When you want to measure the "real impedance" at the antenna, isn't it
important that you have a feeder (coax) that has the correct Zo=50 ohms all the
way?
I guess that the bridge will not read a correct antenna Z if the coax is not
perfect.
Hans - N2JFS
-----Original Message-----
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk <towertalk@contesting.com>
Sent: Tue, May 31, 2016 1:30 pm
Subject: Re: [TowerTalk] Change in Frequency As Antenna Height Rises
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
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