Message-ID: <16439.200001211229@gemini>
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Fri, 21 Jan 2000 12:29:55 +0000
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Subject: LF: LF front end selectivity
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Reply-To: rsgb_lf_group@blacksheep.org
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Dear LF group,
	I don't have any strong local signals close to the 136kHz band - 
the problem comes more from the MF Transmitters across the 
road, which deliver a watt or so of BBC radio 5 on 909kHz to an 
untuned 50ohm load connected to my LF antenna, so clearly some 
front end selectivity is desireable.

 My receiver front end uses an RM6 pot core inductor of roughly 
3mH tuned with a 1000pF variable capacitor, with a Q of 100 or 
so. This combines with the loading coil resonating with the antenna 
(Q also of the order of 100) to give a second order bandpass filter, 
and MF signals are attenuated enough not to affect the RF amp, 
which is a very ordinary JFET affair. I can confirm G3XDV's 
suggestion that this type of arrangement produces a useful degree 
of in-band selectivity - it's worth at least 10dB from one side of the 
band to the other. 

Cheers, Jim Moritz
73 de M0BMU