Grounded frequency selective surface array as millimeter wave beam splitter

Abstract

ABSTRACT This paper presents the design, construction and testing of grounded Frequency Selective Surface (FSS) array as millimeter wave beam splitter. The phase dependence on slot length of grounded FSS demonstrates that the reflection phase of coherent mm-wave can be altered by using FSS array with different slot lengths. A beam splitter was designed with slot FSS array where the slot length is the main design pa rameter used to optimize the ph ase properties of the array. We simulated the FSSs with commercial CST Microwave studio software, fabricated them with etching technique and characterized with a free space MVNA an d BWO with motorized detector setup. Keywords: Frequency Selective Surface, Phase delay, Beam Splitter, Random Phase pattern, Speckle contrast, Stick FSS array, Millimeter Wave technology, Coherent sources. 1. INTRODUCTION Active free-space millimeter wave (mm-wave ) systems have gained more and more attraction during the last few years due to their indoor security application [1]. There are no incoherent mm-wave sources, and coherent illumination leads to speckle due to interference phenomena [2]. Random phase mm-wave illumination patterns can reduce speckle. One of the ways to produce random phase pattern is to introduce diffe rent phase delays (i.e. time delay) in the constant phase coherent mm-wave. Our investigations demonstrate that grounded Frequency Selective Surface (FSS) can be used to design such a system to get incoherent reflection from a coherent source. To explain the random phase distribution system we made a deterministic function realization to go inside the design. The beam splitter as the deterministic function splits the reflected beam into two directions which actually helps to prove the phase delay behavior of composite FSS array. One of the motivations of this work is to overcome the limitations of commercial simulation software to simulate non-uniform infinite FSS array. The simulation of an infinite array composed of finite stick arrays is not possible with commercial software. In that case individual FSS can be si mulated and to characterize the compose array some well known function can be realized. We presented the FSS array structure which splits the coherent beam in two directions. By realizing such structure actually we in vestigated how well the phase mixing process can be achieved from such a composite array compose of finite FSS arrays. Th is paper also considers the building block of a simpler and more versatile architecture where the reflection phase varies with the slot lengths of FSS cells. Rectangular slots were etched in Aluminum on top of grounded Roge r 4003C substrate to fabricate such FSS array. Using ground plane to the back side of FSS and also by choosing the proper slot dimensions and unit cell dimensions such structure can be designed for full W-band (75-110 GHz) application[3,4]. The resonant frequency and the phase of such slotted FSSs can be controlled by using different slot lengths [4, 5]. This property of the grounded FSS is important to design a beam splitter to split the coherent of mm-wave beam. So it is a novel idea to use FSS cells with different slot lengths to control the individual phase element of a beam splitter array. The beam splitter array is an antenna array which receives coherent plane wave and reflects after phase splitting as the array introduces different phase delay in the reflected wave in a controlled way. To design such a beam splitter array differen t phase delay elements are needed. In this paper the delay elements are replaced with FSS cells of different slot lengths. Micro-strip patch with variable stub length or patch with variable size as the individual delay element for such array has unacceptably high loss due to feed network. They are not suitable for free space app lications and require tight fabrication tolerances to achieve desired phase values (i.e. phase delays), as the patch size versus phase curves are extremely non –linear [6, 7]. By using thick substrate the phase slope of such design can be reduced but the quality factor (Q) and the total phase range decreases with the increasing of thickness. As explained in [8] the dual resonant response of a two