Near-earth RF propagation - Path loss and variation with weather

Most of the current radio frequency propagation analysis is focused on signals travelling well above the earth's surface. A recent surge in low power communication networks having RF signals propagating close to the earth's surface has resulted in some networks using antennas operating at heights less than one metre from the surface. The resultant near ground RF propagation and the effect of obstructions that impinge upon the first Fresnel zone for short ranges is not well understood. It is unclear as to which propagation model to apply in this situation. Knowledge of the propagation conditions is essential to the design of radio communications for distributed radars such as those used for perimeter protection. DSTO is investigating RF propagation and evaluating the effects of weather for these propagation conditions. Propagation loss measurements were made at UHF frequencies using antennas set at heights varying between 0.3 m and 1.5 m in rural line of sight environments. Measured RF propagation loss results for these conditions are in very good agreement with the two-ray RF propagation model. In addition, the dew and cloud coverage can significantly affect the RF propagation loss.