[Amps] Power/VSWR readings

[Steve Thompson]

-----Original Message-----
From: wlfuqu00@uky.edu <Wlfuqu00@uky.edu>
To: Kim Elmore <elmore@nssl.noaa.gov>; AMPS <AMPS@contesting.com>
Date: 02 April 2002 17:45
Subject: [Amps] Re: [Amps] Re: [Amps] Re: BirdŽ 43 Manual


snip
>         Beyond 1:1 SWR  the forward and reverse power measurements are
>only an approximations. Closer at lower SWR's and worse at higher SWR's...
I find it helps to consider this as two separate issues:

What the readings tell you - two things, the degree of mismatch at the
output of the meter (relative to the meter Zo) and the amount of power
absorbed by the feeder and load downstream of the meter. If the meter has
perfect isolation between forward and reverse readings (infinite
directivity) then I think these are valid regardless of the VSWR.

Whether they mean anything - a practical meter has finite directivity, some
of the forward 'power' leaks into the reverse reading. If the leakage is
equal to the real reverse reading, then at some phase they will cancel,
180deg different they will add. The reading you see on the meter could be a
false zero, or indicate that the reflection is higher than real.

Accurate measurement needs about 40dB directivity or better, and this comes
expensive in bought gear. For narrow band amateur use it's easier to tweak a
coupler to be this accurate, but only if you have a load that's guaranteed
1.01 VSWR! A coupler or meter with 20dB directivity is best used as a door
stop. Most instruments I've measured come out between 25 and 30dB, amateur
brands at the poorer end. 30dB gives you a useable instrument. Somewhere I
have a QST page with a chart of what meter readings you might see for a
given real VSWR for different directivities, and it will put you off
measuring VSWR for the rest of your life.

Steve