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Date: Fri, 20 Oct 2006 14:08:14 +0200
From: Wolf DL4YHF <dl4yhf@freenet.de>
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Subject: Re: LF: Re. TA Last Night
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Hello Markus,

Just a suggestion...

Instead (or in addition to?) of the "old" noiseblanker (inside SL's 
"DSP" blackbox), try the noise-limiter in the FFT-based filter, with a 
small FFT size for the filter. I tried it a few weeks ago at the club 
and found it worked even better than the classic noiseblanker. The trick 
is to set the limiter values (in the FFT filter control panel) just a 
few dB's above your usual noise level. Then everything above that level 
will be cut off, before transforming the signal back into the time 
domain. Note that this must happen with a low FFT size (256 points or 
less) to let it react fast enough for sferics. After this, the signal 
will be fed to the main spectrum analyser which of course runs with a 
larger FFT size (16384 points, plus decimation).
This "spectral limiter" also works nice for other kinds of local QRM 
which is "normally not there", and would otherwise spoil the 
spectrogram. Of course, when the noise is very strong, it will deafen 
the receiver.
If you like, I can search for some screen shots taken in dual-channel 
mode (with and without limiter+blanker). They must be somewhere on the 
club's "LF computer" ;-)

Cheers,
  Wolf .


MarkusVester@aol.com schrieb:
> Dear John and LF,
>
> yes my grabber seems to be plagued a lot. Most of the QRN is currently 
> emanating from the Mediterranan sea area. I believe the unilateral 
> directivity of the K9AY E-H-loops does help a lot for Hartmurt and 
> Jean-Pierre. However up to now my own attempts on H-field reception 
> have always resulted in decreased SNR due to all sorts of local QRM.
>
> The QRN was probably emphasized a little by the increased dynamic 
> headroom which was required here to cope with the Budapest telegrams. 
> Apparently these have also affected the software noise blanker 
> performance, which I have tried to improve now by inserting a bandpass 
> filter before the trigger.
>
> Judging by the fat CFH trace last night, LF propagation looks quite 
> favorable in the aftermath of the recent period of geomagnetic 
> activity. On the 100 kHz monitor, all stations from the the Far East 
> Loran chains (including Minami Torishima island) were well visible 
> this evening.
>
> 73 and best wishes
> de Markus, DF6NM