that would otherwise cross at their nominal length of 20 meters. The included angle between radials must be preserved. I used Cebik's method here, just as I did in the earlier article. All connected radials connect at a line orthogonal to and that bisects the line between element centres.
• Segment counts for the connected radials must be adjusted so that segment length in all radials is as close to equal as possible. This assures best accuracy from NEC2. I normalized segment length as 1.25 meters. I was able to get within a few percent of this for all of the truncated (connected) radials.

The element spacing range I ended with ran from 0.15λ to 0.30λ. Modelled gain, F/B and SWR (at 50 Ω) from 3.5 to 3.8 MHz are shown in the adjacent set of charts. In all cases the ground loss is approximately -5 db, a value that will vary with ground type and radial count. I use the same 16 radials per element and EZNEC medium ground (0.005, 13) as in the earlier article.

From studying these charts I believe that 0.25λ is the optimum spacing. You can get more gain at 0.20λ but the gain bandwidth is narrower. Better F/B can be had at smaller spacing, but again with narrower bandwidth and with higher SWR. SWR increases at smaller spacing due to decreasing feed point resistance (~25 Ω at 0.15λ spacing) and higher Q (more rapidly changing resistance and reactance with frequency).

In all cases the gain and F/B curves can be moved up or down the band by adjusting monopole length (standalone element resonance). SWR curves can be shifted with a matching network as simple as a series capacitor. Impedance, and thus SWR, will change with different ground, wire type and gauge, and radial count, though probably not by a lot.

Gain relative to a single vertical of the same construction is greater by 0.7 db than shown in the charts since its gain is -0.7 dbi. All gain and F/B values are for an elevation angle of 15°, a good median value for typical DX paths on 80 meters. Over medium ground and the specified radial system the gain peaks at an elevation angle of about 25°.

Step 2: Double the gain

Now that the 2-element array is optimized we can stack them. That is, put an identical array beside it and try for 3 db of additive gain. The idea is to exceed the gain of a 4-square with the same quantity of verticals, and to accomplish it with the same simplicity as each 2-element antenna.

Use of term stack is deliberate and accurate. It is the same as we saw in my article on the basics of stacking yagis. The differences are that the two arrays are ground-mounted, 2-element vertical parasitic antennas and that they are stacked horizontally rather than vertically. In other respects the stacking arrangement is the same.

For this to work we must space the two antennas far enough apart that mutual coupling does not grossly interfere with the additive nature of the array. This is complicated by the radials which would touch, and therefore need to be connected for predictable behaviour, at a spacing of 40 meters or less. Thus the minimum spacing is 42 meters, or 0.50λ. There is no theoretical maximum spacing, though land use and transmission line loss are constraints. We ideally want the minimum effective spacing. So let's try 0.50λ and see what we get.

Gain peaks at 6.33 dbi at 3.65 MHz, and again standardizing on 15° elevation. The gain remains in a narrow range over the band segment of interest. In comparison to the single 2-element vertical array the gain increase is proportional to frequency, from 3 db at 3.5 MHz to 4.5 db at 3.8 MHz. The mutual coupling between the close-spaced antennas is having an effect, and that effect is in our favour in that the gain is for the most part higher than the 3 db expected with zero mutual coupling.

Mutual coupling is also having an effect on F/B, and again it is mostly beneficial. We are now seeing some reasonably good F/B, although only higher in the band (SSB segment). SWR has actually improved, staying below 2 from 3.5 to 3.8 MHz.

The forward lobe in the azimuth pattern has become somewhat narrow, spanning just less than 60° at the -3 db points. This is what happens in stacks: the gain comes from narrowing of the forward lobe in the plane of the stacking direction. Vertically stacked yagis narrow the elevation beam width and horizontally stacked antennas narrow the azimuth beam width.

The feed system is included in the model, composed of several length of coax. Performance will not be accurate if two in-phase sources are instead used in the model. Power splitting is accomplished by tying together (T connector) two λ/4 70 Ω transformers, and placing the single source there. I specified 14 meters of solid dielectric coax (0.66 VF), for which RG-11 is an example. The far ends of the transformer sections must be equal lengths of 50 Ω coax. The transformer lengths alone will not reach from the power splitter to the antennas.

I made the transmission lines loss-less in the model. Real (lossy) coax of the builder's choice can be substituted when selected. In any case the loss in the cabling is not high and would not substantially affect the modelled performance.

0.75λ spacing between the 2-element antennas

As already mentioned, 0.50λ is the minimum spacing. If it is desirable to increase spacing there is a constraint we need to discuss. Unlike a multi-element yagi the beam width of each 2-element antenna in the stacking plane is so wide that there is substantial radiation off the sides (see pattern in the previous article). One advantage of the chosen minimum spacing is that off the sides the radiation from each antenna is 180° out of phase. This is perfect for improving F/S of the array. Until you increase separation to 1.5λ spacing the cancellation can be poor.

The pattern at right is for a spacing of 0.75λ (63 meters). This is not a pattern you are likely to favour. In an array of this type the spacing between antennas is an important parameter.

Step 3: More directions

While 0.75λ is not a particularly effective way to get more than two directions from this array it is possible to steer the forward lobe of the optimum 0.50λ-spaced array. We do this by switching in (or out) a length of 50 Ω coax between the 70 Ω transformer and just one antenna's feed point. However, exact phasing of feed points is not possible since the unequal lines alter the mutual coupling resulting in what might be unexpected results.

In the skewed pattern shown I have "switched" out 10 meters of 50 Ω coax going to the left antenna. While heavily distorted you can see how the main lobe has turned about 30° to the left and partially filled the side nulls. The same can be done in the right direction by the same change to the right leg of the feed system.

While far from the directional performance of a 4-square it is one way to compensate for the fairly narrow main lobe. Yet it is no substitute for getting good performance in the missing 2 quadrants. SWR changes little for the range of cable lengths differences I experimented with in the model. The 10 meters of coax I cut to skew the pattern was arrived at by trial and error; other lengths, I tried gave poorer results. Keeping the 50 Ω feed sections as short as possible (before switching a section out of one leg) gives better patterns.

Step 4: Direction switching and cable management

Direction switching and cable management in the elevated 40 ground plane array requires some care to avoid the coax and relay lines from degrading performance. The same is true here though with some welcome reduction in complexity. In fact the methods used have much in common with typical construction of a 4-square.

For an individual 2-element ground-mounted antenna the presence of the coax (transmission line to shack and between elements) can be put to use rather than cut out of the picture using common-mode chokes. Relays for direction switching are also simpler. This can be done since these conductors are in the same plane as the radial systems, and several of the radials are connected.

First, let's look at the coax and switching line between elements. Unlike the ground plane array we can permanently connect the ends of the coax outer conductor to the radial origins at both elements. It is only the centre conductor that needs to be switched. We are in effect turning these cables into radials. If the cables are buried deep (for weather and rodent protection) they can be complemented with a separate surface radial above the cables and running directly between the elements per the previous article. No chokes are needed. An SPDT relay at the far element switches the monopole between the radial system (reflector element) and the coax centre conductor (driven element).

The element where the transmission line connects still requires two relays, though these can now be SPDT rather than DPDT. One switches the monopole between the second relay (driven element) and the radial system (reflector element). The other switches the transmission line centre conductor between the first relay (driven element) and coax to the far element (reflector element). The transmission line outer conductor connects to the radial system and to the outer conductor of the coax going to the far element.

The transmission line can also become a radial if run where a radial would normally run and it is (optionally) terminated out 20 meters with a common mode chock. In the 2-antenna array these runs would terminate at some convenient point between the 2-element antennas where power splitting takes place. It is likely that the coax-radial will include a transition from 50 Ω coax to the 70 Ω transformer so there may be a need for superior sealing of that joint. If DC relay lines are run the same way (central DC/RF splitter rather than one for each antenna) similar common-mode precautions are needed for the DC lines.

All transmission lines in each half of the array must be identical for the required symmetry. This includes the 50 Ω lines from the 70 Ω transformers to the feed points and the 50 Ω line between elements.

As with stacking yagis there is the alternative of running 50 Ω coax from the power splitter to the individual antennas by using a 2:1 transformer rather than λ/4 transformers. This has a few benefits worth considering:

• There are fewer joints to protect from weather and burial, especially the ones at the far end of the λ/4 transformers.
• One antenna can be easily disconnected from the array. This is a simple way to broaden the forward lobe (135° beam width versus 60°) and thus reduce the need for array steering. Although we give up 3 to 4.5 db gain when doing this it is one way, for example, to cover almost all of the continental USA from my QTH. In contests this can be highly desirable.
• Pattern skewing as described earlier can be done at the power splitter by switching in (or out) a length of 50 Ω coax feeding either of the antennas.

Conclusions

As I stated up front I am willing to sacrifice ultimate performance in two of the directions provided by a 4-square if I can get better performance in the two directions of greatest importance to my operating. This array can accomplish that and do so with electrical simplicity and a little more land to accommodate the greater separation required between the 2-element antennas.

The cost is one of time and effort to roll-your-own rather than buying a commercial 4-square splitter/phasing box. However the tuning effort in my opinion is simpler and quicker with this array than with a 4-square. You also won't find one day that the "dump" resistor of a 4-square warns of failure or burns up hundreds of precious watts due to weather or other adverse event that will occasionally upset the sensitive tuning of the array.

To me this is good food for thought. Since I am not yet in a position to build either antenna there is time to consider my options. Your situation and objectives may be different and so should your choices.

Buying a Used Yagi: Hams Ripping Off Hams

I described my research into choosing a tri-band yagi in a series of articles last year (2014). The antenna I eventually purchased was not the first one I looked at. Originally I preferred to locate a TH3 since that seemed ideal to my needs and tower capacity. The Explorer 14 I bought has the same boom length and more efficiency (4 less traps) but more wind load.

While shopping for a TH3 I encountered something unfortunate. There was a transaction involving an antenna where, in my opinion, one ham ripped off another. That story is the subject of this article. No calls or names are mentioned, and indeed I don't know the identities of certain parties. While stories like this are atypical in the ham community, fraud and frayed relationships sometimes just seem to happen when money is involved.

Inspection

I found a TH3 on the local used market and contacted the seller. He explained he didn't know much about antennas, especially yagis, and would willing to negotiate the already reasonable asking price. I drove over to have a look. Problems were already evident from the ground, which is not a good sign. For example, one element tip dipped downward from the outermost trap.

I climbed the tower for a closer look. More problems appeared. He wanted the antenna down and since he was a decent fellow I gladly performed that small task for him. It was already becoming clear to both of us that I would not buy the antenna.

With the antenna on the ground I began to disassemble it. It quickly turned into something like a forensic analysis at a crime scene. What I found shocked both of us. He soon realized he'd been had by the ham who sold it to him.

How we got here

Someone he connected with in a local club offered to sell him a yagi when he was looking for something small and inexpensive to work some DX on the higher HF bands. That individual was friendly and willing to help put the antenna on his tower. A deal was negotiated.

The seller delivered the antenna and assembled it. In retrospect the buyer realized he should have been suspicious since the antenna looked odd and the seller gave him no opportunity to consult the manual that he'd downloaded in advance. He quite sensibly wanted to make measurements and just to have a closer inspection of his first HF yagi. The seller explained he was pressed for time and wanted to be quick with the job. So up the antenna went and coax attached.

Signals were heard and the transmitter seemed happy. Money changed hands. But it was soon clear that not all was as it should be. The SWR was quite high in many places. When contacted the seller told him that was nothing unusual and just to use the rig's ATU to match.

He had no basis for comparison and so could not check gain and I don't believe he checked the F/B. He only knew that when he turned the antenna the desired signal peaked and he was usually able to get through. Of course even a dipole shows directivity, and a yagi is like a dipole in its basic azimuth behaviour: broadside gain and a null off the sides.

This is a real story and a real antenna so I am able to provide physical evidence. You'll soon see why I was outraged by what had transpired.

Busted trap

The cause of the dipping element tip was discovered when the mass of tape holding the tube to the 15 meter trap was removed and the trap opened.

Pretty, eh? The plastic coil form is severed and the tab connecting the trap shell (capacitor) to the element was sheared off. The stand-off toroidal insulators are heavily scored. A violent impact would account for this degree of damage.

Trap covers

All the trap end covers are taped. The few I uncovered were severely damaged by UV radiation. This is a case of the shutting the barn door after the horse has bolted. Tape (the necessity of which is debatable) should have gone on earlier to reduce sun exposure.

The decay of one cover was so bad that the tape was actually holding the cover together. In another case removal of the tape revealed an ill-fitting plastic plumbing cap rather than a Hy-Gain part. Hy-Gain sells replacement kits which are reasonably priced. There really is no excuse for what I found.

Parts substitution

One tube joint could not be taken apart. There wasn't even a clamp at the joint. A closer look (see upper right of above photo) showed no compression slots in the larger tube and that the tube was not square cut. This is a press fit that was likely seized by corrosion or an oversize wall in the larger tube.

I can only presume the original tube was damaged beyond repair and replaced by another tube that was not properly selected or prepared.

Hy-Gain multi-band yagis have few trap varieties. This is good design and good business. All the 10 meter traps are identical. The 15 meter traps are different for the driven and parasitic elements. This allows kilowatt power without heating the driven element traps to destruction. In a conventional yagi the driven element current is higher than the parasitic elements. These traps are wound with copper wire to reduce resistance and therefore I²R loss. All the other traps are wound with cheaper aluminum wire.

The difference can only be discerned by the part number printed on the affixed label (too faded to read in the present case) or by peering through the drip holes to inspect the wire colour. The wire in the trap wound with copper should be brownish instead of silvery gray.

Element-to-boom clamps


I found two cases of substitution of proper Hy-Gain parts. Have a look at the adjacent picture.

On the left is a clamp made from half of the correct part and half that homemade from sheet aluminum.

First, the alloy is unknown and therefore of uncertain strength. Second, it is improperly shaped. The tubes inserted in this clamp had their ends severely crushed.

Why someone go through so much misspent effort when a replacement part is readily available and inexpensive is mystifying. I can only imagine that it was someone with more time on their hands than good sense.

On the right is the clamp for the driven element. This clamp must be larger to accommodate the plastic inserts which insulate the driven element from the boom. This is needed for the beta match feed. Except I found that the wrong clamp was used. To get a compression fit to the boom the clamp had to be tightened so much that the plastic inserts were crushed and split (not visible in the photo). Further, the anti-rotation set screws were replaced with longer hardware to bridge the gap.

Boom

The TH3 boom is a little over 4 meters (14') long. It comes in 2 identical halves that are joined at the centre with a clamp.

The ends of the two halves (off the left side of the photo) are aligned. It is readily apparent their lengths are unequal. The longer is the correct length for this antenna. The other must have come from another antenna since it shows no signs of having been cut. My guess is that it is from a TH3jr (12' boom) since the length is correct for that antenna.It would also explain the lack of copper-wound 15 meter traps in the driven element. That is, this antenna may be a mix of parts from multiple antennas.

Not only is the resulting antenna physically unbalanced the tuning of the elements will be sub-optimal for the altered inter-element spacing. The result will be poorer performance and SWR behaviour. Even if the difference is small it makes no sense to put up a 3-element yagi that and not get the performance you paid for.

Notice the deep indentations in the longer boom half. It's worse than it appears in the photo. Similar damage is present at the position of every boom-to-element clamp. The set screws (2 per clamp, to help prevent element rotation) were overtightened, in a few cases piercing the boom wall. There are more dimples than set screws which tells me that this antenna was improperly assembled at least twice, with the boom partially rotated the second (or third) time.

Although this is not a large antenna there is no excuse for this abuse, which will reduce survivability in severe weather.

Hardware

On a somewhat positive note most of the hardware -- hose clamps, bolts, washers and nuts -- were stainless steel. However the sizes were often incorrect. This is the best I can do to say something positive.

Prescription

There is great fellowship among radio amateurs. We help each other out with antenna-raising parties, sharing expertise and software, advice and training, and even simply offering pointers to help out the novices among us. Elmers -- those hams who mentor others before and after they first join our ranks -- are rightly venerated.

Yet hams are only human. It is a mistake to imagine that we are all cut from better cloth. Just as in the general population we have our misfits, anti-social miscreants and worse. The number in the latter group is small but can have devastating impact when they prey among the novices in our hobby or those who are overly trusting of fellow hams.

We like to believe we are good judges of character. It is not so easy, though it can become easier as we grow older and wiser, often by the "benefit" of bad experiences. Getting ripped off not only cost time and money it can also cause acute embarrassment. I have seen disputes come close to fisticuffs in flea markets. I have looked sellers in the eye and asked hard questions about an item they were selling, and then watched them squirm. Even the dishonest have consciences and you may see it on their faces.

Too often we keep those incidents quiet, fearing that other will think us foolish. That can be a mistake, one that the criminal class counts on for the continuation of their careers. Perhaps my story can, in a small way, shed some light where it's often absent.

In any transaction where you lack expertise about a product or don't know what question to ask, bring a knowledgable friend along. It won't hurt and it can help avert a bad experience. When we lean on each other we are all stronger. Pass along the same favour when you have a chance. Just don't go overboard and become suspicious of everyone; happily the bad apples are the exception not the rule.

Radial Topology Options: 2-element Parasitic Vertical Array for 80

In this article I want to look further at radial topology of vertical arrays for the low bands. In an earlier article I looked at the impacts of base height and radial arrangement of an above-ground mount for a 2-element vertical (ground plane) array on 40 meters, one that had 4 radials per element. That antenna performed well in the model, although I did note the potential pitfalls regarding the precise way in which the radials interlace, parasite tuning and the expectations for ground loss that may differ in the real world.

Although doing that model on 40 meters makes the antenna more mechanically friendly there are better alternatives for gain, directivity and low radiation angle on that band, such as a small yagi that is 20 or more meters high. So it was perhaps more of a modelling convenience than a desirable antenna project. On the other hand, hams without large towers and a disinterest in large yagis have used vertical arrays, including 4-squares, on 40 to good effect.

Dropping down to 80 meters and the situation is markedly different. Even among the big guns of the world a yagi is rare and raised vertical arrays are, while not rare, not common either. Ground-mounted vertical arrays are more typical, with the 4-square being pretty much the big-gun standard.

Standard element

The vertical element I will use here is a ground-mounted monopole tuned to resonance (X = 0) at 3.6 MHz over EZNEC medium ground (0.005, 13):

Top view of vertical; wire 1 is the vertical monopole

• Monopole is 20.2 meters long and 50 mm diameter aluminum, fed at the bottommost wire segment. The chosen diameter is an average value assuming a tapering schedule for telescoping aluminum tubes.
• Radials are 20 meters long, 16 AWG aluminum wire. This is commonly available and economical electric fence wire.
• There are 16 radials. This quantity is a compromise among computation time, ground loss reduction and NEC2 model reliability.
• Mounted at a height of 10 cm, which is just above the 0.001λ minimum W7EL recommends for reliable NEC2 emulation of radials lying on the ground.
• Use of a common mode choke at the feed point and other places is assumed in order to remove the need to model common mode current on the outside of the transmission line. Burying the coax does not eliminate this requirement.

At resonance the antenna has a feed point resistance of 37.6 Ω with a broad SWR bandwidth that is a good match to 50 Ω coax. Its gain is -0.7 dbi at an elevation angle of 15°. This is pretty typical performance for a vertical of this type.

I have chosen 15° as the comparison standard for 80 meters since that is a median value for medium length DX paths, which are the most productive on this band. The longest paths can have angles well below 10°, though not always. Since low angles are the most difficult to attain and are needed to best DX results and high angles are easy with a second, horizontal antenna I choose suitably low angles as the standard of comparison in the majority of my models. In my many articles about 40 meters antennas I used 10°, and lower angles for progressively higher bands.

Adding a second element

I will continue with λ/4 element spacing, which on 80 meters places the identical second element at 21 meters distance. The optimum spacing may be different though, I expect, not by much. My interest here is to evaluate radial topologies. Optimization can be performed later, should one of these arrays be built.

The parasite will be a reflector element. As we'll see identical elements work well to achieve the desired effect in this configuration. Switching between either end-fire directions is straight-forward, being no more difficult than for the design I proposed for the 40 meters array.

With 21 meters spacing and 20 meter long radials the two radial systems overlap. We have a few strategies to deal with this:

• Place one radial field above the other, allowing capacitive coupling between them. Doing so can require making the parasite a reflector since there is near-critical coupling between elements when the height separation is small.
• Lay them in the same plane with electrical continuity where radials cross. This is sometimes done in 4-squares and similar arrays though there is limited benefit in extending radials further than the first crossing. It is usually better to use the extra wire to make a mesh ground plane at the monopole base to reduce near-field ground loss.
• As above but terminate the radials at the first crossing.

I built models for the first and third cases, rejecting the second since it is mostly redundant to the third.

Overlapping, capacitance-coupled radials

The most critical parameter of a Moxon antenna is the distance between the turned-in ends of two elements. That is where the capacitive coupling is strongest since that is where voltage is highest.

Strong coupling drives the parasite current higher than in a conventional yagi. It also restricts the phase relationship between the elements such that above a critical level of coupling the parasite can only operate as a reflector element. This is why I commented in my earlier article on the 2-element ground plane model that it is sensitive to precise placement of the (interlaced) radials.

As I noted above when the second element is added to make a 2-element ground-mounted vertical array the radial systems must be vertically separated and the amount of separation is a critical parameter, as in any critically-coupled array. I modelled the array with a range of separations, each small enough that the vertical offset of the monopoles would not significantly alter the far-field pattern.

With a 10 cm (4") separation (driven element on the right is 20 cm above ground) I achieved the best performance. However I did not try to fully optimize the array: my aim is to generally characterize the array to decide on whether its merits motivate further investigation.

Wire currents at 3.6 MHz, 10 cm radial separation; reflector at left

To demonstrate radial/element coupling I have plotted the modelled currents on the array's top view. The driven element current is normalized at 1 A. The driven element is to the right (red) and the reflector is to the left (blue). Radial currents are roughly maximum about 2 meters out from the monopole: the ground's dielectric constant makes the 20 meter long radials electrical length slightly more than 0.25λ. Strong coupling is evident in the high current in the parasite monopole: 70% that of the driven element at 3.6 MHz. It only appreciably declines when the radial system separation grows to at least 30 cm (12").

Radial currents are not close to equal or sum to that of their respective monopoles. Currents are higher where a radial crosses another, and highest where the far end of a radial is close to another radial. Only where a radial stands clear is the current close to the theoretical 1/16 of the monopole current. It should be evident that radial placement is critical to antenna performance, as it was in the 2-element ground plane.

The forward gain at 3.6 MHz is  about 4 db more than the single element at an elevation angle of 15°. Relative gain of 4.2 db is maximum at 3.5 MHz, and slowly declines to 2.2 db at 3.8 MHz. F/B is poor, ranging from 8.4 db at 3.5 MHz to 9.7 db at 3.8 MHz, and reaching a maximum of 10.9 db at 3.7 MHz.

The SWR is surprisingly good, staying below 2 over most of the band of interest to DXers and contesters. Unlike conventional parasitic arrays the SWR bandwidth is excellent and a good 50 Ω match. The change in SWR with frequency is mostly due to the feed point resistance since the reactance changes more slowly.

Increasing the radial system separation to 20 cm leaves the SWR and F/B nearly unchanged. However the gain at 3.5 Mhz is 1 db lower and the frequency of maximum gain rises to 3.6 Mhz. Even a small change in radial coupling can have a significant effect.

For a simple antenna on which I spent so little time this is good performance. But it comes with some important catches:

• Maintaining the required radial field separation is difficult. Not only is it mechanically challenging it is a safety hazard to have 20 meter long wires 10 cm above ground, whether for pets or wildlife that will inevitably wander into the area. The radials, if bare wire, must never touch any other.
• Weather will alter radial coupling. Winters with snow and ice will almost certainly destroy the radial behaviour, and the structural integrity of the raised radials.
• The antenna is very sensitive to changes in radial coupling. Those changes are usually for the worse.

The study of this radial topology has been interesting and instructive. Since it is not promising for an 80 meters vertical array it is time to move on to the second case.

Radials tied at the mid-point

Terminating and tying radials of adjacent verticals in an array is an old idea, and has been used in commercial broadcast arrays for years. It is also an obvious one, so that it is unsurprising that I independently thought of it before discovering it in the literature. In the amateur field you can read, for example, a discussion by the late W4RNL (Cebik).

There is some coupling in this array, mostly between radials whose ends are close together. However this is dominated by monopole coupling and direct connection of the radial systems. At first blush this would appear to be an array more suited to having all elements driven, with a power splitting and phasing system to achieve the desired result. Antennas such as 4-squares are of this type, though so are 2-element end-fire arrays such as described here.

Connected radial; reflector (blue), drive (red), connected pairs (black)

The radial currents (defined above) vary less than in the array with overlapping radials. First, the reflector current is 58% that of the driven element, which is lower than with overlapping radials though still more than in a conventional yagi. The sums of the radial current for each element are roughly equal to that of the monopole, approaching the ideal situation of a single element where radial currents are equal and sum to that of the monopole (assuming radials lengths near 0.025λ).

Radials that had been capacitance coupled are now directly connected: 16-21, 15-22, 14-23, 13-24 and 12-25. While the currents in these radial pairs are (necessarily) equal where they connect it may seem surprising that currents are quite different at their origins. Power is flowing between the elements, but with a 133° phase difference (at the radial origins). Current nodes occur on driven element side of the radial pairs, closer to the driven element where the current differential is greatest. On wire #23 the node is adjacent to the radial origin (driven element monopole junction).

Despite the relative lack of control over parasite current and phase this array has gain and F/B. However performance is not quite as good as the array with overlapping radials. Nevertheless this is a more realizable antenna since the radials can all be on or just below ground, and coupling requires less fine tuning.

Since the elevation pattern is indistinguishable from the one above I will instead show the azimuth pattern, which is also the same. The plot is at right.

Maximum gain is 2.82 dbi at 3.575 MHz and 15° elevation, which is 3.52 db better than a single vertical. At 3.5 MHz the relative gain is 3.2 db. Going higher, relative gain gradually declines to 2.9 db at 3.8 MHz. F/B is poor. Its maximum is 9.8 db at 3.8 MHz, and an especially bad 3.5 db at 3.5 MHz.

Although I have spoken against the importance of high F/B, particularly for the high bands in a small station with a single yagi, the situation is different on 80 and 160. The bigger problem here is QRN, not QRM. Directivity is needed to improve SNR enough to copy DX stations. If this antenna is built it ought to be supplemented with a low-noise receiving antenna such as a Beverage or compact, rotatable loop. Otherwise be prepare to not hear many stations that call you, especially if you run a kilowatt.

SWR is sufficiently broadband to allow no-tuner use from 3.5 MHz to 3.8 MHz. The above SWR plot is for equal height monopoles (identical elements). This worked well in the array with overlapping radials, though here the resonant frequency drops a little lower than is ideal. A 2,500 pf capacitor should be switched in series with the driven element to bring the SWR below 2 across this frequency range. The value isn't critical, 2,200 pf or 2,700 standard values can be used, but use a ceramic knob capacitor if possible to reduce loss (and potential failure when running a kilowatt).

My take on this antenna:

• It has the makings of a good, simple, broadband switchable gain array without the burden of a power-splitting and phasing system. Even a commercial product to perform these tasks requires work to tune and match the elements and adjust array performance.
• F/B is so bad that a high-directivity receiving antenna be seriously considered.
• Altering the radial quantity and length will change array behaviour. Model first before adding radials to reduce ground loss, since wire lengths may need adjustment.

Again, this is an experimental model and not a design to be rushed into construction without further evaluation.

Disconnecting the common radials

As a final experiment in this phase of modelling 2-element parasitic vertical arrays I will take the previous design and disconnect the 5 radials pairs that are joined at the midpoint between elements. My aim is to see how much capacitive coupling can be achieved and whether it can be used to raise parasite current and thus hopefully improve F/B without the tribulations of overlapping radial systems.

I modified the connected radials model by disconnected the 5 pairs of connected radials and various the separation distance. The results were disappointing. Mutual coupling between elements was low, as evidenced by parasite current (reflector) only around 36% that of the driven element. Gain and F/B performance was, not surprisingly, poor. There was little benefit found by varying the separation distance; I tried values from 10 to 100 cm. More drastic measures would be needed to increase coupling, such as in the overlapping radial case.

The adjacent elevation plot is typical of the performance. Gain is only about 1 db better than a single vertical! F/B was typically below 6 db. Like the connected radials array, gain peaked at 3.5 MHz and F/B peaked at 3.8 MHz.

However SWR performance mimicked that of the connected radials array, being almost indistinguishable and so not worth showing the curve again. I also declined to mark up the antenna diagram with radial currents as it was not worth my time for this unpromising antenna.

One interesting difference was ground loss: it modelled from -1 to -1.5 db worse than the connected radials array. That is undoubtedly where some of the missing gain went. My guess (I didn't look at it more closely) is that the additional loss is due to a third of the radials being about half the length of the others.

Conclusions

Of these experimental models the only one that shows promise is the one with connected radials. It has good power flow between elements, accomplished with a robust physical design. I expect that it can deliver the modelled performance when built.

To get improved F/B from this array it would be necessary to feed both elements and use a power splitter/phasing system to architect the required electrical parameters in each element, in part by "taming" the mutual coupling to do our bidding. That system could also be used for direction switching. Designs are readily available, with perhaps the most comprehensive treatment found in ON4UN's Low-band DXing book (5^th edition), chapter 11.

My approach would be to either build a 4-square or build the 2-element end-fire array described here. In the latter case, going with simplicity and maximum reliability rather than the best F/B performance. A simple, low-cost directional receive antenna would complement the parasitic array. That is, if you have the land.

It is possible to use elevated verticals to reduce ground loss and deal with any local topography and obstacles that impede the view of the horizon. While more challenging on 80 meters it is not necessary to raise the base 20 meters (λ/4) to get the benefits. Half that height can be effective, though probably no lower. There are ample resources on elevated radials, of which I'll point to two available on the internet: by VE2CV and another by N6LF. ON4UN's book also has many ideas in this regard.

But if you do so the antenna must be carefully modelled so that it can deliver the desired performance. You cannot simply lift the arrays discussed in the article and expect that they'll work.

13 db

In this past weekend's ARRL DX SSB contest I did something I have not done since returning to the air over two years ago: I entered a contest using more power than QRP. For the first time I put my recently-acquired FT-1000MP to work in a contest. Running 100 watts is 13 decibels more than the maximum 5 watts allowed to qualify for the QRP category. This is a brief recounting of how it played out.

I entered 15 meters single band since my time was limited and I expected to be competitive by focussing that time on one band. Based on conditions before the contest I chose 15. My choice turned out well. While the band was not open right through the night it performed well up to 3 hours past local sunset, and came up again with the sunrise. The solar flare on Saturday had little impact on me; it had more effect on those further west aiming at Europe and Japan through the auroral zone, and of course those in Scandinavia. I'm far enough east to have consistently good openings to Europe. That makes all the difference.

Run

Unlike with QRP it is possible to make most QSOs by running with 100 watts. True, it is nowhere near as impressive as what a kilowatt and big antennas will accomplish, but still a very effective way to run up the score. It helps that in Canada we can operate SSB below 21.200 MHz, away from the wall of US super-stations. You could hear me and other Canadians lined up from 21.2 downward running Europe for hours on end. Some US stations called me down there, which I could not work both for being out-of-band and worth no points.

This strategy does not work for other areas of the world, even where they can operate SSB lower in the band. But other than Japan the possibilities of runs of stations outside Europe are slim. I did some limited running of JA in the early evenings, and I'd have done more if not for a new local QRN source that peaks to the northwest.

Another advantage of running is multipliers. While working Europe I had a number of callers from Africa, the Middle East and Central Asia. These are multipliers I otherwise never heard. I ended up with 97 countries, just shy of a weekend DXCC on 15 meters. About 10 of those are uniques that I would not have worked but for running. That's what 13 db buys you.

Diversity

With just the one small yagi I am at a disadvantage when trying to work stations off the side or back. It costs time to rotate the yagi to work one or two Caribbean or South American stations when the bulk of QSOs are towards Europe.

As in previous contests I have some diversity in the form of an inverted vee hanging off my house-bracketed tower. The 40 meters element of the cage inverted vee has a somewhat complex azimuth pattern on its 3rd harmonic but does effectively fill the holes in the yagi's pattern. The strategy is to switch to the inverted vee to work those odds and ends that are not always worth rotating the yagi.

With QRP this often did not work out, and I had to rotate the yagi if I wanted the points. Add 13 db and I could make the needed QSOs on the inverted vee. In fact I could often work them when they were directly off the back of the yagi since the F/B is not very high. As I've stated before, this is reason to question the need for high F/B for contests. A notable exception is stations with multiple yagis per band which do benefit from high F/B, since unlike single-yagi stations they can simultaneously achieve diversity and QRM rejection.

Rotation

Concentrating on one band means that you will at times exhaust the pool of available stations to work. If you have a kilowatt and a big antenna you can keep running, seemingly without end when the band is open to Europe by attracting (and hearing!) more of the majority who have small stations. That doesn't work for me, even with 100 watts.

For a all-band effort, as I always have done with QRP, it is best to make frequent band changes. This allows time for a rotation of stations to occur. When, for example, you return to 15 meters there will be a number of new stations to work. Even when the strategy is to run this focus on rotation can work well.

Rotation works since most stations are either single-operator or casual participants, and they can only be on one band at a time. Although this is increasingly less true with the emergence of SO2R entrants (single-operator, two radios), it is not enough to make much a noticable difference. By spending time away from a particular band, or even from the shack entirely, a different bunch of single-ops will fill the band as they QSY from band to band. The available pool of casual operators who will answer your CQ also rotates. You will benefit from this rotation whether you S & P or run.

In my case -- single band and intermittent operation -- my frequent breaks made rotation work for me. If I'd operated full time my rate would have suffered unless I were to operate all  bands. An extra 13 db can do wonders, but not perform miracles.

Working QRP stations

My small antenna and 100 watts was enough to attract quite a few QRP stations while I was running. They are easy to identify in the ARRL DX contests since, for non-VE/W stations, power is part of the exchange.

Some of the 5-watters from Europe, even far-eastern Europe, had good signals that were copyable through the QRM and QRN. Some were a struggle to pull through, but then so were many 100-watt signals. So while SSB QRP is a challenge it should not be dismissed as not worth the effort involved. Being on the other end I was only too happy to pull them through and earn the points. Hearing them also made me smile since I know what it's like to be in their shoes.

It also pays to listen closely to the weak ones as you S & P across the bands. I worked one multiplier this way, a VP5, who was running 5 watts. He was CQing to little effect. I heard no other VP5 that weekend, so it's a good thing I was paying attention to every weak signal I ran across.

On an amusing note someone mused on the N1MM Logger group after the contest about how to log the station who gave his power as 500 milliwatts. Apart from the logging challenge it goes to show just how far QRP (or QRPp) can go, even on SSB. The lowest power station I logged was running 3 watts.

The exclamation point

Later Sunday evening while exporting my contest log and reporting my results I tuned around 20 meters. I ran across the fierce North American pile-up on E30FB, Eritrea. Of course I jumped in. I was still running 100 watts. Mix pile-up tactics and a heaping load of good luck and I got through in only 5 minutes. That was the exclamation point on my +13 db contest effort.

Next up

I have no immediate contesting plan except, perhaps, CQ WPX SSB later this month. So after 3 straight posts about contesting I am likely to return to antennas in my next article.

I know from web site statistics that antenna articles are by far the most popular, especially those that are about a particular antenna and not on theory or other general aspects of the topic. Since antenna articles take some time I can't produce them at the rate of one per week. So you will keep on seeing many articles that are about other topics that interest me, such as the one you are now reading.

I intend to cover some more general points about vertical antennas that I touched one previously. This will set the ground work for specific antenna designs.

CW Skimmers: The QRP Contester's Friend

Having been a QRP contester for almost 2 years now I have concluded that this is a really good time in the hobby's history to be doing this. The reason is technology. We often tend to think of recent technological progress assisting the more serious big-gun contesters (networked logging, global spotting, remotes, antenna design, etc.). Of course the technology also helps average and even smaller stations. What I want to argue here is that QRP contesting may be getting a better-than-average return on modern technology.

The technology I want to look at is not stuff like transceivers and station software since both big and little guns often use the same stuff. It's networking where I believe the benefit lies.

First we need to understand just how challenging QRP can be in a contest environment. My antennas are typical of suburban hams -- tri-bander & wires -- so we're equal in that. But when I run 5 watts I am -13 db weaker than the more typical 100 watts (barefoot) station. This grows up to -23 to -25 db below full legal limit stations. That's a lot when you consider, as hams, we'll endlessly argue about or waver on spending money over 0.5 db of transmission line loss or 1 db more yagi gain.

Listen to me

Let me give you a concrete example of how weak my QRP signal sounds at the other end of the QSO. 3V8SS in Tunisia recorded all his contacts in the recent ARRL DX CW contest. I placed well among the claimed scores in the QRP category of that contest. Go to 3V8SS's contest page and click on the link (upper left) to bring up a search box where you can enter my call (VE3VN) or any other call. Then select the found QSO (by band) to listen to 1 minute of streaming audio containing the selected QSO.

When you finish listening (and stop laughing) you'll understand what I'm up against. When calling other stations I am usually unable to get through when someone else simultaneously calls the other station. Even then I frequently need to repeat my call or exchange. Calling CQ is a particular challenge since on a busy band my signal is an easy one to overlook, and that's on CW with narrow bandwidth filters.

This is where technology comes to the rescue.

Spotting

Spotting only helps if you're sitting on a frequency and calling CQ (running). Someone who hears or works you spots your call and frequency via one of the hundreds of gateways to the global spotting network. Other see the spot and, if they haven't yet worked you, QSY to your frequency and call. Both general logging and contest software make this as simple as a single click. The software may even highlight whether the station is needed and a new multiplier.

I was only spotted once during the contest, which is a little disappointing. Yet I was still quickly besieged with callers many of the times I started calling CQ on a new frequency. This brings us to another bit of technology that brought callers to my frequency.

CW skimmers and the Reverse Beacon Network (RBN)

Spotting networks only work if someone somewhere makes a conscious decision to spot you (note: never self-spot, and it can even get you disqualified from a contest). CW skimmers take that element of uncertainty out of the picture. There are a many stations around the globe running CW Skimmer by VE3NEA, or similar software, on a spare receiver. They may or may not be located at stations active in the contest.

Unlike spotting networks skimmers automatically scan the bands and report on activity. Primarily this is stations calling CQ or otherwise holding a frequency and inviting callers. While a standalone skimmer node has some value to its operator it becomes far more powerful when combined with other skimmers. This is where the RBN comes in.

Since the linked sites on CW Skimmer and RBN describe these technologies in detail I will skip their descriptions here and jump directly to showing why skimming and RBN are so useful to the QRP contester.

Let's do this with an example. The adjacent picture is an image of the RBN search on my call in the final hours of the ARRL DX CW contest last weekend.

As you can see I am getting a multitude of reports of my running attempts, often many every minute. It should be obvious that this is far more productive than relying on other hams to spot me. Most are not motivated to do so in my case since VE3 is hardly an attractive catch for anyone. Besides, most human operators will pay little attention to a signal as weak as mine. Not so the software.

You can see where I spent the final 2 minutes of the contest by calling CQ on 40 meters. It was a way to spend the time since I knew I had little chance of finding someone new in the time remaining. I was quickly answered by 9A8M, which turned out to be my final QSO of the contest. As I ran down the clock the skimmer spots just kept coming on RBN.

Notice the posting of the SNR (signal-to-noise ratio) on each spot. As with the 3V8SS recording referenced earlier you get an idea of how weak I am at many stations. I have few skimmer spots on 40 from the west coast or Europe. That absence of spots most likely indicates that I was below the noise or covered by QRM. This is useful feedback. On 20 and higher bands I fared better with my CQs, as you can glean from the earlier RBN spots.

Call CQ

Even if you're running QRP or small antennas you must spend time calling CQ in a contest if you are to build up your score. Many little guns or casual participants only call other stations, so if you never call CQ you won't work them. That costs you points. In the ARRL DX CW I made a point of calling CQ as often as possible in the final 12 hours of the contest since by then even the big guns are prowling the bands searching for contacts.

With a small number of human-operator spots the existence of skimmers and the RBN is a boon to little guns. Although many S & P operators may pass you by because you are so weak the multi-operator stations and those entering in "assisted" categories will pay you a visit when you appear on RBN. Thus you get a bigger boost from RBN than the big guns, who are stronger and more often spotted or called by the S & P crowd.

So call CQ and let the technology out there help to boost your contest score. Well, at least in CW contests. SSB skimmers will take a little longer to come along.