Blazed 3-D Cells With Application to Full-Metal Dichroic Mirrors

A novel frequency selective surface (FSS) architecture for dichroic mirrors is presented in this work. The solution employs a full-metal unit cell, which was conceived as the transmission equivalent of the blazed grating opaque metallic structures operating in retrodirective reflection. The unit cell is based on a transverse electromagnetic (TEM) topology, while basic filter analysis for a bandpass response has also been employed. This TEM topology brings significant reduction of the 3-D cell’s periodicity allowing operation in the subwavelength regime, so that grating lobes are avoided. This, in turn, brings enhanced angular stability in terms of amplitude and phase transmission for both TE and TM incidence. The principal application of the proposed dichroic mirror relates to the Deep Space antenna. Owing to the blazed grating concept, the unit cell was optimized for an incident elevation angle of $\theta = 30^{\circ }$ . The blazed architecture, the filtering synthesis, design details toward a 3-D-printing compatible solution, and equivalent circuit analysis of the unit cell are described in detail. Numerical results show that the proposed solution exhibits a very robust RF performance. The characterized reflection coefficient remains below −10 dB, the cross-polarization discrimination (XPD) above 16.5 dB, the low-band (i.e., $0.4f_{0}$ ) rejection below −26 dB, and the transmission losses below 0.5 dB for both TE and TM incidence over a wide frequency bandwidth ( $\approx 25$ %) and angular range ( $0^{\circ } \le \theta \le 40^{\circ }$ ). The simulation results of a linearly polarized horn that illuminates the proposed dichroic mirror have been verified by measurements, which included two prototypes fabricated in selective laser melting (SLM) using aluminum alloy. In light of the favorable performance, the proposed solution constitutes an appealing candidate for dichroic mirrors of Deep Space missions.

I. Introduction

Frequency selective surfaces (FSSs) are widely used in various microwave applications [1]. Among many implementations, FSS can be used as spatial filters providing low-pass, bandpass, high-pass, and bandstop frequency responses. In contrast, however, to microwave filters [2], the polarization of the electric field and the angle of incidence affect their frequency response.