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RE: LF: RE: 8270.0025 Tuesday Apr 19

To: <[email protected]>
Subject: RE: LF: RE: 8270.0025 Tuesday Apr 19
From: <[email protected]>
Date: Wed, 20 Apr 2016 14:58:36 -0400
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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).



-----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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