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

Jim and the group,

  I used with good results the following schematic to match a random piece of wire
to an LF Rx.  Below also the Spice simulation. Gain is high, and is given only by the
passive impedance step-up, so there are no risks of distortion, as long as the Rx
does not saturate...

C1 is absolutely not critical, can range from 1nF to 4.7nF with no practical changes
in the performance of the circuit.

73  Alberto  I2PHD