A long time ago when King released there KX99 hand held radio one of the accessories was a mechanical relay.
King understood the shortcomings of hand held radios as a back up system and so sold the mechanical relay that could be fitted into the panel and connected between the main aircraft radio and the external antenna. When needed you plugged into the device with a cable ( supplied with the unit ) And connected your and held to the external antenna ( as well as disconnecting the aircraft radio ).
The hand held using the supplied antenna was around 12 miles at best with poor signal quality, using the King mechanical relay system I tested it out to 60 miles at 3000 ft and had good quality audio, I have no doubt the maximum range was beyond 60 miles but considered further testing to be unnecessary for an emergency back up system.