On Jun 13, 11:42*pm, Owen Duffy wrote:
Owen Duffy wrote :

...

In the measurements of an IC7000 that I made, the measured output
power on one VSWR(50)=1.5 load was 82.5W when it would have been
104.6W had the source been 50+j0, an error of 0.8dB. I opined that
this test did not support the proposition that Zs was not 50+j0

Too many "nots", isn't there?

It should read:

I opined that this test did not support the proposition that Zs was 50+j0..

Apologies, Owen.

I suppose this will be buried where nobody will read it...

I realized that with the nice instrument-grade directional couplers
that came with a new 100W RF power amplifier, and with the other
equipment on my bench, I can measure RF amplifier/transmitter source
impedance relatively easily and with good accuracy. I strongly
suspect the accuracy will be limited first by how well the setup of
the transmitter/amplifier can be duplicated, and not by the
measurement instruments.

I won't go through the whole test setup, but just say that
substituting an open or short for the connection to the transmitter
yields the expected amplitude return signal, and terminating the line
in a precision 50 ohm calibration standard yields a 47dB return loss,
for the frequency I was measuring (nominally 7MHz, for this first
measurement). The measurement involves sending a signal offset from
the nominal transmitter frequency by a few Hertz at about -20dBm
toward the transmitter, and looking at what comes back.

Measuring a Kenwood TS520S, set up for about 70 watts output, ALC
disabled, operating as a linear amplifier somewhat (about 30 watts)
below its maximum output: result is 56+j16 ohms at the output UHF
connector on the TS520S. That's about 1.4:1 SWR, and at some point
along a lossless line, that's equivalent to about 70+j0 ohms: not
terribly close to 50 ohms. I'm not going to bother with a detailed
error analysis presentation, but I'm confident that the amplitude of
the return loss is accurate within 0.1dB, and the phase angle within
10 degrees, to better than 99% probability.

I may make some more measurements with different amplifier setups and
at different frequencies, but for now, that's it...

Cheers,
Tom