# How to estimate or measure the max voltage of an air variable capacitor

I have a big box of air-variable capacitors I inherited and want to see if I can use one of them for a low power magnetic-loop antenna for 30m digital.

• How can I figure out if any of them have a high enough maximum voltage?
• If I do this by trial and error I risk damaging my radio right?

From the ARRL Handbook...

Spacing
inches ___ V_peak

1. 0.015 ___ 1000
2. 0.02 ____ 1200
3. 0.03 ____ 1500
4. 0.05 ____ 2000
5. 0.07 ____ 3000
6. 0.08 ____ 3500
7. 0.125 ___ 4500
8. 0.175 ___ 7000
9. 0.25 ____ 9000
10. 0.35 ___ 11000
11. 0.5 ____ 13000

Also note that these are mere recommendations. Actual high voltage you will see of course depends on the reactance and input power to the antenna (a little circuit analysis may be in place) along with frequency needed to determine the reactance of the capacitor.

• Air starts to become conductive at around 3 x 10^6 V/m, so that's your maximum - if you have a 3mm separation of plates, you won't be able to go over 9000 volts. It looks like the ARRL handbook is being properly conservative by halving the value (or taking something besides arcing into account). Commented Jun 15, 2016 at 22:33
• The breakdown voltage for air gap is dependent on frequency. As the frequency increases (above 30 KHz roughly) the minimum air gap per voltage breakdown gets smaller. I am not sure if the ARRL posts are frequency dependent or not. I could look again but my handbook is not available as I am now traveling away from home. Here is another reference though: library.ul.com/wp-content/uploads/sites/40/2015/02/… Commented Jun 16, 2016 at 2:02
• perhaps it's time to read this book ;-) been saying that for 24 years now ... Commented Jun 20, 2016 at 17:24
• Do the thickness of the plates, and the metal itself make a difference? For instance, if the metal is copper rather than aluminum, or if the plates are aluminum and paper thin with a plate spacing of 0.5 inches? Commented Jul 13 at 13:35