Design of FSS for Wideband and Wide-Angle Coverage—Challenges and Possible Solutions Raj Mittra 1,2 and Nathawut Homsup 1 1 EMC Lab, University of Central Florida, Orlando, FL, USA 2 EE Department, KAU, Saudi Arabia [email protected]Abstract – This paper examines the challenges encountered in the process of designing Frequency Selective Surfaces (FSSs) for applications that call for simultaneous coverage over very wide frequency band as well as wide angle. The paper begins with a review of existing literature and concludes that none of them can provide, even with the suggested modifications, a design that meets our needs of 3:1 bandwidth over an angular range of 60deg or higher. Next, we propose some new designs and examine how well we can meet the stated goals Index Terms — Frequency selective surfaces; Wideband FSS; FSS with wide-angle coverage. 1. Introduction Even though there are literally hundreds of publications, both journal papers and books which provide the theory of operation of Frequency Selective Surfaces (FSSs), designing them remains a challenging task when the specifications call for coverage over a wide band of frequencies, say 3:1, and over a wide range of angles going up to 70 0 , as is sometimes desired. What makes the design problem even more challenging is that while many publications describe FSS designs over a relatively wide band, say 2:1, which cover angles up to 30 0r 40 degrees (at most), they do not provide a clue as to how to extend these limits of their designs. In this work we describe the results of parametric studies that we have carried out of a number of existing wide-band and wide-angle FSS designs in an attempt to cover the following specifications: a. Passband: Frequency range f 1 to f 2 (ratio of f 2 /f 1 equal to 3:1) b. Coverage up to 60 deg., or higher TE and TM polarizations c. Low ripple in the passband d. Better than 10 dB in the stopbands, at both low and high ends, past the shoulder regions. It is interesting to note that the strategies followed in these works, whose designs we have investigated, are quite different from each other and, hence, the proposed physical structures are also somewhat unique to these designs, despite the fact that their stated goals for meeting the wideband and wide-angle coverage are the same. We did find several papers which proposed a universal approach (see [1], for instance), based on filter theory, which has been well developed for microwave circuits, and has been extensively used for waveguide filters for satellite communication. However, we found that it is not possible, in general, to transition from lumped circuits to distributed circuits representing FSSs, especially in designs where the higher-order Floquet modes are considered, as they must be in the FSS designs we have studied. Thus, while the filter theory was somewhat helpful in leading us to the right direction, we found that it was quite inadequate for our designs; hence it was necessary to develop new design strategies, which we will describe in the presentation, and which enabled us to extend the boundaries of the existing designs to help our stated goals. Fig. 1. Third Order Frequency Selective Surface (FSS). (a) (c) (b) Fig. 2. A unit cell of Frequency Selective Surface (FSS) (a) Top view, (b) Bottom View, and (c) Isometric view. 2018 International Symposium on Antennas and Propagation (ISAP 2018) October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea [ThG1-1] 209
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Design of FSS for Wideband and Wide-Angle
Coverage—Challenges and Possible Solutions
Raj Mittra1,2
and Nathawut Homsup1
1EMC Lab, University of Central Florida, Orlando, FL, USA