BTW, i am now preparing for first tests on ULF, the range between 0.3
... 3 kHz. Tests on 2970 Hz have started. This is a completely new
league. 2970 Hz is almost 100x 'harder' than 8270 Hz. It is not easy to
generate a high voltage on my wire on that frequency. Transformers
saturate and i need 6.1 H of inductance! As a first step i am using a
large ferrite transformer with 2 E cores and a coil body. I have wounnd
3000 turns on it by hand! :-) The ferrite material is N27 and it can
handle 1V / turn at 2970 Hz without a problem. I took 0.22 mm diameter
wire. 15+ layers and each one is isolated by Kapton insulation tape,
like the old TV transformers :-) See
https://dl.dropboxusercontent.com/u/19882028/VLF/ULF%20loading%20coil%206.1H.jpg
and
https://dl.dropboxusercontent.com/u/19882028/VLF/6.1H%20ULF%20loading%20coil.jpg
I can run just 3 kV, not more. Otherwise the 'transformer' which is just
a coil in fact, is risked to flash over.
At 3 kV my ERP is about 100 nW!
So far i managed 3.5 km to my own remote grabber, you still can see the
last races at
http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_VLF_Grabber2.html.
You still can see last traces from a previous 2 kV test there. The SNR
was 25 dB in 424 uHz. But this is just a near field test... The far
field starts at 16 km distance. It is a hard way to get that distance.
But i have built a portable E field probe and stereo audio recorder with
a Raspberry Pi. So i can take a walk in the forest and receive in really
quiet places, far away from any electricity. My next plan i to receive
my own signal in 7.9 km distance, there is a nice place :-) I have also
ordered a second cheap Neo 6M GPS module for pps and NMEA signals on the
other channel during the /p experiment. Then this will be a ideal
portable monitoring system for post processing with absolute time stamps
and even location informations are stored.
We will see. I'll come back to you when i think there could be a chance
to see a trace on 2970 Hz, maybe in a 1 uHz BW :-) The QRN is MUCH lower
there, relative to 8.27 or 6.47 kHz! Maybe this is an advantage that we
underestimate now! But the man made noise can be more problematic there,
you know. So it is good for people who like to take a walk in the forest
and like to install a hidden wire there to mount their portable RX to it
and come back a few hours later to bring it home and post process what
they got :-)
73, Stefan
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