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*