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.

No comments:

Post a Comment

All comments are moderated, and should appear within one day of submission.