Re: LF: Cats among pigeons

["James Moritz" <james.moritz@btopenworld.com>]

Dear Graham, Eddie, LF Group,

> There  is  3 dB gain  in each   X2 time  step
One would expect so, but Eddie's results apparently contradict this. But, if 
the transmitted signal power and noise power spectral density remain the 
same, one would expect the indicated SNR to be the same also, independent of 
the mode selected, since the SNR is referred to a constant bandwidth 
(2500Hz), rather than the actual bandwidth occupied by the signal.
> I think  without using  path simulator  or  similar  these  are a little 
> subjective ..
This is certainly true; 3dB variation in either signal or noise level can 
occur very quickly on 500kHz. Some time ago, watching G4JNT's WSPR beacon on 
500k, the signal level would sometimes vary by more than 30dB during a 
single 2 minute TX period. That was in darkness over about 120km as I 
recall; a shorter path during daylight might have much less variation, but 
3dB could still easily occur.
What is needed is some independent way of determining the SNR during each 
transmission. But a bit tricky when the signals are very weak. 
Alternatively, transmit both signals simultaneously, from the same 
transmitter, over the same path, and decode both simultaneously, which, 
conveniently, Opera can do with some modes at least. This type of test seems 
like a good application for the G0NBD dual-channel beacon...
Cheers, Jim Moritz
73 de M0BMU