----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
> >
> > 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.
>
> No... consider the noise as just another signal, so increased gain in a
> particular direction (of the signal) increases the noise AND the signal by
> the same amount. SNR stays the same.
> -> Exceptions:
I don't think what you are saying is correct, Jim. If the noise
is uniformly distributed, increasing antenna directivity will
not change the total noise power received. The higher
directivity antenna will collect more noise power, but from
a smaller section of the sky, so the net noise power collected
stays the same. This will be true as long as the noise source
is distributed over a larger area than the beamwidth of the
lowest directivity antenna being compared. In radio astronomy
lingo, they refer often refer to these kinds of noise sources
as "extended" sources. Thus even for uniformly distributed
noise, increasing antenna directivity can and often does
increase SNR.
73 de Mike, W4EF.......................................
> 1) VHF or higher where receiver noise dominates...
> 2) Where the antenna is physically large enough that you can coherently
> combine two paths with the signal, while the noise is uncorrelated. An
> array with antennas separated by LARGE distances (many wavelengths) might
> achieve this (imagine combining the receive signals from separate antennas
> in Los Angeles and San Francisco... the noise probably isn't identical)..
> The improvement in SNR is sqrt(Nantennas) in this case. (What this really
> is is a way to make the original assumption (noise uniformly distributed
in
> azimuth) not true any more)
>
> >
> > 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.
>
> Yes... and, consider that "noise" isn't usually what you're worried
about..
> it's some other station(s), which come from a single (but different)
> direction
> >
> > Am I on the right track here or totally out to lunch?
> >
> > 73 de Jim Smith VE7FO
> >
> > Tom Rauch wrote:
> >
> > >This is actually pretty important, because MANY people think
> > >7dB of additional antenna gain improves the receiving and
> > >transmitting 7dB. The actual receiving change can be
> > >anything from zero to dozens of dB. If for example you have
> > >a 15 meter Yagi with 3dB of feedline loss and improve system
> > >gain 2dB by replacing the feedline, the receive S/N will
> > >remain the same.
> > >
> > >73 Tom
> > >
> > >
> > >_______________________________________________
> > >
> > >See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
any
> questions and ask for Sherman, W2FLA.
> > >
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> > >
> > >
> > >
> >
> > _______________________________________________
> >
> > See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
any
> questions and ask for Sherman, W2FLA.
> >
> > _______________________________________________
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>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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