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Message-ID: <5.1.0.14.0.20020426114318.00a7c6f0@gemini.herts.ac.uk>
Date: Fri, 26 Apr 2002 12:35:45 +0100
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Subject: Re: LF: May 2-4 KN97LN Activity
In-reply-to: <11498.020426@dx.ru>
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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