[Elecraft] K3 SWR Accuracy - reprise

[David Woolley]

Phil & Debbie Salas wrote:

> The impedance changes, but not the SWR.  Looking at this on a Smith 
> Chart, you can see that you just rotate around a constant SWR circle.  

You don't even have to introduce Smith Charts.  The basic definition of 
SWR involves distributed measurements along the feeder, That means that 
  a different SWR cannot exist at different points on the cable, because 
it cannot be measured, by its proper definition, at a single point. 
Unfortunately a lot of mystique has arisen over "SWR"s as a figure of 
merit, when it was originally just a pragmatic measurement.  Modern SWR 
meters actually have to do excess calculation in order to get the "SWR" 
from equally good measurements (i.e. |reflected voltage / forward 
voltage|) of the goodness of match to the nominal impedance.

The original definition of SWR is (RMS) voltage at a voltage maximum on 
the cable over (RMS) voltage at a voltage minimum (and can only be 
directly measured for a cable that is at least a quarter wave long).

SWR meters don't actually measure the SWR on the line, but rather the 
SWR that would appear on an ideal 50 ohm line on the input side of the 
meter.  A PA will be killed by excess voltage or current, not by "SWR", 
so  the same "SWR" may or may not be dangerous, depending on the phase.

[Cutoff point for thread kills was Digest 67:10.  At that time the 
fundamental definition of SWR hadn't be raised.)
-- 
David Woolley
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