Actually, several people (W8JI among them) have measured the output
impedance of common amateur linear amplifiers by at least a couple of
methods. The most credible measurements show, interestingly, a value
very close to 50 ohms when the amplifier is adjusted for normal operation.

Of course, it doesn't really matter, but people continue to make a big
deal out of it.

Roy Lewallen, W7EL

Tarmo Tammaru wrote:
"Richard Clark" wrote in message
...

On Fri, 18 Jul 2003 09:28:07 -0400, "Tarmo Tammaru"
wrote:

I've read for years that the common RF rig is NOT a 50Ohm source, and
absolutely none dare commit themselves to just what value it is (much
less offer their own measure). Being a physical reality, the rig must
present some real value, but vacuous theory seems to bar that
discussion.

73's
Richard Clark, KB7QHC


Yeah, seems to be a deep dark secret. If you look at the specs of RF power
transistors, they will give the output impedance vs frequency - BUT you have
to look at the footnote. In virtually all cases what they mean is the
conjugate of the load impedance. It is the jX of the transistor (1/jY), in
parallel with
((VCC-Vsat)**2) /2P.

I have never gotten around to doing this, but I believe the data sheets for
tubes like the 811A and 813 do give the plate resistance, which should make
it possible to calculate the output impedance at the lower frequencies like
160m.

Tam/WB2TT