Rob Atkinson, K5UJ wrote:
"I also assume that
reflected power, or vswr, is being measured in the shack. One thing that
bothers me about this is that this doesn't guarantee that the Z at the
feedpoint is actually 50 j0. It's probably close enough on the lower bands
but on 10 or 12 meters, where the feed is likely to be mulitple lambda in
length, I wonder if using the antenna as a tuner gets the user into
something like the "my feedline tunes my antenna!" type of situation i.e.
the meter in the shack may show true unity along the line, or maybe you just
happen to have adjusted the antenna so that the length of feedline you are
using and your frequency result in a nice looking match at the shack but the
feedpoint, if you could measure there, would tell a different story?"
Rob, I'm no expert on transmission lines, but I believe you are correct
that, with one exception, the Z measured at the shack is not necessarily the
Z measured at the antenna feedpoint. However, the one exception is a biggie:
If you are using a 50 ohm transmission line, then the only way you can
measure a 50 ohm resistive impedance at the shack end of the line is if the
antenna feedpoint is also 50 ohms resistive (at the measurement frequency).
That's why tuning for 1:1 VSWR (which is equivalent to tuning for a 50 ohm
resistive load) guarantees that the antenna feedpoint is also 50 ohms
resistive.
If I'm wrong, then I hope some of the experts on the reflector will set me
straight. Thanks!
73,
Jim W8ZR
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