TRANSMISSION LOSSES IN A FOREST FOR ANTENNAS CLOSE TO THE GROUND

Abstract
During the last few years, measurements have been made to determine the electrical characteristics and the predominant mode of propagation in a forest environment. This work was essential because of the severe attenuation of radio communication signals which occur when operating in a forest medium. Theoretical investigations have shown that for distances greater than 1 km and for frequencies between 2 and 200 MHz the forest can be represented to a first approximation as a conductive slab bounded on one side by air and on the other side by the earth. Using this conductive slab as a model, the transmission losses between two elementary dipoles located close to the ground in a forest were calculated for both horizontal and vertical polarization. Those parameters which would affect the transmission losses are examined and then the sensitivity of the respective parameters is evaluated in detail. It turns out that the change in input resistance caused by the ground proximity produces a loss which may be considerably larger than the other losses. Various data are presented which show the theoretical transmission losses vs frequency for different parameters which include the height of the antenna above ground, the height of the forest, and the electrical characteristics of the ground and the forest.

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