Cecil Moore wrote:
I vaguely remember something about 50 ohms being good
for transmitting and 73 ohms being good for receiving.
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50 ohm line is about optimum for power handling, given the ID of the
outer conductor, and 75 ohm line has about the least attenuation for a
given ID of the outer conductor. I think a European compromise was to
choose 60 ohms as a standard impedance.

RF