> I'm trying to understand your statement below.
>
> If the received noise is uniformly distributed in azimuth
I would think
> that additional gain, presuming that it comes from a
narrower azimuthal
> pattern, would increase the SNR of the desired signal.
Presuming it does, you are correct. Any additional
directivity (not gain) increase in the dorection of signal
would result in the same improved S/N when the noise is
uniformly distruted.
> If there is a lot of noise from one direction and the
increased
> directivity is such that there is now a null in the
direction of the big
> noise then the SNR of the desired signal would increase a
lot.
> Am I on the right track here or totally out to lunch?
No, that's how it works. Directivity and the direction of
signal and noise determine the change in S/N ratio.
Gain is directivity times efficiency, so we have to sort out
if a gain change moved response away from noise and towards
signal (better S/N) or gain increased without greater
attenuation of noise and/or focusing on signal.
Let me give you an example:
I have a receiver, like most non-microwave receivers, that
limits on propagated noise. I change the feedline and remove
10dB of loss. I'm 10dB louder in Europe. Do I hear more DX?
Not at all. I hear exactly the same.
I have a two-meter repeater antenna in a clear field and the
primary noise comes from the horizon at zero elevation, from
all the junk in the distance around the antenna. I change
the 1/4 wl groundplane to a 6dBd collinear, do I get better
S/N? Maybe a tiny bit, but nowhere near 6dB worth. The new
antenna still primarily focuses on the noise. To obtain
gain, the collinear removes response at angles other than
the horizon where there is very little noise. Since the
noise mostly comes from 0 degree angle, S/N barely changes.
It is a common myth that is repeated over and over again
that any improvement in antenna gain pays a double payback
as an improvement in receiving. A recent QST article
assigned a FOM to improvements, and the author incorrectly
always assigned a receiver improvement to antenna system
gain improvements in his FOM. The incorrect idea that more
gain=better receiving is so woven into our minds that no one
caught or corrected the error.
An increase in gain can change S/N any amount from a
decrease to a huge improvement.
73 Tom
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