UHF Repeater Antenna Gain?

Our school district has a repeater system in place for custodial and maintenance operations. The repeater is a vertex 7000 with duplexer using a unity gain UHF antenna mounted at 80ft on top of the water tower on the high school property. We are having trouble receiving portable radios inside other district schools located (2) 1.5 miles away and (1) 3 miles away. Would an upgrade to a high gain antenna help receive the weak signals from portable radios?



Thanks,

Tom

UHF Repeater Antenna Gain?