Re: LF: RE: The next experiment on 970 Hz - Cracked the far field border below 1 kHz

[Markus Vester <markusvester@aol.com>]

Hi Stefan,

congratulations also from my end! The fact that you did receive the signal well on one loop but not on the E-Field remains a big riddle.

One thought just crossed my mind: The Rhine river might behave as a large conductor, extending South-to-North more or less in the middle between the antennas. There are probably also railway and electric power lines running along the valley. At this frequency, magnetic fields are penetrating deeply into the ground, and might encircle and magnetically couple to this conductor from either side.

One question is whether the H-field is stronger than expected or the E-field is too weak. This might be answered by calibrating the TX and RX antennas and measuring absolute levels. The loop area (or "effective depth") of the TX antenna might be obtained in receive mode by comparing induced voltage with a small test loop, using manmade VLF signals or spherics at different frequencies and extrapolating to 970 Hz. ZEVS might provide another data point (if it's azimuth is not too far off the maximum from the earth antenna).

Best 73,
Markus (DF6NM)


-----Ursprüngliche Mitteilung-----
Von: hvanesce <hvanesce@comcast.net>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Mo, 1. Okt 2018 1:32
Betreff: LF: RE: The next experiment on 970 Hz - Cracked the far field border below 1 kHz

Stefan,

 

Congratulations! A great milestone for many reasons.

 

Amazing to think of far-field at that frequency without tons of antenna; a credit to design, test planning, and test execution.

 

Below 2 KHz is new territory for many reasons including markedly different sky propagation and changing earth propagation which also affects the character of noise*.

 

Your (more than) year of considerable effort toward this goal probably kept many on the edge of their seats, me included, I’m celebrating.

 

In free space the electric field at a receiving antenna 0.18 wavelengths away from a transmitting loop antenna should be very good compared to the magnetic field at the same receiving location.

But for a ground loop transmitting antenna at 970 Hz I wonder if anyone knows which field (E or B) and which polarization (theta and phi) predominates at a receiving antenna near the ground at 0.18 wavelengths distance. A very interesting topic and perhaps your E-field (monopole) and B field (loop) receiving antennas have provided another milestone in this respect: perhaps a first documented data point for E vs. B at any polarization, near 0.18 wavelengths (an interesting distance on its own) from a ground-loop transmitting antenna at 970 Hz?

 

Another fascinating realm now opened, much appreciated.

73,

Jim AA5BW

 

 

*(noise characteristics including noise E strength vs. B strength vs. noise polarization vs. distance from noise source vs noise-source-type could be interesting at 970Hz)

 

 

 

 

From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of DK7FC
Sent: Saturday, September 29, 2018 5:00 PM
To: rsgb_lf_group@blacksheep.org
Subject: ULF: The next experiment on 970 Hz - Cracked the far field border below 1 kHz

 

Hi ULF,

Today i run a carrier transmission on 970.01 Hz. Start time was 08:05 UTC. The carrier run for 3 hours without an interruption. I got 1.9 A antenna current on my ground loop antenna, about 320 W DC input to the PA.
The signal was received on my 3-axis RX on the tree in JN49IK. The distance was, as usual, 55.6 km.

So far the results were disappointing. I expected a strong SNR (at least 10 dB in 424 uHz) from the new E field antenna which seems to perform very well in the complete ULF range. However the analysis of a two hour segment of the carrier showed an SNR of 6 dB, i.e. just noise. However, as i routinely calculated the SNR from the N-S loop i got a surprising SNR of 13.03 dB in 139.5 uHz.
At about 08:40 UTC the reception on the tree was disturbed by its own solar charger, just for a few seconds but that was enough to create a bright vertical line in the 424 uHz spectrogram showing the E field.

Anyway, i just analysed the full 3 hour segment, only from the N-S loop (which actually points rather to 30/210 deg) and got the following result (including processing command line):
vtread -T2018-09-29_08:05,+3h /raw | vtcat -p | vtmix -c0,1,0 | vtfilter -a th=6 -h lp,f=1500,poles=8 | vtblank -a27 -d0.0005 -t100 | vtmult -f970.01 | vtresample -r240 | vtresample -r1 | vtraw -oa | ebnaut -dp8K19A -r1 -c2 -v -f15 -f16 -M'***' -N3 -k20 -S24
carrier phase: -111.2
carrier Eb/N0: 1.9 dB
carrier S/N: 14.25 dB in 93.0 uHz, -26.07 dB in 1Hz, -60.05 dB in 2.5kHz

This is the first far field detection of a 970 Hz signal generated by amateurs on the 309 km band! A true milestone for me. Since more than a year it was my goal to cross that far field border on that band. :-) The efforts were immense.
The result is just preliminary. I want to tweak the parameters for a higher SNR and try to filter out the short QRM from the charger.

So now, why does the E field produce such bad results? At http://www.iup.uni-heidelberg.de/schaefer_vlf/ULF/ULFSLFELF.png you can see what it receives in the range of interest. The day/night QRN difference is higher than on the loops, so the loops seemed to be rather deaf. Does it maybe mean a steep reflexion on the ionosphere, so that the E field antenna doesn't see it, but the loops do?

Later i've done a DC measurement and got 1 A at 86.5 V, quite much this time. Maybe a bad contact somewhere. Will check that.

My 120 Ah LiFePo4 accu is fully recharged and i plan to do a new experiment on monday morning, 3 hours before my solar charger starts to work :-) The SNR seems to be promising, i plan to send a 5 character EbNaut message.

More results and a spectrum peak image will follow.

73, Stefan