PURPOSE:To prevent a leak signal from being inputted to a receiver by providing a coupling erasing circuit in the device. CONSTITUTION:The coupling erasing circuit 9 consists of directional couplers 10a and 10b, a variable resistance attenuator 11, and a phase shifter 12. Then, a transmitting antenna 4 and a receiving antenna 5 are directed to the zenith and when a reception level monitor device incorporated in a receiver 6 is viewed while no reflected wave from an object of observation returns, a certain monitor level is indicated. Then the attenuator 11 and phase shifter 12 are adjusted finely mutually while the level is viewed so that said monitor level is zero. Consequently, a leak signal leaking from the antenna 4 to the antenna 5 is erased and then both antennas 4 and 5 are directed to the object of observation to measure the back scattering coefficient of the object of observation in the apparent absence of coupling between the antennas.
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