Hi ULF,
Yesterday i finally did another enjoyable experiment on my 900m ground
loop antenna. The idea was to transmit a carrier at 470.1 Hz,
the 638 km band, as long as my 120 Ah LiFePo4 battery supplies the PA.
After modifying/extending my DC/DC converter i managed to run 460 W
DC power which resulted in 2.2 A antenna current. At this power
level, the accuy hold for 156 minutes and still provided 440 W
at the end. So i pumped about 1.2 kWh into the ground.
BTW at 1 A DC antenna current i measured 85.3 V at the feed point. The
soil has been quite wet.
The receive site was, as usual, my tree in 57.6 km, with its
two loops and the E-field antenna receiving/recording using vlfrx-tools
and a Raspi+Octo-soundcard.
This frequency and distance means that it is a near-field test! The far
field begins at 101 km distance, so the experiment is in 56 % of that
distance.
Since the transmit antenna is a loop, the best RX antenna is a loop too.
The post-processing of the transmissions clearly shows that the best
SNR comes from the loops and just a small portion of the E-field
antenna improves it further. However my loops are still quite deaf
below 1 kHz whereas the E-field line should meanwhile have full
performance at 470 Hz. With a lot of tweaking i managed to reach just 12
dB SNR. A capture showing the processing chain as well as the
residual spectrum peak is attached. So i failed to reach the 5 sigma
criterium (14 dB SNR) and so it is not a successful detection. However
it is very close and for me personally it is a success. I cannot
imagine that it is random noise. And i am sure that it will be possible
to improve the H field reception on the tree to be at least 2 dB better
for the next experiment. This is the goal for the next week! Then, in
the next experiment, i'll try a 5 character EbNaut message which needs
about the same SNR to decode.
73, Stefan
PS: If it's true that the near field signal levels will not drop with
frequency then will it be possible to detect SLF and ELF signals from
the same systems, if the noise is constant and the performance of the
RX will not drop towards DC? With the TX antenna i can run any
frequency towards DC...
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470Hz.png
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470Hz_peak.png
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