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