gravity wrote:
i've been considering an antenna for the audio frequency range plus 1 to 20
hz. basically a big coil would work.

i've thought about using a steel drum, but i don't have room for that in my
new QTH. mu metal rods would be very expensive, even ferrite would be
expensive for an 100,000 turn antenna.

not sure what to use. maybe steel core.

High permeability ferrite (manganese zinc) is fairly cheap in bead
form, and these can be glued or clamped together (with nylon
all-thread) to make arbitrarily long rod forms. For example, 5000u,
5/8th inch diameter, by 1 and 1/8th inch long beads (Steward part #
LFB159079-000) cost less than a dollar each from Digikey, in quantity
of 10. This will make an 11 inch long rod with very low loss at low
frequencys (less than 100 kHz). And the hole down the center eases
the problem of mounting the rod in a drill press to make the winding.
You just slip a chunk of 1/4" all-thread through the glued rod and
run a nut against each end.