[Elecraft] Bifilar wires

[harry17b]

Several coils for the K2 use bifilar windings, made by twisting two
wires together. I've developed a technique for these that may be
generally useful.

I have seen a reputable workshop guide suggest that a bifilar pair is
made by anchoring the wires at one end in a vice and twisting the other
end with a hand drill brace. The trouble with this sort of method is
that the wire is springy and tends to untwist again a little when
released, so you get a loose and untidy coil.

You can make a bifilar pair that stays tightly twisted by employing the
trick used by rope makers. The separate strands that make up a rope are
themselves twisted so that as each individual strand tries to untwist,
it actually tightens up the rope as a whole.

Visualising this is mind boggling, but doing it is dead easy. Practice
with two pieces of string. Tie them at one end and attach a weight to
the knot. Hold the free end of each string between the thumb and
forefinger of each hand, one string in each hand that is, and twiddle
them in opposite directions. You'll find that the string and weight
twirl around and form a neat bifilar pair that still stays firmly
twisted after you let go. The pitch of the bifilar can be adjusted by
moving your hands apart or together, and by the amount of weight you
use.

You'll soon get the knack, and can then apply it to the wires. A further

tip. I find the wires for the coils too thin to twiddle easily, so I
solder the free ends temporarily to two short pieces of thicker wire.

I don't know if this technique is already generally known, I certainly
have not seen it in print. I got the idea from watching a rope making
machine at work.

73, Hugh M1ETU