"Mike Spence" wrote
Is there a recognised way to connect a radio that just has a
telescopic aerial to a coax cable so that an external aerial can be
used?

Mike, a receiver that does not provide for external antenna connection, will
probably not benefit from forcing the input of one. The front end design of
many portables uses double-conversion only, this allows for reception of
fairly weak signals from the provided whip antenna. But it does not always
bode well when powerful inputs such as an external antenna can provide,
overloading the little portables and making it more difficult to discern
between stations close to each other. Recommend you use your portable for
what it was intended for, and look into adding a tabletop receiver if you
want to input external antenna feed.

Best regards,

Jack Painter
Virginia Beach VA