[TowerTalk] Combining antennas

[Pete Smith]

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?


73, Pete N4ZR
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