Markus,
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.
I just zoomed into the image to determine the single frequencies. Here
i noticed that the drift of the resonance is higher for higher
harmonics, somehow logic :-) One reason to decide to use lower
frequencies. Another one is that i have been detected over 904 km by
SQ5BPF with the 300 m kite. Now it must be even lower! Thus i would
decide to use 3.4 kHz in first tests. Also, the QRN is lower there :-)
I will need more C then. And a preamp for the RX loop!
73, Stefan
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