46 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 17, NO. 1, JANUARY 2018 Flexible and Portable Textile-Reflectarray Backed by Frequency Selective Surface Muhammad M. Tahseen and Ahmed A. Kishk Abstract—A center fed C-band portable and rollable textile- reflectarray (TRA) using frequency selective surface (FSS) is pre- sented. The radiating elements are embroidered on the textile samples using conductive thread. The TRA is made of 15 × 15 elements (8.25 × 8.25λ 2 at 5.8 GHz) for broadside radiations. The embroidered flexible FSS aims to replace the solid ground plane. The element size variation backed by the FSS provides a 330 ◦ phase variation, which is adequate to design the TRA. Wide- band multilayer stacked circular patches are utilized as a feed for the TRA. The antenna provides a measured 7.3% of 0.5 dB gain bandwidth, -15 dB sidelobe level, -25 dB cross polarization, and 29% maximum aperture efficiency. Index Terms—Conductive thread, flexible, frequency selective surface (FSS), portable, reflectarray (RA), textile-reflectarray (TRA). I. INTRODUCTION R EFLECTARRAY (RA) is an alternative to overcome the limited scanning capabilities of the parabolic reflector and the high losses of the cooperative feeding network of the planar array antenna. RA is inexpensive, lightweight, usually designed as a planar structure, and can provide wide scanning capability. However, the narrow bandwidth of RA is one its disadvantages [1], particularly with large reflectors and relatively small focal- to-diameter ratio (F/D). The RA elements are used to compensate the phase errors due to the varying spatial delay. Several methods are employed to perform the proper phase compensation. For linearly polarized (LP) RA, the methods employed are element size variations [2], the size-varying air vias [3], variable-size dielectric resonator antenna [4], and fragmented elements [5]. For designing circular polarized RA, the element rotation technique is used in [6] and [7], which shows some advantages as well as some trade offs concerning the limited bandwidth and the cross polarization. In [8], novel broadband elements have been used to increase the RA bandwidth. A progressive phase compensation technique “True Time Delay” is employed in [9] to increase RA band- width. Different FSS elements are analyzed in [10] to design FSS-backed RA while the element size varying tecd for phase compensation. For LP dual band, size-varying cross dipoles are Manuscript received October 6, 2017; revised November 9, 2017; accepted November 9, 2017. Date of publication November 13, 2017; date of current version January 10, 2018. (Corresponding author: Muhammad M. Tahseen.) The authors are with the Department of Electronics and Computer En- gineering, Concordia University, Montreal, QC H3G 1M8, Canada (e-mail: m_tahse@encs.concordia.ca; kishk@encs.concordia.ca). Digital Object Identifier 10.1109/LAWP.2017.2772919 used in [11], while two different-sized square rings are used as an FSS element to operate at each band. Similarly, for the circularly polarized (CP) dual band, an FSS-backed CP RA is designed in [12], where double circular rings are used as an FSS, and the slotted double circular rings are used as RA ele- ments at the front. The element rotation technique is utilized for phase compensation. In [13], it is demonstrated that correcting the phase errors due to the angle of incidence, the RA gain in- creases. A new FSS-based flexible RA is proposed in this letter. The radiating elements at the front side and FSS elements at the back side are embroidered on the textile material using conduc- tive threads. The flexible dielectric is durable and can be rolled, but it is difficult to return it back easily to the required planar form after rolling when it is of considerable size. On the other hand, fabrics can be folded and can also be stretched back to a planar form regardless of its size. Therefore, flexible antennas are preferred and designed extensively using textile materials for medical and wearable applications [14], [15]. Before designing an FSS-based textile-reflectarray (TRA) (FSS-TRA), the electrical properties of the textile materials have been extracted using the resonance method [17] and veri- fied with a ridge-gap waveguide method [16]. A textile element is designed based on the study presented in [18]. These textile materials are used to design the present RAs, which provide an advantage over nonflexible RAs, in portability and low cost. In the first TRA, designed in [19], a conducting copper tape was used as a ground (GND) plane. To ease the folding the rolling of the TRA, a conductive shielding fabric did not per- form as it was expected and the power penetrates through it. Therefore, the shielded fabric is replaced by a conducting tape that provided the required performance of the TRA in [19]. However, the conducting tape made it difficult to fold the TRA. Therefore, here, we solved this problem. The conducting tape can be replaced by a textile surface with conductive threads. This will be an expensive solution because of the large area required, and the folding will not be as easy because of the hardness resulted from the high thread density. However, replacing the TRA back by FSS periodic surface embroidered with conduct- ing threads will make it possible to have a complete, flexible TRA. A flexible and portable RA based on the textile material has been achieved and tested. II. FSS-BASED TRA ELEMENT DESIGN The RA element is of the size 0.55 × 0.55λ 2 . A comprehen- sive investigation of the element characteristics of the RA using textile materials is explained in [18]. Here, Sample 1 and Sample 1536-1225 © 2017 IEEE. 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