Re: ULF: New distance record (?) on the 101 km band: 31.3 km

[Markus Vester <markusvester@aol.com>]

Hi Stefan,

very well done! Thanks or sharing these experiments.

The daytime SNR in your 238 uHz spectrogram seems to be around 13 dB (maybe slightly better at the best times). Considering a bandwidth factor of 1.5 for the Hann window, this is equivalent to 0 dB in 7.14 mHz, or -21.5 dBHz (or -55.5 dB in 2.5 kHz). This implies that using EbNaut at 0 dB EbN0, each of your 200 minute dashes could have theoretically transferred 12000*0.00714 = 85.7 message bits, or 15 characters!

Despite the railway QRM-band around 3 kHz which is visible in the wideband spectrogram, the noise background at your frequency seems to be dominated only by distant QRN. With the known ERP and distance, we can try to quantify the fieldstrength and the daytime noise level. Using 1/r groundwave, 250/1.83 nW = -38.5 dBm EMRP should have produced about 0.11 uV/m (or 0.37 fT) at the receive location. Thus the noise background (after spherics blanking) would be around 1.3 uV/m (or 4.4 fT) in 1 Hz for your loop. Depending on the dominant direction of the QRN, an omnidirectional antenna might have picked up 3 dB more noise, arriving at 1.9 uV/m/sqrtHz - the same order of magnitiude that we have seen around 9 kHz.

At larger distances, ionospheric effects will become significant, so you may start to see some diurnal fieldstrength variation. Unfortunately for my distance (180 km), the geometrical path difference between single-hop skywave and groundwave is approximately half a wavelength at 3 kHz, presumably resulting in a propagation minimum similar to the one we've been observing at 8.3 and 9 kHz.

All the best,
Markus (DF6NM)

-----Ursprüngliche Mitteilung-----
Von: DK7FC <selberdenken@posteo.de>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>; Renato Romero <contact@vlf.it>
Verschickt: So, 18 Sept 2016 4:21 pm
Betreff: ULF: New distance record (?) on the 101 km band: 31.3 km

Hi ULF,

Now the results of the post-processing of my 3 day recording are available. It shows very clearly that my 250 nW ERP (average 46 mA antenna current), 2970 Hz signal was copied in 31.3 km distance. The recording location was in a more or less quiet location, in http://no.nonsense.ee/qth/map.html?qth=JN49IQ37AM. The transmission path is shown in http://no.nonsense.ee/qth/map.html?qth=JN49IQ37AM&from=jn49ik00wd

In this second attempt, everything worked fine, stable GPS reception, accurate orientation of the vertical, single turn receive loop and the recording of a single 30 GB .raw file at 32 kS/s (covering 0...16 kHz). However the QRN could have been lower. But there was no QRM by electrical fences...

Here's a spectrogram showing the complete, unfiltered recording: https://dl.dropboxusercontent.com/u/19882028/ULF/31km_wide_3days.jpg
Of course there is nothing visible of the transmission in this resolution, it is just an overview to get an impression about QRN and QRM in that location. QRM from trains and mains harmonics is the critical parameter on ULF.

The transmission consists of 3 parts: A long dash on 2970.000 Hz, then the message "73" in DFCW-12000 (2.5 mHz frequency shift) with the lower dash sent on 2970.000 Hz and then another long dash on 2969.990 Hz

After filtering and noise blanking, here are the resulting spectrograms in 476 uHz and 238 uHz:
https://dl.dropboxusercontent.com/u/19882028/ULF/31km_476uHz.jpg
https://dl.dropboxusercontent.com/u/19882028/ULF/31km_238uHz.jpg

The experienced visual human spectrogram interpreter experts may find a significant part of the message :-)

Just as a reference, here are spectrograms showing the same transmission received in 3.5 km distance (http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_VLF_Grabber2.html).
https://dl.dropboxusercontent.com/u/19882028/ULF/ref_424uHz.jpg
https://dl.dropboxusercontent.com/u/19882028/ULF/ref_212uHz.jpg

I'm not sure if this is the highest distance ever crossed with a message by radio amateurs on ULF (0.3...3 kHz), but it is certainly one of them :-)

A new experiment in about 65 km distance is in preparation. Probably it will be done in a few weeks. The location will be quieter (the Pfälzer Wald, JN39VH) and there will be less QRN!

73, Stefan/DK7FC



Am 11.09.2016 16:35, schrieb DK7FC:

Hi ULF,

Since yesterday (10.Sep), 11 UTC, i'm running a new experiment on the 101 km band. It is in 31.2 km distance to my transmitter-site.

The last experiment did not produce satisfying results due to various reasons.

Now i improved the loop cross section area, the loop orientation, GPS reception (for the 1 PPS + NMEA reference signal). Furthermore the recording sample rate was reduced to 32 kS/s which allows about 3 days of stereo recording.
I'm still using the Raspberry Pi2 for therecording. The system consumes just 150 mA from a 12V battery. 3x 7 Ah lead acid batteries are in use now.

The transmission contains stable carriers and a short DFCW-12000 message, "73". The transmission can be followed on my 3.5 km distant forest grabber at http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_VLF_Grabber2.html , as usual.

Relative to the last experiment, the average QRN has dropped significantly now, another advantage. So i'm quite optimistic to get a good result!

The next experiment in about 60 km distance is already in preparation. Then i will use my new coil which is still in preparation. The last measurement, after completing the 3rd stack, showed a resonance frequency of 4.27 kHz. So it should have 2.8 H now...
In the next experiment, i will use 100 mA antenna current, again a about 6 dB stronger signal.

The current experiment will stop on Tuesday afternoon. Results can be expected in the end of the next week.

73, Stefan/DK7FC