VLF: Tweek mode resonances

["Markus Vester" <markusvester@aol.com>]

Waking up early this morning, I took a look at Stefan's garden grabber http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_remote_Grabber.html and was greeted by a fascinating display on his VLF panel.

 

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.

 

A (somewhat late) screenshot from Blitzortung http://df6nm.bplaced.net/VLF/spherics/image_b_de_150718_0324.png shows the last red crosses between Wiesbaden and Stuttgart passing over Heidelberg at around 1:30.

 

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)