Building a high voltage air variable capacitor

[duffy]

Yes, linear adjustment. You want the same surface area introduced between the plates for each degree you turn the shaft.

 

[Roff]

Yeah, the formula is dead simple, it's just area/distance = capacitance. Area is in square centimeters, distance in cm, capacitance in µF. If it was anything but air you would need a slightly more complicated formula with a dielectric coefficient.

Your minimum plate spacing is going to be based on the peak voltage and dielectric strength of air, of course. If it's too small, you add more plates - but you got lots of plates!

It's not quite that simple. You forgot to include the permittivity of free space (air is approximately the same).

If the dielectric (the material between the plates) is a vacuum, Capacitance C = e0 (A / l) where A is the area of the capacitor plates, and l is the distance between them.
e0 is the permittivity of free space (8.85X10-12)
If the dielectric is another material, capacitance is given by:
C = ere0 (A / l) where er is the relative permittivity, which varies between materials.
I plagiarized this from Electronics 2000 | Capacitor Theory.

 

[duffy]

You're right, I'm sorry. I get a little too cavalier with the equations I think I remember off the top of my head.

 

[Roff]

You're right, I'm sorry. I get a little too cavalier with the equations I think I remember off the top of my head.

Yeah, I've done that. Sometimes a sanity check helps. Using your equation, a 1cm² cap with 1cm spacing would have a capacitance of 1uF. Tiny cap values might be tough to build, but power supply filtering would be a piece of cake!

 

[Andy1845c]

Just an update, I haven't got the antenna installed yet, but I did get my LCR meter today. I purchased the one Duffy posted from Ebay. Its certinly no Fluke, but seems okay for the price. I measured a bunch of electrolics I have laying around and its mesures them within normal tolerence.

My air cap measures 1130 pF when fully meshed, so as long as it dosn't arc over, I think it will work!

 

[johndm]

v-caps

hi i have builta good many of these they are not to difficultto make if you send me your em i will send you a copy of my aticle that goes through it step by step jdm
johnmorrisonsnr@aol.com.