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Author Topic: Hand winding coils for L-match  (Read 15025 times)

Posts: 70

« on: March 20, 2013, 07:28:14 PM »

Is there some magical trick to hand winding coils for a match box?  I am building a tuner for a random wire and attempting, without success, to wind the inductor. 

I am using a 2" PVC core and 14g bare copper.  So far I have mastered the copper slinky. 


 Huh Huh Huh Grin

Posts: 17477

« Reply #1 on: March 21, 2013, 08:52:34 AM »

Chucking the former in a drill or rotisserie so it rotates as you guide the wire
onto it does make things a bit easier.  For some 160m loading coils I slipped
the former over a board stuck in a vise and put a screwdriver through a pair
of holes in the far end so I could turn the former with one hand while feeding
the wire with the other.  (But that was using a much thinner wire.)

There is a method described in the Technical Topics column of the RSGB
journal RadCom that sounds good, though I haven't tried it.  Put a bolt
and nut on each end of the PVC former.  Secure one end of the wire under
one nut with several inches to spare, wind on the wire, and secure the other
end in a similar manner.  If the wire isn't snug, start at one end and twist it
on tighter with your hands, pulling up the slack at the far end and resecuring
the wire end.

Then put the whole assembly in the freezer for a few hours.

The PVC shrinks more than the copper,  so the wire will be more loose.
Repeat the process of tightening the windings, pulling more slack out of
one end.  Repeat a couple times, and, when warm, the copper wire will
actually be indented slightly into the PVC former, making a reasonably
permanent coil.

On the other hand, if you want a self-supporting coil without the former,
then the secret is to wax or oil your coil form (or coat it with wax paper),
wind on the wire, and run 4 strips of epoxy the length of the coil to act as
spacers.  Once the epoxy is dry you can remove the coil former (that's why
the wax or grease - so the epoxy doesn't stick to it) and the coil should
hold its shape.

Other methods:  drill holes in a flat sheet of plastic for the turns of the coil
(making sure that those on one side are offset half a turn from the other side)
and take one of your slinkies and thread it through the holes as if you are
screwing it into something.  That gives you a flat mounting plate with minimum
plastic in the field of the coil.

One other approach if you want to be able to tap the coil in various places:
mount 9 dowels or nails evenly spaced around a circle (other odd numbers
may work as well).  The easiest way might be to mark the circle, drill holes
at each point, and slip a box nail through from the back.  The posts have to
protrude higher than the length of the coil.  Then, starting at one point, wind
the wire around every every OTHER post and inside the others.  This makes
a lumpy coil with reduced self-capacitance.  When you get enough turns, you
can run a bead of epoxy along the points where all the wires cross each other.
When it dries, remove the posts.  This gives you convenient "lumps" to tap to.

Posts: 165

« Reply #2 on: March 21, 2013, 10:19:17 AM »

Wouldn't go for PVC myself - a bit lossy.

The plastic sheet with holes is probably the easiest unless you have a lathe.

Posts: 8911


« Reply #3 on: March 21, 2013, 11:22:09 AM »

Wouldn't go for PVC myself - a bit lossy.

Wasn't able to do an actual differential measurement yet, but I've been seeing Q peaks of 600-700 with PVC form coils here:

Someday I hope to do the same coil with and without PVC.


Monkey/silicon cyborg, beeping at rocks since 1995.

Posts: 1054

« Reply #4 on: March 21, 2013, 12:29:11 PM »

BYU's recommendation of a slow-turning electric BBQ spit is good, and use a key ring to keep the turns separated. Another way is to wind the wire side-by-side with weed whacker line (it's almost the same diameter as AWG16 bare wire). A final method is to use plastic grommet material, the kind used for insulating electrical wires that runs through bulkheads and firewalls. I think it's nylon U channel that has a notch in it about every eighth inch or so. Once you've got the coil formed, just run a hot glue gun down the U to finish the job. It is best to let the wire go slightly slack and realign the grommet strips before gluing to keep them straight. Just get them slack enough so they'll slide off the PVC former, and you'll have a nice air core inductor. GL.
« Last Edit: March 21, 2013, 12:31:41 PM by W5FYI » Logged

Posts: 3160

« Reply #5 on: March 21, 2013, 02:29:35 PM »

Homebrew Your Own Inductors!
Can’t find the inductors you need for an antenna, a tuner or amplifier? Build your own—it’s easy!
By Robert H. Johns, W3JIP, QST, Aug 1997, p 35.

Robert H. Johns, W3JIP, “Coaxial Cable Antenna Traps,” QST, May 1981, pp 15-17.
Robert H. Johns, W3JIP, “Dual-Frequency Antenna Traps,” QST, Nov 1983, pp 27-30.
Robert H. Johns, W3JIP, “Home-Brewing Large Antenna Coils,” QST, Oct 1992, pp 45-49.
« Last Edit: March 21, 2013, 02:39:41 PM by W9GB » Logged

Posts: 165

« Reply #6 on: March 21, 2013, 03:35:29 PM »

The ones I wound on both grey and white PVC pipe had Qs of less than a hundred.

I guess YMMV, and winder, Beware!

Posts: 6748

« Reply #7 on: March 22, 2013, 09:52:03 AM »

Bill:  When I found it necessary to wind a coil for an antenna tuner I used the suggestion that RZP made:

The plastic sheet with holes is probably the easiest unless you have a lathe. 

I used 1/4" Plexiglas sheet of appropriate size and drilled parallel holes on each edge.  The holes are offset by the size of the spacing.  (In other words the holes are not straight across from one another but on an angle determined by the spacing)

I wind the Formvar or enameled wire on a form slightly smaller than the size of the coil (PVC pipe is great) keeping the turns up against each other.  When I have the correct number of turns I let it go which will let the coil spring open some.

I then thread the coil through the holes in the Plexiglas form.  When finished and all the windings are the way I want them, I glue the wire in place with either hot glue or 5 minute epoxy. 

This has always worked fine for me.


A Pessimist is Never Disappointed!

Posts: 165

« Reply #8 on: March 23, 2013, 07:13:16 AM »

Fast setting epoxy seems generally fine from a Q point of view but many of the older long setting ones are poor. However, YMMV. Plexiglass isn't too bad froma Q viewpoint when just a sheet of it is used, but polystyrene is slightly better.

There was an article in the old RSGB Bulletin (now RadCom) in the 60's on making airwound coils like  B & W Miniductor or Airdux. You need a lathe and  a mill, though. There was another article more recently in RadCom on making your own ceramic formers - that guy's XYL was into pottery making and so had the kiln.

Posts: 6748

« Reply #9 on: March 23, 2013, 10:22:08 AM »

Hi Peter:  Down through the years I've read many "How to" articles" and one of the first things I look for is to see if specialized equipment is needed.  If it is, I pass.  I've never been blessed with anything more than hand tools and a drill press. 

Consequently, I always try to figure out a way of doing things the simple way, although it might take a great deal more time.

Someone once said, "If time was money, we'd all be millionaires!"    Grin

A Pessimist is Never Disappointed!

Posts: 165

« Reply #10 on: March 23, 2013, 10:51:26 AM »

I've set myself up with a workshop (with a lot of support from the XYL). Lathe, mill, shear, small bending  brake, small brazing hearth, pillar drill press, grindstone. Plus a load of test gear. Now just need the time and the energy to use it.....
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