Air variable capacitor plate material CCB/PCB vs Copper Sheet

Discussion in 'Homebrew and Kit Projects' started by S21RC, Apr 12, 2021.

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  1. S21RC

    S21RC Ham Member QRZ Page

    Hello all,
    I wish to homebrew a high voltage air variable capacitor (around 10-120pf, 5KVA minimum) for a STL loop antenna (40m and 20m), I know Vacuum Variable is best but due to Covid situation international shipping to my country (S2) is not available now.

    Trombone type would be easy to make but heard lot of bad things for that type, so thinking of making butterfly type. I understand the gap required and size of plates from the various calculation.

    My question is about the plate material, what if I use PCB instead of 1ml copper sheet? The dielectric combo of the board and air between will increase the capacitance and increase the breakdown voltage as well if I am not wrong.

    It would also be easy to etch and cut the CCB/PCB (etch to keep few ml of extra board without copper as margin to reduce coronal discharge from corner). Easy to solder all plates at both side using another strip of PCB set perpendicular

    What am I missing with this approach? Working with sheet metal and CNC will cost more for me here, mainly the cutting. Any effect due to the small thickness of copper on a PCB compared to sheet metal?

    ** I understand I may not achieve the minimum capacitance when increasing the plates. but then I will make it for single band.

    Please advise.
    Thanks in advance, 73' S21RC
  2. KK4NSF

    KK4NSF Ham Member QRZ Page

    I've made a number of air variables, and solid dialectric variables for my own antennas. Without a doubt, copper sheet is the best material (or copper tubing for trombones). I use 18ga plates made for printmakers. It is nicely sturdy, but soft enough to work easily. I get mine here:

    Several other sources also carry it.

    Aluminum also makes great caps.... and if you use the right rods, it is easy to braze / solder with a small torch.
  3. W7HV

    W7HV Ham Member QRZ Page

    Need more detail on what you're doing. In a capacitor made from two parallel plates, all the action occurs in the gap between the facing surfaces. The backing of those plates doesn't matter. If you're making a butterfly type with a number of interleaving plates. It's what's in the gap between the facing surfaces. If each plate of the butterfly is made from double sided PCB with both sides electrically connected, it will behave like a solid sheet of the same thickness. If you're using single sided PCB in a butterfly cap with all plates facing the same way, There will be combination of air and a single thickness of pcb substrate in each gap. As you mention, this will increase the capacitance and the breakdown voltage. Both are easily calculated using the properties of the board substrate you're using and air. Another factor to consider is the dielectric loss of the PCB material, which will convert your precious RF to heat. In this case, select a board substrate material that has the best properties for your

    Thin copper cladding will, of course, have more resistance than a solid copper sheet, but the skin effect comes into play, as does the geometry. Current from a wire attached a plate must first go through the small cross section of the plate immediately surrounding the connection. That cross section area increases in proportion the distance from the connection point, so quickly increases. I'd choose PCB material with thick cladding, not the type with thin cladding intended to be built up by plating after etching.

    The dielectric properties of the PCB substrate, and the skin effect both depend on the frequencies being applied, with higher frequencies making things worse. Something that works well in the low HF bands will not work as well in VHV and certainly UHF.
  4. PU2OZT

    PU2OZT Ham Member QRZ Page

    What about humidity on the edges?
    In case of arcing, easy around 5kVA, how will you spot the faulty PCB sheet before it becomes too nasty? How will you fix it?
    Has it been done already? Other than vacuum and air, isn't playing with... fire?

    AD5HR likes this.
  5. VK4HAT

    VK4HAT Ham Member QRZ Page


    You would have to do the math to calculate the arc over voltage, but the as for making them sure its kind of easy enough and cheap enough to do using PCB material and manufacturers in China. If you were in Australia rather than Bangladesh, I would send you the bits I have here because I have no use for them. But the gerber filed for parts are on my website if you think this is a useable thing.
    S21RC likes this.
  6. S21RC

    S21RC Ham Member QRZ Page

    Thanks Oliver, David and Louis for the feedback. I haven't made it yet, still in planning stage and was asking to see PCB vs solid coper plate.

    I guess I will spend a bit more and go for copper plate and CNC cutting. Waiting for the lockdown to be lifted which just started again as another wave of COVID hits the country bad.

    Thank you Rob for the link, and that you have already done it is a great inspiration.

    I will update here once I am ready with all parts.

    73' s21rc
    PU2OZT and VK4HAT like this.
  7. KK4NSF

    KK4NSF Ham Member QRZ Page

    keep in mind that for your desired capacitance, you only need two small plates. If you use a piece of glass or polystyrene as your dielectric, you can make your cap as small as 9 squ inches (3" x 3"). I've got one made from two plates of copper, with a piece of 1/8" glass between them..... constructed so the top plate can slide back and forth. It works like a charm, and was easy to make.
  8. WA9D

    WA9D Ham Member QRZ Page

    This is only relevant because of the mentions of trombone capacitors! No connection to high voltage caps or tuning loops. I apologize for that!

    When I was a kid I had a lot of electronic parts my father and grandfather had used back in the 1920's and maybe into the 30's. Among them were a couple of variable caps that worked "back and forth" rather than rotating, hence the connection to the trombone caps.

    But these had flat plates, and a neat mechanical hookup so you could rotate a piece and through a sort of crank it would push/pull the moving plates, with one set of plates sliding back and forth between the other. The plates were (from feeble memory...) about 2" by 2.5", aluminum, mounted on castings that were probably also aluminum but had an enamel-like finish on them. With the mechanism a cap, overall, was about a foot long, and the rotatable part was probably about 5" across. Clearly for a day when miniaturization did not rule the world.

    When we moved, my parents apparently trashed all that. (One of the few things they ever did that I am very mad about.)

    For decades now I have been trying to find some trace of those. Pictures, old ads, descriptions, ...? I have had no luck looking online, searching hamfests, talking to hams even older than I am (almost 80), etc.
    Any clues would be very welcome!
  9. KK4NSF

    KK4NSF Ham Member QRZ Page

    those sound very much like the ones I mentioned above.... but probably had air-dielectrics. Go to Google Books (the free ebook section,) and search for crystal radio. There are several books there from the mid-1920's that show how to make those.... and my favorite: the Book Capacitor, where the plates are hinged like a book.... opening it up decreases the capacitance, closing it increases it.
    Last edited: Apr 13, 2021
  10. WA9SVD

    WA9SVD Ham Member QRZ Page

    Just a note, particularly if you use PCB material: even if you leave a bit of unetched material around the margin, be SURE to round off the corners of the unetched copper. Even with a margin of bare PCB material (ie. without copper) a sharp corner will still be conducive to corona discharge under some circumstances. (The same goes for rounding the corners of ANY material...)
    S21RC and PU2OZT like this.

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