Proceedings of IEEE 2008 6th National Conference on Telecommunication Technologies and IEEE 2008 2nd Malaysia Conference on Photonics, 26-27 August 2008, Putrajaya, Malaysia Phase Range Analysils of Reflectarray Patch Cells Prilnted on LIiquid Crystal Substrate Yusof Ismail, Ahmad Faizal M. Zain, Jiwa Abdullah Faculty of Electrical & Electronics Engineering, Unniversity oF Tun Hussein Onn Malaysia, P.O. Box 101 86400 Parit Raja Batu Pahat, Johore Malaysia E-mail: u uthm.edu Abstract - In this paper we demonstrate that the anisotropic property of liquid crystal can be exploited to control the phase of signals that are reflected from a reflectarray cell. Numerical and measured results at X-band are used to compare the plane wave scattering from two reflectarray cells which are constructed on liquid crystal film of thickness 200 gm and 500 gm. The phase agility, bandwidth and reflection loss are shown to be dependent on both the thickness and the voltage controlled permittivity of the tunable substrate. A tunable phase range greater than 2500 is achieved over a 6.1 % bandwidth. Index Terms -reflectarrays, liquid crystals, phased array antennas, non linear dielectrics. I. INTRODUCTION A reflectarray antenna consists of a planar array of radiating elements which are printed on a grounded substrate and illuminated by a primary feed source [1]. The dimensions of the individual patches are designed to synthesize a progressive reflection phase shift across the aperture in order to create a plane wavefront. Although many aspects of the antenna performance are superior to microstrip arrays and conventional reflector antennas, the main disadvantage of the reflectarray is the limitation that is imposed on the bandwidth of the radiating structure [2]. In the past five years there has been considerable interest in exploiting the anisotropic property of nematic state liquid crystals (LC) in order to produce tunable microwave components [3], [4]. These can be integrated into single material substrates thus combining the key benefits of full integration, low weight and cost and continuous fast electronic control using a small DC or low frequency AC bias voltage. More recently proof of concept studies have demonstrated the possibility of creating a phase agile reflectarray antenna [5]-[7], where the reflected signal is phase modulated by a thin LC film which is inserted between the ground plane and the periodic array. This feed strategy can be used to overcome the instantaneous bandwidth limitation and/or to create phase shifting devices for electronically beam steered reflectarray antennas. In this paper we use numerical results at X-band to study how the anisotropy value and the thickness of the LC film impact on the dynamic phase range, bandwidth and the loss of the reflected signal. Waveguide simulator measurements are employed to validate the computer model using two reflectarray cells structures which are filled with commercially available liquid crystals. II. PROPERTIES OF LIQUID CRYSTAL Liquid Crystals exhibit mesophase between the physical transitions from the solid crystalline phase to the isotropic liquid phase [3]. When a thin LC film is inserted between two conductors, a thin rubbing layer can be used to produce the electrostatic attraction which is necessary to align the molecules parallel to the conductors [4]. However the torque which is needed to rotate the molecules perpendicular to the surfaces can be obtained by applying an electrostatic field in this region. For a reflectarray cell the permittivity will therefore vary between two extreme values ie, c11 when the fringing field is parallel to the one dimensional LC molecules and _1 when these are oriented perpendicular to the RF field. The effective dielectric anisotropy is defined as: AFc= c11 - ci (1) The permittivity of the tunable dielectric layer and hence the electrical size of the patches can therefore be controlled by varying the voltage that is applied between each patch element and the ground plane. This produces large changes in the phase of the reflected signal around the resonant frequency of the patches. The maximum phase range at a given 978-1-4244-2215-9/08/$25.00 ©2008 IEEE. 140