> That's exactly the case in which it I think it DOES apply. The signal
> comes from one point, but the noise comes from all over. So as you
> narrow the beam width you reduce the sum total of noise that is picked
> up. Then you multiply this lesser amount of noise by the gain of the
> beam.
Noise does not multiply by peak antenna gain, unless the noise is
in the peak response of the antenna. My web page outlines how to
do this, but what you do is take the ratio average gain to gain in the
desired direction to determine antenna system signal to noise
response for evenly distributed noise. If noise is concentrated in
one direction you compare gain in that direction to gain in the
signal direction.
> I would guess that if you had a non-directional antenna with the same
> capture area as a beam, at the same location, with both antennas
> perfectly matched, etc., then the general atmospheric noise that you
> would hear would sound exactly the same on both antennas!
That is not quite true. Effective aperture (what we often wrongly
hear is capture area is related to antenna physical size) is a direct
product of the frequency and maximum antenna gain, while
receiving S/N relates to directivity and the direction of noise.
Capture area (correctly called effective aperture) has nothing to do
with S/N, unless the system is noise limited in the feedline and
receiver, a case we almost never have below VHF or with terrestrial
aimed antennas.
> difference would be that the noise on the beam would be reduced
> because of coming from a smaller area, and then multiplied by the gain
> of the beam. The signal would be multiplied by the gain of the beam
> but wouldn't have been reduced.
>
> Does this reasoning make sense?
No, because noise power delivered to the receiver is not multiplied
by antenna gain unless all or the vast majority of noise comes from
the direction of maximum antenna gain.
Of course neither is the receive signal multiplied by antenna gain,
unless it comes from the gain maximum and has the correct
polarization.
Noise power of noise evenly distributed around the antenna relates
only to average gain of the antenna compared to absolute gain in
the direction of the desired signal.
A -20dB gain antenna can improve S/N ratio over a 10 dB gain
antenna or can make it worse, and either one can deliver more
noise to the receiver if noise is not evenly distributed in direction
and polarization.
73, Tom W8JI
W8JI@contesting.com
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