What I did was use a MFJ RCS-8 switch box, and have a short, open, and load at the antenna. So I
can recalibrate my feedline (or test it separately - did the squirrels eat something?). You really
only need the short and load. Open sort of comes for free. I just soldered a 50 ohm
non-inductive resistor for the load to one of the relays, and shorted another. You could probably
do this with the RCS-4, but I'd have to think about how you'd set it up so the short or load
doesn't foul up the control signals (yeah, there's sort of a bias T scheme). Potential side
benefit on some switch boxes - you can have the power off state be "grounded" for the
antenna and feedline.
On Tue, 10 Feb 2026 12:59:34 -0500 (EST), JAMES WOLF via TowerTalk
<towertalk@contesting.com> wrote:
Tom,
Ideally, you would include the feedline in the calibration of the test device.
That's not usually practical either. What you can do, is to calculate how the
feedline transforms the measurement. To do so, you need the impedance of the
line, the velocity factor, and the loss at the frequency of measurement. Then
you can model the feedline as a transmission line. attached to the source
(antenna). You should be able to use a program such as TLDetails to do the
math. https://ac6la.com/tldetails1.html
Jim - KR9U
On 02/10/2026 5:00 PM GMT towertalk-request@contesting.com wrote:
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Today's Topics:
1. Antenna Analyzer Question (Tom Hellem)
----------------------------------------------------------------------
Message: 1
Date: Tue, 10 Feb 2026 07:37:48 -0700
From: Tom Hellem
To: towertalk
Subject: [TowerTalk] Antenna Analyzer Question
Message-ID:
Content-Type: text/plain; charset="UTF-8"
In order to make measurements of impedance, etc on an antenna with any of
the various analyzers on the market, one must connect the analyzer to the
antenna
with some length of coax. And unless the antenna being measured has a
feedpoint
impedance of 50 +/- j0, the impedance as seen by the analyzer is going to
vary
continuously all along the length of the coax, a phenomenon easily seen
with the use
of a program like TLW, etc.
Question is, unless I can attach the analyzer directly to the feedpoint,
how can I
obtain a reliably accurate measurement?
I just looked at the user manual for one of the popular analyzers on the
market.
It makes no mention of this whatsoever. And I don't remember it being in
any other manual I have seen in over 50 years of being in ham radio except
for one:
I have a manual for a simple and inexpensive VNA built from a kit several
years ago, sold by a German company, that makes a big deal out of doing
what it calls an SOL compensation for
the coax and any other connectors used between the VNA and the antenna.
This
supposedly removes any of the impedance transforming effect of the test
setup from the
equation and allows for a precise measurement of the antenna under test.
So what gives? Has anybody here ever wondered about this?
Am I overthinking it or is there something to it?
Tom
K0SN
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