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Hi Stefan, LF,
we're not just
playing around... I would rather consider this an original scientific
experiment, perhaps worth proposing as an research project in
environmental physics ;-)
Frequency stability will not be much of an issue.
Throughout the night, the 5th order resonance moved around by two or three
pixels, approximately 100 Hz or twice its own width. So after
having found the right frequency in the first place, we could just let the
transmitter sit on the exact QRG for many hours without loosing much signal
(even more so for 1.7 kHz).
Here are some
back-of-the-envelope calculations for your institute antenna, grounded at the
other end: Assuming a loop area of around 2000 m^2, radiation resistance at
8.5 kHz would be 0.17 microohms. Given the cross section of your
horizontal wire and the lightning protection system, you could perhaps
run up to 100 A loop current (maybe too optimistic?), which would give 1.7
mW radiated power - comparable to your first kite experiments and surely
enough to be received here easily even without any resonance enhancement. At 1.7
kHz, radiated power would be 625 times smaller (2.6 uW) but probably still
detectable under quiet conditions. Estimated inductance would be around 110
uH or 5.9 j ohms at 8.5 kHz.
Depending on the quality of the grounding
installation you will probably require some kilowatts drive power, and a
suitable array of parallel FKP's to cancel inductance. Alternatively, the
company I work for produces some cool fat MRI gradient power
amplifiers, designed to deliver hundreds of amps into inductive loads
;-)
On the receive side here, my biggest problem at low frequendcies is
railway interference - - this is one reason to
start at higher frequency before going to 1.7 kHz. As the incident wave will be
circularly polarized I could try to rotate the receive loop to minimize
local QRM. An interesting and favourable point is that your TX loop will be
pointing north-south, which will minimize "contamination" by direct
groundwave to my direction. However I
do not know what the lateral extent of the resonance mode is, in other
words how much enhancement we will still get at 180 km distance or 45°
elevation.
Alternatively you could try
to detect the tweek resonance using your garden receiver, close enough to
provide true vertical skywave incidence and full cavity resonance enhancement.
To be able to see the skywave, you will only have to turn the receive
loop carefully to null direct nearfield inductive coupling.
One last point for dreamers: The counterrotating
polarized component which is not reflected will penetrate to the magnetosphere
and may propagate as a whistler wave. This could theoretically be received
at the magnetic conjugate point, somewhere near Madagascar...
73, Markus (DF6NM)
Sent: Saturday, July 18, 2015 10:51
AM
Subject: Re: VLF: Tweek mode
resonances
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 frequencies. 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 be 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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