Dear Markus,
unfortunately, the situation is as poor as you wrote. Data from DCF39
disturb on ham LF band a lot. It is not easy to receive on 137.7kHz, when on
138.83 is DCF39(-41dBu/from ant) and on 135.43 is TX from Budapest(-21dBu/from
ant). Bud then enormus problem is DCF39, nowadays. I would like to believe, it
will change for the better.
73 and Happy Easter!
Lubos,OK2BVG,JN88KS
**************************************************************************************************
-----Original Message-----
From:
[email protected] [mailto:[email protected]] On Behalf Of [email protected]
Sent: Thursday, April 05, 2007
1:42 PM
To: [email protected]
Subject: LF: More QRM from DLF and
DCF39
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
