Joe,
good to read this!
Regarding
the loading coil, I would agree with Stefan that a large
multiturn air coil is the best option. A laminated iron core
would suffer from excessive eddy current losses, and the effect
of ferrites is limited by saturation and hysteresis losses.
My
1.3 henry coil consumed 2.3 km of 0.4 mm enameled wire, using 7
buckets with 480 turns on each:
df6nm.bplaced.net/VLF/VLF_110304-06/coil_legospacers.jpg.
The advantage is that due to magnetic coupling between layers,
you will need less wire for a given inductance. And the
inductance is adjustable across a wide range, using spacers.
Disadvantages are the high electric field between layers
limiting voltage capability, and less effective heat removal
from the inner buckets. Stuffing the buckets too tightly into
one another is surely not a good idea:
df6nm.bplaced.net/VLF/pictures/arced_coil_140601.jpg
