Dear LF,
Yesterday you gave me some ideas about which filter can be used befor
the final stage of my active antenna for 137 kHz. Maybe there has much
to be optimised but today i got a first result that i want to test now.
I choose 2 band pass filters (33mH parallel 3,3nF + 1,5nF, coupled by a
47 pF capacitor). On one of those LC filters i applied another 100pF to
spread the resonance some 10 Hz... This is the result, measured on the
future QTH:
DLF (not visible here) is as strong as the LF background noise now(!)
and DCF39 just about 30 dB above the LF noise, that should be not a
problem for the soundcard or the amp. The soundcards noise of the
netbook in this mode is -110 dB and the LF noise abt 20 dB above which
looks ok but could be 10 dB more since the noise could become much
lower in winther. The filters slopes are sharp enough but the bandwith
is unstatisfying :-( Thus i will choose another filter type since i
want to have the whole band.
Today i got the idea to use 2nd order low pass filter (L as shunt, C
against GND and the gate of the FET) The peak should be above the LF
band and above DCF39, so maybe on 145 kHz. Additionally i will use a
sharp filter to reject DCF39. This should give a flat level in the
passband and will cancel out the high levels (DLF,DCF39) as necessary
as possible.
Anyway, i want to try the active antenna and its location as soon as
possible, which means TODAY ;-) Thus i have set up the webpage and run
the grabber first on the PC of the VLF grabber with the internal
soundcard, sampling at 44100 samples/s. The window is already running
and the antenna will be connected in some minutes...
So this is the first day of my LF grabber :-) Much has to be improved
but i am looking forward to some nice DX in the winther :-) So, i
invite YOU to produce a signal trace there, knowing that 137 kHz isn't
very exciting any more for some people... It could be possible that it
performs rather bad in the first days since the power supply,
soundcard, noise blanking, accuracy isn't optimal but each recorded bug
will be cancelled step by step.
This is the link:
http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_LF_Grabber.html
Best 73, Stefan/DK7FC
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