Richard Clark wrote:
"I keyed down and made sure the excitation was disconnected---."

Supposing you were measuring the impedance the source shunts its output
with, and supposing your amplifier is linear so it can be used for AM
and SSB, and supposing that it is Class AB or Class B for more
efficiency than Class A, and supposing that it has a small amount of
forward bias to reduce crossover distortion, you should measure a much
higher impedance while idling than when the transmitter puts full power
into a load.

The output impedance has a meaning at maximum power output. This can be
determined by trying different loads to find the load that gets the most
power from the transmitter. The source impedance is its conjugate.

Best regards, Richard Harrison, KB5WZI