The radios won't talk Modbus or Profibus. I suppose you could encapsulate the serial into a Modbus packet but would be a bit silly. It's easier to talk plain serial (RS232) to them.
It wouldn't be too difficult to wire the outputs to some relays for antenna switching. It would be neat to hook up a motor with an encoder so that could do some positioning of a rotor. Of course all of this can be done in a cheaper Arduino unless you have a PLC lying around (I have 2 SLC 504s waiting for a project)
It may be easier to talk Modbus from the PC to the PLC since Modbus' documentation is out there and there are some open source programs to talk Modbus RTU to the PLC from a PC.
Good luck in your endeavour
Electricity takes ALL paths back to the source, not just the least resistive. -Guido