Now I suppose somebody will drag in the irrelevant matter of SWR on the
feedline.

Actually, you brought up the subject. :-) The feedline's power transfer
efficiency is just as important as the antenna's radiating efficiency.
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Cecil, you forgot the efficiency of the PA DC power supply. There's much
more energy wasted there than what's lost in the feeder.

Sort out your power budget. ;o)
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Reg.