[TowerTalk] TowerTalk Digest, Vol 278, Issue 4

[Jim Lux]

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 at 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 at contesting.com wrote:
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> Today's Topics:
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> 1. Antenna Analyzer Question (Tom Hellem)
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> ----------------------------------------------------------------------
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> 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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> End of TowerTalk Digest, Vol 278, Issue 4
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