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Date: Thu, 09 Feb 2012 23:15:53 +0000
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Subject: Re: LF: Cats among pigeons
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I have looked back at some other results this evening. There was QSB at 
this time. This was using my signal to Wootton Bassett 104km.

17:32    500 G3ZJO de G4MDH Op4 104 km -26 dB in Wootton Bassett with 
0.0002w + 5m inv.L - rx
17:27    500 G3ZJO de G4MDH Op8 104 km -33 dB in Wootton Bassett with 
0.0002w + 5m inv.L - rx
17:12    500 G3ZJO de G4MDH Op4 104 km -25 dB in Wootton Bassett with 
0.0002w + 5m inv.L - rx
17:04    500 G3ZJO de G4MDH Op8 104 km -30 dB in Wootton Bassett with 
0.0002w + 5m inv.L - rx

Eddie


On 09/02/2012 22:23, James Moritz wrote:
> 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
>
>