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Re: [Amps] Alpha Input SWR

To: Amps group <amps@contesting.com>
Subject: Re: [Amps] Alpha Input SWR
From: Paul Christensen <w9ac@arrl.net>
Date: Sun, 11 Jan 2015 11:24:09 -0500
List-post: <amps@contesting.com">mailto:amps@contesting.com>
> 
>  The standing wave ratio is the standing wave ratio and it refers only
> to the match between any transmission line impedance and it's load's
> impedance.  (never anything else)

Almost never anything else.  Example:  100 ft of line comprised of two or more 
sections whose characteristic Z are different.  

Let's say half the line is a Zo=50 and the other half is 70. The SWR is 
different along the line. It's one value for the 50-ohm section and a different 
value on the the 70 section. 

Getting back to the original issue, I'm intrigued as to why some amp 
manufacturers specify a length of line between the transmitter and amp.  

As others have pointed out, SWR should remain constant when the load Z is 
constant -- as long as there no current flow on the outside of the outer 
conductor.  Any mismatch at the amp input results in a Z change in a circle 
around the Smith Chart as also previously mentioned.  But our amps are used on 
many bands so what's the mathematical computation to get to a specific length 
of interconnect cable when there no wavelength association?  

In the case of the 30L-1, I had read that Collins specified a length that was 
used to stabilize the amp on one band.  In essence the interconnect cable 
becomes a line stretcher to move the complex Z to stable point on the Smith 
Chart.

For highly swamped, resistive amp inputs, SWR between the transmitter and amp 
should see little change with amp class over the RF cycle.   This is the case 
with the tetrode based ACOM 2000A.  I had thought the Alpha 8410 had a similar 
swamped input structure. 

Paul, W9AC
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