Dear Bob, LF Group,
I guess as far as LF frequencies are concerned, the Butternut antenna is
just a 10m vertical whip (provided you disconnect the coil across the feed
point - otherwise it will be nearly a short circuited whip...). This will
give you plenty of signal, provided the antenna is reasonably matched to the
RX input. Matching requires tuning it to resonance. The simplest form of
tuner is just an inductor connected between the feed point of the antenna
and the low impedance RX input. The inductance needs to resonate with the
antenna capacitance at 136kHz. A 10m vertical piece of tubing probably has
about 100pF or so of capacitance, so quite a large inductance is needed,
roughly 13mH. For receiving, small ferrite-cored chokes are OK as loading
inductors - probably the easiest way to get this much inductance to to would
be to get a small selection of 1mH and 2.2mH chokes and connect them in
series - this makes it easy to adjust the inductance. This sort of
arrangement gives a very sharp resonance, with a bandwidth of only 1kHz or
so. For fine tuning, you can connect a variable capacitor say 100pF or so
between the antenna and ground - this will reduce the available signal level
a bit, and the required inductance, but is desirable due to the critical
tuning.
The signal level from an antenna this size should be enough for reasonably
sensitive receivers - as you tune the antenna to resonance, you will hear a
dramatic increase in signal levels and band noise. However, some receivers
have very poor sensitivity at LF, so a preamp is sometimes needed.
Hope this is helpful - good luck with LF!
Cheers, Jim Moritz
73 de M0BMU
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