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
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