Winter 2010 Issue

ANTENNAS

Stacking Yagis

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.
In figure 1 are two identical antennas. While I drew the capture area as a line, in reality it is a tapered area with very fuzzy boundaries. You want to stack the antennas at a distance where their capture areas overlap just a bit. A little more overlap and you have a cleaner pattern and less losses in the phasing harness. Farther apart and you get a little bit more gain, but more masts and longer phasing lines are needed, and in the end you lose most of that extra gain. Also, for those of us with EME arrays, stacking for optimum suppression of side lobes is taking this exercise to an entirely new level.
In figure 2 I chose a 144-MHz and a 432-MHz Yagi. Of course, any two bands could be used, and a higher gain Yagi would have a larger circle. To prevent interaction between the two antennas we need to stack them as shown in figure 3, right? No, because we have to consider the capture area of the Yagis on the same frequency at the same time. In figure 4 you can see how the capture area of the 144-MHz Yagi is very small at 432 MHz, and the 144-MHz capture area of the 432-MHz Yagi is next to nothing. This means we can safely stack the antennas much closer together. Just think of some of the VHF rover stations with over a dozen antennas on their roof racks.

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.

Pitfalls in Helix Antenna Construction

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 calculations.
The helix in photo B looks good, but it doesnít work. Iíll be going into the assumptions and construction pitfalls next time.

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