Summer 2008 Issue

ANTENNAS

ATV Antennas plus Antenna Ranges

By Kent Britain, WA5VJB

Photo A. Cheap Yagi for 915-MHz ATV.

This issue of CQ VHF covers several ATV (Amateur Television) topics, so it’s a good time to update and revisit a pair of ATV antennas for 915 MHz (one 6-element and one 10-element) and one for 2.4 GHz. We will also go over an interesting myth about antenna ranges.

Quite a few ATV systems use 915-MHz FM video input, and for low-power video systems 2.4 GHz is the favorite band. Therefore, in this column we will cover both a 915-MHz Yagi and a 2.4-GHz patch antenna. But keep this quiet: These antennas also work well for digital, FM, SSB, 802-11, and other 900-MHz/2400-MHz services.

915-MHz Antennas

We start with a 915-MHz version of the “Cheap Yagi” shown in photo A and figure 1. The boom can be any non-conductive material. Yes, PVC pipe can be used, but I find that a good hard wood about 1/2 to 3/4 inch in width is stronger and seems to last longer, especially if you give it a coat of paint. All elements are 1/8 inch in diameter. I have used ground-rod wire, #10 and #12 electrical household wire, welding rod, and hobby tubing from the model-airplane shop. Again, almost any 1/8-inch diameter metallic material can be used for the elements. However, for the driven element it is good to use copper or brass so that you can directly solder the 50-ohm coax to the driven element. Small clips have been used to clamp the wire to the driven element, and these can be fun to make, but often they corrode quickly when mounted outside.

The same driven element shown in figure 1 is used for both the 6-element and 10-element versions of the 915-MHz Yagi (see Table 1).

Patch Antenna for 2.4 GHz

Next we have an easy-to-make patch antenna for 2.4 GHz, shown in photo B. This antenna works well with the various 2.4-GHz video senders, Bluetooth®, and other wireless products. For AMSAT it can be used to listen for many of the stronger transponders, or placed at the focus of a small dish for even more gain. Gain is in the 8- to 9-dBi range, and the antenna easily has over 200 MHz of bandwidth.

Construction is simple. The back plane can be almost any sheet of metal. Of course, aluminum is easy to find, and in this case I used some old PC board. For the patch element you want to use something that is easy to solder. Sheet brass, sheet tin, sheet copper, or another piece of PC board work well. Just build per the dimensions in figure 2 and go. If you need to make your patch more rugged, note that the very center of the patch is a null point. You can drill a hole in the middle of the patch and support the patch on a long bolt line as shown in the patch in photo C.

Antenna-Range Mistakes

“But I put both antennas in the same spot!” I heard one antenna tester say that because he puts the reference antenna and the test antenna in exactly the same spot, then his gain numbers are perfect.

Not exactly. This is only true when both the reference and the test antenna are identical. In the real world, using the same spot without making sure the test area has a uniform signal strength can result in some pretty big errors. The problem is that when you have two antennas with different gain, then they also have different capture areas, or aperture areas. The test signal must be uniform over the aperture area of the larger of the two antennas. (See figure 3.)

In figure 4 we have a typical pattern for a source antenna on an antenna range when the source antenna is mounted several wavelengths off the ground (also see figure 5 for a typical pattern of the source antenna when mounted low). Note all the peaks and nulls in the test area.
 

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