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

Richard Harrison wrote:

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

The question you have to ask yourself is, does it intercept all of the
energy reflected toward it, or only some fraction of it.

We should always be cognizant of the limits imposed by the absence of a
free lunch.

ac6xg

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

See comment below.
--
J. Mc Laughlin; Michigan U.S.A.
Home:

"Richard Harrison" wrote in message
...
Jim Kelley, AC6XG wrote:
snip

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.

----- ... not if one wishes to maximize SNR. Best SNR requires a (slight)
mismatch. Of course, this issue is only significant if SNR is due to the
noise figure (NF) of the receiver and the SNR is small.
73 Mac N8TT