Dear Ed, LF Group,
At 11:58 26/04/2002 +0400, you wrote:
I continue to have the big QRM from:
Alfa(Omega) (azim~196 degr, fq ~12kHz, QRB~200km, pwr~300KW)
Chajka(Loran) (azim~240 degr, fq~100kHz, QRB~450km, pwr~800KW)
Radio Chechnya(AM) / Radio Rossii(AM) -
(azim~168 degr, fq~170/1190kHz, QRB~280km, pwr~150KW/2MW)
Comparing to the situation in the south UK, we also get QRM from the Loran
station at Lessay in France - this is visible on a spectrogram display as
many horizontal lines spaced a few Hz apart, and makes a sound like
"galloping horses". Because this QRM is generated by the Loran transmitter,
the only way to eliminate it is by using a directional antenna such as a loop.
There are also several VLF transmitters in the region, such as GBR on
16kHz, but I am not aware of any QRM problems caused by these signals.
The Radio 4 signal from Droitwich (198kHz, 400kW?) sometimes causes
intermods in the LF range when combined with other LF signals, eg. 198kHz -
60kHz(MSF) = 138kHz - it is usually possible to eliminate this by having a
fairly high Q tuned antenna, or a simple bandpass filter. I believe in some
parts of DL, significant intermods are radiated by transmitters on
different LF frequencies using the same site.
At my QTH, I have 4 broadcast transmitters within 1km (Radio 5 909kHz,
150kW, Talk Radio UK 1089kHz, 150kW, Virgin 1215kHz, 50kW, and Sunrise
1458kHz 50kW). Naturally, I have to take care to avoid intermods - to use
my TX antenna for RX, I find that the series antenna loading coil, plus the
"half wave" TX low pass filter (2 x pi sections) give adequate filtering -
I usually use 10 - 20dB attenuation as well. I sometimes find intemods can
be generated by the antenna or the earth connections, if there is poor
contact between the metal parts - I have a 10m mast that is useless for RX
because of this, but a similar sized vertical wire a few metres away works
well on receive. The MF transmitters themselves do not seem to cause many
problems, although there is an intermod on 126kHz (1215 - 1089kHz).
I would be interested to know what form of QRM occurs at your QTH? Hope to
see you on the air again soon,
Cheers, Jim Moritz
73 de M0BMU
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