----- Original Message -----
From: "Jim Smith" <jimsmith@shaw.ca>
Cc: "towertalk reflector" <towertalk@contesting.com>
Sent: Friday, July 30, 2004 1:46 AM
Subject: Re: [TowerTalk] Funniest thing I've seen in weeks
>
> 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:
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.
> >
> >_______________________________________________
> >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.
>
> _______________________________________________
> 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.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|