• Extends from surface of earth to height of 10 km
  • Refraction of VHF and UHF waves is caused by the varying dielectric constant of air due to water vapour (humidity) and temperature.  This causes waves to bend and follow curvature of the earth.  This tropospheric or space wave is the primary mode of propagation at VHF and UHF.  Tropospheric refraction of waves increases radio horizon to 1.15 times visual line-of-sight
  • Gradients in the index of refraction due to turbulence and temperature changes cause scattering, creating over-the-horizon paths (troposcatter)
  • Temperature Inversion – humidity at low levels together with increased temperature at higher levels increases refraction significantly.  This allows the wave to be ‘ducted’ for considerable distances with very little attenuation – tropospheric ducting