Topband: noise

[k3bu at optonline.net]

> 
> We used two receivers, and used the IF outputs from those to 
> drive a 
> comparator. In one input was N; and in the other input was S+N, 
> and the output was 
> the difference or S. We got good recordings and could not have 
> without that 
> system. It requires the same phase thru both paths for the 
> comparator to 
> work.
> 
> I have two TS-850's, which have an 8.83 MHz IF output. I wonder 
> about the 
> comparator circuit and what circuitry to use beyond that, or 
> whether to try 
> and insert that back somewhere into the 850?
> 
> Bob W7LR 

I have been using "el-cheapo" version of this by using two radios, two antennas and feeding it into headphones and "my comparator" - my own scrambling brains. Even if not scientifically perfectly balanced cancelling, it helped in many situations when I would hear and copy weak signal with both rigs going, where either of them alone produced no detectable signals. I couldn't even tell which one was supplying S+N and which just N or ?. Switching to different antennas helped to optimize for the best confirguration, but needs few repeats to zero in on best combination. This is why it helps on first ? or AGN? that stations repat their call or reports few times. Couple that with rolling polarization shift (tens of seconds) the topband stalwarts know that it pays to repeat and don't go away after the first ??.

The coming wave of SDRs is poised to take advantage of the effect by using multiple antennas, with multiple RXes and replacing our sclerotic brains with better DSP algorithms. Less manufactured noise in RF and mixer circuits, absence of mushing crystal filters will also help and days of digging out signals -30 dB under the noise are around the corner. 

73 Yuri, K3BU