IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009 1367 Improved Cross-Polarization Characteristics of Circular Microstrip Antenna Employing Arc-Shaped Defected Ground Structure (DGS) Debatosh Guha, Senior Member, IEEE, Chandrakanta Kumar, Member, IEEE, and Surendra Pal, Fellow, IEEE Abstract—Application of defected ground structure (DGS) to suppress cross-polarized (XP) radiation from a microstrip patch antenna has been reinvestigated using a new DGS geometry for much improved characteristics. Arc-shaped defect has been used in pair, symmetrically located under a circular patch. A number of optimization parameters have been examined using simulated results, leading to a design indicating improved XP behavior. A set of identical prototypes, with and without DGS, have been experimentally studied. The presence of the DGS shows as much as 30 dB isolation of the XP level from its peak radiation, and that compared to an identical patch without DGS indicates an improvement by as much as 12 dB. The relative suppression in XP values are found to be around 7–12 dB over elevation around the boresight of the patch. Index Terms—Cross-polarized (XP) radiation, defected ground plane, defected ground structure (DGS), microstrip antenna. I. INTRODUCTION M ICROSTRIP patch antenna (MPA) has become a major component in most of the modern wireless and microwave communication systems. It, resonating in its fundamental mode, is supposed to radiate linearly polarized fields aligned in a plane of reference, designated as E-plane. However, some degree of orthogonally polarized fields is always associated with the radiating energy, resulting in cross-polarized (XP) radiation [1]–[3]. Experimental studies indicate considerable XP level in H-plane particularly, which typically appears around 15–20 dB below the peak gain. Considerable XP radiation appears to be a major limitation to frequency reuse in a microstrip radiator using polarization diversity. Different techniques to reduce the XP radiation from a MPA have been explored earlier, showing their individual merits in open literature [4]–[10]. Some investigations in [4] and [5] used dual feed with 180 differential phase between them. Modified feed geometry [6], [7] and shaped ground plane (GP) [8], [9] have also been explored to achieve reduced XP level, but all of them involve enhanced cost and complexity in fabrication. Manuscript received November 18, 2009. First published December 31, 2009; current version published January 12, 2010. D. Guha is with the Institute of Radiophysics and Electronics, University of Calcutta, Kolkata 700009, India (e-mail: dguha@ieee.org). C. Kumar and S. Pal are with the Communications Systems Group, ISRO Satellite Centre, Department of Space, Government of India, Bangalore 560017, India (e-mail: kumar_chk@yahoo.co.in; pal_surendra@hotmail.com). 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.2009.2039462 In an investigation [10] by the same group, they noticed that orthogonal resonance may cause considerably high XP radiation from a circular patch and also successfully employed defected ground structure (DGS) for the first time to suppress the same. A pair of small dot-shaped DGSs, each of diameter , was strategically located on the H-plane to weaken the orthogonal resonance and resulted in as much as 5 dB suppression in ei- ther plane. Although DGS appears to be advantageous and less expensive compared to other techniques mentioned, no further advancements or novelty in design have been explored so far. In this letter, a new DGS geometry has been explored for a special reason. A probe-fed circular patch is not always subject to inherent orthogonal resonance, and is thus not seriously af- fected by high XP radiation over E-plane. In such a case, the dot-DGS [10] is found to marginally interact with the fringing electric fields present on the H-plane, indicating minimal effect in reducing XP radiation. Therefore, an elongated arc-shaped defect extended over a relatively wider area following the patch boundary has been intuitively conceived and examined for im- proved XP in H-plane. The new geometry, schematically shown in Fig. 1, uses two arc-shaped defects symmetrically located in H-plane (i.e., yz, orthogonal to xz or E-plane) and having a relatively larger number of optimization parameters compared to that in [10] to obtain improved performance. Optimized design has been obtained using [11], and some representative measured results are presented. Suppression in XP level by about 12 dB is experimentally demonstrated for a circular microstrip patch (CMP) operating in C -band. II. THE DGS GEOMETRY The configuration of a CMP with a pair of arc-DGSs is shown in Fig. 1. The substrate chosen for the present investigation is RT-5870 having dielectric constant and thickness mil. For operation around 6 GHz, the patch radius is found to be 9 mm. The size of the ground plane has been chosen as being the free-space wavelength corre- sponding to the operating frequency of the patch. Two identical patches, one with DGS and the other with a normal GP, have been studied to examine relative improvement and as such, a unique matched location of the feeds has been maintained at mm to avoid any variation in XP radiation, whether in- fluenced by the feeding probe [3]. Two arc-shaped slots are etched on the GP symmetrically in the H-plane. The dimensions of the defect are determined by its angular breadth , width , and location . Relative change in XP values has been studied theoretically by varying the values of , and using [11]. For the present study, the values 1536-1225/$26.00 © 2010 IEEE