Cecil Moore wrote:

Can a doorknob cap rated at 20 KVDC be used to
resonate a small loop?

Sure, but I think you'd end up with a pretty poor antenna.

I don't know for sure, but guess that doorknobs are typically made from
Z5U or similar ceramic. If so, the capacitance is a strong function of
both temperature and voltage. Therefore, the loop tuning will change
with both those factors. I'd also guess that the Q wouldn't be too hot,
so you'd also incur a lot more loss than you would if you used an air
dielectric part.

Roy Lewallen, W7EL