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
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
But currently the QRN levels are very high, which is good for studying
lightnings but not for doing transmissions at ULF :-)
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:
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
we still don't know why the 50 Hz harmonics are stronger at night. but
probably not due to a resonant reflection.
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.
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.
Von: DK7FC <[email protected]>
An: rsgb_lf_group <[email protected]>
Verschickt: Di, 11. Jun. 2019 17:56
Betreff: Re: ULF/VLF: Tweek resonances - also on 50 Hz harmonics?
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
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.
Am 11.06.2019 17:30, schrieb Markus Vester:
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
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.
if others might be seeing similar effects on their loop antennas?
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
, which becomes visible in this spectrogram and plot:
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
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 ;-)