Use this tool to calculate how far a UHF radio can transmit.
Enter the antenna height and pick the units (meters or feet).
d = 4.12 * √h
where h is the antenna height in metres and d is the distance in kilometres.
For a height of 6 feet, the range of a UHF radio system is 4 miles or 6 km. Note that this assumes no obstructions and no radio interference from other UHF systems. These two impairments would reduce the range.
UHF (ultra high frequency) is the name for radio frequencies in the range between 300 MHz and 3 GHz (3,000 MHz). UHF signals are used for television broadcasting, cell phones, satellite communication, wireless LANs, Bluetooth devices, two-way radios and other applications.
Wi-Fi systems operating in the UHF band at 2.45 GHz have an outdoor range of 6 km or 4 miles. However this is under ideal conditions. For a more realistic estimate of distance with an indoor Wi-Fi network try this calculator.
 Line of Sight Propagation on Wikipedia