Free space method with parallel electromagnetic wave beam by using dielectric lenses and horn antennas for reflectivity of electromagnetic absorbers in millimeter waves

A free space method is in wide spread use for the reflectivity measurement of electromagnetic wave absorbers (EMA) in the VHF and UHF ranges. In the free space method, the reflection levels from both the EMA and a metal plate of the same size are measured, and the reflectivity is calculated by their ratio. The incident angle, such as normal or oblique, must be specified, and the polarization of the electromagnetic wave must also be specified as TE, TM, or circularly-polarized mode. A method using only a parallel beam made by placing a dielectric lens in front of a horn antenna in the millimeter wave band is studied experimentally. Electromagnetic wave absorption was measured for normal or oblique incidence using this parallel beam. The following effects are expected: the measurement system is compact because the wave plane becomes equiphase after shorter range running than for no lens use; it is made possible because the scattered wave can hardly enter the receiving part; it is possible to enlarge the incident angles to large degrees when measuring at oblique incidence.