Thank you Markus and Stefan, all quite fascinating.
Attached: image of roughness of ionosphere, illuminated by sprite plus ELVES. LWPC and FDTD do not account for roughness at the spatial scale shown in the attached image.
I would be happy to lend a Crown 4002 (rated 3200W @ 4 ohms L+R bridged) power amplifier (or two) for your prospective NVIS-resonance signal enhancement experiments. The Crown 4002 PA works well for resonant ULF and VLF TX loops of the size that you mentioned.
73,
Jim AA5BW
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...
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...
