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.
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.
 Line of Sight Propagation on Wikipedia