Three days ago (May 22), the powerful French
Navy station HWU shifted its operating frequency from 21.75 to 22.6 kHz.
This is closer to 23.4 kHz from German DHO, and consequently the
intermodulation bands created by joint ionospheric heating have moved and become
more prominent due to the lower audio beat (0.8 kHz).
http://df6nm.bplaced.net/LF/vlf_lf_icm_2013/test1305242330.jpg shows crossmodulation
impressed on HGA22, a fat red band centered on 136.33 kHz (ie. 135.43 +
23.4 - 22.6). There is also a weaker band visible on 136.73 kHz
(135.43 + 23.4 - 22.1) from DHO with GQD, which appears much more rarely here.
Observers in Western Europe might also see crossmodulation from other VLF
combinations, eg. GQD - HWU = 0.5 kHz.
There are some more explanations in this
Sent: Sunday, February 10, 2013 2:07 AM
Subject: Ghost bands created by VLF ionospheric
On my LF grabber http://www.df6nm.de/grabber/Grabber.htm there
is again a ~ 200 Hz wide red noise band centered on 137.08 kHz. It has
been observed several times before but has been absent for a few months.
Sometimes it is accompanied by a weaker "brother", centered on
137.18 kHz with purple (ie. northerly) colour. The structure is
reminiscent of an MSK signal ("the ghost of CFH"). The bands were seen regularly but are stronger in nights with
strong Luxemboug effect.
After a lot of head scratching, a while ago I
have found a really astonishing explanation for this effect: It
is ionospheric cross modulation, with a pair of strong VLF transmitters
acting together as a modulated heater.
DHO38 on 23.4 kHz and HWU (which has recently
reappeared on 21.75 kHz) have a difference frequency of 1.65 kHz, with
the sum of the two FSK shifts. Similar to an AM
demodulator, the beat frequency between the two transmitters modulates the
electron temperature in the D-layer, and the variable absorption
creates crossmodulation on the carriers of HGA22 (135.43 + 1.65 kHz) and DCF39
(138.83 - 1.65 kHz). This constellation is rather unusual not only
in that the heating is done by VLF
signals. What is even more peculiar is that unlike a simple AM
transmitter, the two non-colocated heaters must produce an
inhomogeneous and fast moving heating pattern.
If you look at the Twente WebSDR http://websdr.ewi.utwente.nl:8901/ now,
you can currently see and hear prominent 1.65 kHz FSK sidebands around
DCF39 and also DCF49 (128.93 kHz). However HGA seems less affected at
Twente, presumably because the easterly reflection area is not so well
illuminated for that path.