Roelof Bakker wrote:
> I am looking for a simple way to detect the proposed test signals
> at 8.970 kHz from Germany.
2 metre whip or wire, short wire connection to low noise
JFET pre-amp, 10MOhm bias resistor, battery powered, ground
resistance < 500 ohms, coax to house, transformer to break
the ground loop. If possible the pre-amp should have enough
gain to allow line input to be used, not the mic input.
If preamp oscillates, lower the ground resistance. Test with
Alpha sigs, make sure noise floor is natural VLF noise floor,
not rx hiss or domestic digital mush. The floor should go up
a few dB at night. To check rx noise floor, replace antenna
with 20pF capacitor.
Stefan Schäfer wrote:
> I know the LT1028 as the best audio op amp...
Yes, given a low Z source - no good for E-field rx.
> what is your locator or the locator where your Rx antenna is
> placed?
53.703N 2.072W, I think that is IO83xq.
Marked by the white cross in http://abelian.org/vlf/100105a.jpg
> What do you recommend to those, who just receive with a wire in
> the sound card?
Maybe adequate if you are monitoring the strong MSK signals
for SIDs but for natural radio, it is usually no-go. Easy to
try so give it a go: check how Alpha is rxed and look for the
natural noise floor bobbing up and down. Probably interference
will be bad though because the rx ground = domestic/PC ground.
> What are your experiences about the needed distance between
> antenna wire and the house and what about hight above ground?
For E-field, 10 or so metres from the house. A colleague in
Sheffield obtains good results with a vertical rod in a suburban
back garden. Height helps to get clear of interference but is
not necessary just for signal voltage. Buildings, trees, walls,
etc produce a shadowing effect so keep away from them if possible.
If your neighbourhood power lines are above ground, you have a
big problem. Avoid phone lines, fences, washing lines, other
antennas, etc. Antenna base > 1 metre above long grass (static
hiss as wind blows).
> Is there principally the need for amplification or can it just be
> reached by increasing the wire length?
Out in the countryside, a long wire can work well with minimal
amplification, it might only need some passive filtering to avoid
intermod. Nearer civilisation, a short antenna carefully sited is
better. With modern op-amps it isn't too hard to obtain an rx
noise floor 20dB below the natural VLF floor at audio frequencies
with just a 2m vertical antenna. The difficulty is finding an
interference-free site and then making sure you don't bring any
interference out to the rx along the cable.
Usually E-field reception is easier than H-field. The mains
hum is easier to filter because the E-field has fewer sidebands
and the hum is more stable. For narrow band 9kHz that may not
be an issue.
> And what about a simple band pass filter to supress the 50Hz
> and it's harmonics? Necessary or dispensable?
If you're only receiving above 5 or 10kHz or so, you can ignore
the hum providing it doesn't clip anywhere. But for lower
frequencies an automatic notch filter that seeks out all the
harmonics is essential unless you're really out in the wilds.
At 9kHz hum should not be a problem, but digital gadget noise
can be, eg the PC itself, PSU, USB leads, etc: transformer
isolation essential.
> Tnx for motivation ;-)
Well, isn't this the most perfect kind of thing to inspire
radio amateurs? It really hits the mark. The reason: it is
just about feasible - but only just. If it were harder, you
would not bother to try. If it was easier, well you would try
but it would be not so rewarding - it would be just a formality,
like ticking another country.
No, this is just the perfect challenge for the constructor and
operator. It can, just about, be done! The live spectrum
of the band gives you something to shoot for - a nice long shot.
--
Paul Nicholson
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