In message , Bob
writes


If the Sangean external antenna input were designed for 50 ohm coax,
it would probably have a coax jack, not the 2.1 mm jack.

The 'type' of connector - especially on something like a small portable
radio - is absolutely NO guide to the input impedance (which, actually
could be almost anything). It's often determined by the need to keep it
small. I have a small VHF/UHF TV set which use a 3.5mm audio jack.
--
Ian