how is port C typically terminated in microwave circulators?
I suspect I'm not understanding the question. Here are a
couple of stabs in the dark:
You can make an *isolator* starting with a circulator and
terminating one of the ports with an off-the-shelf terminator,
i.e. terminating it in the characteristic impedance. Typically
in the guts of the terminator is a chunk of "carbon composite"
resistor material.
If it's a *circulator* strictly speaking, port "C" is not
terminated when you take the circulator out of the box; it
is an as-yet-uncommitted terminal just like the others.
There's nothing special about port "C"; the three ports are
cyclically interchangeable.
The canonical generic plain-vanilla application for a circulator,
as hinted previously, is
transmitter power stage --> A
B <--> antenna
C --> receiver front end