[Amps] Power/VSWR readings

[Steve Thompson]

On Wednesday 03 April 2002 12:27, Tom Rauch wrote:
> > 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.
>
> Sometimes 20dB or so is close enough to perfect. 20dB is 1%.
1% power, but 10% voltage or current.
>
> That is not a significant error when the meter itself is 5% or so of
> full scale anywhere on the scale.
>
> Read the measurements I posted, or better yet make some
> measurements of your own.
>
> The Bird meter works just fine if you subtract reflected power from
Agreed - and I thought your measurements demonstrated the reality very well.

To give an example of what I was meaning to convey, consider measuring a VSWR 
which is really 1.22:1. That's 1% reflected power, or 10% reflected voltage. 
Assume the meter has 20dB directivity, so 10% of the forward voltage is added 
into the reverse reading. Depending on phase, the meter can indicate powers 
that equate to anything from 1:1 to 1.75:1 VSWR. Change to the 30dB 
directivity you might expect in a Bird, the VSWR error falls to about +/- .05 
at 1.2:1, and the power error becomes insignificant.

Steve