PURPOSE: To protect antennas and to measure distances with high accuracy by thermally shielding the antennas with a heat resistant dielectric material at the thickness integer times the half wavelength of the microwaves to be used.

CONSTITUTION: A protecting case 4a is made of a dielectric material such as high temp. resistant ceramics or fused silica. Bar antennas 3d are contained in the case 4, and air is fed therein through an air inlet 5a to cool the inside with the air. The thickness in the bottom part 4'a of the case 4a is selected at integer times the half wavelength of the ratio waves in the part 4'a.