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PATTERN RECONFIGURABLE SPIRAL ANTENNA AND ITS CURRENT DISTRIBUTION Amit Mehta, 1 Dariush Mirshekar-Syahkal, 2 and Hisamatsu Nakano 3 1 Institute of Advance Telecommunications, University of Wales- Swansea, SA2 8PP, United Kingdom; Corresponding author: a.mehta@swan.ac.uk 2 Department of Electronics Systems Engineering, University of Essex, Colchester, CO4 3SQ, United Kingdom 3 Department of Electronics Informatics, Hosei University, Koganei, 184-8584, Tokyo, Japan Received 14 March 2007 ABSTRACT: Analysis of spiral antenna with open circuits on its arm is presented for beam adaptive applications. It is shown theoretically and ex- perimentally that when the open circuits are appropriately positioned in the arm, the current distribution varies, causing the beam to steer. This effect can be exploited to design various novel beam switchable antennas. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2567–2570, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22738 Key words: spiral antenna; beam steer-ability; switched beam antennas 1. INTRODUCTION A beam steerable antenna has the potential to improve the overall wireless communication system performance. In mobile voice communication, it can ensure that the link is always intact and locked on to the maximum signal direction, irrespective of the user movements. It can also facilitate the system to steer away from electronic jamming, noisy environments, and improving sur- face absorption rate, effective isotropic radiated power, etc. Presently, some smart antenna systems for the base-stations using phased arrays are being investigated and tested [1, 2]. However, the same cannot be implemented in handsets or small wireless transceivers because of limited space, processing power, and cost. To accomplish this, an innovative spiral antenna design is proposed in this paper. Single arm rectangular spiral antennas radiate a tilted beam (beam cock) from the active regions [3]. In [4, 5], the authors showed that a single element spiral antenna, can provide steerable radiation pattern under electronic control without need of a com- plex feeding network. The direction of the antenna beam is con- trolled through a set of switches shorting the spiral arm to the ground plane at selected points. In [6, 7], an implementation of MEMS in the arm of single and multi-turn adaptive beam spiral antennas has been successfully demonstrated. In this paper, an investigation is presented encompassing the current distribution and radiation pattern for the spiral antenna with open arm at two locations. Qualitative reasoning approach is then offered for pattern reconfigureability. A total of three antenna configurations are analyzed, namely, SPRL (spiral antenna with no switching element activated), One-point open spiral (1 switching element activated) and Two-point open spiral (two switching ele- ments activated). The antenna characteristics in the S band, including the radia- tion pattern, gain, and VSWR, are obtained using the finite- difference time domain method, and are validated experimentally. 2. ANTENNA CONFIGURATION Figure 1(a) shows a single arm rectangular spiral antenna, similar in shape to that in [5]. The substrate for the antenna is composed of two dielectric layers (for ease of construction) stacked together. The substrate has side length L = 51.3 mm, net thickness h = 12 mm and effective dielectric constant  r,eff = 3.41. The spiral is composed of multiple straight filaments, with the first filament end-to-end length a = 4.5 mm. The total arm length of the spiral conductor is  = 291 mm and its width is w = 1.35 mm. The antenna is fed by a coaxial line from point o. The diameter of the feeding conductor is d = 1.3 mm. The junction point of the top end of the feeding conductor and the starting point of the spiral arm is referred to as o'. Two open circuit switches SW1 and SW2 are located at dis- tance of 125.3 and 155.3 mm from the starting point of spiral arm, o'. When both switching elements are in deactivated state (off), i.e., (SW1, SW2) = (off, off), the antenna configuration is referred to as SPRL (Fig. 1). The antenna configurations with (SW1, SW2) = (on (activated), off) and (SW1, SW2) = (on, on) are referred to as One-point open spiral and Two-point open spiral, respectively. In the experimental study, to simulate the short circuiting (off) action, the gap is physically short-circuited by soldering the two parts of the spiral arm at the position of a switch. Figure 2 shows the prototype of the One-point open spiral and the Two-point open spiral configurations. In this study the open circuit gap is 1-mm wide. DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 10, October 2007 2567