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.
*(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)
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.