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home made air variable capacitors	Reply
by KJ4KET on October 30, 2009	Mail this to a friend!
I've decided to build an HF linear amplifier on an absolute shoestring. I saw an interesting article online on how to make your own air variable capacitors and thought I would give this a try because it could save a ton of money. My question is: Does it matter what kind of sheet metal I use to make the fixed and moving veins for the capacitors? I have a sheet of aluminum that I could use, but it's not cheap. I also have a leftover sheet of galvanized steel from a household air conditioning project. My question: Do you see any downside to using galvanized steel sheet metal to make air variable capacitors? Regards David KJ4KET

RE: home made air variable capacitors	Reply
by KA5N on October 30, 2009	Mail this to a friend!
While it is possible to homebrew variable capacitors, I would haunt swap fests for old variables. They are always available and much easier to work with and usually pretty cheap. Most variables have aluminum plates (not "veins", the word is vanes)simply because it is cheaper and easier to work with. You many find some old units with brass plates. I would not use galvanized steel. Most of it is thin and flexible and when it is cut you will have an exposed edge of steel that will rust and cause problems. Good luck Allen

RE: home made air variable capacitors	Reply
by W8JI on October 30, 2009	Mail this to a friend!
Another thing to remember is air variable plates are radiused and micro-polished. We used to tumble plates for 24 hours in walnut shells. The slightest burr or sharp corner can greatly reduce voltage breakdown.

RE: home made air variable capacitors	Reply
by W5FYI on October 30, 2009	Mail this to a friend!
My fairly-comprehensive 41st edition Handbook of Chemistry and Physics gives lots of formulas for capacitors, and none of them mention factors for the type of metal they are made of. What is important in all of the calculations is the dielectric and it's thickness (as well as number and size of plates/conductors). Purists, especially the magnetic loop crowd, would like their capacitors made of solid copper with vanes welded to the stator and frame, but that's primarily to minimize losses, and has nothing to do with the capacitor's other parameters. Trombone capacitors seem fairly easy to build, and the internet has some pretty interesting do-it-yourself capacitor projects, but I'd hunt around at hamfests or look at places like www.tubesandmore.com to see what I could find "ready made." Still, this is the Homebrew forum, and we'd like to know how your project turns out. I'd even give it a whirl if it's not too complicated! GL

RE: home made air variable capacitors	Reply
by KE4DRN on October 30, 2009	Mail this to a friend!
hi, QST Issue: Jun 1983 pg 25 Title: Homemade High-Power Tuning Capacitor, A Author: Thomas Stephens, KD6ED 73 james

RE: home made air variable capacitors	Reply
by W8JI on November 2, 2009	Mail this to a friend!
Don't under estimate my warning about plate surface points. Many people don't understand how important having polished edges and surfaces without scratches is to voltage rating. A sharp edge you can't even feel can reduce voltage breakdown 25% or more. If you can't micropolish the plates and radius all the edges, you can get around it by using very conservative spacing. Tom

RE: home made air variable capacitors	Reply
by N2EY on November 2, 2009	Mail this to a friend!
What value capacitor(s) do you need? What plate voltage will the amp use? What bands will the amp cover? The reason I ask is that there are a considerable number of surplus variable caps that may be able to do the job IF the ratings match. For example, if you're building an amp with 811As running up to 1500 volts, the variables in old ARC-5 transmitters and BC-191/375 tuning units may be adequate. Particularly if you don't insist on covering all bands from 160 through 10. OTOH if you want to run a pair of 3-500Zs at 3000 volts, those caps will be inadequate. 73 de Jim, N2EY

RE: home made air variable capacitors	Reply
by KJ4KET on November 12, 2009	Mail this to a friend!
My current plan is to build an amp using 4 811As with maybe 1500v. I am on the lookout for surplus parts as well. If I build, I have decided I will use aluminum and conservative spacing. However, it should be relatively do-able to and polish the plates and make the edges as smooth as possible. I will probably try to make one while looking elsewhere for a surplus one.

RE: home made air variable capacitors	Reply
by VR2WWI on November 19, 2009	Mail this to a friend!
I have an idea by using a lot of obsolete aluminium harddisk (old 9G hdd) platter to make a high power variable cap. The harddisk platter have perfect surface, hard surface, and standard size. 1. Collect enough amount of platter and alunimium "O-Ring" (around 2mm width) from a lot of small (9G or less) harddisk. 2. Cut the aluminium platter into half, and string up the HDD and O-ring by telfon tube or PVC tube. It will be used as moving part of var cap. 3. For fixed part, I perfer using the aluminium foil to do this....(eg: from Coke can....). I am not only consider using harddisk platter, but also study the possibility to make a variable capacitor by using old CD-R (Not CD-RW/ DVD-RW... Most CD-R are coated with thin silver).....

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