Line of Sight Distance Calculator

Radio line of sight (LOS) is the maximum distance at which two stations can maintain a direct, unobstructed communication path through the atmosphere under standard conditions. It differs from optical line of sight — the distance you could see with a perfect telescope — because radio waves in the VHF and UHF bands are refracted slightly downward by the troposphere, effectively bending them around the Earth's curvature and extending usable range. This atmospheric bending is the reason a 2m FM signal from a hilltop repeater reaches farther than you could see from the same height on a clear day.

The standard model for VHF and UHF propagation uses an effective Earth radius equal to 4/3 of the actual Earth radius (6371 km), giving an effective radius of about 8495 km. This 4/3 Earth model, specified in ITU-R P.452 and embedded in the ARRL Handbook microwave chapters, yields the familiar rule of thumb: radio horizon distance per station equals 4.12 × √h(m) kilometres, or equivalently 1.23 × √h(ft) miles. The optical equivalent uses k=1.0 and the coefficient 3.57 × √h(m) km. The total two-station LOS distance is simply the sum of the individual station horizons.

For practical amateur radio path planning, line-of-sight distance sets the hard upper bound on reliable terrestrial communication at VHF and above under normal atmospheric conditions. Beyond the LOS horizon, signal levels drop steeply — typically an additional 20–40 dB of loss per decade of distance beyond the horizon, compared to free-space loss within LOS. Knowing this boundary tells you whether a proposed path is feasible with the available transmitter power and antenna gain, or whether you need a repeater, a taller tower, or must rely on anomalous propagation such as tropospheric ducting or aircraft scatter.

LOS distance is a function of antenna height above local ground (AGL), not above sea level. A station on a 2000 m mountain plateau is primarily constrained by its AGL height — how high the antenna is above the immediate ground — for computing its local horizon, though the plateau elevation does raise the antenna above surrounding valleys and can block terrain features at a distance. When planning paths across varying terrain, combine this LOS calculation with a terrain profile tool to identify the actual limiting obstacle rather than assuming flat Earth.

The 4/3 Earth model assumes standard temperate atmospheric conditions with a specific lapse rate of refractivity. In coastal areas, desert climates, or during summer high-pressure events, the effective k-factor can rise to 1.5–2.0 or drop to 1.0 or below. When k rises above 4/3, ducting can extend VHF/UHF signals hundreds of kilometres beyond the calculated LOS — something many 2m and 70cm operators notice during tropospheric enhancement events. When k drops below 1.0 (sub-refractive), signals may not even reach the calculated horizon. Use the Radio Horizon Calculator with an adjustable k-factor to explore these conditions.

Antenna height has a square-root relationship with LOS distance, which means the first few metres of height gain yield the biggest benefit. Going from 1 m to 4 m AGL doubles the radio horizon from 4.1 km to 8.2 km — a 3 m gain. Going from 100 m to 104 m only extends the horizon from 41.2 km to 41.8 km — the same 4 m gain but far less return. This is why low-budget EmComm deployments focus on using a lightweight 6 m mast rather than a taller tripod: the first few metres of height dominate the coverage improvement.

Line-of-sight distance is not the same as reliable communication range. Even within the LOS horizon you need adequate link budget — enough EIRP, receiver sensitivity, and antenna gain to close the link at the required signal level. A path at 90% of the LOS distance with a 6 dB Fresnel obstruction and 3 dB of feedline loss can require 9 dB more transmitter power than a clean free-space budget suggests. Always pair an LOS calculation with a full link budget analysis using measured or estimated antenna gains and cable losses.