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Re: [TowerTalk] Combining antennas

To: Pete Smith <>,
Subject: Re: [TowerTalk] Combining antennas
From: Jim Lux <>
Date: Tue, 03 Feb 2004 12:16:55 -0800
List-post: <>
At 02:02 PM 2/3/2004 -0500, Pete Smith wrote:
At 10:51 AM 2/3/04 -0800, Jim Lux wrote:
So, the SNR will be 0.5Sa/.707(Na+Nb). If Na=Nb, this is .5/1.414 or about 4.5 dB worse than it was before.

If the noise is correlated, then it will either be as bad as 2x or completely nulled out, depending on the phasing.

If you have a good hybrid junction, you can get more of the power to flow to the radio, rather than the other antenna (assuming everything is "well-matched"). Assuming an ideal hybrid, then, the SNR at the radio will be:

Sa / (.707 *(Na + Nb) ) for uncorrelated noise (a 1.5 dB hit) , or, Sa/(Na+Nb) for correlated noise (a 3 dB hit)

In more practical terms, what you're doing is creating an antenna with more "capture area" with two lobes.

If I understand correctly, in domestic contests where signals most often are a LOT above the ambient noise level, the receiving penalty will be acceptable. Once in a while, for example when working a weak back-scatter signal, it will be important to find the single antenna that gives the best copy. Happily, this is also probably the situation where you would concentrate all your transmit power to the best antenna too. But for maximizing signal over the widest possible geographic spread, being able to feed two or three antennas at once will be a good thing, as Martha would say.

Are these fair conclusions?

I think so.. What you've basically got is an antenna with "adjustable" beamwidth, which can be very handy. Add adjustable phasing, and you could have a "interference nulling" system, which is even handier.

I have to say that all that I said above is nice and theoretical, and I've never actually tried it. I suspect that more than one person on the list has tried this and can tell you if theory and reality meet.


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