John Popelish wrote:
. . .
It may not be the approach you are familiar with, but I think it is
valid. The transformer just has a lot of core loss, if you use a
ferrite optimized for a much lower frequency. And that loss shows up as
if it were a resistor connected across the ends of the two windings. If
a low loss ferrite (at the operating frequency) is used, then the
impedance across the windings is dominated by inductance, as one
normally expects with a transformer. But the analysis handles the whole
range of cases.

Broadband transformers, which can operate well over several decades of
frequency, commonly use ferrite cores which are essentially resistive
over most of the operating frequency range. The sign of the impedance is
unimportant to the transformer's operation; all that's necessary is that
its magnitude be adequately high over the operating range (and of course
that the core's permeability be adequately high). The wide band high
impedance requirement is virtually impossible to meet with an inductive
core whose impedance is approximately proportional to frequency, but
easily done with cores whose impedance is essentially resistive.

Roy Lewallen, W7EL