[TowerTalk] Funniest thing I've seen in weeks

[Jim Brown]

On Fri, 30 Jul 2004 07:29:54 -0700, Jim Lux wrote:

>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 you describe sounds a lot like a diversity receiving system if you assume 
two receivers at each end. But one really big caveat is that you must account 
for the arrival time and polarity differences when you add the two signals!  If 
there happens to be a polarity difference between the two signals, they cancel 
rather than add. Also, the improvement is SNR assumes that they two 
receivers get equal signals from the transmitter. In the real world, there will be 
differences due to fading, so I would expect diversity to be the more dominant 
factor. 

If we're talking detected audio, it's quite easy to achieve this result with means 
as simple as running the earlier arriving signal through a digital delay line. If 
there are multiple desired signals arriving from multiple directions and 
locations, this delay line will need to assignable to either receiver output, and 
will need to be very easily and quickly adjustable. DSP products that do this 
are readily available off-the-shelf in the pro audio world. Achieving this variable 
delay at RF may be a lot tougher! 

Jim Brown  K9YC