W5PROTOOLS / PATH PROFILE

Line of sight · Fresnel zone · Terrain

VHF/UHF path profile

Check whether two stations can see each other over the real terrain. You get the ground profile with Earth curvature, how much of the first Fresnel zone is clear, free-space path loss and a rough knife-edge diffraction estimate. Terrain comes from the USGS 3D Elevation Program, so it covers the US only.

Stations

Antennas & band

Band presets fill in a representative frequency inside each US amateur band; type your own if you like. Heights are to the antenna, above the ground at that spot.

Map

Tap the map to set:

How it works

This is a line-of-sight check, not a coverage prediction. It tells you whether terrain and the curve of the Earth get in the way, and how much free space is left around the direct ray. It ignores buildings, trees, ground reflections, tropospheric ducting and everything else a full propagation model handles. For signal-strength predictions use a Longley-Rice (ITM) tool such as Radio Mobile, SPLAT! or NTIA's ITM.

Terrain. The great-circle path is cut into 32–128 equal steps (about one every 300 m, wider on long paths) and the ground height at each point is fetched from the USGS Elevation Point Query Service (3D Elevation Program). It only covers the US and some border areas; open water offshore returns no data. A narrow peak between two samples can be missed.

Earth curvature. Radio waves bend slightly in the atmosphere, which is modelled by an effective Earth radius ae = k × 6371 km (k = 4/3 is the standard median). The ground at distances d₁ and d₂ from the two ends is raised by d₁·d₂ / (2·ae) relative to the straight line between the antennas.

Fresnel zone. The first Fresnel zone radius is r₁ = √(λ·d₁·d₂ / (d₁+d₂)). ITU-R P.530 says a path needs at least 60 % of r₁ clear for free-space conditions. Clearance is measured vertically, which is accurate for the small path angles found on real VHF/UHF links.

Radio horizon over smooth ground is √(2·ae·h), which is 4.12·√h km (h in metres) for k = 4/3. Free-space path loss is 20·log₁₀(4πd/λ) dB, i.e. 32.45 + 20·log₁₀ f(MHz) + 20·log₁₀ d(km).

Knife-edge loss (approximate). The terrain is reduced to a single equivalent knife edge using the Bullington construction of ITU-R P.526, and the loss J(ν) is taken from P.526's approximation. Real terrain is rarely a knife edge, and this is only the first step of P.526's full terrain method, so treat it as a rough guide. It is zero when the path has enough clearance (ν ≤ −0.78).

  • Rec. ITU-R P.526-16 (11/2025), Propagation by diffraction: §2.1 eq. (2) Fresnel radius; §2.3 60 % clearance; §4.1 eqs. (26), (30), (31) knife-edge ν and J(ν); §4.4 ae = k·6371 km; §4.5.1 eqs. (49)–(56) Bullington construction.
  • Rec. ITU-R P.530-19 (09/2025), §2.2: median k ≈ 4/3; §2.2.2: 60 % of the first Fresnel zone for free-space conditions.
  • Rec. ITU-R P.525-5 (11/2024), eq. (5): free-space basic transmission loss.
  • Terrain: U.S. Geological Survey, 3D Elevation Program, via the Elevation Point Query Service (epqs.nationalmap.gov/v1). Callsign locations from FCC data via callook.info (mailing address, approximate).