On Mar 1, 4:11�am, "Dave" wrote:

*I am new enough at winding toroids that I am probably going t use
the bifilar winding technique, which seems like it might be more forgiving
if I need to make adjustments afterward (correct me if I'm wrong.) *But I
understand what you are saying here, and appreciate your input. *Thanks
much. Once I get more adept this is what I will do.

Here's a suggestion that's an old one when I first tried it
(successfully) 30 years ago on multi-filar windings: Get
an old hand-operated drill (electric if it has slow speed),
lay out equal lengths of magnet wire, tape one end of
the bundle securely, chuck the other end in the drill
and do the twisting. Pause several times while doing so
to make sure there aren't any sharp twists along the
way, "iron" those out with fingers. You can get an idea
of the number of twists per inch (or meter or whatever)
as you pause...for wideband RF transformers the number
of twists per unit length need only be approximate. What
you wind up with (no pun intended) is the equivalent of
multi-wire balanced transmission lines of relatively low
impedance and the total length would be roughly a
quarter-wavelength at the highest frequency of such a
transformer.

However, if you are doing a tap for a lower-impedance
point such as an oscillator feedback or impedance in/out,
the physical tap position doesn't have to be precise or
absolute (lots of other factors will be involved there).
Just get as close as you can to published data/article
on tap position and you'll be okay. Winding toroids may
be new to lots of folks but all it takes is more patience
than with old-style cylindrical forms.

I might add that, once a circuit is working with a toroid,
it can be secured by using some petroleum-based
varnish lightly brushed over it. Most "boat" or "spar"
varnish types are good for that but one is stuck with
having to buy quart cans of it (rather expensive). The
reason I mention marine varnish is that it stands up to
moisture in the air. If you use "Q Dope" (liquid form
polystyrene) it looks very nice when done but a year
from now the moisture normally in room air can "lift"
it from non-porous surfaces. On testing with MF to
UHF Q Meters, there is no more degredation of Q
using varnish than with liquid polystyrene or lacquers.
I use the McCloskey brand "Gym-Seal" varnish because
that is also good for wood surfaces around the house;
it was originally formulated as a surface varnish for
gymnasium floors and the like. Takes at least an
over-night drying time with a complete cure in two
days.