IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008 85 Half-Mode Substrate Integrated Waveguide (HMSIW) Leaky-Wave Antenna for Millimeter-Wave Applications Junfeng Xu, Wei Hong, Senior Member, IEEE, Hongjun Tang, Zhenqi Kuai, and Ke Wu, Fellow, IEEE Abstract—A novel leaky-wave antenna is demonstrated and developed at Ka-band in this work based on the newly proposed half-mode substrate integrated waveguide (HWSIW). This an- tenna is accurately simulated by using a full-wave electromagnetic simulator and then fabricated through a single-layer printed circuit board (PCB) process. Wide bandwidth and a quasi-omni- directional radiation pattern are obtained. The proposed antenna is therefore a good candidate for millimeter-wave applications. Measured results are in good agreement with simulated results. Index Terms—Half-mode substrate integrated waveguide (HMSIW), leaky-wave antenna (LWA), millimeter-wave antenna, quasi-omnidirectional radiation pattern. I. INTRODUCTION P RINTED leaky-wave antennas (LWAs) have been inves- tigated based on microstrip [1] or coplanar waveguide (CPW) [2]. The microstrip-based LWA may suffer from higher loss and surface-wave modes existing at millimeter-wave band. Recently, a substrate integrated waveguide (SIW) LWA was proposed in [3]. The SIW [4] or post wall waveguide [5] is a planar waveguide technology suitable for millimeter-wave ap- plications due to its advantages of easy manufacture, low cost, small size, low loss, and easy integration with planar circuits. A more compact guided wave structure called half-mode sub- strate integrated waveguide (HMSIW) has recently been pro- posed [6], [7], which preserves nearly all the advantages of SIW whereas its size is nearly reduced by half. In this letter, a novel LWA based on the HMSIW technique is presented. This an- tenna features wide bandwidth and a conical or quasi-omnidi- rectional radiation pattern suitable for millimeter-wave applica- tions, including road-vehicle communication [8], [9] and indoor wireless communication [10]. Similar radiation characteristics can be obtained in those printed antennas [1], [2], [10]. How- ever, the HMSIW LWA has its own advantages. It is fabricated on a single-layer PCB and no special precautions against sur- face-wave mode are needed compared to the antenna proposed Manuscript received January 14, 2008; revised January 25, 2008. This work was supported by the National Science Foundation of China under Grant 60621002. J. F. Xu, W. Hong, H. J. Tang, and Z. Q. Kuai are with the State Key Lab- oratory of Millimeter Waves, School of Information Science and Engineering, Southeast University, Nanjing 210096, China (e-mail: jfxu@emfield.org). K. Wu is with the Poly-Grames Research Center, Department of Electrical Engineering, Ecole Polytechnique (University of Montreal), Montreal, QC H3C 3A7, Canada. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2008.919353 Fig. 1. Configurations of HMSIW and HMSIW LWA. (a) structure of HMSIW and (b) configuration of HMSIW LWA (top view). in [1]. Also its structure is simpler and its design method is more direct compared to the antenna proposed in [2]. And its band- width is much wider than that of the antenna proposed in [10]. II. ANTENNA STRUCTURE AND DESIGN The center symmetry plane of an SIW can be equivalently re- garded as a magnetic wall when it operates with its dominant mode ( -like mode). Therefore, if the SIW is cut into two parts along the symmetry plane, each of the half SIW structures is called as the HMSIW which supports the half —like mode as shown in Fig. 1(a) [6], [7]. The parameters , , and denote via diameter, via period, substrate thickness and HMSIW width, respectively. The proposed LWA topology is shown in Fig. 1(b), in which a transition between 50 mi- crostrip and HMSIW is used for impedance matching with a width of and a length of . To improve the return loss, a section of HMSIW with gradually taperd width from to is adopted. The antenna is terminated by a 50 matching load. Key parameters affecting the LWA characteristics are inves- tigated specifically by using the EM simulation software CST. The substrate thickness has a noticeable impact on the far-field radiation pattern, depicted in Fig. 2. In H-plane there is a beam other than the main one emerging due to the small substrate thickness compared to the guided wavelength. The maximums of two beams become considerably close to each other with the thin substrate while they become imbalanced 1536-1225/$25.00 © 2008 IEEE