That formula is: Horizon in kilometers = 3.569 times the square root of the antenna height in meters . For example, if the antenna height of a radio is at 6 feet (1.82880 meters tall) the horizon is 2.99 miles away (4.83 kilometers) and another radio with the antenna at 6 feet, that equals approximately a 6-mile range.
Read moreHow far is radio line of sight?
So according to our rule-of-thumb, 2 people around 6 feet tall using a 5 watt handheld two-way radio, used on flat ground with no obstacles, will have a maximum range of approximately 6 miles .
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