[600MRG] Vacuum variable capacitor, low-pass/band-pass filters

[Frank O'Donnell]

Thanks to everyone for the great responses. Those sound like promising 
ideas on both the capacitors and filters, and I will check them all out.

One question for now. I mentioned my interest in a vacuum variable 
capacitor to a local ham friend who has a background in the AM broadcast 
industry. His reaction was that the voltages for my 630m station 
couldn't be more than what is seen in AM BC equipment, and he thought I 
might be able to scrounge a cheaper air-dielectric variable cap that 
would be adequate. Is the preference for vacuum caps due to the fact 
that they are more compact? Or more available? Or are there other 
reasons for going with them?

Actually, make that two questions for now: Regarding voltages. What sort 
of voltage rating should I be looking for in any of the caps (variable 
or fixed) to tune the ~120-foot-circumference vertical loop?

Thanks again, and 73,

Frank K6FOD