Tom Rauch wrote:
>
> 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 don't disagree with this. I was trying to explain intuitively why a directive
antenna has a better s/n ratio than a non-directive one. I was assuming that the
noise was evenly distributed. Then I reasoned as follows: if the antenna is
directive, then the noise that comes from the direction in which the antenna is
pointed increases while the noise from other directions decreases.
Then -- other things (i.e., what you refer to as 'average gain') being equal --
I would expect to hear the same noise level when switching between a full size
dipole and a beam at the same height. The difference is that the beam would be
hearing more noise from the direction in which it was pointed than the dipole.
However the dipole would make it up by hearing noise from other directions. Of
course a signal coming from the direction the beam is pointed would get
stronger. That's why a directive antenna improves s/n ratio.
> 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.
This is exactly what I was trying to say. If you divide the pattern of the
antenna into little slices, the total amount of noise presented to the receiver
is the result of integrating the noise level in each slice multiplied by the
absolute gain in each slice. So you could imagine two antennas, one that was
highly directive and the other not, with the same average gain. The directive
one would have a better s/n ratio, but both would hear the same absolute amount
of noise, always assuming that the noise is evenly distributed.
73,
Vic, K2VCO
Fresno CA
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