Main page Propagation caused by variations in the refractive index of the atmosphere in the lower levels (troposphere).
Tropospheric propagation is an interesting mode of propagation which does not involve the ionosphere. The troposphere is the bottom 10 km or so of atmosphere and is where the weather patterns form. In this part of the atmosphere, it is possible to refract radio signals in the VHF part of the spectrum as a result of variations in the refractive index of the air. These variations may be caused by ice crystals, atmospheric pressure changes and temperature inversions (where warm air lies on top of cold).
Tropospheric propagation is frequently responsible for co-channel television interference in which ghosts of another channel can be seen beneath the normal signal.