Winter 2004 Issue

Less than half of WA5VJB’s horn antennas.

Horn Antennas

 By Kent Britain, WA5VJB

Horns areamong the simplest gain antennas. In simple terms, you are collecting radio waves and funneling them down to a monopole antenna. They are difficult to mess up electrically, and no matter how you mess them up, they usually still have gain.

Photo A shows one of three 15-dB gain horn antennas. In photo B, on the horn end of the large antenna in the previous photo I have two more 15-dB gain horn antennas. Of course, the big one covers the 2.3- and 3.4-GHz bands, while the little ones cover the 24-GHz band at the low end and 35 GHz at the high end. It shows how simple a horn antenna is.

In figure 1A we show the waves coming into the opening of the horn and reflecting down into the waveguide. The waves hitting the edge of the horn have to travel farther than the waves going straight in. They all don’t arrive together, or in phase. The easiest way to minimize this problem is to make the sides of the horn longer, as in figure 1B; now you have less phase error. If I make the opening wider, I’ll catch more signal, which is certainly a trade-off. An optimum horn is one in which these two design parameters are compromised for the best trade-off. It also means that you can make it bigger, or longer, without changing gain very much. As I said, horns are pretty “idiot resistant.”

This time I’m going over a downloadable spreadsheet program that you can use both to design horn antennas and to analyze an existing design. To download the program, go to CQ VHF website, <www.cq-vhf.com> and highlight this issue of CQ VHF magazine. The program is also available from the North Texas Microwave Society site; just visit <http:// www.ntms.org>.

Start with the waveguide size you plan to use. The program contains a lookup table if you can’t remember the dimensions of WR-90 (waveguide). Now enter the frequency range in which you are interested. Go ahead and make it a pretty wide frequency range, because horns are very broad band antennas. The frequency range of your waveguide is a good starting point. Next, you’ll need to enter the length of the sides along with the width and height of the opening.

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