Jim Kelley, AC6XG wrote:
"The question you have to ask yourself is, does it intercept all of the
energy reflected toward it, or only some fraction of it."

Nothing is perfect as Jim observes. "Imperfections" are sometimes
exploited to improve an antenna pattern. To a first approximation
though, we assume that all the parallel rays intercepted by a dish are
focused on the radiator and aid, adding in-phase. Received carrier power
excites the antenna and this causes a minimum of 50% of this power to be
re-radiated if the antenna is perfectly matched to to the receiver load.
The antenna`s radiation resistance in this case becomes the Thevenin`s
source resistance for the receiver load on the antenna. This requires a
conjugate match between the antenna and receiver input impedances.

50% of the received power to the receiver is the best that can be done
under optimum conditions, that is , with a perfect match. With a 100%
mismatch, a short-circuit, 100% of the intercepted power is re-radiated
by the antenna. If the antenna is open-circuited, it accepts none of the
power focused upon it.

Best regards, Richard Harrison, KB5WZI