Re: ULF: New experiment on 970 Hz, results (3/3)

[Markus Vester <markusvester@aol.com>]

Hi Stefan,

thank you for sharing these inspiring results! I'm surely looking forward to see your first step into the far field below 1 kHz, and I am dreaming about receiving your ULF signal at even further distance one day. I also hope that the upcoming safety inspection will not make your work harder than it needs to be.

Best 73,
Markus


-----Ursprüngliche Mitteilung-----
Von: DK7FC <selberdenken@posteo.de>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>; Renato Romero <contact@vlf.it>
Verschickt: Fr, 16. Jun 2017 1:04
Betreff: Re: ULF: New experiment on 970 Hz, results (3/3)

Hello dear ULF friends,

Here is my summary of the latest experiment on ULF which took place from 16...20 of May 2017. The experiment was to transmit for 90 hours on near 970 Hz, a wavelength of 309 km.

The transmitter site was my normal inv-L antenna, 30m above ground, about 70m long. A modified HV mains transformer, driven by a homemade linear mode class-AB PA, produced 5 kV on the antenna wire. This results in 15 mA antenna current and an ERP of about 3 nW. The signals were generated by SpectrumLab software. The transmit signal was locked to a GPS reference.

The receiver position was in JN49LN91GB, that's a distance of 27.2 km to the transmitter site. This is the path between TX and RX: http://no.nonsense.ee/qth/map.html?qth=JN49LN91GB&from=jn49ik00wd
This time, an active E field antenna was used for reception because 27.2 km distance is still in the near field for that wavelength! It is about 55 % of the far field border distance.
The signal was recorded by a Raspberry Pi running a USB stereo soundcard. The second channel was recording a reference signal containing PPS pulses and NMEA data from a NEO-6M GPS-module for compensating sample rate drift and sample losses. The data was written into a  ~ 30 GB file on an USB stick connected to the Raspi.

The data was post processed afterwards. Spectrograms in bandwidths of 424 uHz, 212 uHz, 106 uHz and 47 uHz were generated. A few optimisations were done to find suitable settings for a band pass filter and noise blanker.
The first time, two EbNaut messages were transmitted, see the transmission plan:

May, 16th:
14:43:08 UTC: Start of recording. Carrier on 970.005 Hz for the rest of the day.
May, 17th:
Continuing of the carrier transmission, for the whole day.
May, 18th:
00:00:00 UTC: EbNaut, 5 characters, 16K21A, CRC4, 100 second symbols. Taking 24 hours exactly. The message was "DK7FC"
May, 19th:
00:00:00 UTC: Carrier on 970.01 Hz, for the whole day.
May, 20th:
00:00:00 UTC: EbNaut, 2 characters, 16K21A, CRC3, 60 second symbols. The message was "73".
09:20:00 UTC: End of EbNaut transmission.
10:27:05 UTC: End of recording.

Here are all spectrograms and images: https://www.dropbox.com/sh/n7k0il4pp6vqbbc/AACbfwbN4TQ1RrS6YIEDYzMpa?dl=0

The signal shows up to 15 dB in 47 uHz. There is no QRM in +- 10 Hz but the QRN was extreme, including local thunderstorms on the 19th. Several times there were lost samples into the recording but SpecLab corrected that nicely, so the spectrograms look as expected.
Fortunately both EbNaut messages were decoded after a longer series of decode attempts. So there can be no doubt that this was a successful experiment.

The next attempt will be to reach the far field border distance in > 49.3 km. I will need about 10 dB more radiated power, which is possible with some effort. Maybe an improvement on the receiver site will help to rise the SNR a bit more!

73, Stefan/DK7FC