Hello Markus,
Thanks for your observation and article! :-)
Most interesting.
I can indeed confirm there were a number of cloud-cloud lightnings last
night, up to 20 per minute were visible at times.
My VLF loop in the garden is a vertical hula hup circle with 80 turns
of wire inside. About 80 cm diameter. The receiver is the stereo
soundcard, making the stream for MF+VLF.
This observation reminds me (of course) on our idea to try to transmit
(clap our hands!) on about 2 kHz, the 150 km band :-) with a vertical
loop and receive with a vertical loop! I should check if it is possible
to reach the roof of the other building, where my TX-antenna is
mounted, to ground the wire, i.e. to build a vertical loop!
BTW, yesterday i officially requested a permission by the chief of the
local forest district to put some electronic equipment on a high tree
in the forest!! Solar, modules, batteries, WLAN-antenna and a box
including electronic equipment. The location is 3 km distant from the
institute, much more distant to man made noise sources then my garden
and still in a direct view to arrange the WLAN-link! No answer yet...
The 5th mode near 8.5 kHz? Well, that resonance isn't really stable
over the frequency, so it is not possible to try modes like DFCW-6000
or even 600. So the possible distances would be rather small. But woth
to try playing, obviously :-)
73, Stefan
Am 18.07.2015 06:50, schrieb Markus Vester:
The screenshot http://df6nm.bplaced.net/VLF/spherics/dk7fc_VLF_150718_1326.jpg shows a
number of narrow tweek-mode resonances at multiples of 1.72 kHz. These
are obviously spherics from nearby lightnings, bouncing multiple times
vertically between the ionosphere and ground (much the same as clapping
your hands between two parallel brick walls). The resonances are rather sharp indicating a high Q-number
(ie. around 100 bounces until decay). They are visible up to about 20 kHz, showing unusually small
damping of vertical incidence reflections at these ferequencies. There
is a small variation of resonance frequency over time, reflecting the
variable height of the ionospheric ceiling. The fundamental resonance
at 1.7 kHz is probably not visible due to the frequency response of the
loop and receiver.
The tweek resonances were received
on the loop antenna in the garden but not on the E-field antenna of the
(somewhat whitened out) city grabber http://df6nm.bplaced.net/VLF/spherics/dk7fc_wideband_150718_0330.jpg.
This corroberates the notion of near vertical incidence and horizontal
H-field polarisation. According to the literature, tweek tails are
usually circular polarized as only one sense of rotation exhibits a
high reflection coefficient. They are predominately excited by
horizontal current components in intra-cloud lightnings.
Of course the resonances will also
there in quiet nights without spherics, so they could probably be
employed to enhance fieldstrength (up to a factor of Q) for
medium-range VLF communication experiments using magnetic transmit and
receive antennas. When Stefan still had his earth dipole we already
discussed a 2 kHz tweek-mode experiment, which for various reasons
hasn't taken place yet. Now it looks like one could even employ the
fifth mode near 8.5 kHz...
All the best,
Markus (DF6NM)
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