Antennas

Antennas are what get your signal out, and let you receive signals as well. Antennas are probably the most smoke-and-mirrors part of radio, and the source of endless experimentation. This experimentation has become improved with the availability of inexpensive electromagnetic modeling software designed with the experimenter in mind. The ability to model the performance and characteristics of an antenna on your computer, before mechanically constructing one, has eliminated a lot of wasted time for the amateur experimenter. Most of the plots you’ll see in this section come from data produced by EZNEC 4 (www.eznec.com), one of the amateur versions of such programs. Mind you, this doesn’t mean that antennas are hard to build, or necessarily expensive, or that it is a requirement for you to learn how to build antennas or to model them. They are, however, probably the easiest and cheapest things to experiment with in amateur radio. There are seven actively used antennas at my house that cover 11 amateur bands. Over the years I have tried close to two dozen different antennas here, and countless others at other locations What kind of antenna is best for you? The choices can be both simple and endless. The current ARRL Antenna Book is over 900 pages long, and although it also covers a large number of related topics that aren’t specifically about antennas, the ARRL has also just published the seventh issue of the Antenna Compendium – each issue containing an average of 200 pages of articles about antenna systems.

Lets start with the simplest of antennas, the half wave dipole, and its variant, the inverted vee, which is a dipole whose ends slope down toward the ground. Pictured below, these are simple wire antennas fed with coaxial cable. The majority of the radiation from these antennas is perpendicular to the direction of the wire, so if you want best performance in an east-west direction, mount the wire so it runs north-south.

The free-space pattern of a dipole can be seen in the plot to the left. The dipole is shown in blue, and the 3-D doughnut-shaped pattern is shown in grey. The red curve is a slice in the horizontal direction through the 3-D pattern, and shows the classical figure-8 pattern typical of this class of antenna. The problem is, most antennas aren’t used in free space, they are used near the ground. The presence, and even the characteristics, of the ground influence the performance of the antenna. A horizontal dipole in free space is horizontally polarized (its electric field is oriented horizontally, there is no vertical component to the electric field). The plots below show what the azimuth patterns of a dipole and an inverted look like in the presence of the ground, where vertical fields get created off the ends of the antenna. We see a standard figure eight pattern in blue from the horizontally polarized field, and a smaller (dashed red) pattern due to the vertical field.

You’ll notice that both patterns are similar, with a larger contribution to the vertical field coming from the inverted vee. In both cases the null in the horizontally polarized field in the direction of the wire is “filled in” by the vertical field response. While this will have little effect for local contacts with others having horizontally polarized antennas, long-distance contacts may still be possible in the null direction, as reflections from the ionosphere tend to “scramble” the polarization.

The elevation pattern of an antenna is influenced by its height above the ground, as can be seen in the figures below. For long distance contacts, it is important to get more energy at lower elevation angles, so you can see the advantage of increasing the height of the antenna above ground. While this is usually not a problem at VHF frequencies and above, it certainly is a concern in the HF bands, where even a ¼ wavelength on 80 meters requires a height of 66 feet.

As mentioned earlier, the characteristics of soil itself can influence the performance of an antenna. On the left is a family of plots of expected dipole performance over different types of ground.

The black line is the response over salt water, the next best thing to copper
The blue line is the response over good soil, like that found in mid-west farmland
The green line response is typical of soil found in the mid-Atlantic states
The pink line shows what happens in a desert climate like might be found in AZ.