Last weekend, we set up a multi-transmitter station to participate in the Michigan QSO Party. The goal was to have two transmitters operating during the contest, one on 40m and the other on whatever band was open.
One of the antennas we put up was a “Slinky antenna,” so named because it uses two big Slinky coils as the radiating elements. To tune the antenna, you vary the number of windings that are active. You do this by shorting out a number of them on the end of each element.
When we first put it up, we used the tuning chart in the documentation to adjust the length of the antenna and the number of windings to expose. When we measured the standing wave ratio (SWR) with an antenna analyzer, it was about 7:1, not very good even for amateur operation. So, we increased the number of windings on each leg by 5, and the SWR dropped to about 3:1. We let out five more and the SWR dropped to about 1.25:1.
BUT, the guy who was reading the meter said the readings were jumping all over the place.
Hmmmm, I thought. What could be wrong? Did I perhaps not solder something correctly? Is the coax bad? Is the antenna inherently unstable?
I went down into the basement to see for myself, and sure enough, the readings would settle down and then bounce around for a while. It took me a couple of minutes to figure out that the problem was that while we were making these measurements, our second transmitter was busy trying to make contacts on another band. What we were reading as a reflection was actually RF being output by our other transmitter!
After we all got a good laugh out of that, we hooked it up to one of the transceivers and it worked pretty well for the remainder of the contest.
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