The following models can use an external antenna:

Models:  MR HH300, 325, 400, 415, 425

The antenna connector on your radio is a type “SMA female”. 

This is the adapter you will need:  SMA male to SO239 adapter cable

One end of the adapter should have an SMA male, which will attach to the radio.  The other end of the adapter should have an SO239, which will mate to the PL259 on the antenna cable.

Models:  MR HH350, 450, 500, 600

The antenna connector on your radio is a type “SMA male”.

This is the adapter you will need:  SMA female to SO239 adapter cable

One end of the adapter should have an SMA female, which will attach to the radio.  The other end of the adapter should have an SO239, which will mate to the PL259 on the antenna cable.

IMPORTANT NOTE

To minimize mechanical stress on the antenna connector, it is important to get an adapter cable.  One end of the cable will attach to the radio and the other end will attach to the antenna cable.  Get an adapter cable with the appropriate connectors (as described above).