I'm simply trying to establish the radiation resistance of a non-ideal
antenna so I can reasonably match it to the output impedance of the
transmitter PA stage.
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The input impedance of a 1/2-wave resonant dipole is about 70 ohms. But
this may be at a considerable distance from the transmitter output
terminals. What do you have in mind to put in between?

Then all you have to do is find the purely resistive load the transmitter
would be most happy with. Almost certainly it will not correspond to an
impedance match.
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Reg G4FGQ