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[TowerTalk] Isolation and SWR

Subject: [TowerTalk] Isolation and SWR
From: Wilson Lamb <>
Date: Mon, 10 Sep 2012 12:21:53 -0400 (EDT)
List-post: <">>
The isolation is in the number, but so is the line loss.
I'd have to study to get the numbers, but I'll bet someone pops up to tell us.
With a very short open coax, you have 100% reflection, so infinite SWR.
With an very long coax, all the power will be lost on the way out and none will 
come back.  Presto, SWR=1:1!
So a perfect box out there will look just like the line is open.
If there's some coupling (non isolation), the Z at the end of the line will be 
something other than open circuit and SWR will be different.
I guess it will go down, since some energy will go to the nonselected port.
How's that for barnyard analysis?

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Today's Topics:

   1. isolation and SWR (Pete Smith N4ZR)


Message: 1
Date: Mon, 10 Sep 2012 11:03:04 -0400
From: Pete Smith N4ZR <>
To: TowerTalk <>
Subject: [TowerTalk] isolation and SWR
Message-ID: <>
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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.

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?

Be gentle - I was a liberal arts major.

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


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