Some Thoughts on Antenna Planning for Competitive QRP Field Operations

The key for many QRPers operating in the field is finding effective antennas that are easy to transport, quick and easy to erect, yet are efficient. While many compromise systems have their place, they’re not well suited for competitive outings.

The most popular outdoor operating event is the ARRL’s Field Day, so it’s probably the best one to look at when trying to decide on antennas for a competitive portable station. Many of the stations near the top of each category have a long history of field operations, and over the years they’ve settled on effective antenna systems.

Many of the competitive Field Day groups like to use small tri-band Yagis. Here in New Mexico, putting up a tribander and pointing it toward the major eastern population centers can provide several dB gain in that direction when compared to a dipole. But a large number of our contacts can potentially come from stations to the west and northwest of us; unfortunately, that’s off the back of the tribander, anywhere from 10 to 20 dB weaker than the front. To compensate, some Field Day clubs put up two tribanders, one pointed east and one pointed west, and split the signals between them. That’s a lot of work and hardware for something that’s essentially no better than a dipole at the same height.

You can convince yourself of that a few ways. The first is a simple argument: The small 3-element tribanders that are popular on Field Day have at best about 4 dB forward gain over a dipole (see, for example, the tests by N0AX and K7LXC); if you split that in two different directions (-3 dB), you’re left with 1 dB improvement over a dipole. Is it worth the effort? We think not.

The second argument uses modeling. For a given antenna, we wish to determine in which directions we will be strong and in which directions we will be not so strong. Ideally, the directions in which we are strong should coincide with areas of large ham population. That will maximize our rate.

We modeled our expected performance by superimposing antenna patterns on to a ham population density map of the USA. The map is an equiazimuth or great circle map centered on Cedar Crest, near our expected QRPTTF site. An equiazimuth map has the advantage that the shortest distance from the operating QTH to the station being worked (a great circle) is a straight line, and is the same distance on the map for stations the same distance from the operating QTH in all directions. Also, the angle between two points on the map, measured from the operating point is independent of their relative orientation from the operating QTH, Hence the name equiazimuth (equal angle). These great circle maps are ideal for determining antenna pointing. On other map projections, such as the Mercater found on the wall of your 7th grade geography class, the map’s shortest distance between points is a curve, and angles, as well as distances, between two points can be grossly different depending where on the earth they are located. Thus, these maps are difficult to use for predicting antenna coverage.

When the great circle map also contains information on ham population density, it becomes an invaluable antenna-planning tool. Such a map can be generated for free centered on any location you like by pointing your browser to http://www.wm7d.net/azproj.shtml. The map we used, centered on Cedar Crest NM, is shown in Figure 1. There are a wide variety of options to choose from when generating a map on the web site, including ham population by grid square, orientation, scale, and geographic detail. It will take a few tries to get one that you are happy with.

Next I generated patterns for the various antennas we were considering, namely the dipole and the Yagi-Uda. One can use EZNEC to generate these patterns, copy them from “The Antenna Book”, or one of several web sites containing antenna patterns such as www.cebik.com. In generating these patterns, be sure to use patterns for antennas above real ground, not free space patterns. You may also wish to use patterns for several take off angles as well. We then reduced these patterns xerographically to the same size as the great circle map. After reducing them to the proper size, we copied them onto transparency material. The antenna pattern on the transparency is laid directly over the great circle map to determine if the antenna coverage coincides with major ham population centers. The pattern can be rotated to determine optimum alignment. One can also do this in software that allows pasting with a transparent background. The plots for this article were generated by software.

We used EZNEC to generate the antenna patterns. The demonstration (free) version of EZNEC will handle enough segments, 20, to adequately model the patterns of dipoles, inverted vees, and 2-3 element Yagi-Udas. The entire planning project can be done with free software.

We show the coverage of the 20 M inverted vee at 55 feet in Figure 2. All of the major ham population centers lie with the 3 dB points of the inverted vee when it is oriented North-South (major lobes in the east-west direction). There is a notch to the north, but only Colorado, Wyoming, the western Dakotas and eastern Montana are affected. With the exception of Colorado, these are not major population centers and are often inside the first skip zone on 20 M here. They can be picked up easily on 40 M for contest multipliers.

The pattern of two Yagis fed with a 3dB power splitter, one pointed east and one pointed west is shown in Figure 3. This has been suggested as one way of obtaining maximum coverage from NM in FD contests. The much sharper pattern begins to exclude some major population areas. Seattle is down nearly 8 dB on the Yagis, while it was only 3 dB down on the vee, Minneapolis is 10 dB down, as is Houston. Chicago is 6 dB down. The notch has grown to include much of Minnesota, all of the Dakotas, much of Nebraska, eastern Washington, Idaho, and all of Montana. The dead zone now excludes a significant number of hams, which will lead to a reduction in rate.

Clearly the inverted vee is the best choice from NM.

We performed similar modeling on 40 M, reaching much the same conclusion. On 40 M the antenna is much closer to the ground in terms of wavelengths. The notch towards the side fills in. In the contest we were able to work CO, WY, and MT easily on 40 M.

The choice of antennas is clearly dependent on location. If one were located in Maine, Seattle, Miami, or Los Angeles, the Yagi would be a clear winner, as it would be able to cover most of the major ham population centers without problem.

And the third argument in favor of dipoles versus a pair of small yagis: The proof is in the pudding. The AA5B+K5TA+K5TQ Field Day in 2002 used only dipoles, and finished very near the top in the very competitive 2A category. The N2UT/5 operation in the 2003 North American CW QSO Party also used only dipoles to the tune of over 1700 QSOs in 12 hrs (the winners had 1900 QSOs, and huge stations). Would we prefer to have the huge stations? You bet. But in terms of bang-for-the-buck, dipoles are the clear winner from this part of the country, and still very competitive with the big boys.

Our choice of inverted vees for QRPTTF 2004 turned out to be a good one. Read the accompanying article in this issue of the Sojourner to find out how we did. Remember, if you are in a different QTH, other antennas may be optimum for you.