V.H.F. radio propagation by double-hop reflection from a tropospheric layer

A propagation mechanism is discussed which comprises two reflections from a horizontally-stratified, partially-reflecting layer. Such a layer located appreciably below the `crossover height¿ (i.e. the height of intersection of the horizon rays from the terminals) may cause an enhancement of signal strength which exceeds that obtained with a single reflection from a similar layer at the crossover height. This mechanism appears to give a satisfactory explanation for the experimental results obtained by Lane and Sollum in a study of layer-mode propagation over two radio paths, of length 139 and 309km, the shorter being over land and the longer partly over sea.