Vic Rosenthal wrote:
> 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
>
The only thing to remember is that you can not have gain in an antenna unless
you
have directivity. Gain in a particular direction comes from reducing it in other
directions.
73
Gary K4FMX
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