I’m building a homemade small magnetic transmitting loop antenna that I want to ultimately operate at up to 1500 watts over the HF ham radio bands. One of the challenges with these antennas is the high voltages present at the variable tuning capacitor, which range from thousands of volts at 100W to tens of thousands (up to 20kV) at 2KW and vary based on frequency, which can range from 1 MHz to 30 MHz.
The prevailing designs use vacuum variable caps, which are expensive and air variable caps, which are quite large. I need a variable capacitor capable of 10 pF to around 650 pF that can handle 25 kV or more. It must be tunable via a stepper motor to quickly adjust the antenna to resonance.
My design consists of 5 copper plates of at least 16 sq in (4” x 4”) spaced 0.06” apart, using 24 gauge (0.025”) copper sheet with oversized, 0.03” thick plastic sheet (PET) as dielectric, producing variable capacitor with up to 669 pF capacitance at 22.5 kV dielectric strength. The design assumes oversized plastic sheets are glued to each copper plate after plates are attached to copper plate base via brazing and solder. The plastic sheets act as insulators and capacitor dielectric material between each copper plate electrode element. PET should have dielectric constant between 2.8 and 3.4 from sources I have found.
The idea is to then use a 3D printer style stepper motor with a slide screw linear actuator or equivalent to adjust the capacitor plates in and out. The assumption is that the plastic (PET) insulated copper plates can rub closely against one another and should pose no real issue. I believe this design will be smaller and less expensive than the alternatives and provide a much higher voltage rating vs vacuum or air variable caps in the same or less space.
Will this design meet the requirements as expected? Am I on a good track here?