On Jun 14, 2:56*pm, K7ITM wrote:
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

Tom, you stated earlier that you measured the source impedance of a
TS520S transceiver by inserting a somewhat off-resonance signal into
the output terminals when the rig was delivering 70 watts, and the
source impedance was measured as 56+j16 ohms. However, you chose not
to describe the setup or the procedure for obtaining this data.

I'm hungering to learn of the setup and procedure you used, because
I'd like to know what reflection mechanism gave a return signal that
could be discriminated from the 70w output signal from the
transceiver.

In his Nov 1991 QST article Warren Bruene, W5OLY, used what I believe
is a similar procedure, in which he claims he measured the Rs that he
called the 'source impedance' of the RF amp. He used his measurements
in asserting that because his Rs didn't equal RL there could be no
conjugate match when the source is an RF power amp. I have never
believed his procedure and measurements were valid, and I still don't.
So if your setup in any way resembles what Bruene presented in his QST
article I would like to know how you can justify a procedure that
involves inserting an off-set frequency signal rearward into an
operating RF power amp to determine the source impedance.

Walt, W2DU