Monday, November 24, 2014

Possibilities of Moxon Yagis

I bought and read the book HF Antennas for All Locations by Les Moxon, G6XN, over 30 years ago. I learned a lot from it but never put any of the ideas into practice. (Well, except for one time which I'll come to at the end of this article.) I must admit that I did not fully appreciate what I did read because my knowledge of antenna fundamentals was relatively poor at the time. With my interest in getting gain on 40 meters from a simple antenna I decided to have another look.

I was especially taken by the steps taken by others to exploit his work to design rotatable and fixed 2-element yagis on 40 that claim to perform favourably in comparison to other 2-element designs. Not everyone agrees. All of this was enough to grab my interest now that the weather has turned wintery and I am mostly restricted to planning for the future.

Some computer modelling is called for to investigate the matter further. There is enough conflicting information to be found in publications, including on the internet, that it becomes impossible to believe everything I read. This is not about incompetence or exaggeration on the part of others (many of whom are more skilled at this than I am). It is just that suitably comparably modelling is often lacking, or at least are not explained well. We are after all amateurs! A little bit of personal investigation can help to resolve the difficulty and set my mind at ease, and perhaps point the way to other antenna designs that are attractive.

The basic idea

The basic idea behind Moxon's strategy to improve yagi design is founded on the following sequence of ideas, which is my paraphrasing of what he has written:
  • A yagi is a subtractive array that primarily achieves gain by cancellation of fields in unwanted directions. Since conservation of energy applies field reduction is one direction must show up as gain in another direction.
  • Maximum cancellation, and therefore the best prospects (not a guarantee) for gain, requires equal amplitude and opposite phase. Although this is impossible to achieve over a broad range of directions with a 2-element yagi (in fact, only in one or a few points) it can be improved. Conventional yagis with parallel elements cannot achieve equal currents with element configurations that perform well in other respects.
  • If the mutual coupling can be increased without reducing element spacing it should be possible to get improved results. Therefore turn the ends of the elements towards each other to increase coupling. Then tune the antenna for best results.
Before jumping into quantitative measures the above set of EZNEC current plots for an ordinary 2-element yagi (full-size wire elements) and a Moxon rectangle (adapted from a model by DF9CY) get across the basic idea. The current in the driven element is higher than the current in parasitic element (a reflector in the present case). The currents on both yagi types vary with frequency, but they vary less and remain more equal for the Moxon.

Current equalization and less variation with frequency means that the Moxon is able to improve F/B bandwidth. As we have seen before a conventional 2-element yagi does develop effective gain and F/B figures, though typically only over a narrow frequency range in comparison to yagis with 3 or more elements. There are good analytical discussions of yagis designs by W2PV (to pick a venerable example) and others, which I do not need to get into.

Conventional commercial yagis must be built to handle higher driven element current, or to advertise lower power rating. For example, the 15 meters traps on Hy-Gain tri-band yagis are different on the driven element. The higher antenna (and circulating) current is dealt with by winding that one trap coil with copper rather than the aluminum used in all other traps (same wire gauge). Otherwise there is a risk of excess heating at legal limit power.

Comparison of the Moxon and standard yagis

We'll use the same EZNEC models shown above. Both are constructed from 12 AWG copper wire, which results in a loss of about -0.3 db in both cases. This is negligible in the majority of applications. Rotatable yagis made from aluminum tubing have lower loss.

The models are in free space to remove the small skewing of results due to the presence of ground. Since the effect of ground is nearly identical for these antennas this is a good approach.

The difference in F/B is the obvious feature of the performance comparison. It is much higher across the band. Although higher peak F/B can be had with a 2-element yagi (different boom length) that does not alter the results by much.

Gain is less favourable for the Moxon which is 0.5 to 0.6 db lower than the yagi. Plotted current is that of the reflector element referenced to a source current of 10 A.

The Moxon is a Moxon rectangle, which is sensitive with respect to a few parameters:
  • Element spacing, along the "boom"
  • Element tip separation
  • Ratio of centre segment to tip segments
  • Wire (or tubing) diameter
The yagi has fewer parameters, with performance mostly related to element separation and wire (tubing) diameter. The Moxon model has element spacing of 0.13λ, centre to tip ratio of 5.4 and 36 cm tip separation. The yagi has an element separation of 0.14λ. Other parameter choices will change antenna performance. Even so the key distinctions would remain. However do keep in mind that the element tip separation in a Moxon is critical, such that a small change can have a larger than expected effect on performance.

As mentioned earlier, both use 12 AWG wire, insulated wire in the case of the yagi. If made from tubing the SWR bandwidth would increase, although that is only of advantage to the narrow-band yagi.

SWR (not shown in the plot) favours the Moxon. The Moxon achieves an SWR below 2 across the band with a direct feed with 50 Ω coax. The yagi requires a matching network (beta, λ/4 transformer, 2:1 balun, etc.) and does not match as well across 40 meters.

Tuning for Gain and F/B

In both antennas the frequency of maximum gain is close to that of maximum reflector current. Indeed the shape of the current and gain curves are similar across the band. I took the plot down to 6.9 MHz to where the current reached a maximum for the yagi. Both antennas can be tuned to shift performance higher in the band. SSB operators may prefer to make the parasite a director in order to reverse the performance curves.

Before doing the modelling I guessed that the F/B would peak where the currents peak. This is not the case. I was perhaps misled by focussing on the precise reverse direction rather than the total field outside the main lobe.

The patterns are plotted at the same frequency (in this case, where the F/B is highest on the Moxon) so that the gains are fairly compared; the yagi has maximum F/B 40 kHz higher than the Moxon. You can see from whence the yagi gets its gain with respect to the Moxon rectangle. Although the radiation off the forward lobe of the Moxon is much less than the yagi its forward lobe is wider and shallower. There's no free lunch when it comes to antennas.

Gain in the Moxon is somewhat reduced by the length of the tips, and F/B in the yagi is reduced by moderate element coupling. Although the amount of current in the Moxon element tips is low it is still high enough to account for much, if not all of the gain difference.

The intensity of the radiation of an antenna is in rough proportion to the product of element length and average current over that length. Moxon himself points this out, and finds that the trade-off is beneficial. I suspect he is largely correct with respect to what most hams want. On 40 meters especially since most who do have a 2-element yagi choose one with loaded elements and so are therefore prepared to give up the small gain difference possible with full-length elements.

Where F/B and SWR are important to an individual case the advantage becomes greater on the low bands, where the width of the band is high in relation to frequency. There is less advantage on 20 meters and above where mostly-good SWR performance can be achieved across the band with a 2-element yagi. Then it's a matter of F/B. Even there it may come down to choosing a 3-element yagi which has better SWR, gain and F/B performance. Going with a larger yagi is less challenging on the high bands.

Considering all of the above, which antenna's performance do you prefer? There is no right answer, just a matter of personal preference. My own priority is gain, with match and F/B of lesser importance if I cannot get all three. However, I suspect most hams would rather sacrifice 0.5 db of gain to get a broadband match and F/B.

High mutual coupling in other antennas

It is not only the Moxon rectangle that exhibits improved current balance and therefore improved match and F/B. We've seen examples before in this blog.
  • Spiderbeam: Although the elements are vee-shaped there is substantial element coupling from bringing the parasite tips close to the driven element. Notice that the F/B and SWR bandwidth are excellent and the gain is less than a full-size or trap yagi. Gain can be improved with a trade-off in the match. The manufacturer correctly notes that performance will suffer if measurements are not closely followed since, as already mention, element tip separation is critical to performance.
  • 2-element diamond vee: When I built a version of this antenna over 25 years ago (late 1980s) I had Moxon element coupling in mind. At the time I had read somewhere that this design (no unlike the more recent Spiderbeam) can work very well. The antenna did work well although I had no means to measure gain or even to do a proper comparison against a reference. What I do remember well was the excellent F/B when I switched between northeast and southwest directions. I now know that the tip separation was probably not optimally chosen. Both in free space and over ground the modelled diamond vee yagi has less gain than the two antennas discussed in this article. The diamond vee antenna in any case will do worse than a fully-horizontal antenna over real ground.
The W6NL 2-element rotatable 40 meters yagi (either in its pure form or as a modified XM-240) has some advantages. I've made my own EZNEC model of the this antenna to better understand it. You can also check out W8WWV's model analysis if you are interested. I will probably say more about this one in future. It is certainly distinctive in its use of oversized capacity hats to achieve high element coupling.

Where am I going with this?

I'm not sure. Although there are advantages with respect to F/B and SWR bandwidth I have yet to discover a model that achieves better gain than a more conventional 2-element yagi, both in maximum gain and gain bandwidth. That reduces my level of interest, even if many others would disagree. Perhaps I am being overly focussed on losing 0.5 db of gain. That is at least true for 2-element designs; 3 or more elements present additional opportunities for investigation.

Another difficulty is making a version of the Moxon rectangle reversible. This is worth some attention. It might be a superior alternative to one of the 2-element switchable wire yagis I designed in 2013. I see some possibilities to be explored.

So I'll do some modelling of alternative designs and share the results. I think it's a good idea to further my understanding of these antennas and not prematurely dismiss them. I have the luxury of time since none of these antennas will go up at my station in the near future.

Monday, November 17, 2014

Antenna Season Wrap-up

We've had our first snowstorm of the winter. It's more a symbolic event since the snowfall amount is small. Real storms are sure to follow over the coming weeks.

I rushed to finish work on the tower this weekend in advance of the foul weather. This included removing the 40 meters sloper, re-establishing the 80 meters half sloper, cable dressing and other weather-proofing. I have now pretty much brought to a close the 2014 antenna season at VE3VN.

Looking back it's almost surprising how much I got done this year. I know it cost me a lot of time even though it doesn't feel that way. Highlights follow. You can compare these to the plans I made at the beginning of the year.
  • Removal of the house-bracketed antenna mast, multi-band inverted vee, 30' tower, multi-band dipole and 40 meters delta loop
  • Bracket the 30' tower to the house and installed a new antenna mast to 14 meters height
  • Purchase, prepare and install the DMX-52 guyed tower
  • Reconfigure the multi-band inverted vee to include 40 meters, make mechanical improvements and raise it on the house-bracketed tower and mast
  • Refurbish a Ham-M rotator and mast bearing, and install them on the DMX-52
  • Purchase and install an Explorer 14 tri-band yagi at 15 meters height
  • Design, build and install a loaded half sloper for 80 meters
  • Design, build, install and ultimately remove a loaded sloper for 40 meters
While the contesters are away the mice will play

As a contester I suppose I ought to have participated in Sweepstakes SSB this past weekend. I opted not to since SSB QRP is pretty tough, despite how good my results were in CQ WW two weeks earlier. I did however scan the bands to be sure my section (ONE) was well represented. I could see that I wasn't needed.

While the contesters were busy there was some DX to be had for the taking on CW. That is, until the high geomagnetic activity ruined propagation. I was able to quickly work 3B9HA on 40 CW since the pile-up was so small. This again demonstrates that QRP and a bit of wire can do wonders, if everyone else stays away! After the storms hit I found VU4KV on 20 meters CW, but they were far too weak for me to work. It was interesting that their signal peaked to the northwest rather than the direct route just east of north. This is an excellent example of skew path propagation, going around rather than through the auroral zone.

Winter antenna work

Doing antenna and tower work during our cold winter is possible if unpleasant. It's something you get used to when you live in this climate. When I was young and living in an even colder climate (VE4) it was enough that on a Saturday or Sunday the temperature rose above -10° C, was sunny and not windy. That type of winter day was rare, so we made use of them.

Winter tower climbing was rarely used for new construction. More often it was to perform repairs in advance of contests. Careful planning is essential when doing this type of work so that the absolute minimum amount of time is spent outside and on the tower.

Once your hands or feet start going numb you have been up there too long. Some dexterity in the extremities is needed to get down safely. It isn't always possible to dress as warmly as you'd like since all that movement-impairing clothes is a safety hazard. It's easier now with lightweight and thin synthetics. Then there's the detail work that requires bare hands.

We usually did the work in stages, warming up at intervals. It helped that our towers back then were rarely over 15 meters height, allowing for rapid ascents and descents. Once in Ottawa I found that winter tower work became easier due to the warmer temperatures. It was just necessary to get used to doing with dull, overcast skies. I've even done work as high as 100' while it was snowing. With snow the biggest problems are getting wet and cold from snow melt and slippery footing. Towers with horizontal cross-braces are much safer than those with diagonal braces (DMX and Trylon).

Perhaps the biggest problem with winter tower work is wind. If you live in a warm climate you may be unfamiliar how deadly even a moderate breeze can be when the temperate dips below -10° C. The 20 kph breeze I had to deal with this weekend quickly stole away heat during the 10 or 15 minutes I was atop my tower this weekend despite being well dressed and the temperate at a relatively balmy 0° C. It was also snowing, making the tower a bit slippery. I adjusted by using a slower and safer climbing technique.

The blog in winter

Although I will not be building new antennas over the winter the blog will not be quiet. There is operating, equipment and, very importantly, software modelling of new antennas and old. There is a lot on my mind that I want to work through, the interesting bits of which I'll share.

There will be some focus on bigger antennas and low-band antennas that may play a part in the coming years. Forward planning is always a good idea, whether or not I act on those plans. At the least I can offer useful ideas to those of you who follow along.

Friday, November 14, 2014

40 Meters Loaded Sloper

Earlier this week I had the opportunity to exploit unseasonably warm weather to climb the tower (several times) and install the newly-constructed loaded half sloper for 40 meters. My intention is an antenna that will outperform the 40 meters inverted vee to Europe, my most productive DX and contest points path. This article will cover antenna modelling, construction and on-the-air performance.

Here it is fully built, ready for installation on the 14 meters tall tower:

Starting from the upper left and proceeding clockwise:
  • 3.02 meters wire (measured to centre of coil)
  • 11 μH coil
  • 3.75 meters wire (measured centre of coil to centre of feed point insulator
  • 1.5 meters of  ½" Schedule 40 PVC pipe, with 2 U-bolts
  • Centre insulator with SO-239 feed point
  • 6.7 meters wire (measured from center of dog bone insulator), plus 30 cm extra for trimming
  • Egg insulator bottom termination
This antenna is intentionally asymmetric. Symmetry is overrated. This particular arrangement gave the lowest ground loss in the EZNEC model, and required winding only one coil instead of two.

Another change I made was to place the sloper and tower in the same plane, so that the sloper runs directly away from the tower towards the northeast. This gave the best forward gain (no surprise). In the original model I offset the sloper so that it started 2 meters to the tower's side and ran parallel to the 80 meters half sloper. The new arrangement reduces variables for this experimental antenna. The 80 meters half sloper wire was pulled out of the way, at least temporarily.

Symmetry is in any case a mirage since the antenna halves interact unequally with the tower and ground. A common mode choke tuned for 40 meters is made by coiling 12 turns of RG-213 (air core, 7" diameter) on the transmission line, positioned about 5 meters from the feed point. This arrangement avoids feed line radiation while allowing the weight of the coil to be supported by the tower rather than the antenna.

The coil is wound on a 5" length of 2" O.D. plastic pipe cut from a remnant left over from the installation of a built-in vacuum cleaner. It's a bit soft but strong enough for the job. Four holes are drilled along a line, 1" and ⅜" from both ends. The coil length is 3", the distance between the inner holes. The insulated 12 AWG stranded wire I used is not ideal for the application but it was handy and the turns spacing was a perfect fit. By threading through the holes as shown ensures that the wire is mechanically stable. A bit of plastic wrapping tape keeps the outer turns of the coil from spreading outward.

Coil ESR (equivalent series resistance) can be somewhat high and still achieve negligible loss. This is generally true of loading coils on a single element antenna. More care in coil design and construction is recommended for traps and for loading coils in yagis. As built the coil can probably handle a kilowatt without excessive heating. However this cannot be guaranteed without testing.

The feed point is bit odd looking. Two tie wraps secure the wire ends, which are soldered to the SO-239 centre pin and solder lug. The lug is secured with a stainless steel screw and nut. The synthetic twine and third tie wrap hold the connector to the insulator so that the weight of the coax doesn't stress the soldered connections.

At right you can see the PVC pipe angled upward and secured to the uppermost X-brace on the tower. This places the far end of the pipe at about tower height (14.1 meters) and 1 meter distance when tension is applied to the antenna and simple rope stay.

The 80 meters half sloper was released from its bottom anchor and left dangling vertically for this experiment. The model shows some negative interaction so I wanted to remove that influence for the present. If you're interested this photo of the half sloper feed point complements the one shown in the 80 meters half sloper article.

For now the ground anchor for the 80 half sloper is used for the 40 meters sloper. I've included a photo of the anchor here since I only showed the temporary anchor in the 80 meters half sloper article. The stake is 42" long, with 12" buried adjacent to the retaining wall and then nailed for lateral stability. A screw at the top rear acts as a retainer for the nylon rope holding the bottom of the antenna. It works remarkably well considering its simplicity, able to take a lot of tension.

Modelled performance

As stated above and in earlier articles my objective is a 40 meters antenna that does better toward Europe (northeast direction) than the inverted vee. It is forward gain I want, with rejection of signals from other directions not a priority of the design. However some rejection (F/B, etc.) comes along for free since if you add gain in one direction it must come from other directions -- conservation of energy.

From the EZNEC antenna and current view of the sloper at the start of this article you can see that the sloper has a deliberately selected separation from the tower at the top. The induced current on the tower shows that it acts as a parasitic element, a weak reflector element which is not specifically tuned for the purpose. The modelled gain due to the tower is only ~0.7 db. The 80 meters half sloper is rotated 90° in the model since there are interactions that the model suggests would result in up to -1 db loss of forward gain on 40 meters.

The diagram to the above left is extracted from an ARRL document that plots measured elevation angles for various paths and frequencies. It is data like these that I relied on to establish 10° as the standard of comparison between antennas designs for DX paths on 40 meters.

The gain of the sloper only exceeds that of the inverted vee below 19° elevation. At 10° elevation the difference is 2.2 db in favour of the sloper. The sloper should reject (have less gain) than the inverted at most other elevation angles and directions. This reduces QRM from North America, which can be helpful at times. The modelled ground loss of the sloper is -4 db for medium ground. The inverted vee has almost negligible modelled ground loss.

The feed point impedance is a little high for 50 Ω coax but good enough for my purposes. It turns out that the measured SWR of the installed antenna has a similar curve but is acutely sensitive to the bottom end's distance above ground. Before trimming the SWR dipped to 1.4 at resonance, then rose to a minimum of 1.9 when resonant at the bottom end of the band. The KX3 seems happy with this.

Measured performance

The antenna was hooked up just before sunset, which allowed for immediate feedback on actual antenna performance. It was with some anticipation that I hooked up the coax, measured the SWR and trimmed to antenna to resonance. Then I listened to the European station that were just beginning to roll in. I switched back and forth between the inverted vee and sloper, comparing receive levels (including SNR) for various DX and domestic signals.

Let me be blunt: I was disappointed. The F/B was as expected but not the forward gain. Europeans were consistently stronger on the inverted vee than on the sloper. I stepped away from the shack to wait a couple of hours. It is often the case that elevation angles at sunset can be higher because absorption in the D layer of the ionosphere continues for a while due to the sun still shining at those altitudes.

After waiting the results were unchanged. Even on the longer paths in the same direction (4X, 4L, etc.) I saw the same thing. The inverted vee was consistently ~1 S-unit better. The atmospheric noise level was nearly the same on both antennas, suggesting that something other than higher ground loss on the vertically-polarized antenna. What was going on?

Let's look at what I actually measured. Keep in mind that I did not go for precision in these measurements, which would have required some sophistication in test equipment or hooking up the I-Q output to a PC and writing some DSP software. I know how to do that but I don't believe it's worth the time investment.
  • System loss: Two major and easily modelled losses are near-field ground loss and transmission line loss. With medium ground the modelled ground loss is -4 db. Transmission line loss of 150' of RG-213/U at 7 MHz and a 1.9 SWR measured at the source is -0.9 db. Total system loss is therefore -4.9 db. This compares to -0.8 db for the inverted vee. The relative system loss of the sloper is therefore -4 db. This will affect both noise and signal, but not SNR since the atmospheric noise at 7 MHz far exceeds receiver noise. Coil loss (ESR) is not a factor, which was confirmed by elevating it to an unnatural level in the model.
  • SNR: I picked a clear frequency (mostly near 6.990 MHz) and set the receiver bandwidth to give a noise level of about S-9. Tests were only done when there was no man-made QRN (typically lighting system power supplies), and were done at a variety of times over two days. Atmospheric QRN here in November is low and there is little chance of electrically-active storm systems that would favour one antenna due to directionality or polarity. The averaged comparison has the inverted vee at just under 1 S-unit more efficient than the sloper.
  • F/B: The modelled F/B of the sloper is in the range of 10 to 15 db. I tested this with many stations on the air and estimated F/B using only the S-meter, with signals near the standard calibration point of S-9. It varied by station (direction and path angle) but was borne out overall. This confirms the model in that the tower is acting as a weak reflector parasitic element.
  • Gain: Conditions to Europe on 40 have been good much of this week, providing many test signals. To Europe, west Asia and the Middle East (all within the antenna's main lobe) the sloper did worse than the inverted vee on every station. The difference ranged from 1 to 2 S-units. Since the noise level (see above) typically declined by less than 1 S-unit on the sloper this resulted in poorer SNR for the majority of signals on the sloper. Measuring relative levels was complicated by Faraday rotation since the period of rotation can be slow at 7 MHz and the antennas are of opposite polarity. When one antenna reaches peak amplitude on a particular signal the other antenna is usually at minimum amplitude.
Analysis and conclusions

Was this experiment a failure? No! The antenna did not meet expectations but it did so in an interesting and instructive manner. I now know something I didn't know before. That knowledge is valuable. Too few hams have, or take, the opportunity to compare antenna performance. Yet antenna performance is the major factor between doing well or poorly, whatever your operating pursuit, for the typical suburban amateur with a small station or with QRP. High power obscures antenna problems at many stations.

There is also the psychological factor, where many hams convince themselves that the antenna they built or bought must be doing well since to consider the alternative calls into question their decisions or abilities. Better not to compare and remain blissfully ignorant of the truth. I think this explains why so many hams claim to be happy with their low-performance multi-band commercial "no radials" verticals.

I prefer to know the truth even if the truth hurts. So I experiment, measure, assess and learn to do better. Every experiment is a successful experiment to my way of thinking. None of which says that this sloper doesn't work, only that it doesn't work as well as I'd like. I think that's important to know and, if possible, to know why.

Now it's time to speculate on what might be going on and what that might mean.
  • Ground loss: Let's assume ground loss is worse than the stated -4 db. If we take the worst soil characteristics that EZNEC has among its options the loss worsens to -7.5 db. Since loss affects both signal and noise the SNR would not change and the noise measurements I made (see above) contradict this possibility. This does not appear to be the cause of the problem.
  • Radiation angles: The European paths at the time of my measurements could have been higher than 20°, thus favouring the inverted vee. The lowest MUF on the Europe path during my measurements seemed to fall between 11 and 14 MHz (full nighttime path); solar flux is high which keeps the MUF high. Elevation angles for the optimum path decline as the MUF falls closer to the operating frequency. But the measured results would require angles of at least 25° if this were true, and the empirical data (see diagram above) indicates that this is unlikely. Further, the difference between antennas did not change as the MUF changed during the listening hours.
  • Low angle disruption - the Suburbs Hypothesis: The near field of vertically-polarized antennas interacts more strongly with local ground than horizontally-polarized antennas. It get worse as the average height of antenna current declines. The sloper's average current height is ~8 meters versus ~12 meters for the inverted vee, and the antenna comes as close as 1 meter to the ground. But all of this is accounted for in the model if real ground behaves something like the idealized flat ground in the computer model. In a suburban (and urban) environment this is not true. Metal (flashing, eaves, wiring, utilities, clothes lines, etc.) within 1λ are in the near field and metal further away will affect the far field. Any near-resonant or large conductor (or lossy dielectric material) has the opportunity to "steal" and dissipate energy from, or diffract or deflect the radiation directed downward at low angles, the pure reflection of which is crucial for establishing the low-angle far field radiation pattern. How serious this effect can be is not clear to me, nor how it differentially impacts polarization or average current height.
  • Delta loop and half sloper: It's impossible to look at the present results and not wonder about my previous vertically-polarized delta loop for 40 and the newly-installed loaded half sloper for 80. I have already pointed out the possibility of higher ground loss than modelled for the 80 meters antenna. Might this also have been true of the delta loop? Unfortunately I can't do a proper comparison test now since the tower is resonant on 40 and the guy wires make installation difficult. I wish the 40 meters inverted vee had been up at the same time as the delta loop.
  • Distrust modelled gain comparisons: If the suburbs hypothesis described above is true you should distrust the low-elevation angle gain figures for all the vertically-polarized antennas I surveyed for 40 meters earlier in 2014. If you lop a few db off those figures it is the horizontally-polarized antennas that shine more often. Unfortunately there may be no reliable way predict the actual performance in every station (unless you live on a flat rural acreage), except by building two antennas and comparing them.
Assuming that my negative assessment of vertically-polarized antennas for 40 meters in a suburban area are poor DX performers is true then my best bet for gain would be a wire yagi. That is not an option at present since it would destructively affect the performance of the tri-bander at the top of the tower. For now I will have to make do with the inverted vee. Although it's a fine DX antenna some gain would help my QRP signal.

What I will do now is take down the sloper, coil up the coax and put them into storage. The 2-element sloper is also out of consideration for this QTH since it is now obvious that it will not perform as modelled. The 80 meters half sloper will be restored to its former position.

There's no reason to disassemble the sloper after removal. Perhaps in future I'll have an opportunity to test the suburbs hypothesis, by installing the antenna in an open area. I'm still curious to discover what is going on with this antenna rather than relying on speculation.

Monday, November 10, 2014

One Gets Away

QRP DXing is difficult. Even though my DXCC results are quite good since I returned to the hobby in 2013 with a KX3 for a rig, it remains a challenge. That's a good thing. I always have to wonder (and sometimes I directly ask) why others feel a sense of accomplishment from working a rare one with QRO and a big (or not so big) antenna. All it takes is some dedication of time. You will get through.

There is nothing wrong with QRO DXing, and indeed I did so for many years and likely will again in the future. For me, at least, other than the low bands and 6 meters I got bored by it. I like to think there is more to DXing than country counting. When I do work a rare one with QRP I feel a sense of accomplishment that is missing from the DX exploits I achieved earlier in my ham career. Yet there are times when it can seem like a futile quest, that I am wasting my time in the pile-ups, getting few results.

If you're an active DXer you can probably guess where I'm heading with this narrative. No, I did not work the FT4TA (Tromelin) DXpedition. Although I did not expend a great deal of effort to work them I still did try for several hours spread over close to a week. I would have loved to work them, but I didn't and I'm okay with that. It's even possible that I did work Tromelin in years past, except that finding out would require digging through old paper records, a task for which I feel no motivation. If I did work it back then it would not have been with QRP.

I have discussed in the past how I have worked some rare ones with my QRP station. Now I will say a few words regarding the most likely reasons I did not work FT4TA.

Duration of DXpedition

While not a brief operation the rareness of FR/T meant that even those who failed to get through were still in need of it for an all-time new one (ATNO). My usual approach of avoiding the massive pile-ups in the early part of a DXpedition didn't work this time since the pile-ups did lessen enough to allow my little signal to be heard.

I may need to wait for a subsequent Tromelin operation to find more amenable pile-ups.

Stations spread across modes and bands

There appeared to be fewer simultaneous stations on the air in this DXpedition than some others. When spread across many bands and modes (CW, SSB, RTTY) this left fewer opportunities for me. These included both openings and band-mode combinations.

I have no good capabilities on 80 and 160; I could hear them but not work them on 80. I stick with CW since SSB is far more difficult with QRP, where power outdoes tactical intelligence every time. I do not operate digital modes. CW is my preferred mode, with the added advantage that it offers the best prospects for a QRP operator.

As a result the number of favourable openings (good enough for them to hear me, and when I am able to operate) on CW on bands from 40 to 10 meters were insufficient to give me a good chance. The best opportunities came on 30 meters. Yet I struck out even there.

Pile-up procedures

The DXpedition operators spread the pile-up over a large spectrum on CW, often from 15 to 20 kHz wide. This was likely done to help them pull out individual signals from the large pile-ups they attracted. Such a wide range is detrimental to those in the pile-up, individually, even if it allows more calls to make it into the FT4TA log. The reason it is detrimental is that intelligent pile-up tactics are less effective in this environment.

While there was a chance to practice zig-and-zag techniques, they worked less often. The operators tended to skip further and with less-predictable steps across the wide listening range they established. Sometimes I guessed right but mostly I did not. The same seemed true of other callers I heard. There was also a frequent random factor when the operator would jump to a frequency far removed from the just-completed QSO. It was especially difficult to figure out where they were listening.

It seemed that many callers took the easy way out of picking a frequency and making all their calls from there. I could have done the same, and did for short periods, but this is more gambling than operating. The more time to call the more chances you get, so time spent calling matter. However this is similar to buying more lottery tickets increases your chance of winning, where the absolute probabilities remain low. At best this strategy favour QRO.

The strategy of picking a reasonably-quiet frequency in the listening range is problematic. You cannot hear everyone who is calling the DX so there is no way to know if your chosen frequency is otherwise quiet at the other end. On most of the higher bands there is likely a multitude of callers from the US northeast that cannot be heard from my location. They will have similar propagation to the west Indian Ocean, and they are not running QRP. What you can't hear can hurt your chances.

Although spending more time calling can increase one's chances of getting through, this is not always true. For example, if your signal is below the other station's QRM or QRN level the number of times you call is poorly correlated with your probability of getting through. In the case of a DXpedition pile-up you also have to compete against the other callers. With QRP you are unlikely to be heard through the din, no matter how long you keep at it.

Back to antennas

I have completed construction of a loaded 40 meters sloper along the lines described in an earlier article. It will favour Europe. Since it is not yet raised I will defer saying more about it until it is up, has seen some use, and compared to the inverted vee. That should be done within the next week. This may be my final antenna construction project for 2014 since the weather is turning decidedly colder. It has already snowed once and more will be arriving within the week.

The loaded 80 meters half sloper has now achieved its first DX. If the 40 meters sloper works out it is likely that I will redeploy this antenna to optimize my capabilities on 40, which is in any case is a far more productive a band for both DX and contests with QRP. The problem is the modelled interaction between the two half slopers. It's about compromises. I can't have it all in a station this small, so 80 meters may have to be the loser in this antenna dilemma.

Wednesday, November 5, 2014

CW Sweepstakes Experience

This was the first time that I'd operated in a Sweepstakes contest in many years. When I was newly licensed in the 1970s and got into contesting this one was the gold standard for me and my friends. DX contests were taken less seriously since we did so poorly with our small stations (even with towers and tri-banders) nestled so closely as we were to the auroral zone (Manitoba, VE4). It was only after moving to VE3 that I switched my allegiance to DX contests.

The Sweepstakes experience is very different between here and there. Not only is a VE4 prefix attractive in this contest it is also an easy shot on the high bands from there to the main US population centres of the northeast, southeast and the midwest. In this way I made the low-power top-ten back in 1978. It is not so easy from Ottawa. ONE (Ontario East) is not a rare section and high bands propagation skips over many major population centres. I did try the contest once or twice in the 1980s with the relatively good station I had back then. My interest quickly waned when results did not follow.

So it was with some interest and modest expectations that I tackled CW Sweepstakes this past weekend in the QRP category. It was tough going. My results are not especially good even compared against other QRP entries from this part of the continent. Better low-band antennas would have helped. Despite this I did have some fun. However, as noted by many, rates dragged by the time Sunday afternoon rolled around. I frequently stepped away from the radio to relieve the tedium of hunting for contacts (S & P) and long delays between responses to my CQ. In the end my QSO count was 570 (573 with dupes), and 80 out of 83 section multipliers.

But rather than bore you with my contesting antics I will speak to a few items from this weekend's experience that may be of wider interest.

80 meters -- driving-distance DX

I made 31 QSOs on 80 meters in this contest, which is 5.4% of total QSOs. That isn't much. Partly this was due to lower activity on 80 (you don't get to work stations once per band in this contest) and the challenges of QRP and my loaded half sloper antenna. Some strong stations had difficulty copying me well through the QRN and QRM, though I suspect mostly the former. I can hear others well enough. Even FT4TA (Tromelin) is copyable here.

As I like to say, all my 80 meters contacts were with stations within one-day's driving distance. This includes all of New England, Virginia and Ohio. Clearly it will take some effort to work DX with this setup. The real test will be CQ WW CW at the end of November. It is at least encouraging that I can work stations with some reliability.

The only change to the antenna before the contest was to replace the brick anchor with a long wooden stake anchored against the retaining wall, increase wire tension and trim about 40 cm from the end to make it fit. To my surprise these changes were all it took to raise resonance from 3.45 MHz to the desired 3.55 MHz. The arithmetic doesn't directly apply since that would indicate the need to shorten the antenna by at least twice that amount. This demonstrates how sensitive the half sloper is to ground proximity and angle between wire and tower.

40 meters -- the go-to band

40 meters was my best band, with 270 QSOs, or 47% of total QSOs. That's a lot. Most others in this part of the continent who put in a serious effort appear to have had a similar experience. This is unsurprising because the large population centres cannot be reliably hit from here on the high bands. Activity on 40 continues throughout the daylight hours during Sweepstakes, though the best times are early morning, late afternoon and the evening.

My only reservation about 40 is that it can be a difficult band for QRP. I needn't have worried. The QSOs kept coming, even to the west coast. I was able to establish lengthy runs, though usually not with high rates. This helped my score since calling CQ is the only way to work casual contesters who primarily do S & P.

N1MM logging software

The exchange in Sweepstakes is lengthy and complex. That is a challenge not only for the operator but also for the logging software. I had never looked at how N1MM deals with Sweepstakes so I wisely began reading the documentation and practicing with it days before the contest. I chose to use N1MM rather than N1MM+ to lessen the learning curve and chances for problems with beta software.

N1MM uses a single entry field for the entire exchange. This includes, in sequence: serial number, precedence (entry class), call sign, check (year first licensed) and section. The call sign is optional, and need only be entered if the call entered in the call sign field is incorrect. For example, those contacting me would type the exchange as 123Q 72ONE or 123Q VE3VN 72ONE.

The grouping of serial number + precedence and check + section is necessary to avoid ambiguity. That is, the software can reliably parse the entry if done as recommended. Get it wrong and the QSO won't be correctly logged or at all.

This can be a problem since it is easy for the fingers to fumble while tabbing and typing at contest speeds. Make a mistake and the entry screen freezes, demanding that the error(s) be corrected. If you override and log it anyway the software strips away all the unrecognized text. That can make it impossible to go back later to edit the contact.

That was probably the most irritating aspect of using N1MM in Sweepstakes. On occasion I had to delay the other operator while I made corrections or needed to request repeats due to entry problems. I noticed similar behaviour by others I contacted, indicating that I was not alone with these difficulties.

It is also necessary to program the function keys so that exchange can be properly sent. As the documentation emphasizes it is not possible to send the correct exchange by just programming a function key for the standard exchange. I hope that N1MM+ is better in this regard. Upgrading is on my list of new adventures before CQ WW CW.

Age statistics

It is not news that the average age of hams in Canada and the US is quite high. Sweepstakes provides a snapshot of the demographic challenge for the following two reasons: year of first license is part of the contest exchange, and contesters are among the most active of hams.

I stripped out the 'check' data from my Sweepstakes log and turned it into a graph. I grouped the checks into 5-year bins and plotted the number of QSOs per bin. There are many reasons why this is not a reliable statistical analysis, so take it as a more qualitative indication of where our favourite hobby is at.

I grouped the checks before 1950 into one bin. These are statistical outliers since half of those QSOs are with school club stations where the check is not that of the operator. Multi-op checks are typically those of the station owner, and not the individual operators, which obscures some important data. I didn't remove dupes, which is acceptable since there were only 3 out of 573 raw QSOs.

Unsurprisingly this chart is distressing. The large bulk of the contesters are "baby boomers", or their parents. In fact the largest bin -- 1955-1959 -- is that of the boomers parents. This assumes that most hams are first licensed when they are in their teens or early twenties, which is based on my recollection of that era.

When I was first licensed (1972), as I recollect, the distribution was similar to that shown above, but horizontally flipped. There was a long tail of checks down to ~1920 (I can still remember receiving a check of '19' from one old timer). There were of course no checks beyond 1979. Now we must conclude that newer hams are not entering contests or their total number is small. The situation in other countries can be quite different, such as in Eastern Europe and former USSR republic where there appear to be a higher proportion of younger hams in contests.

This bodes only ill for the future of contests, and of amateur radio in general in developed countries. Make of that what you will. Unless something changes Sweepstakes will become extinct by 2035.

Contest plans

After two consecutive weekends of contests I have had enough for awhile. With Sweepstakes so frustrating I see little reason to do the SSB weekend, especially with QRP. I will operate CQ WW CW and then pick and choose a few smaller outings until the ARRL DX contest and, perhaps, WPX.

Unlike serious contesters I burn out easily from too much of it. I didn't feel any desire to make QSOs for a few days post-SS. I needed the break even though there are DXpeditions that are drawing my attention right now -- FT/t and VU4. These can wait until the hordes depart and QRP has a fighting chance.