A year ago I made this 20...200 pF variable capacitor. It is used in a magnetic loop antenna for 80m. The plate distance must be large enough to withstand the high voltages that exist at the magnetic loop antenna's top (levels of over 1 kV are easily reached).
The static plates are made of pieces unetched PCB. The variable plates are aluminium. This capacitor works fine in my magnetic loop antenne. I have been able to made some QRP contacts on 80 m with it without the use of a long wire antenna. Still handcontrolled. Remote control would be a nice improvement.