Sent: Friday, October 18, 2013 11:41 AM
Subject: Re: LF: T/A OPDS DK7FC
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
