High voltage variable capacitor.

[Andrew B]

I have almost gained my radio amateur certificate and thoughts have turned to all things >transmitter<.

One of the things I am going to need is an ATU capable of handling a reasonable amount of RF.
So, my mind was set in making a variable capacitor (or two). It sprang into my mind that I could use double sided copper clad PCB. Easy to solder, fabricate and quite strong material.

I next thought about insulation between the stator and rotor plates and mooted the idea of using wide self adhesive "Kapton" tape stuck on both sides of the stator plate(s) as the insulator/dielectric material.

Does anyone think this idea would be worthwhile?

[Radio Wrangler]

I'd use adequate air spacing between plates. Metal edges should be smooth, not jagged.

Etched-off bare FR4 is OK for structural insulation, so is perspex.

You need to either use 'butterfly' type vanes, or have to arrange sliding contacts.

There's a lot of good ex-military capacitors kicking around in people's junk boxes, and it's not unusual for newly licensed people to get given bits for getting them on the air... at least in the clubs round here.

David

If you make your antenna resonant (one for 20m/14MHz is only 10m long) there will be no need for a tuner and you will put most of your power out rather than losing it in the ground or elsewhere. You can do an awful lot on just 20m, good training for the newcomer there are lots of things going on and enough idiots to make your minor mistakes invisible. It is a joy to listen to (and learn from) a properly conducted QSO (contact*) and one I strive to get to even after 30 odd years of being on the air (I have a terrible memory for call signs, makes portable operation quite interesting, no way to make a note!).

Don't be afraid of putting out a CQ call (seeking you*) saying you are a beginner, lots of help out there. Radio is so much fun, carry on!

*explanations used because not everyone is an amateur radio enthusiast.

[MrBungle]

I gave up amateur radio about a decade ago. Not sure why. Getting back into it again now.

I'd use a real air variable capacitor for a first ATU if you can. If you're not chucking out too much power a standard 365pF dual gang generic one will do the job with some switched inductors. VK3YE has some nice videos on homebrewing these. I've built a few capacitors over the years from scratch and they never work exactly how you expect or perform well over time.

A big note with copper clad - it goes horrible if you leave it a few days or touch it. Look at the state of my current breadboard, only 3 days old:



(naive and low quality 30W-ish switch mode boost converter to get a 36v tuning voltage for varactor diodes and fill up some larger voltage capacitors to test some stuff. 555 on left to supply PWM, LM311 to turn it off when the desired voltage is reached, 100uH inductor at the top being pulled by an IRF510 MOSFET)

Depends what band you are running but there are some usable motor driven ones inside a Clansman/Racal ATU as well as some nice transistors and a cute little switching power supply. I bought one to steal the motors from for another project a while ago. They go for virtually nothing on a certain auction site.

[kellys_eye]

There are quite a few online articles on building your own variable capacitor - not entirely sure I'd go to all that effort given the relative simplicity and low cost of obtaining one via auction sites.

Here's an article on a typical example I found

http://www.qsl.net/n4dfp/buildcaps.html