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Re: LF: Cats among pigeons

To: <[email protected]>
Subject: Re: LF: Cats among pigeons
From: "Graham" <[email protected]>
Date: Sat, 11 Feb 2012 17:02:00 -0000
Importance: Normal
In-reply-to: <[email protected]>
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All  noted  , Jim !

Bob WE6XGR , is running a OP2 / OP4 test tonight (11/2) from 00:00 z , using the 80 meter settings, this should give a play off 2/4 mode , then we should have the full picture of speed, as most of the decodes show above the OP2 limit.
Normal service should resume this Sunday night ... I assume 'we'
need the carrier levels to be the same .. that would reflect the
most 'practical' comparison .. (and the most simplest !)
73 -G.



--------------------------------------------------
From: "James Moritz" <[email protected]>
Sent: Thursday, February 09, 2012 10:23 PM
To: <[email protected]>
Subject: Re: LF: Cats among pigeons

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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