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1970 Гц - QRB 380 km!
« : 17 Май 2021, 02:14:21 »
New distance record in the 152 km band


Hello dear friends of the really low frequencies,

I can report about a further successful experiment at ULF: A 5 character
EbNaut message, transmitted at 1970.005 Hz from JN39WI has been decoded
successfully at DL7NN in JO60IS, a distance of about 380 km.
It is 2.5 wavelengths distance in the 152 km band. I guess this is a new
record distance for hobby radio experimenters.

The antenna current was 2.75 A into a 1130m ground loop.

Details of the decode command line and results can be found in the
attachment. The decode was worked out empirically in a cooperation by
DL7NN and DK7FC. A new 3-axis antenna was used at DL7NN.

73, Stefan


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Re: 1970 Гц - QRB 380 km!
« Ответ #1 : 17 Май 2021, 16:16:49 »
Hello Jim,

Well, you need to have a soundcard with at least 4 synchronous inputs.
One channel is used for a 1 PPS reference, used for drift compensation.
The remaining 3 channels are for 3 antennas, one is E field and two are
orthogonal loop antennas. They need to work on that frequency of course
and you need a reasonable quiet location when working at ULF.
The rest is done by software.
With Paul Nicholsons software vlfrx-tools, you can mix these antennas in
a post-processing of the recorded raw data from the soundcard. When
writing 3 channels at 48 kS/s and 16 bit/Sample to a HDD, you will get
about 24 GB per day. So you can keep the last 40 days available if you
write to a 1 TB HDD...
With the tool vtmix -c0.35,0.1,1/50,0 you are mixing 4 channels into one
channel. CH1= E/W loop, CH2= N/S loop, CH3= E-field, CH4= 2nd E-field
antenna. Here the resulting signal stream is 0.35 E/W + 0.1 N/S + 1 E
(rotated by 50 deg). Quite simple once you understood how it works. The
resulting cardioide antenna can be optimised for a best S/N of the
signal,
In the beginning it all looks very complex when starting with command
line tools, but after some usage it becomes normality :-)

73, Stefan

Am 17.05.2021 00:39 schrieb James via groups.io:

Свернуть
 Please describe the 3-axis antenna or remind us of a link to it if
 already described.
 TU & Vy 73,
 Jim H W5EST
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Re: 1970 Гц - QRB 380 km!
« Ответ #2 : 17 Май 2021, 22:53:05 »
Hello,
the two closed loops, arranged orthogonally to one another, have a diameter of 2 m and consist of 22 mm copper pipe with a wall thickness of 1 mm. Loops are housed in a container made of fiberglass-reinforced plastic and are about 1 m above the ground. The signal is decoupled by means of a ferrite transformer (EPCOS N30) with 2 x 100 turns. A voltage proportional to the magnetic field strength is fed to an instrument amplifier (2 OpAmp NJM2122D) via two 1 k Ohm shunts. A 2.7 m long probe (28 mm diameter) in a 60 mm glass fiber reinforced tube was used for the E-field. The impedance converter (JFET U310) and a bipolar transistor follow-up stage generate a symmetrical output signal. The E-field probe is located approx. 2 m above the ground. The symmetrical output signals of the two H-field loops and that of the E-field probe are then also symmetrically fed to an ADC (OctoSound; max. 6 channels). All modules have been optimized with regard to high common mode suppression (> 80 dB) and minimal phase rotation in the useful range and are dimensioned for the frequency range from 1 KHz to 50 kHz. Further signal processing is carried out using a Raspberry Pi4 and Paul Nicholson's vlftools. A battery that is charged via solar cells serves as the power supply. The project is a collaborative effort with DK7FC (Stefan), DL3JPN (Steffen), DL3JMM (Bernd) and me and other OMs.

73
Albrecht
DL7NN
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Re: 1970 Гц - QRB 380 km!
« Ответ #3 : 18 Май 2021, 17:41:19 »
ADC is an "Octo" (Audio Injector):

https://github.com/Audio-Injector/Octo

The stability of the sampling rate is very important for VLF and especially for EbNaut. Therefore you should either stabilize the sampling rate with a 1pps GPS clock using vlfrxtools or SpectrumLab or, as I did, remove the TCXO and replace it with a GSPDO (e.g. from Leo Bodnar) with a GPS stabilized frequency of 49.152 MHz.

73

Albrecht
DL7NN
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