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Re: [TowerTalk] isolation and SWR

Subject: Re: [TowerTalk] isolation and SWR
From: Pete Smith N4ZR <>
Date: Tue, 11 Sep 2012 07:15:12 -0400
List-post: <">>
190 feet of LDF4-50.  Antenna switch and 80m array switch are in series.

73, Pete N4ZR
The World Contest Station Database, at
The Reverse Beacon Network at, blog at,
spots at, port 7000 and, port 7000

On 9/10/2012 6:34 PM, Jim Lux wrote:
On 9/10/12 8:03 AM, Pete Smith N4ZR wrote:
Just now, I was fiddling with my homebrew 80m array switchbox.  I have
only two dipoles on it at the moment, both reasonably well matched.
Accidentally I switched to one of the unterminated positions, and got an
SWR on the order of 4.5:1 (feeding the dipole switchbox through my
Ameritron RCS-10.  By comparison, if I disconnect the feedline inside
the shack, the SWR is in excess of 25:1, and if I switch the RCS-10 to
an unused position, I get 4.5:1.

So, the RCS10 has mostly good matches on most of the outputs, and one that's unterminated. When you switch to the unterminated one, you get 4.5:1, and 1:1 if you switch to the others? And this is at 3-4 MHz?

4.5:1 is a return loss of about 4 dB. That is.. if you had a 2 dB pad in line and a dead short, that's what you'd see. How long is the coax to the RCS-10? Is 2 dB of loss plausible?

Is there even an approximate relationship between these various SWR
numbers and the degree of isolation afforded by the two switchboxes? And
probably an even dumber question.  If you have two devices in line and
each has (say) 30 dB isolation between a line in use and an unused one,
both of which pass through them both, is the resulting isolation 27 Db?
Some other number?

Are you talking two switches in series?

Be gentle - I was a liberal arts major.


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