We are using a 250Pf Gang Capacitor, i.e. Air Variable capacitor, in an RF Tuning Network. This we use to match a high power ~600 Watts of RF Energy to a load.

Two of 25 Capacitor plates are burnt, and we are not able to match the power due to sparking between the burnt portions.

We want to prepare a new capacitor. Regarding this I have some questions:
Does the material of the plates matter in determining the capacitance (i.e. can I make the plate with Steel or Aluminium or any other metal)?

What happens if I remove the burnt plates and use the capacitor?

 

Removing the burned plates could be an option if the capacitor was not fully meshed and was still able reach the resonant tuning point. Removing the plates will certainly lower the total capacitance available, but if you didn't need it all to begin with, there should not be any problem.

Gerald Beaudoin

 

Arcing is probably due to bent plate that makes the air gap too small for the voltage this capacitor has to withstand. If you cannot bend the plate straight and make the air gaps the same, then you could take the plate out. If there are 12 plates, then removing one plate will reduce the maximum capacitance to 91% of what it was (229 pF). If you never need to adjust beyond 229 pF, then that will work for you. A different metal would not change the capacitance, but it might change the Q. If you can't bend the bad plate straight, then I doubt that you can make a new plate.

Robert Scott
Real-Time Specialties

 

The material of the plates doesn't determine the capacitance if the size and spacing remain the same. That's assuming they are metal. What may vary is the Q of the resonant circuit if the originals are silver plated to lower skin effect or the replacements are significantly higher in sheet resistance.

If you remove the plates you will have proportionately less total capacitance which may or may not matter depending on whether you need the total capacitance, or close to it to tune the circuit. And even then, if you need the high end and not the low end you could "pad" the variable with a fixed 30 pf _RF_ capacitor of suitable ratings. It's getting harder to find these types of things these days.

Regards,
cww Who had a Commercial Radiotelephone License that was never used.