Wedge-Shaped Dual Planar Log-Periodic Antenna with Enhanced Directivity for WiMAX Applications Stamatios A. Amanatiadis, Antonios X. Lalas, and Nikolaos V. Kantartzis Department of Electrical and Computer Engineering Aristotle University of Thessaloniki GR-54124 Thessaloniki, Greece e-mail: kant@auth.gr Abstract—A dual planar log-periodic antenna with a significantly improved directivity over existing setups is presented in this paper for high-speed WiMAX applications. The novel structure consists of four log-periodic arms at a wedge pattern, which are located in pairs on the same plane to preserve the fabrication simplicity of the single-arm configuration. For its radiation characteristics, a thorough investigation via an efficient finite-difference time- domain method is conducted. To this end, the promising behavior of the proposed antenna is compared with that of a regular planar LPA, while an instructive study of directivity enhancement, with- out degrading the other radiation features, is also performed. Keywords-log-periodic antennas; WiMAX applications; planar structures; directivity; FDTD method I. INTRODUCTION Contemporary communication systems, such as satellites, radars or even television, require wide frequency bandwidths to accomplish their high-end goals. Hence, a large variety of pro- tocols, standards and efficient algorithms has been hitherto pre- sented. Amid them the WiMAX (worldwide interoperability for microwave access) technology, has recently gained impressive recognition [1], [2]. In essence, WiMAX represents an entire technology for wirelessly delivering high-speed internet services to large geographic regions, through very compact devices. Ex- hibiting a relatively uniform input impedance and radiation characteristics over a broad frequency range, the log-periodic antenna (LPA) is deemed ideal for the prior applications [3]-[5]. In its simple rendition, an LPA comprises variable-size di- poles, whose dimensions determine the response bandwidth. Despite their versatility, typical LPAs are often proven inade- quate, due to the constantly raising quality needs. Thus, it is the objective of this paper to introduce a novel LPA for broadband WiMAX systems with high directivity requirements. The planar antenna is constructed by four log-periodic arms, which are in pairs located on the same substrate side in a dual way to form a wedge shape. So, the radiator is unidirectional and offers ad- vanced directivity levels of the main lobe, well above the typical values of conventional LPAs. It should be stressed that the pro- posed design does not affect the other radiation characteristics, retaining its fabrication simplicity, as well. These advantages are substantiated by means of a rigorous finite-difference time- domain (FDTD) method for several realistic implementations. II. DESCRIPTION OF THE WEDGE-SHAPED DUAL LPA The new antenna consists of four single-arm LPAs ar- ranged according to a wedge pattern. Each of the arms is desig- τ d d τ α y x σ Figure 1. Top view of the dual planar LPA (dark grey arms are on the front side and light grey arms on the back side of the substrate). ned by placing similarly-shaped elements in electric contact with a central conductor. Following the log-periodic concept, adjacent elements differ in their dimensions by a constant scale factor IJ, while their distance is specified by a spacing factor ı. In this manner, every arm of the antenna has an opening angle IJ α . The relation of the above parameters is given by 1 4 tan       . (1) Consequently, only two of them are necessary to define the log-periodic radiator. Furthermore, the maximum and mini- mum length of the elements, l max and l min , respectively, deter- mine the overall response bandwidth of the antenna. x y z h Figure 2. A perspective view of the dual planar LPA and its substrate. Subsequently, the four log-periodic arms are placed in pairs on the front and back side of a substrate, where the angle be- tween the arms of the same side is IJ d , as depicted in Fig. 1. To enhance the radiation efficiency of the antenna, the two pairs are arranged in a dual regime with respect to each other (dark and light grey arms in Fig. 1), thus forming a wedge-shape setup. Moreover, distance d between the arms of both pairs is selected to be constant in order to avoid undesired coupling or crosstalk effects due to the proximity of nearby elements. The device is fed by a port located at the narrow apex of the arms. A perspec- tive view of the structure is shown in Fig. 2 for a substrate of thickness h. The key asset of our antenna is that the resulting