2nd try sending my message, with attachment splitted (audio file)...
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Hello Roger, LF,
Today i got the idea to check how my eartha antenna behaves at ultra low
frequencies. I am currently planning to build a ULF active antenna for a
ULF grabber window (300 Hz...3000 Hz). Knowing that mains hum gets a
real problem when coming down to ULF and most antennas have less
sensitivity in the ultra low frequency range (most active antennas have
a cut off frequency of some kHz).
As you know, my 600m spaced earth antenna is well outside (> 1 km) of
any mains application and cables. Thus i have taken my netbook and drove
on the hill and checked the situation. The earth antenna is coupled to
the PC by an audio transformer.
The whole band was disturbed by the DLF (153 kHz) transmitter which is
40 km apart from there. But DLF is reducing the power at 20 °clock local
time. This greatly reduced the noise. So i will apply a low pass filter
for the next test (cut off at arround 25 kHz) to prevent overload.
Anyway i have got some nice results: The mains frequency (50 Hz) is just
20 dB above noise in 168 mHz without any notch filter, so not even audible!
All the spherics were audible down to some 100 Hz ("crack", as sometimes
audible in AM radios when someone switches a lamp).
Has anyone some informations about the ZEVS transmitter
(http://www.vlf.it/zevs/zevs.htm) at 82 Hz? Is it still on air and is
there a scheme of transmitting times?
The first time i have taken an audio record of my reception which is a
REAL advantage when receiving weak signals in a /p location. So i can
play the file in SpecLab again when i am at home and apply the optimal
filters and shift frequencies and extract signals. It is possible to
display it again in differnt QRSS modes and one can optimise the noise
blanker settings or can just watch the ALPHA transmitters. If found a
new world using the SPecLab filters and mixer. It is really unbelivable
what this program can do!!
Before some days i have made changes on my VLF active antenna: I have
thrown out the band pass filter that was resonated at 9 kHz. Now the
noise and spherics floor appears quasi as it is. I have learned that it
is important for the noiseblanker to "see" the spherics noise floor to
work accurately! Additionally it is essential to filter out the mains
hum and the MSK transmitters. Thus i have now done all the filtering
within SpecLab. This is possible since the soundcard is not overloaded
without a bandfilter. It is done by applying a 5 kHz band filter
centered at 7 kHz, 500 Hz slope width.
I am still not sure but maybe my VLF grabber is up to 6 dB more
sensitive now! Also the ALPHA F1 transmitter (with its own filter)
appears with an S/N improvement of at least 6 dB. I will chelk this out
on the earth antenna, sending a weak signal at some W TX power. Maybe
others are earilyer ;-)
So, this way of filtering can significantly improve the RX side, not
only on VLF but on LF as well! Also it is important to set the optimal
noise blanker settings to achieve proper filtering. On most HF
transceivers there is a noiseblanker that can be switched on or off but
not exactly adjusted for an optimal result!
In my last RX test on VLF on the earth antenna at 8.97 kHz, Michael
Oexner run a usr file (settings in SpecLab) with my older grabber
settings. Maybe we can do the test again with much better results if
applying an internal SpecLab filter (since the mains hum and MSK haven't
been excluded!). I am looking forward to that!
Attached you find an audio file (sorry for size and format) of what you
see in the attached picture: The RSDN-20 (ALPHA) transmitters, received
on the earth antenna, all 3, shifted by 11 kHz and band filtered and
noise blanked, all in SpecLab! I have no usual VLF RX, just the netbook.
Anything else would be to heavy in the bad on the back when driving on
the mountaion with the bicycle :-) There is still noise caused by DLF
but this will be improved in some days (or hours)...
Best 73, Stefan/DK7FC *impressed*
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