Making Variable Capacitors By Stretching Aluminium Cans

Sometimes when you need a component, the best way to get it is by building it yourself. [North Carolina Prepper] did just that, creating his own trombone-style variable capacitor by stretching some aluminium beverage cans. 

The requirement was for a 26 pF to 472 pF capactitor, for a radio transmitting from 7 MHz to 30MHz. The concept was to use two beverage cans, one sliding inside the other, as a capacitor, with an insulating material in between.

To achieve this, a cheap exhaust-pipe expanding tool was used to stretch a regular can to the point where it would readily slide over an unmodified can, plus some additional gap to allow for a plastic insulating sheet in between. Annealing the can is important to stop it tearing up, but fundamentally, it’s a straightforward process.

The resulting trombone capacitor can readily be slid in and out to change its capacitance. The build as seen here achieved 33 pF to 690 pF without too much hassle, not far off the specs [North Carolina Prepper] was shooting for.

Radio hams are very creative at building their own equipment, especially when it comes to variable capacitors. Video after the break.

[Thanks to Seebach for the tip!]

33 thoughts on “Making Variable Capacitors By Stretching Aluminium Cans

  1. There’s a reason 365pF variables were often specified. They were a value common in am broadcast radios, so they were cheap and available. Strip a radio, or the parts store would have them. If anyone was building AM radios or broadcasters, they were perfect.

    They weren’t great for everything, but you could put a small value capacitor in series for a smaller value. Or pull off plates to reduce capacitance (but an irreversible process).

    There was a period when I’d pull them off radios waiting for the garbage, but kess likely now with digital tuning. There’s probably still a large mass in collective junk boxes.

    1. Yeah but this is fun, doesn’t require ordering/cannibalizing a part, and can be easily tuned on the fly. One could also just stack up bits of scrap metal and paper to custom-tune, but this is like a really cheap variable cap made out of trash. Plus, might be handy for people who need ultra-specific low capacitance that doesn’t happen to be 365pF

    2. Why were they common?
      I heard this tract years ago, but it is somewhere along the lines of the best range from min to max of what what you could get from an air variable cap. They didn’t have Mylar then.
      It is the reason that the AM band (US) is wide as it is. 550 – 1700 kHz.

  2. Huh? That video is more than 7 years old.

    By the way, I can’t imagine that the breakdown voltage of that assembly (thin dielectric, very sharp edges) would sustain even low power for a high Q loop antenna that he says it is for.

      1. Hacks that old aren’t news enough to be very useful, and have almost always been supplanted by newer hacks that are. Otherwise we’d be reading stuff like how to make CO2 by mixing baking soda and vinegar. To my point, there are countless articles and YouTube videos on how to make a trombone-style variable capacitor out of commonly available materials that will actually work for the tuned loop antenna application that author had in mind. His approach won’t unless he’s using milliwatts of power.

  3. I was recalling the expression “hearing it on the cans”… usually just meaning headphones, buuuuut… can we go full can? …
    shoepolish tin/can carbon headphone
    drink can tuning cap
    real tin can for point contact tin diode…
    tuning coil round a cylindrical salt “can” (cardboard)

    Any other way we can use cans instead of just canning them? Yell if you think of anything, I’ll be on the can.

  4. BTW, I have a vague recollection of a construction article from anywhere between 1960 and 1980 (I had a lot of backissues of Popular Wireless etc) of a variable capacitor made from varying diameter slices of concentric pipe, several elements, so they nested, operated in transformer oil maybe, dunno if it was some special high power rig or a “just because” kinda deally.

  5. BTW II .. I would imagine a 3D printed exhaust expander would just be robust enough for this (With a steel bolt/threaded rod) .. orrr in fact you could maybe make one from a hard wood that would work if you greased it up well.

  6. Teflon sheet wrapped copper pipe sliding inside a larger copper pipe creates a variable capacitor capable of handling the very high voltages present when tuning a transmit loop antenna. Has anyone ever seen something I’ve thought about, not necessarily high voltage capable: 3D printed plates covered with aluminum tape and assembled into the structure of an air variable capacitor?

  7. A few months ago I did something similar but with a difference.I wrapped the can with a piece of transparent sheet, cut to the right size, not too tight(the stuff you use on the old projection lamp equipment) Then wrapped it again with alluminium foil(not too tight).leaving a thin tail piece at the end to wrap a connecting wire for measure purposes.So I could slide the the transparency sheet with its foil.It was meant for use in a crystal radio set.It worked good and I could get close to 700 pf with a large beer can.If you like you can glue the foil to the transperancy sheet,just dont wrap the transperacy sheet too tight.The transparency sheet can be wrapped with pvc tape to secure it.

  8. I bet you could cut a bunch of cans, stretch them (flatten) to a bunch of sheets. Solder 2 small wires to each, and an insulating layer between them when stacking.
    Solder each wire to one other layer above and below.

    Then roll em up and stick em inside a can, and have a much larger capacitance.

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