Friday, July 23, 2010

I have a question about an HF/ VHF SWR/power meter I just purchased. I’m using an ADI AT600HP 2m/70cm hand-held transceiver ...

Q Andy, KB1ETK, asks, “I have a question about an HF/ VHF SWR/power meter I just purchased. I’m using an ADI AT600HP 2m/70cm hand-held transceiver. It generates 5 W output on the ‘high power’ setting when I use the 13.6-V battery. When I hook up my home-brew 1/4-wavelength vertical the meter measures an SWR of 1.3:1 and the RF power measurements are correct: 5 W on high, 2 W on medium, 400 mW on low. But when I attach the rubber duck antenna that came with the radio (with the SWR/power meter in between), my measured power output shoots up to close to 10 W on high, 6 W on medium, and 3 W on low. How is this possible?”

A The power reflected at the rubber ducky is re-reflected back down the coax from the transmitter to the antenna. When it reaches the antenna, the power again reflects and the cycle begins again. On each reflection, there is some power lost in the transmission line. However, the net effect is that both the forward and reflected powers will read higher than they actually are. The difference between the forward and reverse power readings is the actual net power. Thus, when the SWR is higher than 1:1, the forward power will rise, but so will the reflected power.

Here’s a good case in point. I used to own an HF QRP rig that did not reduce power for high SWRs. Its maximum output was 2W. When I had a schedule with a station very close by and didn’t have a tuner handy, I ran a random wire around the room and connected the rig directly to it. The SWR meter said I had 10W forward and 8W reflected. The difference between forward and reverse powers was 10 − 8 = 2 W, just what it should have been.

Rubber ducks are poor antennas in all respects save one— portability. I wouldn’t be overly concerned about the high SWR with your rubber duck. VHF/UHF equipment typically transmits into a 3:1 to 4:1 SWR without suffering ill effects. Further, a rubber duck uses the H-T (and the human body holding it) as its “ground plane.” Putting a piece of coax between the rubber ducky and the SWR meter doesn’t yield the same amount of groundplane area and the feed-point impedance of the rubber ducky will change from when it is used in a more traditional fashion connected directly to the H-T.

From QST July 2000