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Plastic Capacitor -- Am I Crazy?

Do not contact (N4ZOU) on November 5, 2004
View comments about this article!

After having major brain surgery to remove a golf ball size tumor deep in the left side of my brain back in June of this year I have discovered that I look at and think about things differently now. I had to take a medical retirement and I do have disabilities and so I have lots of time to think about all kinds of stuff and a lot of stuff pops into my mind from places unknown in it.

Here is an example. Plastic air gap transmitting capacitors! Is that crazy or what? I did a search for plating non-conductive plastic surfaces and found a product that will do just that and to top it off it's for silver-plating plastic! The product I found can be seen here.

RF energy has a skin effect in that it only flows across the surface of the conductive material used. Silver is more conductive than copper and next down the list is aluminum. Variable capacitors used in antenna tuners and amplifiers are made from aluminum. Copper would be much better but it is so soft you cannot punch the parts out on a press. Needless to say solid silver parts would cost a fortune! So all the capacitors today are made from stamped aluminum sheet and hand assembled producing a less than optimum but cost effect product. They still cost a lot of money and are becoming hard to find.

A plastic capacitor could be injection molded and then silver-plated producing a high capacitance, high voltage, and highly conductive product at less cost.

I know that the first thing to consider about this is the problem of incorporating a sliding mechanical contact on a plated surface; it would soon ware through and fail. The solution is to make Butterfly type capacitors, which do not have any type of sliding contact to the rotor. Butterfly capacitors are really two capacitors with the rotor linking them. This is a very simple solution to that problem. Another factor about using a Butterfly type capacitor is the voltage rating is doubled for the gap used. Also going from minimum to maximum capacitance only requires a 90-degree turn of the rotor. A simple and cheap screw jack drive like used on the large TVRO satellite systems can be utilized making a multi-turn knob control system easy and cheap to install in the tuner or amplifier. It would also greatly simplify stepper motor control in automatic systems.

When this crazy idea hit me and I thought about it for a while I decided to build a Butterfly capacitor by hand and having the therapist wanting me to make craft items and not wanting to make something dumb like a decorated paper holder I had my wife drop me by a sheet metal shop in town where I picked up a scrap end peace of 15 gage copper sheet metal that was 5 inches wide and 8 feet long and cost me $6. I cut the following peaces out of the copper by hand using aviation snips.

0x01 graphic

The stator plate

0x01 graphic

The rotor plate

Next came the Plexiglas end plates.

0x01 graphic

0x01 graphic


And here is a picture of my capacitor.

0x01 graphic

The hardware used is 1/4-20 all-thread rod and 1/4-20 nuts as spacers between the plates and the Plexiglas end plates. The drawings were found on the Internet at

Metric sizes are shown but as long as you keep the shapes the same you may make it any size. This capacitor is 10 pF to 100 pF at 12,000 volts. I attached this capacitor to a homebrew Magloop antenna and the difference between the copper butterfly capacitor and an aluminum split stator capacitor was astounding! Silver should be even better.

Now for the question, am I crazy or what? Would a silver plated plastic capacitor work? Later I plan on plating some thin Plexiglas and try it out on the Magloop. If the Plexiglas with silver plating can withstand the voltages developed in this type antenna system it will work for anything. I just need some more recovery time so I can use a Jig saw. The Nurse was really upset when she found out I used an electric drill press on my project!

I would like to thank ON4CEQ for the information on his excellent site.

Member Comments:
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Plastic Capacitor -- Am I Crazy?  
by W3PH on November 5, 2004 Mail this to a friend!
Very interesting & well-written article. You've been busy!

What happens when you get an arc-over between the plates, as would be pretty likely to happen in a transmitting application. Would the plastic melt, or holes start developing in the plating where the arcs happened, etc.?
Plastic Capacitor -- Am I Crazy?  
by KR1ST on November 5, 2004 Mail this to a friend!
Hi Scott,

I'm sorry to hear that you had to have a brain tumor removed. I hope you will fully recover quickly so you can do even crazier things. Yes, I definitely think you're crazy. :-)

I don't know if silver plating would make much difference over the copper material you used. However, I noticed that you did not solder all the plates together. Most homebrew capacitors are made from a material that can not be easily soldered, but since you used copper you have the opportunity to do so. Soldering the plates together would decrease the resistive losses between the plates compared to clamping them together with threaded rods and nuts.

I do not have any experience myself with homebrewing such capacitors, so I don't know how much of a difference it would make in the real world, but it seems an easy and cheap improvement. On the other hand I think that some folks are a bit too obsessed with the resistive losses when it comes to building magnetic loops. Build one with whatever you have and get on the air and make contacts, then worry about achieving maximum efficiency. :)

Thanks for the nice article. I enjoyed it.

--Alex KR1ST
RE: Plastic Capacitor -- Am I Crazy?  
by KA4KOE on November 5, 2004 Mail this to a friend!
RE: Plastic Capacitor -- Am I Crazy?  
by G7HEU on November 5, 2004 Mail this to a friend!
An interesting article.

Some months ago I started to make a large variable capacitor for an ATU project. It was going to use plexi-glass ends, alumium for the vanes and threaded rod to hold it all together - similar to yours.

The whole thing became a test of my engineering skils. A test which I failed because I lost my temper and threw it all away.

Well done on completeing yours and I wish you a speedy recovery to full health.

Steve / M0HEU.
RE: Plastic Capacitor -- Am I Crazy?  
by KC0KBH on November 5, 2004 Mail this to a friend!
That is so cool!!!!!!!!!!!
Plastic Capacitor -- Am I Crazy?  
by K7NNG on November 5, 2004 Mail this to a friend!
My hat is off to you. I salute you. This is real ham radio at its best...I think you are on to something, keep working at it and good luck....
RE: Plastic Capacitor -- Am I Crazy?  
by W9OY on November 5, 2004 Mail this to a friend!
My guess is you will have 2 problems. The first is due to RF heating, and the second mechanical stability over time in the face of RF heating. If a tank is 90% efficient at a KW that is still 100w that is dissapated. Put your peice of plasic on a 100W light bulb and see what happens

For low power applications this should work, but then you don't need a 12,000v capacitor for low power applications. You may be able to get around this to some extent by using a composite of materials, the coated plastic for some parts of the capacitor and aluminum plates for other parts.

Maybe you can figgure out how to make the plastic into a tube and plate a silver coil onto the tube, and you can make a complete tank using this technique.

Good luck

RE: Plastic Capacitor -- Am I Crazy?  
by K5DVW on November 5, 2004 Mail this to a friend!
>Would a silver plated plastic capacitor work?

Yes, it would. But, a couple of things you might not have considered.

1. If you were to have an arcing condition, that would spell the end of your plastic coated capacitor.

2. Any heating due to loss could deform the plates

It could sure work in receiver applications.

Other than that, it's an interesting idea. I've used metal coating of plastic enclosures to sheild against EMI. From what I remember, it's a fairly costly process, so it may end up costing more than regular stamped aluminum in small quantities.

Interesting idea, regardless.

Plastic Capacitor -- Am I Crazy?  
by N4ZOU on November 5, 2004 Mail this to a friend!
Thinking about the comments so far and I think you all for them. I have come up with the answers.
If you have an arc across any plate it would remove the thin silver-plating in that spot. With an aluminum vane you would need to dissemble the capacitor and replace the arc damaged stator and rotor vanes. You would need to contact the company that made the capacitor for replacements or hand make replacements. With the plastic capacitor all that would be required is to clean and plate the damaged areas. Just use the silvaspray on the damaged area and then silver dip plate the entire capacitor. I would think that it would be possible to simply throw away the entire damaged capacitor and order a new one, as it would be so cheap. Also, this capacitor would be so cheap that you could rate it at 3 times the normal use and never arc it. I have never arced a capacitor since I was licensed in April 1990 and I do use an amplifier when needed.
The way to make the plastic capacitor is to injection mold the stator and rotor plates and flat plastic bars that the stator vanes would be glued into. The stator and rotor vanes and spacers are sprayed with the silvaspray and only the slotted side of the stator mounting bars. Copper bolts are molded in the stator-mounting bar for connecting the capacitor to other components or conductors. This is also coated with silvaspray at the same time as the bar it's molded in. The same process is used for the rotor assembly. Then the entire capacitor is silver dip plated to increase the plating thickness to the required voltage and current rating and to bond the all the parts so there are no mechanical joints, the ultimate capacitor! RF energy only penetrates to a very small depth. When using silver plated inductors in amplifiers only the silver plating is transferring the RF energy and not the copper beneath, copper is simply a good material for plating silver on.
On one of the rotor vanes a wire mount could be incorporated so the Butterfly capacitor could be used as a split stator type capacitor. As the capacitor uses only a 90-degree turn from maximum to minimum capacitance no sliding contact is required. Stops would be required to keep the rotor from turning more than actually required.
On my copper capacitor I am not using the steel 1/4-20 hardware as a conductor to the Magloop. A 14-gage wire is soldered to each stator vanes with SAFE FLO silver lead free plumbing solder and to the 3/4-inch copper plumbing pipe used for the Magloop element. All joints on the Magloop element use the SAFE FLO silver solder for no loss joints. Donít tell my therapist I used a torch; She would freq out (pun intended)!
RE: Plastic Capacitor -- Am I Crazy?  
by WB2LOU on November 5, 2004 Mail this to a friend!
If you can, you better get a patent quickly !!
Plastic Capacitor -- Am I Crazy?  
by N3NL on November 5, 2004 Mail this to a friend!
Thank you for your excellent article!! This is ham
radio at its best. Keep producing new ideas and
building new components. Inventing new items is a
lot of fun and very satisfying. This is a great
public service that hams can provide to the World.
Please note the interesting salt water variable
capacitor that Mr. Simes, a ham at the Crystal Set Society, developed.
73, Nickolaus E. Leggett, N3NL
U.S. Patent 6,771,935 - wireless bus
Lighthouse Protocol (QEX July/August 2004 p. 60)
Natural Vacuum Electronics (AMSAT 22nd Space Symposium
- October 2004)
RE: Plastic Capacitor -- Am I Crazy?  
by W6TH on November 5, 2004 Mail this to a friend!

Very good reading and enjoyed every word.

Years gone by we made capacitors using double edged razor blades. They worked great. Also Reynolds aluminum and wax paper for fixed capacitors worked great.

Yes, silver plating over copper would be the way to go, but keep your eye on the production costs.

Should you have an extra one, I can run a test evaluation for you.

Best of health to you and all crazy people are inventors and such.

RE: Plastic Capacitor -- Am I Crazy?  
by W8JI on November 5, 2004 Mail this to a friend!
Nice work! Very few people build things any more.

Just a few things to get you thinking outside the common closed-loops people fall into (sorry).

Silver solder is mostly the same basic resistance as conventional solder. It has a higher melting point and is often stronger.

It isn't high enough silver content to affect resistance, and the current path through the joint should NOT be that long or you are doing something wrong at the metal to metal interface.

Pressure joints aren't bad either, if they have high clamping force. The problem with most joints is people use a tiny bolt or screw and the contact width is very small and often recessed. The ideal joint would be at the OUTSIDE of the fastener since RF current pushes to the outside edges of any path carrying parallel currents. This is called the "edge effect", and is just like skin depth but in another direction.

Fasteners not only have small contact area, the pressure isn't high and people unwisely (stupidly?) put them at the center instead of the edges where the current flows.

ANY small area contact is a potential loss problem....welded, brazed, bolted, or soldered. It is almost guaranteed to be a problem if you use a hose clamp, don't clamp with thousands of pounds of force, or clamp in an area where current is low from edge or skin effect.

This means we either need to have the proper hardware located correctly, or we need to deeply braze or weld. Solder of any type often will not penetrate the metal to metal area inside the joint and provide large contact area, so we need a WIDE contact area. What we have is a very tiny ridge of solder we depend on to carry all the current (skin depth) and so we need to make that ridge very wide. As long as the contact is at the outer surfaces and edges (where skin effect forces the currents) and doesn't break we are all set with solder.

Even lead-tin is fine if it does not tear, but REAL silver solder (not the plumbing stuff)is much better mechanically.

The problem in a conventional dual capacitor is shaft length and stacking distance, not the plate materials. You gain almost NOTHING in capacitor ESR by adding silver plating. Capacitor ESR comes from pressure joints and LONG leads. The ideal capacitor would occupy the smallest possible box, and have large smooth short collecting leads for current that taper into the size required to match the loop element.

Adding a good dielctric is not generally a large problem, but the lowest ESR generally comes from air or vacuum. Air or vacuum is also self-healing.

The plates have to be perfectly smooth and cannot have a sharp edge or radius. We normally tumbled the plates for days in a media starting out coarse and eventually winding up with walnut shells as a final polishing step. One sharp burr, nick, or scratch and the voltage breakdown will greatly deteriorate.

I did a capacitor for a 100kW transmitter and in manufacturing someone omitted a step where they polished a lead that went inside 1" thick Teflon insulation. Over a period of months corona from sharp edges of that lead ate through the Teflon and blew a hole through. The Teflon that would normally handle hundreds of thousands of volts failed gradually over time because of a few sharp burrs on a 1/4 inch conductor. Polish your plates!

99% of low capacitor ESR is in the connections and layout of the capacitor. Dielectrics can be a problem, because they can increase minimum C and concentrate current (which increases ESR). You'd be lucky to see one percent change in ESR by silver plating.

In my opinion, individuals are getting a little too hung up on capacitors. When the distributed loss of the loop, including coupled losses from the media around the loop, is several times larger than capacitor ESR it's time need to look elsewhere for more efficiency. Take for example silver plating. With a conductor 10 feet long making up the loop, why on earth would silver plating 2% of that path where the surface area is wider make a change?

I think a good step would be to increase conductor diameter in the loop and minimize conductor length and surface area in the capacitor.

73 Tom
RE: Plastic Capacitor -- Am I Crazy?  
by KE4MOB on November 5, 2004 Mail this to a friend!
Hmmm...I would change the construction a bit for a commercial application. Instead of injection molded plastic, I'd use a epoxy-type material. Stronger, less flexible and temperature tolerant.

Wonder what would happen if you "doped" the epoxy with metallic powder?
RE: Plastic Capacitor -- Am I Crazy?  
by WB2WIK on November 5, 2004 Mail this to a friend!
What a wonderful article! Makes me yearn more strongly for retirement, one day!

But I probably wouldn't be motivated to build any homebrew variable capacitors because I live close to an enormous surplus electronics warehouse that stocks dozens of varieties of very large air-variable and vacuum-variable capacitors available inexpensively.

The commonly accepted thought that these kind of parts are scarce and expensive is likely held mostly by people not living close to 120,000 square foot (mostly milirary/aerospace) electronic surplus warehouses that are open to the public 6 days a week.

One of the great reasons to live in L.A.!

Keep up the good work, and let us know your final results with the loop.

RE: Plastic Capacitor -- Am I Crazy?  
by WA4MJF on November 5, 2004 Mail this to a friend!
I know ICOM was using plastic
trimmer caps in their equipment
a few moons ago. Most of them
failed and when replaced by
ceramic ones, the equipment
worked great for a long time.

I have Several R-71As that had to
have them replaced.

I don't know if they're like yours
or not.

73 de Ronnie
RE: Plastic Capacitor -- Am I Crazy?  
by K4JSR on November 5, 2004 Mail this to a friend!
Great work, Scott. Excellent article. Not even I
can make a silly comment about your work. Eham and
ham radio in general needs more great thinkers and doers. All I can offer you is thanks and wishes for a total recovery from your illness.
I hope that you kept a good laboratory notebook with
each page's progress dated. I don't know if your idea is patentable, but if it is, the notebook will save your rights in a court case. Of course that is assuming that your work preceeded some other inventor's work.
Best wishes and I hope you make MegaBux!

73. Cal K4JSR
RE: Plastic Capacitor -- Am I Crazy?  
by KC8VWM on November 5, 2004 Mail this to a friend!

I was considering building a similar one using polycarbonate as it might withstand a wider temperature range, won't warp with heat thus maintaining closer operating tolerances. It would be unlikely to break if dropped or crack in the cold if installed in outdoor applications.

The application of teflon spray lubricant on the mechanical movements would ensure for very smooth, precision like tuning movement.

All old trick to prevent arcing on tuning capacitors is to smooth the plate edges and corners using a fine automotive grade sandpaper normally used for paint finishes.

These capacitors are definetly getting harder and more difficult to find.

Great job, nice project !! - It's nice to see another "crazy" person is out there other than myself.


Charles - KC8VWM
RE: Plastic Capacitor -- Am I Crazy?  
by N0TONE on November 5, 2004 Mail this to a friend!

I tried this some time quite a while ago. I used G-10 glass epoxy PC board material. One of the great advantages is that they are copper-plated, so the conductivity is high.

If I used double-sided PC board material, the stuff is nearly free - pennies per square foot. I could use 1/32" stuff, and it was rigid enough to make the butterfly capacitors as you described. No etching of any kind is required, either.

The only thing required is to get the plates cut or routed and drilled to the right dimensions, and any PC board shop can do this in production quantities.

Assemble the stackup and voila! A capacitor.

I built a few dozen like this and they worked well. To make them last, I had the parts silver-plated after fabrication, a fairly small cost. The major cost of the parts was in the PC board processing. I took the glass epoxy stuff to a machine shop and they used CNC machines, more efficient than the machinery used at PC board shops. However, the glass epoxy stuff (mainly the epoxy) caused their cutting tools to get dull quickly, so they had to charge me extra for the constant re-sharpening. At one point, the shop manager bemoaned that it would be so much easier if I were using a material that was easy to work with, like aluminum.

Well, it turns out that in bulk, 0.010" aluminum is cheaper than PC board stock, and stiffer. And cheaper to machine. So, my new capacitors were slightly smaller, still silver-plated and cost about 30% less than when I was using the PC board material.

Hmmm...what did I gain?

Later, I was driven to try it again, this time for size reduction. Instead of having copper on both sides of the PC board material, I used copper on one side only. This meant that I would have glass-epoxy (dk=4) in the air gap, increasing the capacitance, for a given gap size. I could use fewer plates for the same amount of capacitance.

This turned out to work - I could, in fact, use fewer plates. Unfortunately, it also reduced the arc voltage. Now here is where I'd love to see you do some experiments, to see if I went wrong somewhere. I ended up doing some emag field calculations, and my math showed me that by introducing the dielectric in the air gap, I actually increased the voltage gradient in the air portion of the gap. When the capacitors arced, the arc was visible in the air gap and actually seemed to terminate on the surface of the G-10 material.

If you could come up with a variable capacitor where all of the dielectric was plastic or epoxy, I think the voltage withstand would actually increase. But having any air gap in there seemed to make it worse.

At least, that was my experience.

Haven't played with this in some 20 years, would love to see someone else, particularly with a properly modified brain such as yours, to try it and see where I went wrong!

73 & GL

Plastic Capacitor -- Am I Crazy?  
by KE4ZHN on November 5, 2004 Mail this to a friend!
Great article! Very creative. Im not sure how plastic would hold up but I have to give credit where credit is due for the idea. This kind of inventivness is what ham radio is all about. Very nice thread.
Plastic Capacitor -- Am I Crazy?  
by N1CFB on November 5, 2004 Mail this to a friend!

Excellent work Scott. Crazy-Genius whats the difference? My favorite part of Ham Radio is the idea of building parts by hand and fixing, improving with what you have at home. I find it sad that the whole world has turned to Factory made mass production
(radios, tv's etc). You see them in the lanfill. Tv's are cheaper to buy than fix. A repairman came to our office to fix the switch on the microwave oven and told us it would be cheaper to buy a new one.? I could've replaced the switch myself. So, More power too you. Get well soon. Patent your ideas. Keep thinking. 73's KB1LOO Chris
RE: Plastic Capacitor -- Am I Crazy?  
by W2DUG on November 5, 2004 Mail this to a friend!
Sounds interesting. The first thing that occurred to me was whether you'd have to be concerned with oxidation (tarnishing) of the silver plating and what effect that would have on the performance. It seems like it would change the dielectric constant (not sure which direction it would drift), but, more importantly, you might get inconsistencies across the plates where oxidation forms due to varying exposure to the environment as well as fingerprints from handling during assembly, etc. I wonder if that would cause "weak spots" where breakdown could occur. You might have to dip the plates in Tarn-X every so often! I don't know the answers to these questions; I'm just throwing them out there for consideration.
RE: Plastic Capacitor -- Am I Crazy?  
by KB1LKR on November 5, 2004 Mail this to a friend!
I like the idea in general, but it seems to me that silver plated, injection molded plastic is not going to be the ideal material compared to say silver plated stamped/punched aluminum alloy (say 3003 or 5052), for several reasons: cost per piece, structral stiffness, cost to plate, & thermal resistance come to mind.

It is possible that stamping/punching/etc of rigid plastic or perhaps thermal/vacuum forming even, might be attractive compared to molding too. The butterfly configuration however should work equally well whether the plate cores are metallic or nonmetallic. sertaintly it should not be hard to compare the options.

Another possibility (though likely not practical for cost reasons) would be to use pressed alumina ceramic plates w/ a moly-manganese coating which then can be plated.

As for silver oxides/sulfides on the surface, the RF should flow in the conductive layer just below the surface anyway, and the oxides will be such a small fraction of the total dielectric (the ballance being air -- almost as good as vacuum -- that I would not worry about it.

Also, on the topic of silver vs copper vs aluminum how large is the real resistance componant relative to the reactive capacitance, and how much improvement in this ratio is actually seen by switching to increasingly conductive skins? Also, how deep is the skin effect at the freq of interest, the deeper it is the more desireable it may be to use a thicker layer of a lower cost metal (say copper, but 2X as thick as silver, yet at lower cost and lower total resistance.

Just food for thought. Keep the ideas coming, for an itea can lead to another, then yet another, and you never know when you may have the "killer idea" akin to a "killer app" in th ecomputer/software business.
Plastic Capacitor -- Am I Crazy?  
by WD40 on November 5, 2004 Mail this to a friend!
Sir, you're not crazy, you are a genius!! Keep up the good work!
Plastic Capacitor -- Am I Crazy?  
by W4SK on November 5, 2004 Mail this to a friend!
Excellent. Thank you.
RE: Plastic Capacitor -- Am I Crazy?  
by WA5ZNU on November 5, 2004 Mail this to a friend!
W9OY wrote:
> don't need a 12,000v capacitor for low power applications.

I think the application for this butterfly capacitor is a mag-loop antenna, which for a 1M diameter copper tubing loop on 7MHz at 5W QRP needs a capacitor that handles over 1KV. Up that to 20W and you get 3-4KV. A good, cheap source of high-voltage capacitors for mag-loop projects would be excellent!

The ARRL Antenna book lists glass-separated sliding plate capacitors, so perhaps a glass butterfly capacitor might work.
Plastic Capacitor -- Am I Crazy?  
by PA7WWO on November 6, 2004 Mail this to a friend!
Hi Scott .
You are not crazy , I build two of those capacitors myself and they were 50-250 pF .
I used aluminium plate for rotorand stator and plexiglass endplates. Iwent even further and , I hooked up 2 motors and variable resistors , and the both of them are with a coil with taps used as a remote controled T tuner at thebase of my home brew 80 meter vertical . i had great fun fixing the lot and building it andit surely took me alot of time to build it, so i woold saybuild an other one and a coil and you have yourselves a niceantenna tuner. 73 and all the best.
RE: Plastic Capacitor -- Am I Crazy?  
by KF4VGX on November 6, 2004 Mail this to a friend!
After having major brain surgery to remove a golf ball size tumor deep in the left side of my brain back in June of this year I have discovered that I look at and think about things differently now. I had to take a medical retirement and I do have disabilities and so I have lots of time to think about all kinds of stuff and a lot of stuff pops into my mind from places unknown in it.


I can only admire you for honesty,through the pain and suffering you still want to give to your fellow hams!
Having fifteen major operations myself,leaving only half an ear drum to hear with. ( Mastoids ) ( Tumor )
I can respect your courage . :)

Give all !


Plastic Capacitor -- Am I Crazy?  
by WA2JJH on November 6, 2004 Mail this to a friend!
Maybe you should talk to a patent attorny.

The plastic is of some consideration.Most plastics are polymers. High molecular weight poly ethlyn is used in hip replacements.

You have a great idea. I guess the right polymer and silver plate thickness is the major factor.

You can determain how inexpensive you want to make them.

Thin silver plating for low volt apps.

Wow, you have major surgury then you invent something!

Congradulations and speedy recovery

Plastic Capacitor -- Am I Crazy?  
by WA2JJH on November 6, 2004 Mail this to a friend!
You just gve me an idea using UHMW polyethyne and carbon fiber(not for a capacitor)

TNX AGN SO MUCH. Welcome to the hypist technical site on the web.

If your crazy, you found the asylme hi-hi
RE: Plastic Capacitor -- Am I Crazy?  
by W4PC on November 6, 2004 Mail this to a friend!

Patent numbers 6,147,855 and 6,118,651.

Plastic Capacitor -- Am I Crazy?  
by W4KLP on November 6, 2004 Mail this to a friend!
I thoroughly enjoyed your article. I have been doing some plating of non-metallic materials with much success. One of my favorites is a 24K plated Scarab Beetle. The process is called 'electro-forming'. Keep up the good work.You are definately not crazy...Blessed would be more fitting.
73 de Ken - W4KLP.
RE: Plastic Capacitor -- Am I Crazy?  
by G3RZP on November 7, 2004 Mail this to a friend!
Firstly, do the sums for skin depth. This shows that if the usual rule that the current is negligible at 5 skin depths below the surface is followed, then because the plate of the capacitor has current on both sides, it needs to be at least 10 skin depths thick. At 10MHz in copper, that means a plate thickness of at least 0.0085 inches. Thinner than this will lead to a decrease in the capacitor Q, while going down to 3.5MHz, the thickness needs to be square root of 10/3.5 times as thick, or about .015 inches thick. That (or rather half of it each side) is going to be a tall order to plate onto plastic. The difference between silver or aluminium won't make that much difference.

The idea of a butterfly capacitor to avoid wiper currents isn't new - the WW2 SCR522 receiver (BC625) used ganged butterfly caps for that reason, and it's reappeared several times for tuning loop antennas. But a lower than optimum Q tuning capacitor will tend to heat, which might well cause problems with the plastic dielectric. Incidentally, silver oxide and sulphide aren't that bad as conductors - indeed, you'll find silver oxide used in relay contacts.

Many years ago, I believe it was EF Johnson who made air spaced trimmer caps in which, in order to avoid the resistance inherent in clamping or soldering the vanes to support rods, they extruded a brass rod of the correct shape and then milled out the gaps. This pushes up the Q by a worthwhile amount. I shudder at thinking of doing it with a big enough block of brass or copper for a 100pF 12kV cap, though! Especially as copper is pretty awful stuff to machine.

One point to note is to be careful handling thin sheet copper, as minor cuts can easily go septic for some reason - a fact fairly well known in model engineering circles.


Peter G3RZP
RE: Plastic Capacitor -- Am I Crazy?  
by K2WH on November 7, 2004 Mail this to a friend!
Nice idea although I don't know how long it would last in real service. Heat and cold would probably make the capacitor very unstable and prone to breakage (cold weather) and value changes in hot weather.

On another matter, I am making a bottle amplifier. It will be using an empty Coke bottle as a driver and (2) empty pickle bottles as the finals. Should do a couple of KW's without any problems. I just need to figure out how to maintain a high vacuum in the bottles.

Plastic Capacitor -- Am I Crazy?  
by K0TWO on November 7, 2004 Mail this to a friend!
Nice idea, its already being done and a lot
easier by just cutting CDs to the wanted
RE: Plastic Capacitor -- Am I Crazy?  
by TACTIC on November 7, 2004 Mail this to a friend!
Look just go ahead and make it, if it works then you get the pleasure of saying to all the profs and theory buffs "up yours" if it don't work whell we all had a blast trying. Is not FUN a big part of amateur radio?

Great idea, I for one am going to give it a go, will let you all know how it goes. I ordered the silver spray stuff in the UK today should have it by Wed next week. Now curse you, yet another hobby to keep me away from the wife :-)

Another crazy idea, how about making the thing out of glass or ceramic?
RE: Plastic Capacitor -- Am I Crazy?  
by KA4KOE on November 8, 2004 Mail this to a friend!

There is a book out there telling you how to fabricate your own vacuum tubes.

I believe you can find the book on

Plastic Capacitor -- Am I Crazy?  
by WA6BFH on November 8, 2004 Mail this to a friend!
This is a truly wonderful article, and idea! It sounds to me as if you have all of the physical, and fiscal considerations well in place. I too would recommend filing for a patent!

The experiments, or thought constructs, you have already done are impressive. I suspect, just as you stated, that Silver would be wonderful -- look how good copper was! I too have done such experiments, and with High-Q tuned circuits realized an 8/10th dB increase by using these same 'better conductive material.' For one of the responders out there, think in terms of reactive impedance -- not DC resistance.

Also, since current is distributed OVER THE SURFACE of each plate, arc'ing may not be as severe as you cautiously and prudently fear. Perhaps a slightly thicker, better durable, plating is available?

Your best effort and answers I suspect will come from your own tests and experiments. I would suggest you build up a test jig by making a Pi tuned Trans-match with your own "homebrew" capacitors. Run progressively higher powers in the tuner to a controlably bad reactive antenna. You might save some time and trouble, as you should really only need a capacitor for the antenna side but, having your own capacitors on either side may yield some valuable information.

de John WA6BFH
RE: Plastic Capacitor -- Am I Crazy?  
by AB7E on November 9, 2004 Mail this to a friend!
N0TONE's experience with plastic between capacitor plates reducing the withstand voltage is correct. In essence, putting a dielectric between the plates (but leaving an air gap) creates two series capacitors, each with the same plate area but one with air for a dielectric and one with plastic. The plastic gives a larger capacitance, so at RF you get a disproportionate share of the voltage falling across the capacitor with the air gap. For DC, the withstand voltage would be aided by the plastic dielectric, but for AC the opposite is true.

This effect can be a wear out problem where high voltage components are molded in plastic. Any voids in the molding can actually grow over time due to microscopic arc'ing. A common piece of test equipment for insulators, such as between transformer windings, subjects the insulator to high AC voltages and monitors the corona currents across the microscopic internal voids.
RE: Plastic Capacitor -- Am I Crazy?  
by WB3JOK on November 12, 2004 Mail this to a friend!
>I think the application for this butterfly capacitor is a mag-loop antenna, which for a 1M diameter copper tubing loop on 7MHz at 5W QRP needs a capacitor that handles over 1KV. Up that to 20W and you get 3-4KV.

I've been playing with loops for a while now, and a 1 meter loop is better suited for 20 meter operation where it is only about -1 db down. I've managed to work trans-atlantic with an HW-101 and a 4 ft. loop. Yes, you can use it on 40, but you need around 150 pf, at considerable voltages as you noted, and the efficiency is low (about -7 db). My 40m loop is 8 ft diameter, 1-inch copper tubing, and can be used on 75m with a similar loss of efficiency (and 250 pf of capacitance). To run the SB-200 requires a 10+ kv cap capable of handling 30A RMS or more - this is vacuum cap territory.

> A good, cheap source of high-voltage capacitors for >mag-loop projects would be excellent!

Unfortunately there is not one. There are good sources and good caps but they are not cheap :(
The Russian mil-surplus caps now coming on the market (check Ebay) are good deals, usually $100-$150 whereas a new Jennings or Comet cap can easily run $1000 from the factory. Max-Gain ( also has some reasonably priced vacuum caps.

You can make low-value fixed caps out of 1" and 1/2" copper tube, coaxially mounted. Plenty of voltage and current capacity, and about 48 pf per foot if I recall. The trick is the mounting of the small tube centered within the large one. The fields are so intense inside the cap at even moderate (100W) power levels that many materials melt or are so lossy that the Q and minimum SWR are ruined. 5-minute epoxy won't work, I tried it. Even wood shims are too lossy. Teflon works great but is expensive - and once it arcs and carbon tracks is a throw-away. Perhaps "pinning" the tubes with nylon screws would work, but I think even nylon would melt. Teflon screws are expensive. Hand-cut washers of LDPE are working but are a pain to cut to correct size.

Another hint - to cover a ham band you don't need the variable cap to be all the capacitance! My small loop, which I'm only using on 20m, needs less than 5 pf change to cover all of 20m phone. So I have a 20 pf coaxial tubing cap in parallel with a 5-25 pf 20kv variable vacuum cap ($69 from MaxGain) and it works great.

I have discovered, in my experimenting with capacitor materials, that the plastic dielectrics are easily punched-through (and useless for high voltage thereafter, even if they don't catch on fire, which I have also done :). Don't forget dissipation factor too. Window glass, for example, has a nice dielectric constant (7-8) but is very lossy at RF. Lots of info on the various Tesla coil sites. LDPE plastic is supposed to be good, and available in sheets from McMaster-Carr, but I gave up and bought vacuum caps.

Plastic Capacitor -- Am I Crazy?  
by W4ABX on November 14, 2004 Mail this to a friend!
The unknowing general public thinks all inventions are the product of an insane individual, until said invention is proven to be a viable product that will advance technology into the "future".
Wireless communications? Moving pictures in a black box? Photography? Space travel? Hula-Hoop?
Who's to say what is insane and what is imagination?
Best of luck in your recovery, and I'm looking forward to more information on your experimentation. This could be history in the making.Great article!
Mike W4ABX
Plastic Capacitor -- Am I Crazy?  
by N1ATO on November 15, 2004 Mail this to a friend!
RE:Aluminum Capacitor  
by JOHND on February 28, 2005 Mail this to a friend!
I was just looking at a pile of 5" computer hard disk platters. Hmm. Aluminum, highly polished and flat... Looks like something that might make a good capacitor butterfly. Anybody ever done this? The only limitation might be the fact that the hole in the middle is kind of large, about 1". It might require a bit of creative engineering.
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