The AM Forum

OK,
It's probably not a new idea, its bound to have been done by someone somewhere, But....

I was just fooling around with some heat sinks and a plastic sheet. I wove the sheet between the fins and slid two identical heatsinks together. I was able to get about 300-3500pf between plates by sliding the two surface between each other.

Now my idea.. How would it work to use Teflon dielectric woven between plates to prevent shorts, and sliding the plates horizontally.. Basically, a horizontal variable cap.. The plates slid across each other instead of rotating through each other. In theory it should work right?? But heres my question.. How would I figure the voltage rating with a Teflon dielectric instead of air? I am assuming with Teflon the spacing could be closer and not break down and arc like with air but am I wrong?

Why go through the trouble? Well, I've found that I could potentially make a 50~500pf variable cap using less than half the vertical and horizontal space of a 500pf rotary variable. It requires about the same depth, but still I just wonder how to judge the voltage rating with approx 1/6th in. plate spacing and a Teflon dielectric.

Anyone have an idea?

Yep, a little odd.....
But.... if it works......

Do a google search for "dielectric strength of Nylon"

Of course... you need to know what "mix" of Nylon it is (brand name etc)
They vary.
I saw some around 15Kv/millimeter ( NOT  "Mil")

The Crosley 51 circa 1923 uses a book capacitor. It is two pieces of metal that are spring loaded. A cam rotated from the front panel varies the spacing. Sort of a variation of your idea. You can see the cap and the cam in the lower picture on the right hand side of the interior below the grid leak and cap. It is the two pieces of vertical metal

(http://www.crosleyradios.com/pics/51.jpg)

I have one of those old Crosleys. That book cap is mucho cool.

I am able to get the Gray Hair Net on mine, if i use the longwire.

I swear late at night I can sometimes hear Seymour (W2JBI) and George (GAC) in QSO

The variable capacitor idea reminds me of the rotary variable tuning capacitor in some old transistor radios.  The insulating/dielectric material in those appeared to be polyethylene sheeting, but the idea was similar.

If the idea is workable, a relatively small existing rotary breadslicer could be enhanced by the insertion of teflon or other insulation between the plates.  Maybe someone with arcing problems has already done this, and can report on the results.

I'll look more into it.

The Tools For Research:
2 aluminum heatsinks 6"(L)x5-1/2"(W) 1" fins and 1/2" Deck.
A sheet made of plastic shopping bag (because it's kinda got that slippery Teflon like surface)
A digital capacitance meter.

My Findings:
With both heatsinks fully meshed I could get 50~3500pf
With only 2" of the heatsinks meshed I could get 30~550pf. (A Nearly Perfect value for the QIX-500 plans.)

Prognosis:
This would effectively make a variable cap that would take up half the width, half the height, and about 13" in length. My present 3.5KV 30~300pf standard rotary style cap is 4"(W) x 4"(H) x 10"(L). (Including mounting insulators.)

If I could devise a frame 12" long that would allow the two plates to slide over each other with a Teflon dielectric it would be a perfect space saver design for a compact E-Rig RF Deck. It could be done so that both plates slide independently over each other or one static and the other movable. I've got a few ideas how it could be done I've just got to devise a way to make it practical. First thing is to figure out if it would be cheaper to build one this style or buy a standard rotary type. But if someone is looking to design a legal limit E-Rig in a relativity small enclosure  it may work. Particularly because the plate could be thin and short and mounted lengthwise and easily adjusted from a 90 degree angle. Shortening the required depth of a design using a standard rotary type without a reduction drive or gear driven system.

Can't help but mess around with silly projects here while I wait for the PWM PCB's from Steve QIX. (Hint, Hint, Nudge, Nudge.. hehe) I've already got my RF Deck and Power Supply 95% built, now it's just a matter of waiting for the PCB's to slobber together the bag-o-parts I've got waiting to make a modulator. Other than that I'm waiting on a torriod tranny, and my heat sink that a friend is drilling and tapping for me. I should have those parts in the week following the Timonium  Fester. So I'm bored.. LOL

Chinese fan varicaps..... thats the ticket .... ..      klc

I saw an article somewhere where a guy used two sections of copper tubing with one on a lead screw. The one on the lead screw went inside the fixed one to make a variable cap. the older handbooks had a chart of breakdown voltages of different materials.

You might find this useful reading (I have):
Make Your Own High-Voltage Capacitors:
http://www.uoguelph.ca/~antoon/circ/hv/hvcap/hvcap.html (http://www.uoguelph.ca/~antoon/circ/hv/hvcap/hvcap.html)