Perhaps this debate is really a measurement problem ---
Perhaps this debate is really a measurement problem ---
Part of the problem is that some people consider "resonance" and "matched to
the transmission line" (i.e. 1:1 VSWR) as being one in the same when they
are talking about antennas, when they most decidedly are not.
What is needed is an analyzer that can be calibrated with the
measurement plane at the far end of the coax. A VNWA,
AIM4170, SARK110,
etc. can be calibrated with S/O/L on the end of the cable so the
measurement plane is at the antenna. Then when jX = 0 you have
resonance. However, even then real R might not be 50 ohms, not 1:1
SWR. Otherwise some complex Z probably transforms to 1:1 SWR
on an SWR
bridge at some other frequency than Fres depending on the
length of coax.
Grant KZ1W
Maybe I'm not understanding what you were saying, but there's no way a 50
ohm cable can transform a load Z that's not 50+j0 to yield a 1:1 VSWR (ref
50 ohms). Please clarify.
I agree that the most reliable way of making feedpoint Z measurements is to
move the calibration plane to the end of the transmission line. But lacking
that, you should be able to get fairly accurate in situ feedpoint Z data by
starting with an accurate measurement of electrical length of the line
(using TDR) and then rotating the Z measured at the source end of the line
accordingly. Of course you have to factor in line loss as well, which is
best determined via a return loss measurement at the frequency of interest
with the line open/shorted (take the average of RL with the line open and
shorted, and then divide by 2 to get loss), but lacking that, manufacturer's
data is the next best thing.
--- Jeff WN3A
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