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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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