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Re: LF: T/A OPDS DK7FC

To: <[email protected]>
Subject: Re: LF: T/A OPDS DK7FC
From: "Markus Vester" <[email protected]>
Date: Fri, 18 Oct 2013 17:41:25 +0200
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Stefan, Jay, John,
 
the "96%" number shows the temporal overlap between the receive period (ie the duration of the currently used FFT, about 35 minutes) and the identified Op sequence (33 minutes). With 10 minutes spacing between subsequent FFTs, the sequence does not always fit completely into one of the slots. so it says that 96% of the sequence was included in the evaluation. However the missing part has only neglegible impact on the detection sensitivity.
 
Ideally Opera can be treated as an AM signal, with a central carrier and modulation sidebands around it. The "2mHz" bandwidth figure refers to the bandwidth of the carrier, which should be as small as possible. Opds applies some smoothing to the power spectrum and then tries to measure the -10 dB bandwidth of the central peak. Stable and phase coherent signals consistently show less than 3 mHz bandwidth. An intermediate width up to about 30 mHz typically indicates a coherent signal but with a slight thermal drift. Even higher bandwidth (~ 100 mHz) are mostly due to incoherent keying, ie random phase dashes caused by stopping the TX oscillator or divider during gaps.
 
Opds internally uses an "autofocus" concept similar to synchroneous demodulation, where the central spectral peak is used as a phase reference. Narrower carriers produce better demodulated SNR. For fading or incoherent signals, the phase has to be tracked faster or even on a dash-by-dash basis, which is much less efficient.
 
The "dBOp" column is showing SNR according to José's Opera scale, which is approximately based on average power. It shows 4 dB more negative values than the standard WSPR scale, ie. carrier power in 2.5 kHz. A marginally decode with WSPR-15 would need -38 dB, and an Opera signal with same PEP would then show as -42 dBOp. For a coherent signal, the Opds-32 threshold should be around -50 dBOp, which in theory is 8 dB better than WSPR-15 and 11 dB better than standard Opera-32.
 
Please be aware that the SNR figures shown in opds results can sometimes be inaccurate. With an incoherent signal, often not all of the carrier power is captured during the bandwidth measurement, producing a low or invalid SNR reading. The SNR measurement also doesn't work well for strong signals (> -20 dBOp), eg for DK7FC who should really be plus several dB here.
 
So why was Stefan's TA signal not stronger last night? My guess is that TA propagation just didn't extend into central Europe: While UK and duch stations received Bob well on 74 kHz, little or nothing at all apperared on Hartmut's and my 74 kHz grabbers.
 
Best 73,
Markus (DF6NM)
 
PS The weather has improved here, so I have put out the TX antenna for a possible joint TA session tonight.
 

Sent: Friday, October 18, 2013 2:36 PM
Subject: Re: LF: T/A OPDS DK7FC

Hi John, Jay, Markus,

OK. Well, it seems to work. But i can't value the results. Why is it only 96% and what is the meaning of 2 mHz here?
Seems the S/N is rather low. Condx must have been poor.
Were there other reports from US stns?

73, Stefan/DK7FC

Am 18.10.2013 13:05, schrieb John Andrews:
Stefan, Markus, Jay,

Results from last night:

2013-10-18 05:09:25 DK7FC   5981km 137560.016Hz   2mHz -42.8dBOp  96% 20.6dB
2013-10-18 04:29:25 DK7FC   5981km 137560.016Hz   2mHz -44.8dBOp  96% 17.8dB
2013-10-18 03:49:23 DK7FC   5981km 137560.016Hz   2mHz -44.0dBOp  96% 20.3dB
2013-10-18 03:09:23 DK7FC   5981km 137560.017Hz   2mHz -43.7dBOp  96% 19.0dB
2013-10-18 02:29:23 DK7FC   5981km 137560.016Hz   3mHz -47.5dBOp  96% 17.9dB
2013-10-18 01:49:23 DK7FC   5981km 137560.016Hz   3mHz -46.4dBOp  80% 16.8dB
John, W1TAG

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