The way the PCB and schematic are drawn implies it is a two-pole connector with ground on the sleeve and both antenna input and power on the tip. The third connection is a break contact that disconnects the internal antenna when an external one is plugged in.
That is a common type of circuit to allow for amplified or simple antennas.
In the external antenna, power is taken via a choke so the RF is not affected and the signal through a capacitor, so the RF is passed but not DC power.
It's pretty much the same as the feed in the radio, L101 and C110 on page 12.
R110 provides current limiting & short circuit protection, needed at the supply end.
Example circuit that uses this concept - the "coax" connection would be the plug to connect to the radio, the right-hand part is equivalent to what is in the radio.
**broken link removed**