IEEE Proof Web Version IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 5, MAY 2014 1 Width-Modulated Microstrip-Line Based Mantle Cloaks for Thin Single- and Multiple Cylinders Ladislau Matekovits, Senior Member, IEEE, and Trevor S. Bird, Fellow, IEEE Abstract—A thin mantle cloak is described for electromagnetic footprint reduction for arrays of thin cylinders. The cloaking is obtained by covering the conformal geometry with a metasurface having as a unit cell that is a width modulated microstrip line. En- foldings at different angles have been considered and numerically studied both for single and multiple cylinders. Frequency scala- bility and polarization sensitivity of the geometry has also been considered. Experimental results obtained at K-band are in good agreement with numerical data and demonstrate the applicability of the proposed approach for wide-band ( ) cloaking appli- cations. The performance of the proposed solution has been com- pared with available data in the literature, and its superiority has been demonstrated. Index Terms—Cloaking, conformal surfaces, metasurfaces, pe- riodic structures. I. INTRODUCTION T HE hiding of an object by electromagnetic cloaking has received considerable attention in recent years. Cloaking can be achieved by covering the object with a coating of dif- ferent material(s) than the object, on which the former locally modifies the propagation characteristics of nearby scattered waves. If properly chosen, it is expected that in the presence of the surrounding materials, the difference in the interference pattern between the scattered and direct field behind the object (with respect to the incidence direction) will be lower than in its absence. This reduction in the interference pattern opens the possibility of a range of applications in different fields of science such as radio-astronomy or medicine, security and others. Different schemes have been devised for the theoretical anal- ysis of cloaking. One approach involving transformation op- tics/electromagnetics [1] is based on a coordinate transforma- tion, which can expand or compress space around the object. Manuscript received June 18, 2013; revised September 24, 2013; accepted February 01, 2014. Date of publication February 21, 2014; date of current ver- sion May 01, 2014. This work was supported in part by the Marie Curie Alumni Association within a Micro Media Grant scheme and in part by a Marie Curie International Outgoing Fellowship within the 7th European Community Frame- work Programme. (Corresponding author: L. Matekovits.) L. Matekovits is with the Department of Electronics and Telecommunica- tions, Antennas, and EMC Laboratory (Laboratorio Antenne e Compatibilità Elettromagnetica—LACE), Politecnico di Torino, I-10129 Torino, Italy (e-mail: ladislau.matekovits@polito.it). T. S. Bird is with Macquarie University, Sydney, NSW 2109, Australia, and also with CSIRO Computational Informatics, Marsfield, NSW 2122, Australia (e-mail: ts.bird@ieee.org). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2014.2307587 The required coordinate transformation is usually geometry de- pendent, e.g., a carpet-cloak requires a quasi-conformal coor- dinate transformation, reducing the possibility of a general so- lution. In another approach, the method of field equivalence is used in [2] to reveal the boundary values of the fields on the inner and outer surfaces of a cloak that yield zero scattered fields outside the cloak and zero total fields inside the free-space cavity of the cloak. Although in theory many designs can be obtained, practical realizations have to take into account different problems related to the properties of the materials: the existence of materials with feasible values of permittivity and permeability—anisotropic spatial distribution—represents a major concern. However, such materials may not physically exist but can be created electro- magnetically through metamaterials. The use of metamaterial surfaces is one way of dealing with extreme cases such as par- tial covering. The thickness of the surrounding layers with re- spect to the dimensions of the object to be cloaked and achieving broadband reduction in the scattering signature are just some of the difficulties encountered when real-life applications are in- volved. The first work on the use of metamaterials for covering an- tenna structures dates back to the 1990s [3], where periodically repeated microstrip lines were used to cover holding masts in large aperture antennas to reduce their influence on the radia- tion pattern. Hard- and soft-surfaces were defined, which basi- cally mirror the response of the conformal surfaces to different polarizations of the incident electromagnetic field. Since then many other approaches have been proposed by different groups working in this area. The major advances in the field that have occurred are described by [4]. The recent literature is quite ex- tensive, e.g., discussion of numerical modeling [5] and experi- mental validations [6] of blockage reduction in antenna appli- cations have been presented. A multilayer cylinder was studied in [7], and in [8], an ultralow profile cloak was proposed for the suppression of scattering from a finite-length rod in free space. Recently a helix sheath winding was used to cloak a dielectric circular cylinder [9]. The numerical characterization of cloaked devices also presents difficulties, since the exact modeling of the geometry covered by multilayer cloaking structures over a wide fre- quency range requires considerable computational resources in terms of memory and CPU, as well as employment of ade- quate numerical tools. Numerical electromagnetic optimization has been used for bandwidth (BW) enhancement or material selection for example by [10]. A metasurface covering an object is known as a mantle cloak [11]. In this work, a surface pattern that consists of a 0018-926X © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.