>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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