IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008 229 Enhanced Gain Patch Antenna With a Rectangular Loop Shaped Parasitic Radiator Bahadir Yildirim and Bedri A. Cetiner, Member, IEEE Abstract—A new method to increase the gain of a conventional microstrip patch antenna is presented. A rectangular loop shaped parasitic radiator placed a specific distance away from the patch surface increases the gain by about 3.3 dB. The impedance and ra- diation performances of the proposed antenna are presented with excellent agreement between simulated and measured results. Index Terms—Electromagnetic analyses, gain measurement, mi- crostrip antennas. I. INTRODUCTION I N ORDER TO generate a compact, low-profile, lightweight, and low cost receiver architecture, the antenna elements must be suitable for integration with monolithic microwave integrated circuits (MMICs). Microstrip patch antennas with low profile and planar geometries are suitable for integration. However, there are two major disadvantages associated with patch antennas; low gain and a narrow bandwidth. In recent years, various approaches have been proposed to improve the gain and bandwidth of microstrip patch antennas. An extensive survey of all such techniques has been covered in [1]. These techniques mainly rely on stacked patch configurations, where the source driven patch interacts with the parasitic patch placed strategically on top of the fed patch. When the parasitic patch is closely located to the fed patch, the stacked antenna geometry has two near-resonant frequencies, the combination of which results in improved bandwidth performance. When the distance between the patches is approximately one half-wavelength, these patches form a leaky resonator whose resonant fields enhance the antenna’s gain [2]. A gain enhancement method relying on a substrate-superstrate resonance technique, where multiple superstrates with a quarter wavelength thickness are arranged in a prescribed manner, has also been proposed [3]. Yet another method that is based on an electronically tunable dielectric superstrate to improve the radiation characteristics of patch antennas was also demonstrated [4]. For antenna array applica- tions, patch elements placed into a surface mounted horn frame providing 3.5 dB improvement in gain have been reported [5]. The most critical factor, regardless of which of these various techniques or combination of which is employed, is not to lead to a bulky structure, thereby causing the loss of the very basic Manuscript received March 4, 2008; revised March 24, 2008. This work was supported in part by the National Institute of Justice under Grant 2007-IJ-CX- K025. B. Yildirim is with the Department of Electrical Engineering, Fatih Univer- sity, 34500 Istanbul, Turkey (e-mail: byildirim@fatih.edu.tr). B. A. Cetiner is with the Electrical and Computer Engineering Department, Utah State University, Logan, UT 84322 USA (e-mail: bedri@engineering.usu. edu). Digital Object Identifier 10.1109/LAWP.2008.922313 Fig. 1. Cross section of the proposed patch antenna with the rectangular loop shaped parasitic radiator: – (dimensions are in millimeters). Fig. 2. Top view of the patch antenna. The antenna is in the plane: (dimensions are in millimeters). advantage of using microstrip patch antennas. It is also impor- tant to employ material layers with practical permittivity and permeability values without making the antenna susceptible to surface wave excitation. In this paper, we present a new and simple method of in- creasing the gain of a conventional microstrip patch antenna. This technique relies on a rectangular loop shaped parasitic ra- diator which is suspended on top of the antenna. The thickness of the air gap, which is sandwiched between the fed patch and the parasitic radiator, is carefully adjusted to obtain the best gain performance at the operating frequency of 1.6 GHz. Both the theoretical and experimental results show that the gain of the conventional antenna is increased from about 4.5 dB to 7.8 dB by employing this technique. While the thickness of air layer of 30 mm provides the greatest gain improvement, a thickness of 1536-1225/$25.00 © 2008 IEEE