Paul,
Thanks for the excellent and definitive insights. I will enjoy running the
trials that you suggested (30-minute Speclab trial running now with 3kHz
bandpass, hum filter, blanker, and test signal).
73,
Jim AA5BW
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Paul Nicholson
Sent: Wednesday, April 20, 2016 1:37 PM
To: [email protected]
Subject: Re: LF: RE: 8270.0025 Tuesday Apr 19
I should mention that for sferic blanking to be this effective it is essential
to band limit the VLF before the blanker.
A broadly optimum bandwidth is 3kHz centered on the signal
frequency. Too wide and too much noise is modulated into the
signal band by the chopping action of the blanker. Too narrow and sferics are
stretched which gives too much blanking.
It is also important to make sure that the 3kHz band passed into the blanker
doesn't contain any significant constant signals
such as mains harmonics. Weak ones are OK so long as they
don't show above the 3kHz wide noise. Therefore you may have to precede the
blanker with an automatic hum notch filter.
Getting the sferic removal working right is crucial to detecting
weak signals at VLF. I'm sure that many reception attempts fail
because the blanking is nowhere near optimal. It might be very
well worth doing some trials using a weak near field source with amplitude set
so that it is only just visible without blanking.
Record about 30 minutes of raw VLF with the test signal buried in there, then
use the recording as input for trials of various settings with a bandwidth of
1/(30 mins).
In vlfrx-tools a typical detection pipeline is
vtread -T2016-04-19_13:15,+2h /raw | # Select the signal (3 chans)
vtmix -c 0.927,-0.375,0.5/-135 | # Antenna mix and phasing
vtfilter -h bp,f=8270,w=3000 | # 3kHz filter
vtblank -a1.2 -d0 -t1 | # Sferic blanking
vtnspec -f 8270.0025 -r 138.88e-6 -w0.05 > df6nm.160419a.dat
The blanker options are:
-a 1.2 Auto threshold, 1.2 times the moving mean level
-d 0 No dwell time
-t 1 One second time constant for the moving mean
--
Paul Nicholson
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