Dave Shrader wrote:
Lots of good stuff. Reciprocity is applicable to antennas. Antenna gain
and directivity are the same transmitting or receiving. Impedance is the
same as a source or load.

Dave wrote:
"That is, it has the same efficiency transmitting or receiving."

I hadn`t given that much thought but it seems to me there may be a
difference. When an antenna is receiving, it is excited by the received
signal, resulting in voltage and current on the antenna. The antenna
doesn`t care about the source of the signal. If the antenna is
conjugately matched to the receiver, radiation resistance is the source
resistance of the signal feeding the receiver. Half the signal power is
consumed in the source resistance (radiation resistance) and half is
consumed in the receiver. The half consumed in the radiation resistance
is re-radiated. The antenna doesn`t know that re-radiation is uncalled
for. If the antenna is mismatched to the receiver, more than 50% of all
power received is re-radiated, depending upon the severity of the
mismatch.

If we have a Class C amplifier feeding power to the same antenna and
enjoying a conjugate match, we can have a source that takes less than
50% of the available energy. So, the transmitting antenna system can be
more efficient than the receiving antenna system, it seems to me.

Best regards, Richard Harrison, KB5WZI