Richard Harrison wrote:
. . .
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.
. . .

This is a bit misleading. Typically, an antenna extracts only a tiny
fraction of the energy from a passing wave. The actual amount of energy
it does extract is expressed as its "capture area" or "effective
aperture". A recent posting I made includes a way to calculate this if
you know the antenna's gain. Because the "capture area" is usually a
tiny fraction of the wave's total area, only a tiny fraction of the
wave's energy is extracted.

Roy Lewallen, W7EL