I have been kicking around the idea of getting back on the ham radio air waves. I would like to be able to work 40 meters, but don't have 70 feet of clear space to install a dipole. I am thinking of trying a magnetic loop antenna. Here is a site about what I am looking at. Magnetic Loop Antennas, Magnetic Loop Info, How to Build A Magnetic Loop, W2BRI Loops

I don't have alot of extra cash to buy a fancy variable capacitor at the moment. I do however have about 200 3" x 3" x 1/16th aluminum plates.
What I lack is knowledge figuring out the number of plates, shape of plates and spacing I will need to make my own capacitor. Something like 40-600pf seems to be what most plans mention. I can only output 100 watts with my radio. The author of the page in my link says he has used 4KV caps at 100W with no arcing. So possibly something arounf 4 or 5KV would work for me. I don't know how to

Can anyone guide me on how I could start out on this? I should also mention I don't have a meter that will measure capacitors. So I imagine It will be alot of trial and error.

Are there any math formulas that I could possibly use.

Any words of wisdom from the resident radio experts?

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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!

 

I think you will find a lot of good information here:
Plastic Best Choice For High Voltage Capacitors THE PLASTIC SPORK BLOG
Making it variable is another question, a lot depends on the tools you have available.

 

What about shape? Are square plates okay? Commercial built air variables I have seen usually use a half circle shape plate. Do sharp points like corner affect arcing at all?

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Yes, electrons tend to arc off sharp points. A non-semi-circular plate will make the response non-linear with respect to rotation.

 

Just look at one of those old style vari caps used in radios.

I use to work on a 100W HF automatic antenna tuner. The caps were huge. About 3.5 inch diameter and about 8 inch long. The spacing between plates was around .2 inch. It used thick teflon in between plates which increases the dielectric constant.
Even with the rounded plates and large spacing the thing could arc pretty good at
< 10MHz. I would recommend you look for an electronics salvage place and try to find a high power vari-cap.

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Pay it forward.

 

Thanks for the help so far. I know this might not work. But I still want to try. I know a commercial made part would be better, but I dont have alot of money at the moment, but I do have alot materials and time.

I know its likely complicted, but how is the voltage figured with respect to transmitter power and frequency? The 4 or 5 KV I mentioned is just something I saw online for a HF loop project. I don't know how one arrives at that figure.

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Smooth edges are best. The physical shape of the plates will determine the percentage of capacitance change Vs percentage of shaft rotation. Many old caps has strange shapes to help linearize shaft rotation Vs frequency range so the shaft plate would have an evenly spaced frequency scale. If your tuning via something other then a scale printed on the shaft plate then it probably doesn't matter too much and standard half moon shapes should work fine.

Lefty

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Measurement changes behavior

 

The link suggested above is really worth reading.
Plastic Best Choice For High Voltage Capacitors THE PLASTIC SPORK BLOG

 

Hi Andy,

Would you have enough room to put up a 90' long Inverted Vee? If so, I'd like to mention the Barker and Williamson Folded Dipole for 80 through 10 meters. I used it for several years after getting my license in '98 as my only antenna along with a Heathkit SB200 amp' and easily accomplished 7 band WAS (40 thru' 10), WAZ, 7 band DXCC (40 thru' 10), and confirmed over 300 countries with it. It's coax' fed and provides a 2:1 or better SWR on each band without a tuner. It's also extrememly quiet. The down side is that it's very expensive and very inefficient on 80 meters.

After a few years struggling to finish WAS and DXCC on 80 meters I purchased a used 500 watt Dentron SuperTuner for $50 and replaced the B&W Folded Dipole with a ladder line fed 90' wire Inverted Vee the day after Christmas. It wasn't as quiet as the B&W but it worked extrememly well and I finished WAS-80 and worked 67 countries on 80 to finish up DXCC-80 during some amazing conditions within just a fews days of putting it up.

Now about your capacitor. I've been thinking about using a slotted teflon frame to hold both fixed plates as well as sliding plates driven by a stepper motor. Anyone know it this would work? I'd like to use this in a home brew microcontroller controlled 500 watt auto' antenna tuner.

Mike, K8LH

 

I did skim the link over. I will give it a better read through though.

I guess I should say its not so much the lack of room, as it is the lack of room close enough to the house to run a feedline to. Theres alot of open space 500 feet from the house, but not close to it. I figured maybe I could mount the loop on the roof, or mount it on a pole outside.

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My aluminum squares have paint of somekind on one side. This is proving hard to remove. Is there any reason I would need to?

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Not so far as I know. It will affect the dielectric, but not by much if it's a thin layer. If this was a magnet, it would be like a thin layer of steel sitting on a magnet pole next to a big gap - most of the reluctance is in the gap, the steel barely changes it.

 

Okay, thanks.

It is a fairly thick coat of paint. The squares appear to have been some kind of samples/tests for some kind of paint. Its not just a misting or anything.

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Uhh... umm...

Buy an LCR meter. You can get on on eBay for less than $20.
LCR INDUCTANCE, CAPACITANCE, RESISTANCE METER TESTER - eBay (item 330300626676 end time Jan-16-09 16:50:30 PST)