[Amps] Alpha Input SWR

[Paul Christensen]

> 
>  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