Harry wrote:

I know that a half-wave dipole in free space has
a feed-point impedance of approximately 73 ohms.

Can anyone tell me exactly how this number is calculated.
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Simple Ohm's law: If you apply 73 volts of RF, 1 amp of current will
flow.

To calculate the effects of nearby ground a program like EZNEC is
useful, although you almost never know the exact parameters of the
ground, so the results should not be taken as precise.

73, Bill W6WRT