[TowerTalk] Combining antennas
Jim Lux
jimlux at earthlink.net
Tue Feb 3 12:16:55 EST 2004
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.
More information about the TowerTalk
mailing list