Richard Harrison wrote:
SNIP
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.

SNIP: Agree

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.

SNIP: This resistance is the Radiation resistance of the antenna, i.e.
approximately 73 ohms in a thin 1/2 wavelength dipole.

Half the signal power is
consumed in the source resistance (radiation resistance) and half is
consumed in the receiver.

SNIP: Not quite. Half is RE-RADIATED. [It does not dissipate it
radiates!][See your next statement]. The other half is delivered to the
transmission line sub-system then to the receiver.

The half consumed in the radiation resistance
is re-radiated.

SNIP: Agree

The antenna doesn't know that re-radiation is uncalled
for.

SNIP: I wonder if this statement is the root of our misunderstanding? My
understanding is that the antenna does not have to know anything other
than passively allowing the Laws of Nature [Physics] to operate.

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.

SNIP: Have to think about what you are trying to say. If the antenna has
received a 10^-12 watt signal and 4*10^13 watts is delivered to the
transmission line and 5*10^-13 watts is reradiated then 1*10^13 watts
is energizing a standing wave in the antenna.


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.

SNIP: Help me understand what you are trying to say.

So, the transmitting antenna system can be
more efficient than the receiving antenna system, it seems to me.

SNIP: I probably disagree. But, I do not fully understand what you are
trying to say in the previous paragraph.

Deacon Dave


Best regards, Richard Harrison, KB5WZI