On Fri, 21 May 2004 21:52:06 +0200, "Richard"
wrote:


I am looking for a circuit diagam for a MW pre selector, anything would be
of interest, bit if someohw has one of thoese super dooper ones that can
focus donw to a few khz that wold be even better, (like the palomar MW pre
selector)

I have used for LF a parallel resonant circuit followed by a
(Darlington) emitter follower driving the 50 ohm line to the receiver.
Assuming hFE=100 for each transistor, the resonant circuit loading is
about 500 kohm, thus the loading is quite small. Since the antenna was
short compared to the wavelength and thus highly capacitive, a small
10-100 pF capacitor was used to connect to the hot side of the
resonant circuit.

Although the voltage gain is only 1, it works surprisingly well due to
the large (natural and artificial) field strengths in the LF and lower
MF bands.

When using some processor controlled receiver, it is a good idea to
move the preselector (and hence the antenna connection) as far as
possible from the receiver and connect them with a shielded cable, to
prevent the receiver processor radiation from entering the high
impedance antenna connection. A 1-2 m separation helps a lot, without
making it too uncomfortable to operate.

Paul OH3LWR