High voltage capacitors are not as easy to make as inductors, and they can be expensive if you don’t know where to look.

I’ve found two affordable options.

Surplus Soviet transmitting equipment

Available on EBay, these are large ceramic “doorknob” capacitors. These can be found by searching K15Y capacitor on ebay. I’ve had good results with 3.5kV rated doorknob capacitors.

2 K15Y doorknob capacitors will cost between $5 and $10, before shipping.

I’ve used doorknob capacitorswith success. IMO, these are cheaper since they come in packs of four or more.

Parallel silver mica capacitors

I haven’t tried these, but it seems that 3 \(120pF\) 1kV silver mica capacitors in series will provide a sufficient voltage rating for our antenna. Three of these capacitors in series will have three times the voltage rating and one third the capacitance of each capacitor.

For example, 3 \(120pF\) capacitors in series gives us with a trap capacitance of \(40pF\).

At $1.60 each, 6 capacitors for a pair of traps will cost $9.60, before tax and shipping.

I haven’t tried silver-mica caps in series, but I have seen this technique in QST magazine and elsewhere.

Voltage rating

My assumption is that 3.5kV is sufficient for transmitting up to 100 watts. As far as I can tell, I haven’t had any arcing. If I have, it doesn’t seem to have affect capacitance.

Generally, capacitors fail when voltages exceed ratings and they arc over. The arc punctures the dielectric layer between the capacitors’ conducting plates. You’ll know you need a higher voltage rating when there is an arc, your capacitance drastically changes, your antenna is no longer resonant and your SWR soars.