Millimeter-wave propagation in vegetation: experiments and theory

Microwave/millimeter-wave propagation in woods and forests was investigated at 9.6, 28.8, and 57.6 GHz. The experiments were repeated over the same transmission paths, under both summer and winter conditions, i.e. with tree in leaf and without leaves. Of particular interest were the range dependence, beam broadening, and depolarization of millimeter-wave beams in vegetation and frequency dependence of these effects. The experiments have shown, in particular, that the range dependence is characterized by a high attenuation rate at short vegetation depths and a reduced attenuation rate at large depth. For trees fully in leaf, the transition between the two regimes can be abrupt and the change in attenuation rate substantial. Just after the transition significant beam broadening (and depolarization) occurs. A theory of millimeter-wave propagation in vegetation was derived using transport theory.>