Re: ULF/VLF: Tweek resonances - also on 50 Hz harmonics?

[DK7FC <selberdenken@posteo.de>]

Hi Markus,

Thanks for your contributions. I hope that we can get further understanding with the coming vlfrx-tools based system at DL0AO, in combination with data from blitzortung.org. Then we can looks at specific lightnings close to the TX location. And we can compare the tweak resonance spectrum side by side from DL0AO and DK7FC. Maybe 3305 Hz is a better frequency to start on, due to the higher antenna efficiency.
Another question is the phase stability on the top of the resonance peak. At 3305 Hz we could look in a higher bandwidth (lower FFT window time)...
But currently the QRN levels are very high, which is good for studying lightnings but not for doing transmissions at ULF :-)

73, Stefan

PS: I started trying to apply a phase correction table for my RDF spectrograms. An interesting procedure...

Am 15.06.2019 11:14, schrieb Markus Vester:

Hi Stefan, all,

as you say, the fact that the enhancement is also visible on 1550Hz makes it unlikely that the 1650 Hz boost is really due to an ionospheric resonance. This is corroberated by a plot showing both lines through several nights:

https://vlf.u01.de/VLFgrabber/vlf2.png

So we still don't know why the 50 Hz harmonics are stronger at night. but probably not due to a resonant reflection.

Another counter-argument is an observation from 2017 where we used colour-DF mode to plot the phase between the twoo orthogonal loops (attached). Due to Earth's magnetic field, in the northern hemisphere tweek resonances come down with pure left-hand circular polarization (magnetic field rotating east-north-west-south). Due to the 90° phase shift the resonances show up in deep red colour. However the 1650 Hz mains harmonic appeared yellow-green, implying linear polarisation with 180° phase between loops.

Regarding the best frequency for a tweek-mode experiment: It looks like there is some variation of ionospheric height from night to night. In some nights the resonance maximum goes down all the way to 1600 Hz, in others it rarely reaches 1670 Hz. Also there is some upward shift with distance to the source. One might speculate that cloud-to-ground lightnings (like an E-field-antrenna) emit less energy upwards, and would tend to come in from further away, with moderate elevation and less time delay between consecutive hops. Note that the resonance usually has a sharper cutoff on the low-frequency side, so that transmitting a few Hz too low may be worse than too high.

Best 73,

Markus (DF6NM)

-----Ursprüngliche Mitteilung-----
Von: DK7FC <selberdenken@posteo.de>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Di, 11. Jun. 2019 17:56
Betreff: Re: ULF/VLF: Tweek resonances - also on 50 Hz harmonics?

Hi Markus,

Thanks for the images. Interesting!
I checked the spectrograms (i've produced another one from last night) and found that my hum filter was actually NOT enabled, so i do not see a 50 Hz harmonics at 1550 or 1650 Hz, at least not strong enough to measure it's S/N without sferic blanking. Strange...
Also the decribed effect seems to be present at 1550 Hz too, which is unexpected.

Could it be the actual spectrum of the current in the HV power grid? It could be answered by using a stereo soundcard, one channel showing the ULF H field, the other one coupled to a mains transformer (using a resistive divider, dividing down to say 50 mV rms).

>From the observation from last night, which was dominated by cloud-earth lightnings, not by cloud-cloud lightnings, it looks like to cloud-cloud lightnings produce a much narrower maximum near 1640 Hz whereas cloud-earth lightnings have their maximum at higher frequencies, like 1670 Hz, and this maximum is much less narrow. So for amateur tests using H field antennas on both sides, 1640 Hz is the best choice i think.

73, Stefan



Am 11.06.2019 17:30, schrieb Markus Vester:

Hi Stefan,

looking through stored screenshots from the DL0AO grabber, I noticed a fascinating detail: During night hours where resonant spherics appear around 1670 Hz, mains interference on 1650 and 1700 Hz seems to become significantly stronger as well. This enhancement is also present during quiet nights with low QRN levels. Here's a collection of some recent spectrograms:

http://df6nm.bplaced.net/dl0ao/files/dl0ao_vlf2_1650Hz_all.jpg

We had always assumed that 50 Hz harmonics were a local effect, due to the earth antennas picking up stray ground currents from the neighbourhood. However the nighttime enhancement strongly suggests that they may in fact be emitted by more distant sources, like high-voltage transmission lines, radiating upwards to the ionosphere. 

I wonder if others might be seeing similar effects on their loop antennas?

Best 73,

Markus (DF6NM)

-----Ursprüngliche Mitteilung-----
Von: DK7FC <selberdenken@posteo.de>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: So, 9. Jun. 2019 11:14
Betreff: ULF/VLF: Tweak resonance measurements

Hi ULF/VLF friends,

Last wednesday there was a heavy local thunderstorm with cloud-earth but also with cloud-cloud lightnings. The latter produce well visible tweak resonance patterns in spectrograms taken from the H field. The fundamental frequency is at a wavelength where the distance between ground and D-layer is just Lambda/2, i.e. near 1650 Hz. But they also appear at integer multiples. Such peaks can reach in the order of 20 dB above the level besides the resonance frequency.
A reprocessed spectrogram from that night shows such tweaks for a time of about 6 hours. http://www.iup.uni-heidelberg.de/schaefer_vlf/ULF/Tweaks_on_NS-Loop.png
They started in daylight, where the D layer refelction height is lower. Then the night set in and the resonance frequency went down to about 1638 Hz, which becomes visible in this spectrogram and plot:
http://www.iup.uni-heidelberg.de/schaefer_vlf/ULF/Tweak-resonances_detail1.png

Again it is a good result done by vlfrx-tools and SpectrumLab. SpecLabs 'long term average graph' (red colour in the spectrum) was set to different half time values, 10 minutes was one of the best values. The plot shows the frequency of the highest peak of that graph. Unfortunately the time stamps are not correct, but they are correct for the spectrogram.
It looks like the frequency is reasonably stable  from 21 UTC onwards.
Sometimes that frequency is jumping by a few Hz. I guess that is because the location of the lightnings is varying and this will have an effect. However the peak is not to small, so it could work to make use of that resonator for amateur transmissions?

Also the 2nd resonance at 3305 Hz is quite expressed. The antenna efficiency at that frequency is much higher so maybe it is a good idea to start in that range.

With some luck, there will be a time-stamped and streamable VLF station at DL0AO soon. Signals from DL0AO and DK7FC could be joined in vlfrx-tools and then analysed side by side in SpecLab. With the plotter, the resonance frequencys from both locations could be plotted synchronuously, which will give a better impression of the dependency of the peak resonance from the location.

It is the ideal time for analysing such resonances from lightnings, however not for transmitting amateur signals ;-)

73, Stefan