Paul, Markus, VLF,
Some days ago there have been local thunderstorms near DL0AO in JN59.
Two images in attachemnt.
The distance is about 225 km. The thunderstorms were on a small spot only.
The sferics are clearly pronounced with an SNR increase of 10 dB and
more, quite a sharp peak/trace. That's not a new observation but now we
are operating just in that frequency range!
The effect seems to appear in integer multiples of about 1.7 kHz, which
is roughly a wavelength of a 180 km, and the earth-ionosphere height is
just lambda/2.
It would be most interesting to see how a similar spectrogram looks from
Pauls site (time and date given in the images). Is the effect still
there in 881 km? How strong is it expressed. Are the frequencies the same?
If so, we should try to find that region of enhanced propagation, by
using a 2-tone or 3-tone carrier signal. No problem to provide that from
here.
The effect is less expressed on 3.4 kHz in this event. Recently we
discussed about a geometric single-hop path difference between DK7FC and
DL0AO (Markus calculated 63.3 km difference between groundwave and
skywave). 5.17 is at a 1.09 lambda difference on that path, so
groundwave and skywave will add constructively. The 3rd order trace of
the sferic resonance effect is also at 5.1 kHz!
3.4 kHz, or the 88.2 km wave, is then at 1.4 lambda difference, so
skyway and groundwave do not add so constructive. Maybe this explains
the lower expressed resonance effect?
It looks like there can be SNR difference of several dB when changing
the frequency by just a few 10 Hz, at least arround 5.1 kHz and on the
path to DL0AO. We should try to check that!
Could there be a frequency with an advantage on the way to RN3AUS? On a
2000 km path the effects will be less expressed. But worth to think about...
73, Stefan
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