In message tonline,
Roy Lewallen writes
Richard Ferryman wrote:
I am in the process of making a 1:1 balun for QRP use and this has
made me think! I have seen many balun designs and construction
articles. A fair proportion of theses advocate twisting the wires
before winding on a toroid core. I have never seen a clear
explanation as to why they are twisted. Does it improve the balance?
affect the coupling? Is it better to twist or not to twist - that is
the question.
Dick G4BBH

You don't want the differential-mode field (primarily between the
conductors) to couple to the core. Twisting the conductors together
helps prevent this, so that's what I recommend. If you don't twist
them, the two conductors of the transmission line should lie together
as closely as possible.

The third conductor (you mention trifilar) makes the balun a "voltage"
balun, which degrades its performance in antenna transmission line
applications. This is explained in the article at
http://eznec.com/Amateur/Articles/Baluns.pdf.

Roy Lewallen, W7EL

FWIW, in the 1960s and 70s wideband CATV amplifiers, splitters and taps
(typically 50 to 600MHz), the bifilar and trifilar windings of the small
ferrite transformers were almost invariably NOT twisted.

I had a fair amount of experience in making some of these devices, and
can confirm that twisting tended to reduce the performance at the HF
end. With bifilar and trifilar windings, adjacent turns were carefully
interleaved. At least one manufacturer had bifilar and trifilar wire
specially made, with the strands lying side-by-side, and glued together.

If the individual strands of wire are already in intimate contact,
twisting them does not lower the characteristic impedance. Of course, if
the windings are NOT in intimate contact, the characteristic impedance
will be higher, and less-well defined. But, as the length of the
windings will be short compared with the wavelengths of the RF signals,
does this matter?

Twisting has the effect of allowing the increased possibility of part of
one winding randomly lying immediately adjacent to the next turn of the
same winding, so it tends to increase the self-capacitance of the
winding. This is almost certainly what reduces the HF performance.
--
Ian