[Amps] Antenna vs. amp (was: al-1200 question)

[Gary Schafer]

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