John Smith wrote:
"Oh...This is about gain?"

Don`t think so. This is about unequal currents into and out of an
antenna loading coil. The effect a loading coil has on received signals
is due at least in part to its effect on radiation resistance versus
total (radiation + loss) resistance. If you increase radiation
resistance as comared with loss resistance you increase effective
radiated power. Directive gain has nothing to do with loss.

Here is Terman`s comment on gain in Note 2 on page 870 of his 1955
edition:
"Directive gain depends entirely on the distribution in space of
radiated power. The power input to the antenna, the antenna losses, or
the power consumed in a terminating resistance have nothing to do with
directive gain. Such factors are taken into account in terms of the
power gain of the antenna which is defined as the ratio of the power
input to the comparison antenna required to develop a particular field
strength in the direction of maximum radiation, to the power input that
must be delivered to the directional antenna to obtain the same field
strength in the same direction. Unless otherwise specified the
comparison antenna is a lossless isotropic radiator."

Best regards, Richard Harrison, KB5WZI