LF: More QRM from DLF and DCF39

[MarkusVester@aol.com]

Dear LF,

after signon of Rugby Loran and Budapest, interference to the 137 kHz band in Central Europe has again become worse twice. This was apparently caused by recent changes in the modulators of two large broadcast/utility stations, DLF and DCF39.


DLF (Deutschlandfunk Donebach, now 250 kW on 153 kHz) has always been prone to have a pair of spurs about 17 kHz from the main carrier, which are FM-modulated across several kHz by the program content. They were quite strong (up to -65 dBc) for some time in 2002/2003. Though the origin of the spurs is still not understood, one on-site engineer found a workaround to mitigate the problem, by placing a PCB in the 54 kHz switch-mode modulator circuitry on an extender board and thus cooling a certain CD4011 chip.

This year Feb 22, 9:20 the problem reappeared at somewhat lower level, first intermittently but now permanent since Feb. 28. In an SSB receiver tuned to 136 kHz, the lower spur is audible as a shuffling and hissing sound, going on and off along with the modulation on 153 kHz - it actually sounds very much like listening to a FM broadcast transmission in SSB mode. In my colour-DF spectrograms, it shows up as a blue-greenish haze, interrupted by irregular short pauses. The screenshot http://members.aol.com/df6nm2/QRM/DLF_break_070317.png was taken during a short transmitter outage on Mar 17, and really shows the impact on the receive situation on a quiet day.

At 160 km range, the spurious noise is -12 dB µV/m/sqrt(Hz), some 6 dB above my average daytime background. This corresponds to a sideband power density of -72 dBc/kHz at the transmitter. There is also a weak (-93 dBc) line, slowly drifting within about 10 Hz around 136.75 kHz. 

I have contacted the Donebach staff and informed them about the reappearance of problem. Unfortunately they said they can do nothing about it at the moment. They have dismantled one of their two 250 kW units, and are currently in the process of installing a new digital transmitter, which is scheduled to transmit DRM and Simulcast (AM-compatible DRM) formats after June 2007. This statement however did raise even more concerns from Eike (DJ5AO) and myself, regarding potentially drastically higher noise sidebands from digital transmissions.


The other new problem is caused by DCF39 (Burg, 40 kW, 138.83 kHz idle with FSK telegrams every 10 seconds). Normally at 310 km range, the FSK sidebands were just barely visible below 137.8 kHz in daylight. Since April 2nd, 08:55, the bursts seem to have increased amplitude and are extending all the way down to 137.1 kHz! This is probably related to the idle frequency change (normally 138830.03Hz, now 0.4 to 0.57 Hz high), reported by several LF observers. 

The transmitter switched back and forth between "narrow" and "wide" modes a couple of times yesterday, and has been "wide" continuously since April 4, 13:00 UT. For a comparison, please look at http://members.aol.com/df6nm2/QRM/DCF39_sidebands_070404.png - even with the rather inconspicuous violet rendering you can see the significant effect on the QRSS slots around 137.7 and 137.78 kHz.

My unconfirmed assumption is that the modulation parameters of DCF39 have been changed towards even less pulse shaping. But an occupied bandwidth of 3.8 kHz for a 200 bd FSK signal would seem rather ridiculous! I would be keen to know whether others are also seeing the annoying sidebands, and if anyone in the group has contact information to the DCF transmitter operators on site. 


73 and Happy Easter,
Markus, DF6NM