# Air Variable Capacitor Calculator

Discussion in 'General Technical Questions and Answers' started by KC9KXW, Mar 14, 2010.

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1. ### KC9KXWHam MemberQRZ Page

Hello Everone,

Does anyone know of a "Air Variable Capacitor Calculator Download" that gives the size of plates, number of plates, space between each plate, what metal to use for the project at hand, and if you should use a dielectric of some sort between the plates, and the "END CAPACITANCE VALUE" of said capacitor.

73 Jim
KC9KXW

The ARRL Handbook has some good data on this.

But if you use a dielectric other than air between the plates, it's no longer an "air variable capacitor."

And as long as the stators and rotors are metal, it doesn't matter what they're made of. Most are aluminum or plated brass. Capacitance doesn't change with material. If you could deposit a thin film of metal on balsa wood, that would work just as well.

4. ### AE1PTHam MemberQRZ Page

A great item for analyzing unknown components that you have is an old Heathkit Impedance Bridge, such as the IB-2 or 2A. You can find the range of the capacitor relatively quickly--and these items can be had for under \$30.

5. ### KC9KXWHam MemberQRZ Page

Steve,

Thanks for the bit of info on the dielectric, so if that's the case then what kind of variable capacitor is used in something like a transister radio (the ones that are in the little plastic ---> cube)?

Greg,

Thanks for the links, I will be looking at them here shortly.

Patrick,

Moneys tight right now, can you find those impedance bridges on ebay?

Thanks again Guys.

73 Jim
KC9KXW

Last edited: Mar 14, 2010
6. ### VK2TILHam MemberQRZ Page

The calculator that 'GB gave (the Plate Capacitor one) is a good one.

The rest is design that requires the designer to make decisions; not really a subject for a calculator.

Home-building variable capacitors is not for the faint-hearted but there are quite a few examples on the 'net: Google will find them.

What 'WIK was saying is that, if you use a dielectric other than air, it's not an air-spaced capacitor.

The dielectric in the small caps often called Polyvaricons is plastic; its dielectric constant is greater than that of air and smaller plates can be used.

What is the intended purpose of the capacitors you want to build?

7. ### W9GBHam MemberQRZ Page

That is a Polyvaricon Capacitor, photo of one used in the QRP Kits Bitx20 transceiver!
LG Electronics and Mitsumi were 2 large mfg. but these specific parts are on backorder in USA.

Those might be suitable for QRPp rigs, but would not normally be used for real transmitters. Nothing's quite as stable as a vacuum or dry gas, but next best dielectric is air. It's very low loss, stable, and has great withstanding voltage. Most all variable capacitors used in transmitters are air dielectric, although some high powered ones use vacuum dielectric (those are sealed in glass or ceramic).

What are you building??

9. ### KC9KXWHam MemberQRZ Page

Variable Capacitor Fun

There is no real "intended purpose" for the build other than just (GOOD OLD FASHIONED) "Ham Radio Experimentation", but maybe for a QRP CW Rig to take camping or hiking to toss a resonent dipole in the trees and have some fun.

I like to learn about OLD RADIO and how guys way back when used to experiment with what they had (and nothing more), and "MAKE IT WORK".

That's all, It can give you great satisfaction to build something with your own hands and it works. I do have a Yaesu FT-857D, but I don't want to be just another "APPLIANCE OPERATOR". Hey at least give me an ('E' FOR EFFORT). HIHI

73 Jim
KC9KXW

10. ### VK2TILHam MemberQRZ Page

That's great! we need more amateurs who want to dig-down into "real" radio & electronics.

The question that I and others asked was directed towards finding-out just what you want; there's a lot of difference between a 1000pF/20kV unit and a tiny receiving capacitor.

Most homebuilders of variable caps seem to do it because they require something special and/or expensive such as a tuner for a magnetic loop.

Whilst I make many items myself, I would not make a variable capacitor; there are many more-interesting things to design & build.

I think that you will expend much energy in trying to build an effective small (say 100pF) variable capacitor but, if you really want to do it, start by searching something like "homebrew variable capacitor"; that gets 12 400 hits with Google.

One interesting one is this;

http://www.qrp.pops.net/PNJ.asp

It discusses the use of simple PN junctions (diode or transistor) as VVCs (Varicaps).

Cheap and interesting.