Miguel Chezzi, LU6ETJ wrote:
"In a deep focal point of parabolic dish two antennas are mounted...

Which of them does pick up more energy?
An antenna with 180 degree beamwidth or a highly directional sntenna
with 0.1 degree beamwidth (both pointed to dish, of course)?"

I`ll risk being the fool. We sometimes test for illumination of a
reflector. We would not be concerned were it not advantageous to do so.

With 180-degree radiation, we fill the dish, using all its surface.
With 0.1-degree illumination, we might as well remove all but the
illuminated area. It would save dead load and wind loading.

My answer: The 180-degree radiation angle will receive a larger area of
the plane-wavefront and extract more watts from the wave with a given
number of watts per square area.

Best regards, Richard Harrison, KB5WZI