TECHNICAL NOTE / HORIZON / TERRAIN
Long VHF/UHF paths begin with geometry. The straight-line radio horizon, intervening ground and trees, antenna gain, scattering mechanisms and atmospheric refraction all contribute. A path that looks impossible for routine line of sight may become workable during tropospheric enhancement.
Start with mutual horizon
For a first approximation under standard atmospheric refraction, distance to the radio horizon in kilometers is about 4.12 × √h with antenna height h in meters. Add the horizons at both ends to estimate a mutual radio-horizon distance. This is a screening calculation, not a path guarantee.
Terrain and clutter change the result
Antenna height above local ground is not the same as height above the intervening terrain. Trees, buildings and terrain ridges can affect takeoff angles and diffraction. A clear route on a map does not prove adequate Fresnel-zone clearance.
Atmospheric mechanisms
- Standard refraction: bends ray paths modestly under common conditions.
- Enhanced refraction or ducting: some refractivity profiles support considerably longer paths.
- Rain scatter: most established at higher microwave frequencies; do not assume the same usefulness at 902/1296 MHz.
- Meteor scatter and EME: entirely different path geometries and link budgets.
Build a complete prediction record
Record start and target grids, real antenna heights, frequency, polarization, modes, power, line loss, antenna gain, terrain profile and observed contacts. Estimated EIRP alone is not proof of decodability at the other end.