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ULF: Experimenting on 2470 Hz and 1970 Hz

To: [email protected]
Subject: ULF: Experimenting on 2470 Hz and 1970 Hz
From: DK7FC <[email protected]>
Date: Mon, 06 Aug 2018 16:19:26 +0200
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On Sunday, 5th 2018 i successfully crossed the far field border on 2470 Hz for the first time. That's the 121 km band. Later i even crossed it on 1970 Hz, the 152 km band. These are two new records of the lowest frequency signals generated by amateurs and received in the far field. The distance between RX and TX was 55.6 km . The far field for 2470 Hz starts at 19.4 km distance. For 1970 Hz it starts at 24.3 km distance. The RX antenna and the TX antennas were loops!

By running about 100 W (PA DC input) i managed to get 910 mA antenna current on 2470 Hz into the earth loop in JN39WI. Here i transmitted a plain carrier from 06:13...07:15 UTC.
Later i QSYed to 1970 Hz and here i got 910 mA as well. The 1970 Hz transmission took place from 07:19...08:21 UTC.

Despite beeing in the middle of a large forest there was good internet connectivity and so i was able so watch my own grabber window showing the band activity on 2470 Hz in a spectrogram of 424 uHz FFT bin width, which is very wide for that frequency range! The spectrogram uses a Hann window and the FFT window time is about 40 minutes, so it took some time until a peak builts up. But already after 20 minutes i saw that something happens! After 40 minutes the carrier transmission reached an SNR of about 20 dB! It was a relatively quiet morning for early August.
All the VLF stream data is stored into a ~ 12 day covering buffer so i have the chance to optimise the filter settings and antenna mixing in a postprocessing to achieve the best SNR from the system.

For the 1970 Hz transmission there was no spectrogram available but since i had internet access and a Linux notebook available, i processed the VLF stream data (via SSH remote access to the storage PC) during the transmission and followed the peak's SNR building up!

It clearly looks like this antenna outperforms my large inverted L in 30 m above the ground, at least into the ULF range! This opens up a new room for experimentation on the way down to DC! :-) Now i need to get rid of these output transformers since they will become problematic for wide-band experimentation on ULF / SLF.

Now, attached you can find two images showing spectrum peaks from the two bands, out of the 55.6 km distance. The complete transmission time is here integrated in one peak. Since the carrier S/N can also be calculated from decoding a '*' message in EbNaut, i also show the results for such calculation along with the whole postprocessing chain.

Spectrograms will be produced as well, but this will take a few hours here...

73, Stefan

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Attachment: 1970.png
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