Every VHF contact that won't complete, every WISP customer with mystery packet loss, every repeater that covers the valley next to yours but not you — most of the time, the culprit is the same: terrain in the path. Above ~30 MHz, radio propagation is essentially line-of-sight, and the single most valuable thing you can do before raising a mast, buying a dish or programming a repeater pair is to check whether the path actually clears the ground.
The good news: that check takes seconds, not a site survey.
Take a real case from the Alicante coast: Xàbia and Dénia, two towns 13 km (8.1 mi) apart — with the 753 m (2,470 ft) Montgó massif sitting squarely between their antennas. It's the running example for the rest of this article.
The radio horizon is farther than the visual one
Radio waves refract downward in the atmosphere slightly more than light does. Engineers model this with the k-factor: under standard conditions radio behaves as if Earth's radius were 4/3 its real value. The practical result:
visual horizon ≈ 3.57 × √h [km, h in m]
radio horizon ≈ 4.12 × √h
An antenna at 10 m (33 ft) reaches a radio horizon of ~13 km (8.1 mi); at 100 m (328 ft), ~41 km (25 mi). Two stations combine: a 10 m base and a 100 m repeater can theoretically work each other at ~54 km (34 mi) — if nothing in between blocks the path. And that "if" is the whole game in hilly terrain.
Line of sight isn't a line — it's an ellipse
The second thing terrain can break is the Fresnel zone: the rugby-ball-shaped volume around the direct path that carries most of the signal energy. Obstacles that intrude into it attenuate the link even when they don't touch the geometric sightline. The first Fresnel zone's radius at mid-path is:
r ≈ 8.66 × √(d / f) [r in m, d = path length in km, f = frequency in GHz]
Rules of thumb that follow from it:
- Keep at least 60 % of the first Fresnel zone clear for a solid link.
- Lower frequencies bulge more: on a 10 km (6.2 mi) path the first zone's mid-path radius is ~11 m (36 ft) at 5.8 GHz but ~72 m (236 ft) at 145 MHz. That grazing ridge that's fine for a microwave dish can eat a 2 m SSB path.
- Clearance matters most at mid-path, where the ellipse is fattest — a ridge halfway hurts more than the same ridge near either end.
Check any path in seconds
UpToWhere's point-to-point mode answers the core question directly from 30 m Copernicus elevation data, with Earth curvature and refraction applied:
- Open the calculator, drop point A on your station and point B on the far end — the repeater, the customer roof, your buddy's QTH.
- You get an instant verdict — clear or blocked — plus the full terrain profile along the path, showing exactly which ridge is the problem and where.
- Raise either endpoint virtually (antenna height setting) and re-run until the path clears: that's your minimum mast height, before you've bought a single metre of tubing.
For coverage rather than a single link — where can my repeater be heard? which valleys does my node serve? — run a 360° viewshed from the antenna site instead and read the visibility map as a first-order coverage map.
A note on scope: a viewshed is an optical line-of-sight computation with standard refraction — a first, decisive filter. It doesn't model Fresnel clearance, diffraction, ducting or vegetation. If the optical path is blocked by 200 m of granite, no link budget will save it; if it's clear with room to spare, you've earned the right to do the detailed RF planning.
Check your link path free — point to point
Frequently asked questions
How far can a VHF/UHF signal travel?
For practical purposes, to the combined radio horizon of the two antennas: roughly 4.12 × (√h₁ + √h₂) kilometres with heights in metres. Two handhelds at head height manage about 10 km (6.2 mi) over flat ground; a base station working a mountain-top repeater can span well over 100 km (62 mi) — terrain permitting.
What is the difference between visual and radio line of sight?
Radio waves refract downward more than light, so the radio horizon lies about 15 % farther than the visual one (the 4/3-Earth or k-factor model). However, radio links also need Fresnel-zone clearance around the direct path, so a path that is barely visually clear can still perform poorly.
How much clearance does a radio link need?
The standard rule is at least 60 % of the first Fresnel zone free of obstacles. The zone's mid-path radius grows with path length and shrinks with frequency — long, low-frequency paths need tens of metres of clearance above the terrain.
Can I check radio line of sight online for free?
Yes. UpToWhere computes point-to-point line of sight and full terrain profiles between any two points on Earth using 30 m elevation data with curvature and refraction, and 360° coverage viewsheds from any antenna site.