PURPOSE: To improve the compensating capability of reception signal power reduction due to duct fading by varying the transmission direction of a beam radiated from an antenna based on an output of an estimate device estimating the transmission medium characteristic.

CONSTITUTION: An estimate device 3 observes a meteorological data such as air temperature, water vapor pressure and air pressure to estimate the distribution of refractive index in the transmission medium. A control circuit 2 calculates the direction of the transmission beam reaching a reception point based on the refractive index distribution estimated by the estimate device 3 and the information obtained from a reception antenna location information storage circuit 4. Then the direction of the transmission beam of the transmission antenna 1 is directed to a direction obtained by the calculation. Through the transmission above, the reduction in the reception signal power due to a duct is compensated. The direction of the transmission beam is set in the direction to reach the transmission signal to the reception antenna in this way to compensate sufficiently the reduction in the reception signal power due to duct fading.