[TowerTalk] TowerTalk Digest, Vol 278, Issue 4

[Jim Lux]

Cool..
SO that would make a convenient way to deal with multiple feedlines to multiple antennas. Put the RCS4 "receiving end" close to the feedpoint of the antenna.

I'm going to think, too, about the "auto-de-embedding" the coax with a wide sweep.  That's a "no additional parts required" solution.
The only thing the VNA won't do is for those situations where there's a Tx power dependent fault (i.e. something arcing or getting hot and changing).

Of course, a 1kW VNA could be built and used.
 


On Tue, 10 Feb 2026 17:29:02 -0800, Michael Tope <W4EF at dellroy.com> wrote:

Jim,

Everything on the RF side of the RCS-4 remote is DC blocked with a
capacitor relative to the common feed line, so you can place whatever
loads you want on any of the switch ports and it won't impact the switch
control function.

73, Mike W4EF............

On 2/10/2026 11:07 AM, Jim Lux wrote:
>
>
>
> 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  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:
>>
>>
>> Send TowerTalk mailing list submissions to
>> towertalk at contesting.com
>>
>> To subscribe or unsubscribe via the World Wide Web, visit
>> http://lists.contesting.com/mailman/listinfo/towertalk
>> or, via email, send a message with subject or body 'help' to
>> towertalk-request at contesting.com
>>
>> You can reach the person managing the list at
>> towertalk-owner at contesting.com
>>
>> When replying, please edit your Subject line so it is more specific
>> than "Re: Contents of TowerTalk digest..."
>>
>>
>> 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
>>
>>
>> ------------------------------
>>
>> Subject: Digest Footer
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>
>> ------------------------------
>>
>> End of TowerTalk Digest, Vol 278, Issue 4
>> *****************************************
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk


_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk at contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk