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? Does it 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