Ron, W4TQT wrote:
"How about a dish antenna?"

The parabolic reflector converts the spherical waves of its radiator at
the focus of the parabola into a plane wave of uniform phase across the
mouth or aperture of the parabola. Mouth ans aperture are syninymous
when applied to parabolic, lens, and horn antennas. Rays enter and exit
parallel but reflect through the focal point. Reciprocity rules and the
path through the antenna is the same, coming or going. The parabolic
reflector antenna sends and receives to and from a familiar spot on its
axis and at a distance. It is inoperative outside the spot and its path
of travel. The larger the parabola, the smaller the diameter of the
spot, and the higher the power gain.

The beamwidth of a large circular aperture such as a parabolic antenna
is inversely proportional to its diameter in wavelengths. The total
field radiated by a arabola is the vector sum of the fields generated by
the elementary areas making up the aperture or mouth of the parabola.
The directive gain of a parabola antenna is directly proportional to the
area of its mouth and inversely proportional to the wavelength squared.
See 1955 Terman page 899, equation (23-28) as pointed out at the bottom
of page 911.

Best regards, Richard Harrison, KB5WZI