For possible interest (recently published in our local journal
"Amateur Radio":-
MAKING AIR-WOUND COILS FOR HF Drew Diamond, VK3XU
We have all admired those photographs of antenna couplers and power amplifiers with Air-Dux (TM) or B&W (TM) air-wound coils. By using the minimum amount of former for support material, they look, and are, efficient. Not many years ago, we could order AusWe have all admired those photographs of antenna couplers and power amplifiers with Air-Dux (TM) or B&W (TM) air-wound coils. By using the minimum amount of former for support material, they look, and are, efficient. Not many years ago, we could order Aust
ralian made equivalents from the William Willis Co., but sadly, no longer trading. At time of writing, there is no known local supplier. They may be ordered from overseas, although the landed cost can be rather high.
For larger coils of perhaps 10 turns of 12 gauge wire, a perspex rectangle may be drilled with the number of holes necessary to accommodate the helix. The coil is wound onto a slightly undersized mandrel, then removed, and threaded or 'screwed' onto the fFor larger coils of perhaps 10 turns of 12 gauge wire, a perspex rectangle may be drilled with the number of holes necessary to accommodate the helix. The coil is wound onto a slightly undersized mandrel, then removed, and threaded or 'screwed' onto the fo
rmer (see Ref. 1). However, when a larger inductance involving tens of turns of smaller gauge wire is required, this technique is rather awkward and time consuming, and the results are not always as good as one would wish.
Here is a method that allows the home brewer to fabricate high-Q coils to requirements. The coil is supported upon a rectangle of perspex, or other low-loss material, such as fibre-glass circuit board- copper removed. Rather than have the turns running thHere is a method that allows the home brewer to fabricate high-Q coils to requirements. The coil is supported upon a rectangle of perspex, or other low-loss material, such as fibre-glass circuit board- copper removed. Rather than have the turns running thr
ough holes, they are fixed upon a rack or comb cut along each edge of the former. In the example shown, we make a near equivalent to the B&W type 3022, which is a 20 uH coil of 40 turns of number 16 B&S, 8 turns per inch, 1.75 inches diameter. A templaterough holes, they are fixed upon a rack or comb cut along each edge of the former. In the example shown, we make a near equivalent to the B&W type 3022, which is a 20 uH coil of 40 turns of number 16 B&S, 8 turns per inch, 1.75 inches diameter. A template i
s recommended if you plan to make more than one coil of a specific diameter. Use brass sheet if you can get it, otherwise steel, about 18 gauge. Carefully, and as accurately as you can, mark out the cutting points for the two racks. A black felt-tip pen is recommended if you plan to make more than one coil of a specific diameter. Use brass sheet if you can get it, otherwise steel, about 18 gauge. Carefully, and as accurately as you can, mark out the cutting points for the two racks. A black felt-tip pen ma
kes a good background medium for marking out. Remember to offset one rack by exactly one half of the pitch. In this example the pitch is 8 t.p.i., so the offset must be 1/16".
A hack-saw frame fitted with one or two blades, according to wire size, or better still, an Abrafile rod-saw (available from engineer's tool suppliers, comes with three blades and clips to suit an ordinary hacksaw) may be used to cut the rack. Take your tA hack-saw frame fitted with one or two blades, according to wire size, or better still, an Abrafile rod-saw (available from engineer's tool suppliers, comes with three blades and clips to suit an ordinary hacksaw) may be used to cut the rack. Take your ti
me, and cut each slot to exactly the same depth, as evenly spaced as possible. Clean up burrs with a smooth file. Cut a rectangle of perspex to size, then sandwich perspex and template together. Fix in place with two small G-clamps (cramps?). Mount the wime, and cut each slot to exactly the same depth, as evenly spaced as possible. Clean up burrs with a smooth file. Cut a rectangle of perspex to size, then sandwich perspex and template together. Fix in place with two small G-clamps (cramps?). Mount the wor
k in a vice, and carefully cut each rack into the perspex.
Cut a length of suitably sized quad timber (90 degrees worth of round stuff) into four equal pieces, slightly longer than the planned coil length. Plane a small bevel along each edge. From thin plywood, make a pair of spacers to fit between the quads. TheCut a length of suitably sized quad timber (90 degrees worth of round stuff) into four equal pieces, slightly longer than the planned coil length. Plane a small bevel along each edge. From thin plywood, make a pair of spacers to fit between the quads. The
spacers should be about an inch longer to provide a 'handle' and so allow you to remove them when the coil is wound. Rub a little linseed oil into these parts to make them more slippery.
When you are ready to wind the coil, position the four quads and their spacers onto the perspex former, then temporarily wrap a length of tape around the assembly to hold the job intact, in order to receive the coil. Roughly calculate the length of wire rWhen you are ready to wind the coil, position the four quads and their spacers onto the perspex former, then temporarily wrap a length of tape around the assembly to hold the job intact, in order to receive the coil. Roughly calculate the length of wire re
quired. In the example above it will be pi times the coil diameter times the number of turns; 3.14 X 1.75 X 40 = 220 inches. Unwind say, 240 inches (20') of wire and clamp the spool in your vice. With a suitable tool, grip the far end of the wire and giequired. In the example above it will be pi times the coil diameter times the number of turns; 3.14 X 1.75 X 40 = 220 inches. Unwind say, 240 inches (20') of wire and clamp the spool in your vice. With a suitable tool, grip the far end of the wire and give
it a firm stretch to remove any small wrinkles. The ends of the winding must be anchored by passing the wire through a hole, or simply by bending the wire into a U, and seating it firmly down into a spare tooth for the purpose. Whilst maintaining tensive it a firm stretch to remove any small wrinkles. The ends of the winding must be anchored by passing the wire through a hole, or simply by bending the wire into a U, and seating it firmly down into a spare tooth for the purpose. Whilst maintaining tension
on the wire, wind the coil onto the former by walking towards the vice. Remember to observe the winding sense. Make sure the wire seats nicely down into each tooth of the rack. When a few turns have been wound on, you can remove the tape; and complete on on the wire, wind the coil onto the former by walking towards the vice. Remember to observe the winding sense. Make sure the wire seats nicely down into each tooth of the rack. When a few turns have been wound on, you can remove the tape; and complete the
winding. Apply a narrow fillet of epoxy glue along the two racks to cement the winding in position. Take care that no glue finds its way onto the quad (the bevel on each of the quads should dodge this problem). Immediately hang the assembly in the verthe winding. Apply a narrow fillet of epoxy glue along the two racks to cement the winding in position. Take care that no glue finds its way onto the quad (the bevel on each of the quads should dodge this problem). Immediately hang the assembly in the vertical
position. Finally, when the glue has set, grip the spacers in a vice and carefully pull one out, then the other.
If taps are required, form a spade on the tapping wire by flattening the end with a hammer. With pointed long-nose pliers, bend a loop in the spade to match the coil wire diameter. The wire may now be hooked and soldered onto the coil at the requisite spoIf taps are required, form a spade on the tapping wire by flattening the end with a hammer. With pointed long-nose pliers, bend a loop in the spade to match the coil wire diameter. The wire may now be hooked and soldered onto the coil at the requisite spot
without risk of shorting adjacent turns (Ref. 2).
The enamelled wire coil shown in the example has a measured Q of 350 at the 2.5 MHz test frequency. Unless it is done properly, silver plating the coil wire has little benefit, and may actually increase coil losses (Ref 3). Plain, enamelled or tinned coppThe enamelled wire coil shown in the example has a measured Q of 350 at the 2.5 MHz test frequency. Unless it is done properly, silver plating the coil wire has little benefit, and may actually increase coil losses (Ref 3). Plain, enamelled or tinned coppe
r is entirely satisfactory for amateur applications. Wire of 12, 16 or 18 B&S may be obtained from wire & insulation merchants, auto electricians and some electronics suppliers. Ordinary single-strand electrician's wire, stripped of insulation, or 'junkeer is entirely satisfactory for amateur applications. Wire of 12, 16 or 18 B&S may be obtained from wire & insulation merchants, auto electricians and some electronics suppliers. Ordinary single-strand electrician's wire, stripped of insulation, or 'junked-
but-good' power transformers are also a good source.
References
1. Radio Communication Handbook; RSGB, 4th edition, P13.36.
2. Tapping Air-wound Coils; Technical Topics, Rad Comm, May '93.
3. RF Performance of Electroplated Conductors; Fowler, A.M., Electronics Australia July 1970.
4. Constructing Air-wound Coils; Johnson, W7KBE, Ham Radio (USA), Aug. '84.
