on several occasions during the last months I have observed a high noise
level in our LF band, mostly in the afternoon and on many evenings. For
example today the band was rather quiet, with the usual background around
-26dBuV/m/Hz at 137.4 kHz. Since some time about 02:00 UT the noise has
increased by 15...20dB, and I can only hear the strongest signals.
In USB mode, the QRM sounds like a wideband FM signal sweeping across the
audio band. The spectrum of the splatter extends at least from 134 kHz up to
Deutschlandfunk's 153kHz, and maybe beyond that. Its characteristics are
clearly correlated to the modulation of DLF, in brief periods of silence the
noise often narrows to an unstable carrier a few 100 Hz wide.
At first I thought I might have some strange local crossmodulation problem,
but Gerhard (DK6NI) and Walter (DJ2LF), both 20 km from me in different
directions, confirmed the observation, whereas Reiner (DL3FDO) said the band
was still quiet near Frankfurt. My theory is that there may be an
intermittent fault in one of DLF's transmitters (153, 207 or 547 kHz) or in
its (switched-mode?) modulator.
If the problem persists, I would raise the point to our authorities (RegTP),
but before that I'd appreciate some more hints from OMs in South Germany, and
it would be great if someone with a loop could get bearings.
Thank you, 73
DF6NM in Nuernberg, JN59NK