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
