1536-1225 (c) 2018 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. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LAWP.2018.2842109, IEEE Antennas and Wireless Propagation Letters 1 Abstract—This letter presents a novel design of a uniplanar compact electromagnetic bandgap (EBG) for millimeter-wave (mm-wave) wearable antennas. The unit cell of the EBG has a flexible fractal design with self-similar window-like structure, which can be easily fabricated at millimeter-scale. The fabricated EBG is a 3×3 cell array laser-cut from adhesive copper foil on polyester fabric substrate. Results show that the gain and 10 dB bandwidth of a wearable coplanar waveguide (CPW) antenna backed by the proposed EBG are improved by 3 dB, and 40%, respectively, across the frequency range from 20 to 40 GHz. Backward radiation is also decreased by 15 dB, significantly reducing the potential health risk posed by the radiating antenna to the human wearer. Furthermore, on-body measurements show that the CPW-EBG antenna performances are not highly sensitive to human body proximity. Index Terms—electromagnetic bandgap, coplanar waveguide, fractal, millimeter-wave, wearable antenna I. INTRODUCTION HE millimeter-wave (mm-wave) spectrum is attracting an increasing interest due to the abundance of unused bandwidth that can be tapped to meet the demands of future wireless systems for wider bandwidth, lower latency and higher throughput [1]. On the other hand, wearable antennas have been more extensively studied at microwave than at mm-wave frequencies for various human-centric applications [2]. Recently, a wearable end-fire antenna has been developed for on-body communication at 60 GHz [3]. In [4], an inkjet-printed Yagi-Uda antenna was realized on flexible substrate at 24 GHz industrial, scientific and medical (ISM) band. However, these mm-wave wearable antennas suffer from either a high backward radiation (>5 dBi) that increases the radiation hazard to human users, or insufficiently wide bandwidth (< 20%) that may not satisfy anticipated future requirements. Electromagnetic bandgap (EBG) realized on a dielectric substrate has been popularly used for improving performance of microwave antennas. In [5], a textile-based double concentric square EBG is designed to increase the gain and reduce the backward radiation of coplanar waveguide (CPW) antennas at 2.45 and 5.8 GHz. However, this EBG design narrows the bandwidth of the antenna. An EBG based on a Moore space-filling (MSF) curve is proposed to increase the bandwidth of CPW-fed slot antenna [6], and decrease X. Lin, B.-C. Seet, and B. Li are with the Department of Electrical and Electronic Engineering, Auckland University of Technology, Auckland 1010, New Zealand (e-mail: {xiaoyou.lin, boon-chong.seet, brandt.li}@aut.ac.nz). F. Joseph is with the Textile Design Lab, Auckland University of Technology, Auckland 1010, New Zealand (e-mail: frances.joseph@aut.ac.nz). mutual coupling between the patch antennas [7]. However, these EBG geometries are inherently complex, which present challenges to their fabrication at millimeter-scale for mm-wave applications. Moreover, flexibility is essential for wearable devices since they have the requirements of conforming to the shape of the human body and providing softness for comfortable wearing. However, to the best of our knowledge, there is no published literature on using flexible EBG to enhance the performance of wearable antennas at mm-waves. Therefore, the main contribution of this letter is a novel design of a flexible uniplanar compact EBG with simple-to-fabricate fractal geometry for mm-wave wearable antennas. The performance of a CPW antenna backed by the proposed EBG is then investigated. Both the proposed EBG and CPW antenna are constructed of laser-cut adhesive copper foil on polyester fabric substrate. The frequency range of interest is in the mm-wave and quasi-mm-wave bands from 20 to 40 GHz, which covers the unlicensed ISM band (24 GHz), and two candidate frequencies for 5G cellular networks (28 and 38 GHz) [4, 8]. II. PROPOSED EBG AND ANTENNA DESIGN A. Material selection and fabrication technique In this work, a 0.35-mm-thick soft plain-woven polyester fabric is used as the substrate. As the dielectric properties of the fabric substrate are not known a priori, they are firstly characterized using the hybrid microstrip-line method [9]. In brief, the properties of a dielectric material covering a microstrip line can be derived from the variations of its scattering parameters. The substrate’s dielectric constant (r) and loss tangent (tanδ) are found to be 2.2, and 0.004, respectively, which are valid for the frequency range of interest from 20 to 40 GHz. Given the precision requirement of fabricating millimeter-scale radio-frequency (RF) structures, the use of conductive yarns or textiles is not recommended for constructing the conductive patterns [3]. Instead, a thin layer of copper foil with adhesive backing on a fabric substrate is laser-cut to the desired pattern and the excess copper is removed. It is reported that this technique can lead to a geometrical accuracy of 10 μ m while maintaining the flexibility of the created structure [3]. B. Proposed EBG and CPW antenna design Surface waves can significantly deteriorate the performance of an antenna such as its gain and bandwidth. The EBG is a high impedance periodic structure that can suppress the propagation of surface waves at certain frequency ranges, also referred to as stopbands of the EBG. The EBG structure can be modeled as LC Flexible Fractal Electromagnetic Bandgap for Millimeter-Wave Wearable Antennas Xiaoyou Lin, Student Member, IEEE, Boon-Chong Seet, Senior Member, IEEE, Frances Joseph and Brandt Li, Student Member, IEEE T