for magnetic loop usually vacuum capacitors are used. As alternative you can use air capacitor with extended gaps between plates. Also air capacitor should not use moving mechanical contacts like it is used in air capacitors for a usual tube-receivers. For example you can use two sections connected in series through a rotor to avoid moving mechanical contact.
Vacuum capacitor provide you with better Q. But on the other hand very high Q leads to very narrow bandwidth, very sharp resonance and extreme high voltage and current in antenna. So, it will be more hard to tune and requires more expensive capacitor.
By the way, using magnetic loop for transmission is not safe, because it has extreme high field strength near the loop.
Just look at simulation for a magnetic loop at 14 MHz with a 100W power. The black line on the color grade is official safety limit.
As you can see, the magnetic loop has:
- E field strength more than 15000 V/m, while safety limit is just 130 V/m
- H field strength more than 60 A/m, while safety limit is just 0.34 A/m
For comparison, half wave dipole has 8 times smaller E field strength and 15 times smaller H field strength.
I don't recommend to use more than 10-15 W power with magentic loop. And when TX, keep away from antenna as much as you can.
Some years ago I was played with magnetic loops and it was a funny when you keep the gas-discharge lamp in your hands and it glows like it is plugged to the mains. It was just 10 W power and I had a many very painful burns on my fingers when tried to tune air capacitor. But later I read more about electromagnetic flux, antenna design theory and underlying physics and I came to the conclusion that it is not safe for the health. Especially if you use more than 10-15 W power with a shortened antennas.
Using a full size (half wave dipole) antenna is much more safe and give you better results. Full size antenna has much better efficiency , much less noises, it's more easy to match it and it has much more safe field strength near it.