Free Space Measurement Method with Parallel Electromagnetic Wave Beam by Using Dielectric Lenses and Horn Antennas for Reflectivity of Electromagnetic Absorbers in Millimeter Waves

摘要

A free-space method is in wide spread use for the reflectivity measurement of electromagnetic wave absorbers (EMA) in VHF and UHF range. In the free-space method, the reflection levels from EMA and from the metal plate with same size as the EMA are measured, and the reflectivity is calculated from their ratio. The incident angle such as normal or oblique must be defined, and the polarization of electromagnetic (EM) wave must be specified to be TE, TM, or circularly-polarized mode. In this paper, a parallel EM wave beam method using dielectric lenses in front of horn antennas was studied experimentally. Electromagnetic wave absorption was measured with the vertical and the oblique incidence by using this parallel EM wave beam. This measurement system has following features: 1. It is compact because equiphase parallel EM wave beam was obtained in a short distance from the dielectric lens. 2. It requires no anechoic chamber because of little multi-reflection due to high directivity of parallel EM wave beam. 3. It allows a large oblique incident measurement by using high directive parallel EM wave beam.