Line of Sight Calculator

This CalcTown line of sight calculator calculates how far the horizon is from an antenna placed at a particular height. The calculators also calculates the radio horizon. It assumes there is nothing between the radio signals and the antenna horizon. To get maximum line of distance you need to enter the height of the antenna.

Line of Sight Calculator


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h = height of the antenna

R = radius of the earth

dl = total line of sight

dr = radio horizon (actual service range)

Radio horizon

The radio horizon is the locus of points at which direct rays from an antenna are tangential to the surface of the Earth. If the Earth were a perfect sphere and there were no atmosphere, the radio horizon would be a circle.

The radio horizon of the transmitting and receiving antennas can be added together to increase the effective communication range. Antenna heights above 1,000,000 feet (189 miles; 305 kilometres) will cover the entire hemisphere and not increase the radio horizon.

Earth bulge and atmosphere effect

Earth bulge is a term used in telecommunications. It refers to the circular segment of earth profile which blocks off long distance communications. Since the geometric line of sight passes at varying heights over the Earth, the propagating radio wave encounters slightly different propagation conditions over the path. The usual effect of the declining pressure of the atmosphere with height is to bend radio waves down toward the surface of the Earth, effectively increasing the Earth's radius, and the distance to the radio horizon, by a factor around 4/3. This k-factor can change from its average value depending on weather.

The actual service range

The above analysis doesn’t take the effect of atmosphere on the propagation path of RF signals into consideration. In fact, RF signals don’t propagate in straight lines. Because of the refractive effects of atmospheric layers, the propagation paths are somewhat curved. Thus, the maximum service range of the station is not equal to the line of sight (geometric) distance.