Characterisation of depolarisation of radio signals by single trees at 20 GHz

Abstract

Measurements investigating how vegetation depolarises a propagating signal have been performed and presented for a number of different trees. From the measurements it can be seen that significant depolarisation takes place at the rotation angles which coincide with the measured nulls of the 360/spl deg/ attenuation patterns especially for the conifer trees. In some cases, it has been seen that the transmitted vertical linearly polarised wave has been depolarised to a near horizontally polarised wave by the vegetation material. The depolarisation occurs as a result of the various components of the tree having currents induced in them. The various tree components are then re-transmitting the waves at different polarisations resulting in an overall depolarisation of the originally transmitted wave. With the metal tree it can be seen that the amount of depolarisation increases as more and more branches are added to the structure. This shows that the various components of the tree are responsible for the depolarisation of the incident signal.