In your reply you mentioned:
For the usual 500 volt 4700 pf polystyrene ( or the current 'equivalent
material- polycarbonate??) capacitor this means enough wired -
a) in parallel --- to keep the current in each to a safe figure- 1 amp
to be about all the polystyrenes will sustain long term, and
b) in series---- to withstand the applied PEAK voltage.
So a large array of caps in series/parallel is what you wind up with. Not
pretty, but functional.
The other suitable type of capacitor is polypropylene. Andrew ZL2BBJ
operated a single turn transmitting loop for 180 kHz for several years
before shifting QTH late last year, and has yet to set up for transmitting
on LF, but he is thinking of another loop rather than a top loaded vertical.
As you have, he also made a series/parallel network of capacitors, his Mark
1 ATU with polystyrene capacitors, and popped a few till current was fanned
out to be less than 1 amp per capacitor. His Mark 2 network used
polypropylene capacitors, as we found a surplus source of 1.5 and 2 kV
capacitors. For QSY I think he has wafer switches to do coarse and fine
tuning, and noting that several wafers are needed to keep current per
contact within ratings.
73, Bob ZL2CA