Hal Rosser wrote:
"(one acts like a receiver, and the other acts like a transmitter.)"

Back-to-back antennas both act like transmitters. Neither cares if its
energy came from radiation sweeping it or fed to it from its feedline.

The most energy any antenna can extract from a passing wave is 50%. That
is only when the receiving antenna is perfectly matched to its load. In
this case the load is an identical antenna, so the chances for a match
are good. But, at least 50% of the energy captured by the receiving
antenna is going to be reradiated by that antenna right back in the
direction it came from.

Another problem is the rapid decline with distance in the first
wavelength from the antenna. It losses 22 dB in field strength in the
first wavelength from the transmitting antenna. After traveling through
a second wavelength from the antenna (distance doubled from the antenna)
the field strength declines by another 6 dB. It loses 6 dB every time
the distance doubles. So, after 4 wavelengths, total loss will be 34 dB.

At great distances form the transmitting antenna, double the distance is
such a great distance that the field strength hardly varies at all even
when traveling directly toward or away from the transmitter.

It is obvious that path loss is a function of frequency and distance if
only from continuous expansion of the sphere of the electromagnetic
wavefront. There are less watts per square meter when the watts are the
same but the number of square meters is growing. Our initial loss
distance was determined by wavelength which is inversely related to the
frequency.

Best regards, Richard Harrison, KB5WZI