Editor's Note: Question your assumptions
Dan Romanchik, Technical Editor -- Test & Measurement World, 6/1/2003
On a recent weekend, our amateur radio club set up a multi-transmitter station to participate in a contest. One of the antennas we put up was a "Slinky antenna," so named because it uses two big Slinky coils as the radiating elements.
We used a commercially available antenna analyzer to tune the antenna, eventually getting the SWR down to about 1.25:1. But my teammate said the meter readings were jumping all over the place.
Hmmmm, I thought. Did I solder something incorrectly? Is the coax bad? Is the antenna inherently unstable?
I went to see the meter for myself, and sure enough, the readings would settle down and then bounce around for a while. I finally figured out the problem: While we were making the measurements, our second transmitter was trying to make contacts on another frequency band. What we were reading as a reflection was actually RF being output by our other transmitter.
The antenna analyzer works by injecting some RF into the antenna and then measuring the antenna's impedance. In making the measurement, the instrument assumes it is the only significant source of RF energy in the system. A nearby RF source, such as our second transmitter, negates that assumption and throws off the measurement.
When setting up test systems, always question your assumptions; ensure that your measurements are valid and your tests are effective. Failing to do so can create costly errors and expensive delays.
Contact Dan Romanchik at editor@aatr.net .
