Also known as: radio horizon
The radio horizon is the farthest point a line-of-sight signal reaches before the Earth’s curvature gets in the way.1 It lies slightly beyond the visual horizon because the atmosphere refracts radio waves a little, bending them gently around the curve rather than letting them fly straight off into space.
How it works
Because the Earth curves away, an antenna at height h can “see” only out to where its straight sight line grazes the surface. Geometry alone puts that distance at roughly 3.57·√h kilometres for h in metres. But the lower atmosphere is denser near the ground and thins with altitude, so it refracts radio waves downward slightly, letting them follow the curve a bit farther than light does. Engineers model this with the 4/3 Earth radius approximation, which stretches the constant to about 4.12·√h km.2 The distance between two stations is the sum of each one’s horizon distance:
- d(km) ≈ 4.12·(√h_tx + √h_rx), with heights in metres.
- A 30 m tower reaches about 22.5 km to the horizon on its own.
- Two 30 m towers can therefore work each other out to roughly 45 km.
The rule is nearly frequency-independent across VHF and UHF — a UHF and a VHF signal from the same tower reach about the same horizon — so antenna height, not band, sets the range for line-of-sight work.
In practice
This is why repeaters and trunked trunking sites sit on towers, rooftops, and hilltops, and why raising a receive antenna even a few metres can pull in systems that were invisible at ground level. Beyond the horizon, signals fall off fast, but not to nothing: diffraction fills in some of the shadow, and occasional tropospheric ducting can carry VHF/UHF far past the normal limit when atmospheric layers form a waveguide. At lower frequencies, ground wave and sky wave reach well beyond the horizon by entirely different mechanisms, so the radio-horizon limit chiefly governs VHF and above.
Relevance to SDR
For line-of-sight bands, antenna height is often the most effective way to reach more distant systems — usually more so than a higher-gain antenna or a better radio. Estimating the radio horizon from your antenna height and a target site’s tower height tells you quickly whether a system is even reachable before you spend time chasing it. GopherTrunk cannot recover a transmitter that sits below your horizon; the practical response is to get the antenna up and clear, which extends the horizon and improves the link budget at the same time.
Sources
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Line-of-sight propagation — Wikipedia, on the radio horizon, Earth curvature, and atmospheric refraction. ↩
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Horizon — Wikipedia, for the geometric horizon distance and the 4/3-Earth radio-horizon correction. ↩