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Trombone Capacitor building

Discussion in 'General Technical Questions and Answers' started by VE3PCD, Apr 22, 2011.

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  1. VE3PCD

    VE3PCD Ham Member QRZ Page

    I am interested in the trombone capacitor and would like to build one for a mag loop that I am planning on building; I am in the research phase and looking for personal experience. Does anyone have complete details ( pics, drawings etc..) on building one of these puppies?

    Thanks, 73
  2. W8JI

    W8JI Ham Member QRZ Page


    I know people build them and use them, and I have built caps like that myself, but be aware they are very lossy in an application like you suggest and might have reliabilitly issues.

    Personally, if it was my project, I'd buy a surplus vacuum cap or a cap from MFJ. I went through a series of measurements on a commercial loop antenna and concentric tube capacitors were not very good. With a real trombone, you would have two lossy caps in series.

    You might be more successful with series inductance, ESR and loss problems if you used a very large diameter tube, like three inch copper or aluminum, but the dielectric breakdown issues would still exist.

    73 Tom
  3. KE3WD

    KE3WD Ham Member QRZ Page

  4. VE3PCD

    VE3PCD Ham Member QRZ Page

    Thank for the reply Tom...

    The real reason why I was looking at the trombone was form a power perspective; I want to go beyond the 100 watt limit. You also mentioned an MFJ cap, which model specifically would you suggest for about 15-200pf range?

    Having said that Tom, are you willing to provide as an example at least your design info as a reference?

    73 Peter
  5. KA9VQF

    KA9VQF Ham Member QRZ Page

    At one time I had the bright idea of using empty beverage cans as a capacitor.

    It took a long time to gather cans that would fit inside each other and not touch then more time carefully cutting the tops off of them and even more time making a frame to be able to extend and collapse the thing. I thought it would be of very high capacity and work FB on my home brew antenna tuner at least so long as I kept it indoors.

    It was a miserable failure. After reworking it several times I went with a stack of glass and double sided PC board instead.

    I found plans on some website that showed how someone else had built one to tune his attic loop. That fellow used a cheap electric screwdriver to remotely tune his and I did the same.

    While adjusting the thing, there is a lot of static but its short lived.

    The only thing I really did different was to add a pair of nylon ‘gears’ that mesh to turn the shaft. I scavenged these from a toy Monster truck that the neighbor kid managed to run out into the street and got run over by a car. There is a spring on one side and that ‘gear’ can slip if too much toque is applied like when the sheets of PC board reach the end of their travel.

    I have a real cheap {$20.00 at Bog Lots} 18V cordless drill that was intended to be used as a sheetrock screwdriver that has an adjustable ‘clutch’ in it that stops turning sheet rock screws at the point you set it for.

    I’ve been thinking about using it in place of the other real cheap Black and Decker screwdriver that’s already in use should it fail me or on the off chance I decide to built another. The Black and Decker thing I’m using was picked up at a yard sale for only a dollar. The NiCad’s in it were shot but the motor runs just fine with a small home brew power supply.

    Its really a pretty pitiful drill but might be just right for this application.
  6. W8JI

    W8JI Ham Member QRZ Page

    Check MFJ to see if they sell the butterfly cap they use in the MFJ loop as a replacement part or product. It is not 200 pF, but it is a good cap.

    I've never built one near 200 pF. I can't imagine even doing that!!! ESR and series inductance would be terrible.

    I think you are heading down the wrong path. Buy a vacuum variable if you want high Q, high power, and wide tuning range.
  7. W9GB

    W9GB Ham Member QRZ Page

    Peter -

    Here are some additional Internet articles,

    Small Transmitting Loop Antennas
    Steve Yates, AA5TB

    Magnetic Loop Antenna - and measurements to DIY build a butterfly capacitor
    Tony Van Herck, ON4CEQ

    The best material for the front and the back is CLEAR PVC 3 or 5 mm thick as alternative you can use GREY PVC or 2 sheets pcboard together with the copper removed .
    The best material for the washers, nuts (M6) and threaded rod (M6) is brass or stainless steel,( NON MAGNETIC MATERIALS for the losses).
    For the spacing of the vanes you can use 2 washers M6= ( 6Kv) or a nut M6 =( 12 Kv) if you use aluminum plate 1 mm thick.

    If you use a nut then the best thing to do is remove the thread by drilling withØ 6.2 mm.
    The effective area for the vanes is 11.7 cm² and with the formula for 2 washers = (0.0885 x 11.7 cm²)/ 0.1 cm = 10.35 pF for 1 air gap.

    for 1 nut = (0.0885 x 11.7 cm²)/0.2 cm = 5.17 pF for 1 air gap

    If you you make a capacitor with 2 washers as spacing and you make
    5 rotor vanes and 6 stator vanes then you have 10 air gaps.

    10.35 pF x 10 = 103 pF + 10 pF stray capacitance = 113 pF / 2 = 56 pF

    The final result is a capacitor with a value from 5 - 56 pF.

    You can also follow this discussion by Vasile, VA6POP here on QRZ about DIY/homebrew HV butterfly capacitors
    Capacitance of Concentric Cylinders (for DIY build a trombone capacitor) by David Pace

    Cylindrical Capacitor Formula Calculator

    Polyethylene tubing, aluminum rod and aluminum tubing can be used to build a
    piston / trombone or coaxial capacitor. Cushcraft did that for their R5 and R7 vertical antenna capacitors.

    Last edited: Apr 22, 2011
  8. VE3PCD

    VE3PCD Ham Member QRZ Page

    I want to thank you all for your feedback; I will continue to explore all of my options and hopefully will update you all on my decision.

    Oh, one last there any other benefit other than it being easier to buid a hexagon rather than a circular loop?

    73 de Peter
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'm sure there isn't. And a circular loop isn't all that difficult if you have a form to use, to wrap it around.

    As silly as it sounds, I've used 3/4" copper tubing: Cap one end, and pour in dry sand in the other end until the tubing is filled with dry sand. Then, pinch or cap the "fill" end and wrap it around a utility pole used as a form. Viola! A round copper tubing loop. It takes two people to do this unless you're the Incredible Hulk.

    I've also found a lot of sheet metal shops have a bending device called a "roller." By using different steel forms in the roller, they can bend tubing to almost any diameter you want, provided they have a form of the right radius.
  10. KE3WD

    KE3WD Ham Member QRZ Page

    Even the slightest amount of resistance can be problematic in a magloop, so there are indeed afficionados of the use of a single circular piece of soft tubing bent to shape as vs using solder joints and pipe. However, there are likely jiust as many happy loopers out there who use the solder joint method, many of those stipulate the use of silver solder on the joints in an effort to get around that resistance problem. Another methodology that is quite sound is the use of Aluminum and heliarc welding to form loop, square, hexagon, whatever, shape doesn't seem to matter all that much in the grand scheme of the magloop.

    MFJ sells two butterfly caps, unfortunately neither of them will give you 200pF, but here's the links:

    I doubt they'd be able to handle QRO power where the voltages are going to be pretty steep indeed.

    For example, a 2.5' diameter loop on 7.3MHz, the capacitor voltage calculates to be just under a kilovolt when the RF power is 10W.

    At 100W the same loop would require a cap that can handle a tad over 3KV.

    At 1000W the cap voltage soars to just under 10KV.

    At that point, the vaccuum cap would make the most sense, as Tom has already stated.

    If you simply cannot afford the vacuum cap, then the use of hard copper tubing, telescoped, has been done, is being done and there are plenty of examples around the world wide web.

    For QRO power I think I'd look into KAPTON sheeting (McMaster-Carr has it) for dielectric between the two cu pipes.

    Flat glass dielectric, common hardware store window glass, plus cu clad circuit board has also been described and shown in pictures on the web. Shades of Nik Tesla, but it is indeed a variable cap.

    Last edited: Apr 22, 2011
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