[TowerTalk] Stupd question about measuring Z and SWR

[Hans Hammarquist]

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 at audiosystemsgroup.com>
To: towertalk <towertalk at 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