Dear Jordan, Roelof, LF Group,
I have used a very similar circuit to the one in Roelof's schematic for some
time (it is in the RSGB's "Radio communication handbook", and "LF Today"- I
should declare an interest as a co-author of these). With slight
modifications it can be quite versatile. In my version, the input RF gain
pot is omitted, and the antenna is connected direct to the tuning inductor.
Gain is adjusted by a shunt capacitor between the antenna and ground, which
ranges between several 100pF for a short wire to several nF for bigger
antennas, making the input circuit into a pi network. Generally the same
shunt capacitance is satisfactory over the whole tuning range. The shunt
capacitor can be partly or wholly made up of the capacitance of a coax
feeder, so quite long feeders can be used, e.g. one leg of an existing HF
dipole. Alternatively, the antenna can be a large wire loop (say 10m^2 or
so), in which case the shunt capacitor is omitted - again, the antenna can
be fed through quite long lengths of coax.
Because the input tuner is essentially a low-pass filter circuit with a
sharp peak in its response, it works well at rejecting the very local MW
broadcasters here, although I have had problems with very small ferrite
cored inductors saturating - using larger chokes, or series combinations of
smaller values cured this. I have used it with inductors of up to 200mH, and
1000pF tuning capacitance, to tune to the bottom of the VLF range. The
tuning is very sharp, but this is not a problem for receiving the narrow
136kHz and 500kHz amateur bands.
The amplifier section in my version is similar, except instead of the 2N5109
I have used lower-frequency and cheaper transistors such as 2N3053, BD135,
BFY51, etc, which work fine at these frequencies. The biasing in my circuit
is a bit different - I have eliminated the 680R, 2k2, 10k, 470n and 47R
coupling and bias components, and instead connected the source direct to the
base, and a 100R resistor between base and emitter in a "bootstrapped"
arrangement. This compound follower arrangement has been very useful as a
high impedance, low noise buffer in several LF/MF applications.
Cheers, Jim Moritz
73 de M0BMU
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