One of my favorite questions! I get this one at every trade show I attend, without fail. And, it makes sense. My response is always, "Well, how far do you NEED them to talk?"
Radios can talk a very long way, given extremes. One of my pet peeves is when radio manufacturers put on their spread spectrum datasheets: "Range = up to 60 miles point-to-point *" Note the asterisk. Sure, they CAN talk 60 miles, with 1000 feet high gain antenna on either end! In fact, the longest spread spectrum shot I ever managed was 38.7 miles, but that was mountain peak to mountain peak.
In a real world SCADA application, however, ranges shouldn't have asterisks. Here are general range limits by frequency, with realistic gain antenna and realistic antenna heights (say 50-75 feet at the master and 20 feet at the remotes):
VHF, 5 watts = 20 miles
UHF, 5 watts = 15 miles
900 MHz, 1 watt = 8 miles
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