I should mention that the model and prototype measurements suggest that
lossier higher µ cores may produce a more efficient device... it depends
on ther things.

The tradeoff between core size, core material, number of turns etc for a
given load is a complex one, more complex than implied by simple rules
like "#61 is the best material for HF baluns".

Even the low frequency model of such a balun reveals this. If the balun
is analysed using the techniques common used for a 50Hz or 60Hz
transformer, the magnetising current (the current that flows into the
transformer with no load attached) is design point. If the core is a low
loss core, one could choose a relatively high mangetising current yet
still have low H+E losses because the Power Factor of that magnetising
current is quite low... or in the case of the RF transformer, one could
use a relatively lossy material (high magnetising current Power Factor),
but the higher µ of the lossier core means lower magnetising current, and
the losses are acceptable.

The model I have proposed allows exploration of these different
configurations, and the tools that I have developed allows solution of
the problem using the core material frequency dependent characteristics.

I don't want to trivialise designing with magnetics, it is a challenge...
but we can do better than simple rules like #x material is the best HF
balun material... it is a very eHam approach.

Owen

BTW, the commonly held belief that powdered iron the material of choice
for baluns is not soundly based. Such a view seems driven by the belief
that lowest loss core material assures a good outcome. It is interesting
that powdered iron has such a following, yet so little information is
published on the core material compared to the ferrite materials.