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Re: LF: Cats among pigeons

To: <[email protected]>
Subject: Re: LF: Cats among pigeons
From: "James Moritz" <[email protected]>
Date: Thu, 9 Feb 2012 22:23:56 -0000
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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


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