Wednesday, February 21, 2018

ARRL DX CW: Antenna Lessons

I had what I consider a successful operation in the ARRL DX CW contest this past weekend. From early reports my competitive position is good despite not likely to win my category of single operator, all band (SOAB LP). Since my station is new and incomplete and my experience with putting it to best effect is a work in progress it was an ideal opportunity to learn and adjust my station building plans. Lessons were learned, several of which I'll share in this article.

There were of course a series of problems which seem to crop up during contests and not at other times. A few of the important ones I'll briefly mention to get them out of the way; I don't want to dwell on them unnecessarily.
  • Prop pitch rotator: Sometime during the first night a fault occurred and I didn't realize it until I have overturned the rotator by 100°. Since the indicator pot decoupled back in January and it's too cold to climb I got into the habit of counting seconds to set the position. This is more difficult at night since confirmation of direction is more difficult. Luckily the problem was in the shack and I got it fixed Saturday morning, at the cost of 30 minutes of prime time operation.
  • Noise: That power line QRN that was cured a month ago? It came back Sunday. Obviously something out there is still amiss. I'll try to better localize the source before calling the utility to deal with it.
Now I'll move on the what I believe are the important lessons that I learned. I am restricting this article to antennas rather than contest operating techniques. I may get those in a subsequent article. Contest operating with big antennas is not the same as it is with small or modest antennas.

Europe on the high bands

For contests there is no question that for us in North America it is Europe that provides the bulk of the QSOs and multipliers. They are numerous, active in contests and not so far as to be difficult to work. A log analysis shows over 75% of my QSOs were Europeans. It should be obvious that Europe is a priority in my antenna plans.

As I previously noted the lower antenna (Explorer 14 at 34 meters, fixed toward Europe) always outperformed the higher yagi (TH6 at 43 meters) on 20 meters. This has more to do with height (elevation angle) than gain since the lower antenna has lower gain on 20. For the same reason the higher antenna does better to Europe on 15 meters. I am now revising my opinion.

The optimum elevation angle for a particular path varies widely depending on MUF (solar flux, time of day and time of year), absorption at D and E layers (geomagnetic storms, etc.) and other factors. This weekend, especially on Sunday morning, the antennas were more equally matched to Europe on 20 meters. Indeed when I tried the high yagi the run rate increased; this did not happen on Saturday morning. While insufficient as proof it is very suggestive the greater height does have value on 20 meters towards Europe even when the antennas seem to be the same on receive.

A few decibels difference is often not obvious when receiving due to the heavy QSB typical at HF. However on the other end it can draw in more callers.

As expected the high antenna always outperformed on 15 meters to Europe for the limited openings we had during the contest. There was no 10 meter opening other than working CR3W and hearing but not working CU4DX. Unsurprisingly they were favoured by the higher yagi.


On 20 and 15 meters the TH7 at 21 meters always outperformed higher yagis to the Caribbean and Central America. Often the difference was 2 to 3 S-units. This weekend the TH6 at 43 meters was always better on 10 meters, no doubt because the MUF barely reached 28 MHz on this short DX path.

Although there are relatively few stations active from these areas it is important in contests to quickly bust the inevitable pile ups and move on to make other QSOs. I quickly learned this lesson and would keep the TH6 turned south or south-southeast to search for and work those multipliers. Switching from the high yagi to the low one often resulting in just one call to cut through the pile up despite running only 150 watts.

Compared to most of the US we have a better shot at the Caribbean on 20 and 15 meters when the solar flux (MUF) is low; many in the US are in the skip zone on this short path. For the longer southerly path, even as close as the north coast of South America the higher yagi is better on all the high bands. The optimum range for a low yagi is narrow but important.

Longer paths

As ought to be expected the high yagis were the go-to antennas for longer DX paths on the bands from 40 through 10 meters. Whether CE, LU, VK, ZL, JA, DU, UA0, ZS and others the difference was never less than 2 S-units and could be in excess of 5 or 6 S-units. These antennas were a big help in putting many Japanese stations in the log and distant multipliers I would otherwise be unable to work or not be able to work so quickly and easily.

40 meters

As my antennas improve I learn more and more about this band. It can be interesting. It is also important to a contester at this stage of the solar cycle since it is the second most productive band for QSOs and multipliers.

First off let's look at the 80/40 inverted vee (apex at 32 meters). Put bluntly it was almost totally useless. On all DX paths the XM240 at 46 meters was always the superior choice except when the direction was directly off the ends of the elements, including when off the back of the yagi. Two element yagis (other than the Moxon) have poor F/B, yet even with 10 to 15 db rejection it still outperformed the inverted vee.

I only using the inverted vee if it was inconvenient to move the XM240 a few degrees to catch a multiplier. The inverted vee remains valuable for working short paths within eastern North America.

The path to Europe and other points eastward opens in mid-afternoon. I was able to begin working (and running) Europe around 2100Z, which is 90 minutes before sunset. This was not possible when the yagi was at half its current height. Stations running a kilowatt could work Europe a full hour before I could. Height and gain can overcome the pre-sunset path loss and higher received noise in Europe. This isn't possible with the inverted vee.

Speaking of noise, another way I fought QRM and atmospheric QRN on 40 meters after sunset was to use the northeast Beverage. The SNR on received signals was better and was a definite help with the weaker signals. The beam width is too narrow for working anything other than Europe since on 40 meters the 175 meter long Beverage is 4λ.

80 and 160 meters

As everyone knows conditions were poor to middling for most of the contest. Although disappointing it is a situation everyone experiences and so does not mean a great deal. For me the big impact on the low bands was that the extra decibels of path attenuation put my 150 watts below the noise for far too many stations.

Not only were my QSOs and multipliers low on these bands I only improved my 160 meter DXCC count by two. Although both my 80 meter (temporary inverted vee) antenna and 160 meter antenna offer decent performance it is obvious that I can do better.

Another challenge I faced on 80 and 160 meters was skew path propagation to Europe and perhaps other directions. This is reportedly not unusual during a geomagnetic disturbance. With just the one Beverage (northeast) all I could say for sure during the contest was that at times it did no better than the transmitting vertical antenna on receive. I learned about the presence of skew path after the contest from the reports of others.

While running low power the occasional ineffectiveness of a receive antenna is not a disaster. It will prove problematic when I return to QRO operating since my signal will attract weaker callers if I don't have receive antennas for other directions.

Station automation

This is a work in progress. At the moment I have little more a manually controlled remote antenna switch (2 x 8) and N1MM Logger with CAT control of one rig. I still do not have all the equipment to do SO2R and automatic antenna selection.

It is very easy to choose the wrong antenna when you're tired or in a hurry, both of which are common in a 48 hour contest. Since the SWR protection kicks in this is only a time waster rather than a potential disaster. When I go QRO the same event can become more serious. Manual switching is time consuming and deflects my attention from focussing on operating.

SO2R requires more work. The SCU17 interface for the FTdx5000 died so the one CAT cable I have goes to it rather than the (idle) FT950, as I had intended. I do not have a headphone mixer to listen to both radios at once or band pass filters to protect the receivers. All this and more are still to be done. Up until now station automation has been low priority in comparison to antennas. I need to practice doing SO2R, which is a skill I do not yet have.

Once I have SO2R working I will also be in a position to invite others to do a multi-op contest. Without SO2R I was unable to capitalize on many opportunities this past weekend to run on 20 or 40 meters and concurrently hunt for stations on other bands. That put me at a competitive disadvantage.

Antenna conflicts

With a limited number of antennas it is perhaps not unexpected that I would encounter conflicts. For example, in late afternoon I want the XM240 (40 meters) pointed to Europe and the TH6 (20 meters) pointed to Japan and the east Asia. My current choices are to lose time rotating the yagis back and forth or use sub-optimal antennas. Either way QSOs and multipliers are negatively affected.

If I had a second tower such conflicts can be avoided. There is the added benefit of increased isolation between antennas. It is common that serious contesters have at least two tall towers. With tow of them one is typically dedicated to 40 and 10 meters and the other to 20 and 15 meters. Although there are still conflicts they are less serious. A rotatable multi-band yagi or a few fixed yagis at an intermediate height can resolve almost all the remaining conflicts.

Impact on 2018 antenna plans

Many problems can be addressed with a second tall tower. That is in my plans although I am undecided whether to do it this year. My concern is that the effort required to put up a tower of between 120' and 140' will mean little time left to design, build and test antennas. Further, the choice of rotatable, fixed and stacked yagis for 40 through 10 meters depends on whether I have one tower or two.

I definitely plan to stack yagis on 20 and 15 meters for additional gain towards Europe and to match elevation angle to the prevailing propagation as it changes. Now that I know for certain that more height can be beneficial to Europe on 20 meters I am rethinking my plans for side mounted yagis and the rotatable antenna at the top. Depending on whether I go with a second tower the yagis will either be mono-band or multi-band.

On 40 meters I would like a fixed, reversible northeast-southwest (Europe-USA) 3-element yagi up ~25 meters. Regardless of whether I can fit in a rotatable yagi better than the XM240 up top into this year's schedule the added performance and flexibility provided by the fixed yagi will be a help during contests. My preference is for a tubing antenna rather than wire (inverted vee) to reduce interactions, improve performance and avoid additional anchors in the hay field. I am currently investigating designs and material choices.

80 meters is an easy decision: build the vertical yagi. In contrast I remain uncertain how to deal with 160 meters. The vertical I have up at the moment will have to come down by May at the latest due to the arrival of haying season and because it will interfere with work on yagis on the 150' tower. If nothing better comes along in my plans it will go back up in September or October, deferring a decision at least one more year. My expectation is that this is what will happen.

Reversible Beverages remain my preferred choice for low band receive antennas. Paths for two of these have been surveyed and most of the material purchased. On the critical path is the design and construction of a remote switching system. Until I have that ready putting up the Beverages is low priority.

Other than antenna plans I have been getting into the specifics of station automation equipment design. I am closely reviewing commercial products and public designs while also injecting my own ideas on what will work best for me. The final product will combine commercial and custom hardware and software. I don't know how far along I'll get by the fall contest season.

I have a challenging year ahead of me. No matter how much I accomplish I'll be in a better competitive position for next season's contests.

Thursday, February 15, 2018

Twisted Inverted Vee

Several weeks ago a problem showed up with one of my 40/80 meter inverted vee antenna. I first noticed it when the SWR on 40 meters was unusually high. At first I assumed it was due to weather since moisture or ice on the wires will affect resonance, and we get lots of both in the winter. I let the rig's ATU calibrate to the new impedance and continued operating, expecting the impedance to return to normal later.

When the problem persisted after two days of fair weather I decided to look into it. Or, if you like, I looked up at it. What had happened was immediately evident. On one leg of the vee the wires for the 40 and 80 meter antennas twisted. A closer inspection with binoculars revealed that the twist was comprised of 3 or 4 rotations. The picture is annotated due to the poor resolution; I had no intention of climbing up there for a better view.

In an article last fall I explained how this was to be a temporary antenna until I could put up permanent antennas for 80 meters and a lower one for 40 meters. I built it very quickly, using existing 80 meter and 40 meter inverted vees from my my stockpile, tying them together and constructing a set of spacers made from small PVC pipe (3 per leg). Small ropes extend from the bottom spacers, positioned at the ends of the 40 meter wires to the ends of the 80 meter wires. As I said, it's very simple.

Unfortunately I made a bad decision on the ropes used to tie the antenna to anchors on the ground. I went with expediency rather than good sense since I was so pressed for time. I had hundreds of feet of ¼" polypropylene twist rope for which I had no other use. It has been in storage for many years. Why I originally bought it I no longer recall.

I cut 75' lengths and completed the antenna. Since twist rope of this type develops a torque when put under tension we had quite a job preventing the antenna from twisting when first installing it. When it seemed stable I let it be and hoped for the best.

But hope is a 4-letter word. After a late January day with strong winds the twist reappeared in one leg of the vee. The twist is approximately 3 meters from the feed point, between the top and middle spacers. The middle spacer is ~5 meters along the antenna, near the middle of the 40 meter wire.

Although an inconvenience, and not a disaster, the result of this accidental experiment is instructive and merits a brief article. It is worth thinking about should you ever run into a similar problem with one of your antennas. So let me step back and describe where I started.

As with any fan antenna of this type the elements for the lowest frequency are almost totally unaffected by those for the higher bands. It is the higher bands that see the impact. The reason is that the ends of a dipole are most susceptible to capactive coupling to adjacent elements; that is why capacity hats must be placed far along an element to be effective. For this antenna the susceptible band is 40 meters. The resonant frequency on 80 meters was not noticably affected by the fan arrangement. In contrast the 40 meters resonance moved well below the band due to that coupling by increasing the antenna's electrical length.

I knew this would happen and that there would be no time to tune the antenna after raising it. The difficulty is overcome by use of the rig's ATU. However there is some challenge when switching between the inverted vee and XM240 on 40 meters since the SWR curves are so different. For operational simplicity I currently reserve the ATU for the inverted vee and disable the ATU when switching to the yagi. It's an acceptable inconvenience for the short time the inverted vee is expected to be in use.

When the elements twisted together the impedance impact showed up on 40 but not 80 meters. Some change on 80 must have occurred though not enough to require reprogramming the ATU. Interestingly the 40 meter resonant frequency didn't move far. Perhaps that's because the end of the 40 meter element is still properly separated from the 80 meter element. Instead the negative impact is a substantial decrease in the SWR bandwidth.

The resonant frequency moved downward at least 75 kHz and the 2:1 SWR bandwidth decreased to ~150 kHz. When undamaged the SWR at 7.0 MHz was ~2 and ~2.5 at 7.1 MHz. I have done no further investigation to determine why the impedance changed in this particular fashion since it is of limited interest to me. In any case modelling the twisted elements is almost certainly beyond the capabilities of NEC2.

While not an ideal situation it does not hobble its performance. Single element antennas can survive a lot of abuse since even drastic impedance swings due to rain, ice, tangling and environmental coupling do not affect the pattern. If the loss due to a higher SWR is managable there is no need for emergency repairs. The same cannot be said of directive arrays whose patterns and impedance are very sensitive to changes.

I never did fix the problem and frankly I can't be bothered to spend more effort on it. The antenna works and that is what matters for the few remaining months it'll be up there. The anchor for that leg is frozen to the ground (two large rocks) and there is the risk of making the problem worse by trying to untangle it from the ground. It isn't worth the trouble and risk of climbing the tower in winter weather.

Saturday, February 10, 2018

80 Meter Vertical Yagi: Revised

As I write these words it's -21 C and the wind is howling. There is no antenna work getting done. I am well behind schedule with my winter projects due to the severity of the weather. In time it will moderate and work can resume. Until then I am limited to what I can do indoors. One of those is sitting at the computer and modelling antennas.

Since I was planning to begin construction of the 80 meter vertical array in the winter I have revisited my original design. Changes have been made. Please refer to that article for design and construction details not included here, and for additional background. I won't unnecessarily repeat myself.

For the first change the tower (driven element) will be a little shorter. This means that the "stinger" at the top will need to be ~6 meters tall rather than just 1 meter to be λ/4 on 80 meters. Adding a switchable section on top to have a 160 meter vertical is physically unreasonable with the now much longer stinger. I had hoped to use the extensive radial field on 160 meters, keeping land use and cost to a minimum. This plan has also changed.

For contests I do not want to be in a position where I cannot be on 80 or 160 meters at the same time, whether for SO2R or multi-op. For the next few years over the duration of the solar minimum these may be the only two productive bands for several hours at night.

The other reason is that I have been unable to come up with a design to switch between 80 and 160 meters that does not compromise performance on 80 or 160 meters or both. Whether it be a trap or mutual impedance with an isolated pole and top hat for 160 meters there are negative impacts on the 80 meter array's pattern and bandwidth. To be clear, it can be made to work, just not to my satisfaction.

With this exclusion I am free to focus on 80 meter performance. Despite this the physical design still constrains the electrical options. When I first developed the model I did not own this property and could not predict the specific layout of the property and my site plan for towers and antennas. I am in a better position to do so now.

Elements of the redesign

A casual search for additional sections for my small tower -- as the main support and driven element -- didn't turn up anything suitable or economical. Since the tower is currently 14 meters tall (6 x 8' sections, with splice overlap) and a λ/4 on 80 meters is ~20 meters a "stinger" of ~6 meters length is required. This is straight-forward. However the original plan for a 20 meter tall tower allowed for an isolated stringer for a 160 meter vertical with a capacity top hat.

The wire parasitic elements will continue to be supported from the top of the stinger. Due to its lower height (14 meters vs. ~25 meters) the wires cannot simply be wires hanging from support catenaries. I am therefore using angular T-top verticals. The array will closely resemble the original design by K3LR, details of which can be found in ON4UN's Low-band DXing book.

This adds some uncertainty to the NEC2 model due to the odd shape of the parasitic elements. This must be compensated for during construction and testing with an antenna analyzer. I found this with the similarly shaped 160 meter antenna I recently built, which resonated ~80 kHz lower than the model. Interestingly there is a chart in ON4UN's book that  recommends dimensions that appear to be more accurate than what I can model with NEC2 (EZNEC). I expect the same for this antenna.

There are two additional changes I'm making. The first is to exclude SSB. This simplifies the design and construction without giving up too much with regard to my operating interests. I can always add it later. The array will be an omni-directional single element vertical between 3.65 and 3.8 MHz. When receive directivity is needed it can come from the Beverages (still to be built).

A further change is to radial system. Rather than busses at intersections of radials between the 5 elements I will put down 5 independent and overlapping radial systems. This is difficult to model so I can only rely on reports that performance is not compromised. My reason is solely to simplify construction since creating the bare copper busses and the multitude of soldered connections, and not with lead-tin solder, is a lot of work. The price is the amount of radial wire required. I can change to a bus system later if I wish or if necessary to optimize performance.

Developing the model

The array is designed to act as both a 3-element vertical yagi and as an omni-directional λ/4 vertical. Since the yagi performance is narrow band it is designed for CW only. However as a simple vertical it can be more broadband than that, and indeed can be made to work well from 3.5 to 3.8 MHz. Therefore the first objective is to resonate the driven element more centrally in the band; the L-network -- switched in for yagi operation -- is easily adjusted to accommodate the higher resonance of the driven element.

After some modelling work I settled on 3.6 MHz as the resonant frequency for the driven element. The match is very good, deliberately favouring CW and the DX & contest segment of 80 meters which is what matters to me. You can choose another frequency without affecting yagi performance since a matching network is required regardless. Keep in mind that a matching network may be required as the radial system is improved beyond that in this model since the feed point resistance will drop.

The impedance is dependent on the ground system since the ground loss is in series with the radiation resistance. For this model I used MININEC ground in EZNEC and inserted a 5 Ω load at the base of all 5 elements to emulate a very good radial system. This is far easier than creating a radial field for all 5 elements in the model yet gives results that are close to reality. The load resistance can be adjusted to test the antenna's predicted performance with other radial systems. There are tables of approximate equivalent resistances of radial systems (length and number) to be found in several places, including in ON4UN's book. I'll have more to say later about the radial system and its effect on the antenna.

The next step was to design the wire parasitic elements, including their vertical and T-top lengths. Spacing to the driven element in all cases is 10.5 meters, or λ/8 in the CW segment of 80 meters. Since the array is reversible different director and reflector spacing is not possible. Consequently there is some loss of performance (gain and F/B) relative to an optimized (unidirectional) yagi, though not enough to be of practical concern.

As I saw with my 160 meter antenna the model for an element of this style is not accurately modelled using NEC2. Two reasons of which I'm aware are the acute interior angle of ~45° on the low side of the T and the effect of ground.

The error can be corrected during construction by floating all the other elements including the driven element (disconnected from ground) and adjusting the wire element to self-resonance at 3.68 MHz, as a director. Symmetrical trimming of the two halves of the T is recommended. With the 2.1 μH reflector coil in line at the element base the self-resonance is 3.45 MHz. First tune the wire element as a direction and then with the coil in line adjust the coil, not the element, to tune its self resonance as a reflector.

Since the radiation resistance of a yagi is lower than a simple vertical a matching network is required. I used TLW (comes with the ARRL antenna book) to design the network based on the EZNEC reported impedance. The designed network is inserted into the EZNEC model to confirm that the antenna is now matched. In practice you'll want to measure the array's impedance once it's built and then design the L-network to transform that impedance to 50 Ω. As you can see coil Q is not critical as there are no large losses in the small transformation ratio required. I used a "low pass" L-network to help attenuate harmonics for SO2R and multi-op contest operation.

Within reason the director and reflector self resonant frequencies can be adjusted to centre the array on another band segment without going to the trouble of a complete re-modelling. The reflector coil value stays the same. A small improvement in gain and F/B can be achieved by tightening the tuning of the parasitic elements. For example, lowering the reflector coil to 1.8 μH gives several more db of F/B and ~0.2 db of gain. In this case the director self resonance should be lowered ~30 kHz.

The SWR bandwidth will be narrower. That may be a fair trade-off since the SWR bandwidth of this antenna is superior to the original. For this specific case the designed L-network still works well.

I modelled the elements with insulated AWG 14. The vertical length of the wire elements is 10.2 meters and each half of the T is 6.3 meters. The vertical length is a compromise between minimizing the length of the T (capacity hat) and minimizing the distance from the tower than the element must be anchored. I want to keep the anchors within the radial field to reduce the amount of land dedicated to the antenna which would otherwise have to be taken from the haying.

If one is careful the tuning is only required on one parasitic element. The others can then be cut to match it. Even so it is probably wise to measure and trim them all to resonance to avoid surprises. Either way it is done the reflector coils ought to be adjusted to accurately resonate the elements as reflectors.

Model performance

SWR, gain and F/B bandwidth are better in this antenna than in the original design that used straight parasitic elements. Unfortunately the gain and F/B are not as good over most of the operating range. The difference is not severe but requires consideration. Peak gain drops from 4.5 to 4.2 dbi, which is close to negligible and does not overly concern me. Peak F/B drops from well over 20 db to only 15 db. That is perhaps the only negative performance impact of note

Let's look more closely at the numbers, in particular in comparison to the original design. I kept the 5 Ω equivalent series resistance of the ground loss to ensure the comparison is valid. While not charted the SWR bandwidth is superior to the original design. The T-top elements at least achieve that much.

Although the loss of F/B is disappointing the overall performance change is neutral in my opinion. You may feel differently. While the wider bandwidth is not consequential to a pure CW operator it does matter if your interests include digital modes and SSB. With coil switching to support the SSB segment (as in the original design, and which can be added to this one) it can be a good performer from 3.65 to 3.8 MHz. Perhaps one day, but not initially in my case.

F/B performance is less of a concern where this array is primarily devoted to transmit and a separate, multi-direction receive antenna system is available. Those using a 4-square antenna on 80 meters often reporte they only occasionally use their receive antennas since the 4-square's F/B is quite good. That is one comparative disadvantage of the yagi array.

Before constructing the model I speculated that the F/B would improve. The reason is that the top of the T of all the wire elements lean towards the driven element, thus increasing capacitive coupling with the driven element. This is how the Moxon works where critical coupling serves to equalize current, a necessary condition for a high F/B (field cancellation). Of course there are 3 elements, not 2, so perhaps the better comparison is a Spiderbeam style of yagi.

Obviously this didn't happen. Looking at the element currents it is clear that the elements are nowhere close to critical coupling as the tips come no closer together than 6 meters (~0.07λ). Another hope dashed on the shores of reality.

I notice that the March QST has an article on a 3-element vertical Moxon yagi. Unfortunately I don't have it yet since it would be interesting to compare. If it looks promising I may model it and compare to what I this yagi design. Should that happen I'll write a follow up article.

Ground sensitivity

It is no surprise that the quality of the radial system and the conductivity of the ground below have a strong influence on the efficiency of vertical antennas, especially ground mounted verticals. For every antenna we have to find an acceptable trade off between cost & convenience versus performance. Directive arrays such as the vertical yagi and the 4-square are more affected since their lower radiation resistance results in greater ground loss versus a simple vertical for any given radial system.

Compared to perfect ground this vertical yagi and the 4-square have approximately the same peak gain ~6.5 dbi. The 4-square has better F/B and both F/B and have a much wider bandwidth. On the other hand the vertical yagi is simpler, cheaper, more amenable to experimentation, direction choices and the addition of more directors. For me these make the choice easy. The majority of contesters I know choose the 4-square since their primary motivation is competitiveness without undue time spent on experimentation and home brewing the control system.

When ground is imperfect, as it always is, the 4-square has greater efficiency than the vertical yagi for any given radial system. The vertical yagi's radiation resistance is lower due to the closer element spacing -- λ/8 vs. λ/4 -- and the consequent higher element currents lead to higher I²R ground loss. The better the radial system the less the difference. Providing you are committed to an extensive radial system there will be little efficiency difference between the two antennas, even accounting for the dump load (up to -0.5 db loss) in the 4-square.

As ground quality improves the vertical yagi's peak performance moves lower in frequency. As with any yagi the frequency of maximum gain is correlated with minimum radiation resistance. For this antenna that occurs below 3.5 MHz. Over a perfect ground this vertical yagi's gain peaks ~3.47 MHz and the peak F/B rises well above 20 db at 3.525 MHz. If the antenna is built with a superior radial system, one with an equivalent series resistance of 3 Ω or less it can be worthwhile to shift its tuning upward by 30 or 40 kHz to exploit that change.

With the very good but not great radial system in my model -- 5 Ω -- the modelled ground loss is -2.4 dbi, although it varies with frequency, increasing towards the bottom and top of the operating bandwidth. EZNEC reports quite high loss as the frequency increases, exceeding 300 watts at 3.65 MHz. This isn't bad. You can always add more radials over time if desired.

The above chart compares base element currents for the original and new versions of the array, both with 3 Ω equivalent series resistance for ground and 1,000 watts, to match the values I chose in the original article.  It is possible to greatly reduce ground loss by improving the radial system for the driven element. In fact the loss becomes quite low when the driven element ground resistance is lowered to 2 Ω and the parasitic elements to 5 Ω.

Review the original article and note that it is more important to lower the ground loss in the driven element since its current is always higher than in the parasitic elements, which is unlike the 4-square whose elements have no unique identity. In its omni-directional configuration the ground loss is lower.

For a fixed amount of wire the performance can be optimized by putting more of that wire into the driven element radial system than the 4 parasitic elements. This is not a 4-square where the elements should be treated equally! You'll even gain some performance benefit by use of wire thicker than 14 AWG in the parasitic elements; I'll be using 14 AWG wire since that's what I have on hand.

Radial topology

By using MININEC ground the details of the radial system can be glossed over by substituting fixed load resistances between each element and the perfect ground. That detail cannot be avoided when designing the actual radial system. Aside from the size of the radial system to achieve the target ground loss the topology is important since the radials are longer than the distance between elements. That is, they must either overlap or be connected.

I modelled connected and overlapping radials quite some time ago in an attempt to determine whether one is better than the other in phased and parasitic vertical arrays. Although there are measurable and significant differences in the radial current amplitudes and distribution in the end it seemed to be one of small differences rather than one topology being obviously superior. In both cases the currents on radials between active elements can become quite complex, and perhaps not intuitive, due to the superposition of fields of the mutually coupled elements in the return paths through the radials and ground beneath.

Overlapping radials are easier to construct but more expensive. The capacitive coupling between crossing radials is only significant off to the side where currents are lower and voltages higher at the crossing points. Radial interconnection, via busses or directly, is difficult in practice and forces return current to zigzag at the interconnection points. This is difficult to model and compare.

My present inclination is to go with overlapped radial fields for each element, based partly on my (inconclusive) models and not well quantified data (to my knowledge) from experimenters. I intend to keep it simple and create a thick radial field for the driven element where the potential loss is greatest and sparser and shorter radials for the parasitic elements. I want them shorter so that the land impact is minimized. Long radials are not a problem for the driven element since it's at the array's centre.

Should I be unhappy with the results I can revisit the decision and redo the radial field.

What is the reality of performance?

Modelling is not the final word on an antenna of this type. Ground influences, radial topology and the environment have significant impacts that are very difficult to model. NEC4 can do better than NEC2 though even that has limits. Relying solely on models to characterize performance and comparison to the 4-square (this array's nearest competitor) may be unwise. How will they perform in practice?

Perhaps the greatest problems with comparisons are propagation variability and instrumentation for measuring differences. Direct A/B comparisons are typically impossible since no one I know has both a 4-square and a 3-element vertical yagi for the same band.

F/B on both antennas is sensitive to tuning. I wonder how many 4-squares are tuned so well that F/B measures at or near what is theoretically possible. Maximizing F/B is difficult in any antenna since balancing phase and amplitude so that near perfect cancellation of fields in the reverse direction occurs. Consider than 30 db of F/B requires 99.8% field cancellation! That is a challenge even with commercial phasing and switching systems.

Claims of 25 to 30 db or more of F/B should be looked at critically. How was it measured? With an S-meter? That is a sure path to overstatement. S-meters are not linear and follow no standard. There are a few recent model SDR receivers that do better by digitally compensating for the analogue data coming from the receiver. Unless you have and can confirm calibration no S-meter should be relied upon for a dependable measurement.

We have also seen that the vertical yagi F/B is sensitive to ground loss, and therefore the quality of the radial system. More and longer radials improve gain and F/B. Even so it can never reach the performance of a well-tuned 4-square. Even then the performance bandwidth is narrow. Many owners of 4-squares find that no separate receive antenna is necessary other than in exceptional cases since the directivity is quite good. That's persuasive.

A vertical yagi will require resorting to a separate highly directive receive antenna more often than with a 4-square. I believe I can live with that. I may change my mind after building and living with this antenna for a while. Unlike in an earlier article that derided the importance of F/B the viewpoint I espoused is less supportable on the low bands where good directivity is needed to copy under the prevailing low SNR conditions.

Construction plans and testing

The construction and testing sequence is laid out above in the modelling section: tune the driven element after construction it and its radial system and then move on to the wire elements, tuning each of those with all other elements floating. Only then should the system be driven as a parasitic array and the L-network designed and built.

I will initially use 20 meter long radials for the driven element and 15 or 16 meter radials for the parasitic elements, as reasoned above. My aim is 32 radials for the driven element and 16 for the parasitic elements. Doubling the number of radials to improve performance can be accomplished later by placing a new one between each pair of existing radials.

Directions covered by the array do not have to be at 90° intervals, unlike the 4-square. The only constraint is that the director and reflector must be in a line with the driven element (tower). My choices (already surveyed and staked) are: 50° and 230°; and 160° and 340°. The first pair covers Europe and most of the US and Pacific. The second pair covers Japan and east Asia, and the Caribbean and South America. From here those are the most productive directions for contests. The beam width is wide enough that there are few coverage gaps, and one can always resort to the omni-directional mode.

As mentioned at the beginning the weather turned foul very quickly in December. I did manage to get the tower anchors installed mere hours in advance of the initial blast of frigid temperatures. During a brief January thaw I tested the anchors and found them to be inadequate. The screw (auger) anchors have to be longer and/or wider to withstand the wind load. Altering or replacing the anchors is not difficult but it cannot be done in the winter. Hence construction is delayed by a few months.

Once the tower (driven element) is up and the radials rolled out I will compare it to the high inverted vee so that I have data on how they compare. After that the inverted vee will be removed, at least for the time being. The tower has to be cleared of obstructions to raise side mount yagis, a priority this year.

Spring is coming

Sunday, February 4, 2018

CQ 160 With the New Antenna

Last weekend I entered the CQ 160 CW contest with the objective of running up my DXCC total and working whatever else I could find when I wasn't doing that. I not only had no intention of being competitive I didn't even notice the point structure until after the contest started. It seems that this is another contest in which US and Canadian scores are not comparable due to the population asymmetry. In that it is no different from CQ WW.

By not being competitive I was free to operate when I pleased, in whatever manner I pleased, and to walk away from the rig when it stopped being enjoyable. The last occurred when the availability of stations to work dropped off. This is typical of single band contests and in contests like ARRL Sweepstakes where you can only work a station once regardless of band. For that reason I kept to regular meal times knowing that the majority of stations would be there later.

DX results

If you peruse the claimed scores on 3830 you'll notice how it played out. My country total was  relatively high compared to my peers (single op, low power) while my QSO count was low. That is as it should be. Conditions to Europe were especially good the first night (as most participants noted) and I had no trouble running Europeans as the sunrise line swept across the continent. Indeed many stations were worked well after their sunrise. Out of 666 QSOs 130 of them were 10 pointers (between continents), the large majority of which were European. That's pretty good for 150 watts.

The final tally was 51 countries worked (not including VE and K) and boosted my DXCC count to 82 on top band. It has since climbed to 85 (LoTW shows 60). My goal of reaching 100 countries by the spring is well on track.

This is further confirmation that my antenna works, and that it very competitive with other stations. There is no surprise in this since most hams have great difficulty putting up an efficient low angle radiator on 160 meters. Another way of putting it is not that my antenna is great but that others' are so poor. Imagine what 160 would sound like if everyone could put up an effective antenna!

Running up the QSO total

If you peruse the results posted to 3830 you'll notice that many report having been active for only a brief period, some no more than one or two hours. To have a chance of working those casual operators you must be active for the maximum allowed under the rules -- 30 hours in this contest for single op entries. These stations are difficult to work them they do not call CQ and only work the stations they find or want. You must run to have a chance, hoping they find and call you during the brief time they are on.

After the first night when the majority of serious competitors have been worked it can make for seriously low rates and boredom. Yet it's necessary. Operating assisted can relieve much of the boredom since you can take a break from continuously CQing to QSY, work the fresh meat and then return to running. I learned to do this pretty well while operating this contest from a multi-op station.

For unassisted stations such as myself last weekend running can be as exciting as watching paint dry. That is why I kept stepping away from the shack. New stations are always showing up and running can be resumed later with decent rates, for a little while at least. Search and pounce is largely pointless since there are so few unworked stations that are running. Some operators combine running and hunting by going SO2R or SO2V. I didn't do this despite having this capability with the two receivers in my FTdx5000. I'll consider doing so in future.

The point is that running is mandatory, no matter how boring it gets, if you hope to do well. I avoided this for the most part since I was not aiming for a winning score.

The terminator is your friend

The proximity of the terminator, whether sunrise or sunset, at both ends of the path can be critical to understanding propagation on 160 meters. This is because atmospheric noise strongly determines success.

For example, I can hear Europeans on my Beverage antenna well before my sunset even though absorption is quite high in the sunlit hemisphere since noise and signals are similarly attenuated. Unfortunately the reverse is not true in Europe where night is well advanced and they are receiving atmospheric noise from all directions. Hence they cannot copy signals from North America. Even after our sunset terminator passes the inequity of noise levels continues for at least another hour, after which copy becomes equally good (or poor).

As sunrise approaches Europe from the east their noise level drops. At this time they begin to be able to copy weaker signals from North America. This most likely explains our success with working Europe after our midnight, and even after the sunrise terminator has passed for some stations in Europe. This is where directive receive antennas show their mettle by making it possible hear the Europeans who are then better able to copy us.

With my northeast Beverage antenna many of those who replied to my CQ in the contest at that time were barely above the noise level. Without the Beverage they would not have been copied and some might not have been heard.

Many of the so-called gray line contacts on the low bands can be attributed to reduced atmospheric QRN at both ends of the path rather than propagation enhancement. However the latter remains an important factor in many if not most cases.

Future work planned

Spend any amount of time on the low bands and you'll soon learn who the alligators are. Those stations everyone can hear yet they copy only the strongest signals. Sometimes that's due to man-made or tropical/summer QRN while other times they do not have directive receive antennas, even a small one such as pennant or flag that can fit in a small space. In a few case it's because they run excessive (and illegal) power.

I am well set up to receive European signals that are close to the noise level. I now need more Beverage antennas to cover more directions. This will be especially important when I make the move back to QRO operating since a bigger signal attracts more and weaker callers and I don't want to become one of those alligators. I want to be able to work them.

I have been surveying routes for the Beverages and making a list of parts to order to construct a remote switch to select among the Beverages. The next one will be a reversible Beverage made with coaxial cable. It is more complex than a unidirectional Beverage but saves a lot of effort overall. I have several resources from which I am adapting the design. When it's done I'll write an article about it. If it works well I'll build another, otherwise I may go back to unidirectional antennas.

My next article will be about antenna design rather than operating. Those appear to be the most popular of the articles on this blog and are the ones I most enjoy writing.  Spring is coming and I want to be prepared.