Read the range as a planning estimate
Use the result as a first estimate of line-of-sight range between two antenna heights. It is most useful for VHF/UHF planning and site comparisons.
Radio Horizon
Approximate line-of-sight distance between two stations on VHF/UHF.
Heights
Geometric horizon—
Typical 4/3-Earth radio horizon—
Geometric horizon uses 3.57 × (√h1 + √h2). A common 4/3-effective-Earth approximation uses 4.12 × (√h1 + √h2), with heights in metres. Tropospheric enhancement can extend range significantly.
LOS
Geometric: d ≈ 3.57(√h₁ + √h₂) km
4/3 Earth: d ≈ 4.12(√h₁ + √h₂) km
4/3 Earth: d ≈ 4.12(√h₁ + √h₂) km
Useful for planning VHF/UHF simplex or repeater paths. Real coverage also depends on terrain, power and antenna gain.
Why terrain can dominate the calculation
Terrain, clutter, atmospheric refraction, Fresnel-zone clearance and antenna patterns can dominate real-world coverage. The radio horizon is not the same as reliable service range.
What to check before relying on the link
For a real link, inspect terrain and Fresnel clearance and then allow margin for the expected propagation environment.
Use the horizon as a site-planning estimate
The radio horizon is a geometric planning concept for line-of-sight VHF and UHF communication. Increasing antenna height generally increases the distance at which the antennas can see one another over a smooth Earth, and a standard refraction model often extends the radio horizon beyond the purely optical horizon. The result is useful for comparing mast heights, portable locations and relay-site options, but it is not a terrain-profile analysis. Hills, buildings, trees and local clutter can block a path well before the calculated horizon, while tropospheric propagation can sometimes extend communication far beyond ordinary line-of-sight expectations. The calculator therefore works best as a first estimate. If a path is important, use a terrain or elevation profile and consider the actual antenna heights above the surrounding ground. At VHF and UHF, Fresnel-zone clearance can also matter even when the two sites are nominally within the geometric horizon. Treat the number as a planning boundary, not as a guaranteed coverage radius.