Winter 2010 Issue
By Kent Britain, WA5VJB
I would like to start out with a bit of an apology to the readers of both CQ VHF and CQ. I try to avoid the having the same topic in my columns in both magazines, but stacking Yagis has been a hot topic of late. Nothing seems to get a group of VHFers to take off in a dozen different directions than a discussion on how far apart you need to space your 50-, 144-, 222-, and 432-MHz Yagis to minimize interaction. Also, I can go into a bit more detail here than I could in CQ.
First we start with a concept that will make university professors cringe, but it works quite well for our applications. The subject is capture area.
Think of capture area like you would a panel
of solar cells. If you want twice as much power from a solar panel, or 3
dB more DC power, you need twice as much area. In this case we will call
it capture area. An antenna with 3 dB more gain will also have twice as
much capture area.
At the 2009 Microwave Update conference I had an opportunity to run some quick tests on the antenna range. While measuring a 432-MHz beam, 50-ohm terminated 902-MHz and 1296-MHz Yagis were mounted at different heights as shown in photo A. At spacings as close as 6 inches the gain change was only a few tenths of a dB at 432 MHz. I had to space them where the U-bolts were actually touching before the gain dropped a full dB. Also, I think much of that was SWR losses where the elements from the other Yagi got close to the 432ís driven element. After all, on HF it is quite common to build Yagis for two, three, and even five bands on the same boom.
The bottom line is that you can mount a higher
band Yagi very close to a lower frequency Yagi with virtually no
interaction. The next question is what changes are there in the higher
band Yagi? Well, with snow coming down as I write this column, it will
take warmer weather to make those measurements. However, Iím just a
curious as you are, so stand by.
Without a doubt, the UHF+ antenna with the
most myths is the helix. Dozens of websites have simple calculations for
determining the dimensions for your desired frequency. There is a problem,
though. The calculations themselves come out about 3 dB too high. However,
few hams actually build the helix per the assumptions behind those
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