The Doctor Is IN: Antennas for Domestic Contests

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Published on Sunday, 08 November 2009 01:17

Written by ARRL News Editor S. Khrystyne Keane, K1SFA

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Just the other day, the Doctor and I got to talking about ARRL Sweepstakes. I showed him my crystal mug and whisk broom from last year's Sweepstakes running (the W1AW team did quite well), and he showed me what kind of antennas I should look into for domestic contests. Being more of a DX RTTY contester, I really don't know much about the domestic side of things. I dabbled in the February NAQP RTTY Contest last year from K1TTT -- and will do so again in 2010 -- so I made sure to listen attentively. Here is what the good Doctor had to say:

Figure 1: Azimuth pattern of a half-wave dipole at a height of half a wavelength has a -3 dB beamwidth of of 87 degrees on each side -- a close match to the coverage needed by W1ZR to reach US and Canadian stations. Click the picture to enlarge.

Each contest brings its own special requirements to the antenna designer. While many popular contests focus on communications outside North America and require the ability to send signals to all points of the compass, Sweepstakes is different, with a need to cover just the US and Canada. That means generally shorter range contacts and contacts in a limited range of directions, depending on station location.

In addition, points are gathered based on individual contacts multiplied by ARRL Sections. Thus, it is desirable to have the capability to reach all 80 sections on at least one band that will have propagation available. ARRL Contest Manager Sean Kutzko, KX9X, notes that many a contest superstation's secret weapon for Sweepstakes is a 40 meter dipole up between 25-30 feet. He says 40 meters is the Sweepstakes "money band" -- you can get close-in contacts during daylight and rake in the distant Sections when the band goes long in the evening hours. He said he had never put in a serious effort at Sweepstakes without a low dipole for 40, no matter how much aluminum he had up in the air.

Figure 2: By adding a 5 percent longer than the dipole and 6 feet behind it, I reduce -- but don't eliminate -- rearward radiation and provide some gain to the front where distances are longer. Click the picture to enlarge.

Another great solution is a multiband Yagi that can be pointed towards the areas with the best propagation. If possible, have it relatively low -- perhaps at a half-wave length above ground -- to be able to cover the close-in stations, as well as those at the continent's far edge. Obviously, from the Central US or Canada, distances tend to be shorter than they are from the coasts with stations near the edges better able to make use of higher antennas. If you have the ability to try different heights, by all means try lowering your antenna from the optimum height for transcontinental contacts and see what works best for you.

If you're like me and don't have rotatable HF arrays available, all is not lost. First you need to figure out what azimuths you need to cover and then try to match those to fit your location. From my Connecticut location, I would want to cover from the direction toward old friend Don, WT1I, in Ocala, Florida (bearing 214°) up to Mark, KL7TQ, my old Army buddy in Eagle River, Alaska (322°).

There are many ways to compute the bearing to a station. The easy way out is to just use www.qrz.com. If your listing includes your latitude and longitude, bringing up another station and "looking at the details" will provide you with the bearing to their station. If you don't know anyone at the edges of the desired coverage area, just put a city name in the "Name Search" function and pick one that comes up. It doesn't get much easier -- or, if you must, you can use spherical trigonometry.

Figure 3: If I have no need for coverage to the rear, I can optimize the reflector length to achieve more gain by focusing almost all of my signal to the front. Click the picture to enlarge.

Using my station as an example, the range of bearings I want to cover requires a beamwidth of 322°minus 214°, or 108°. A half-wave dipole at a height of half a wave length has a -3 dB beamwidth of 87(see Figure 1). At a width of 108 it's only down to -4.6 dB from the peak. That's pretty close, and might be good if I had a lot of distant stations behind me, as in Central US or Canada, but I don't.

If I were to put a wire reflector, 5 percent longer than the original dipole, 6 feet behind it (for 20 meters), I would have an easy to deploy 2-element Yagi with the pattern shown in Figure 2. To make it resonate in mid band, I need to trim about 4 inches from each end of the now driven element and I'm good to go. Note what I have -- a bit more gain in front, a lot less in the back, but still plenty of signal toward northern New England. My signal at the edges of my coverage area is now stronger than the dipole's -3 dB points.

If I don't have many stations to my rear, an additional 1 dB of forward gain can be achieved at the expense of rearward signals (see Figure 3) and a higher SWR by shortening the reflector a few inches -- about 2.5 percent over the driven element should do the trick. This may be worthwhile if you are right at a corner of the country. For more bands, just use parallel elements and multiple reflectors. See the article by Marcus Hansen, VE7CA, to get the idea. Azimuth plots represented in Figures 1, 2 and 3 represent the output from the EZNEC antenna modeling software by Roy Lewallen, W7EL.

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