RB wrote:
So: does the commonly available choke balun (round cylinder with coax
connectors on each end, and ferrite beads inside the cylinder) allow us to
put unbalanced feed in one end, and get balanced feed on the other?
Or, are they ununs?

Since they function by choking RF on the outside braid, they
can function as either baluns or ununs. If RF is choked and
not flowing on the outside braid, then differential currents
are forced to flow in the two remaining conductors be they
parallel wires or the inside wire and braid of the coax.
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73, Cecil http://www.qsl.net/w5dxp