Oblique incidence optimisation of a pyramidal absorber using a frequency selective surface

This paper presents a topology for improving the low frequency performance of pyramidal radar absorbing materials at oblique angles of incidence. The technique uses an embedded frequency selective surface as an impedance matching layer and improves the -40 dB reflectivity limit from 6 GHz to 4 GHz for 102 mm pyramidal absorber. The frequency selective surface is based on a resistively loaded dipole FSS and simulations of optimised results of the modified RAM structure are presented.

I. Introduction

The design of Radar Absorbing Materials (RAM) for use in anechoic chambers has been a topic of research for many years [1], [2]. The design of the geometry, dimensions and loss mechanism depends on the application (i.e. Antenna/ Radar Cross Section/ Electromagnetic compatibility measurements etc), frequency range of interest and the structure of the chamber. Much of the research to date has been focused on improving the reflectivity performance and bandwidth with application to wedge absorbers [3], [4], arrays of doubly periodic curved pyramids [5], arrays of multi-level wedges based on Chebychev polynomials [6], variation of the geometrical profile in conjunction with the use of permittivity profiles, [7], and two-dimensional wedge absorbers [8], [9]. Research has also been carried out into assessing the quality of absorbers within a chamber design [10].