According to the formula a coil 5" diameter, 10" long, and 35uH would have
to have 26 turns. This would be a coil with ±1/4" spacing between the
turns.
Bob W6TR
----- Original Message -----
From: "Roger" <sub1@rogerhalstead.com>
To: <Amps@contesting.com>; "Dennis Ashworth" <K7FL@arrl.net>
Sent: Tuesday, November 20, 2007 5:59 PM
Subject: Re: [Amps] Winding large inductors
>
>
>
>>I need to construct a 35uH air inductor using .250 copper tubing. D
>> is around 5"; L = 10 inches. I can play with the dia & L a bit and
>> still be in the Q ballpark I desire.
>>
>> Does anyone have a jig or technique they use to make such a coil
>> while maintaining the turn spacing and not kinking the tubing? It
>> needs to be "pretty" because of where it will be located :-)
>
> Much depends on the tubing you'll be working with. IE, "temper" which
> depends on manufacture and it changes with age, heat, and how much the
> tubing has been "worked".
>
> New, "dead soft" 1/4" Copper tubing "should" be capable of being bent to
> a
> diameter of 5" with little problem of flattening. The problem comes from
> what diameter you will have to use for a form to get 5". If the copper is
> dead soft you can wind it on a 5" tube and expect to see it still grip the
> tube. The higher the temper (age, how much it's been worked) the smaller
> the
> form required. Much smaller than 5" and flattening will become a problem.
> As
> has already been suggested sand (or salt) used to fill the tube works well
> when needed. I've wound 1/2" diameter Copper tubing to 5 and 6 inch
> diameter
> although that was over 20 years ago.
>
> Without digging out the books (I don't remember the formula and the
> handbook
> is in the shop) what is the number of turns required in 10"? If space
> wound
> I wind two coils at once. So they serve as spacers for each other. It
> sounds
> easy but you have to "unscrew" one from the other when finished and they
> often don't want to come apart. Wire of various sizes can also serve as a
> spacer being wound with the tubing other than the smaller stuff will just
> go
> under the turns so wire and cord probably won't work for close spacing. A
> 5"
> diameter coil 10" long may want to sag a little even when mounted
> vertically, so it may take a bit of playing to get the turn spacing to
> remain equal along the full length.
>
> When I was working in industry I always had lots of pipe scrap from PVC to
> Stainless to use for forms and a lathe to cut a shallow grove for a guide.
> Probably a piece of rigid 5" plastic drain pipe will work. It might be a
> tad
> large, but you said the dimensions were not critical. You might find some
> PVC couplings of the right OD that could be glued together to make a long
> enough form. Lowe's and the local Ace Hardware store are getting used to
> me
> asking for "something" that will be of a given size and it doesn't matter
> what it's original purpose was. <:-))
>
> Sounds like the beginnings of a tesla coil (as it has to look good) or one
> whale of an antenna tuner.
>
> Good Luck,
>
> Roger (K8RI)
>>
>> Thanks
>> Dennis, K7FL
>>
>>
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