wireless HMI

I have an application where some PLC euipment will be used outside - it will be separated by 300 ft(its approx 100 ft wide) - I am looking to setup a wireless HMI to control the items manually while in this 300 x 100 ft area(sending and receiving discrete I/O). I prefer to only have antennas at the far ends of the 300 ft dimension(nothing on the sides). What is the best, most cost effective way to do this? Would a palm pilot running windows CE do this well? Any sugestions wiould be much appreciated.

P.S. - I have some PLC experience(A.B) but I am not a computer/networking guru......

Thanks