Exactly right, Grant.
I regularly measure Z in the shack with my DG8SAQ-designed VNWA, follow
that with a TDR measurement of the line at VHF, then use Dan's ZPlots
with short and open measurements of a shorter length of the coax to
determine Vf as a function of frequency, and then use SimSmith to
subtract out the transmission line to show the Z at the antenna (and to
design stub matching networks, when needed). It's important to realize
that Vf varies a percent or so with frequency, so to get Z at the
antenna we must use Vf at the frequency of the antenna. The error is not
great with electrically short lines, but can be a significant source of
error with longer lines.
73, Jim K9YC
On Tue,5/31/2016 8:03 AM, Grant Saviers wrote:
Perhaps this debate is really a measurement problem ---
I think the important thing to remember is that a SWR bridge is
probably NOT going to measure resonance when attached to some random
length of coax hanging in the air to an antenna.
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.
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