Cecil, W5DXP wrote:
"Antenna gain over isotropic is an application of constructive
interference."

Yes. An often offered annalog is an inflated spherical balloon. It
contains the same amount of air no matter how it is squeezed. Sqeeze it
one place and it bulges elsewhere.

An isotropic antenna, could one be constructed, would radiate equally
well in all directions. As a radiation pattern becomes lopsided, the
bulge is filled with the energy squeezed from elsewhere.

Directive gain of an antenna is a power ratio. It`s the power that you
would have to put into an isotropic wersus the power you have to put
into the gain antenna to lay the same signal on a point in the preffered
direction.

Other things equal, if a gain antenna radiates twice the power in the
preferred direction as an isotropic, it has a gain of 3 dBi.

Best regards, Richard Harrison, KB5WZI