To uniformize electric field distribution, and to curtail the area required for installing a wave absorber as far as possible by determining the transmission height of an antenna, so that electric field strengths at a stated plural points in an imaginary vertical surface separated from a transmitting antenna by a specific distance may be within a specified deviation.

On the basis of the heights and horizontal distances, etc., of a transmitting antenna 1 and a substance to be tested 5, phase composition of a direct wave 2 from the antenna 1 and a reflected wave 3 from a floor surface is computed, and the relation between the transmission height of the antenna and a frequency being the condition of electric field uniformity is determined by computation. Next, a reference point of an electric field strength is set in a vertical plane of a substance to be tested 5, and by the degree of the variations of the electric field strengths at a plurality of stated points against this reference, the relation between the transmission height of the antenna and the frequency is evaluated. And when the variations are larger than a specified range, the height of the antenna is made higher, and similar computation and evaluation is performed to determine the height of the antenna and the frequency desired, and an electric field distribution is uniformized. When it is impossible to perform uniformization by this method, necessary minimum wave absorbers are laid on the metal floor 4 surface.