Tunable Electric-LC Resonators using Liquid Crystal Pouria Yaghmaee * , Withawat Withayachumnankul * , Ali K. Horestani * , Amir Ebrahimi * , Bevan Bates *# and Christophe Fumeaux * * The School of Electrical & Electronic Engineering, University of Adelaide, Adelaide SA, 5005, Australia # Defence Science and Technology Organisation (DSTO), Edinburgh, SA 5111, Australia Email: pouria@eleceng.adelaide.edu.au Abstract— A concept of tunable electric-LC (ELC) resonators is presented in this work. The voltage-controlled tunability is achieved by using liquid crystal in a micro-fluidic channel running through the central capacitive gaps. To attain the largest tunability, the structure is optimized through a parametric analysis using full-wave electromagnetic simulations. The simulation results predict a 6% continuous frequency tuning for this ELC resonator around the frequency of 4.5 GHz. The achieved results demonstrate the possibility of using these ELC resonators in an array to form a tunable frequency selective surfaces (FSS). The principle can be scaled for operation at higher microwave frequencies, where the dissipation of liquid crystal is low. I. INTRODUCTION Growing requirements in wireless communications have resulted in demand for tunable microwave devices in various applications, from body-centric communication antennas to filters and satellite communications. The ELC resonator has been discussed as a building block for metamaterials absorbers, reflectors, modulators [1], [2] as well as microwave filters [3] and sensors [4], [5]. In this work the anisotropic properties of liquid crystals in their nematic phase [6] are exploited as a voltage-controlled tuning mechanism in an ELC resonator array. The liquid crystals fill a channel running through the ELC resonator capacitive gaps, in an arrangement that maximizes the interaction of concentrated fields with the liquid crystal molecules. To enhance the tuning sensitivity, a parametric analysis is performed to optimize the parameters of the ELC resonator, and as a result a maximum frequency tuning of 6% is predicted using an available liquid crystal and conventional microwave materials. II. PROPERTIES OF LIQUID CRYSTAL Liquid crystals are anisotropic dielectrics materials with characteristics of both liquid and crystal states. They are widely available, can be integrated in flexible microwave printing technologies, and highly anisotropic under low bias voltage. The nematic phase, with thread-like shaped molecules, is a common form of liquid crystal. In this phase the application of an external bias voltage V across a liquid crystal cell can change the molecule alignment from perpendicular, (no bias voltage is applied, V = 0 V) to parallel state (with bias voltage above a threshold voltage V th , i.e. V >> V th ) with respect to the static bias field line direction. The resulting dielectric anisotropy can be defined as: ߝ∆ ௥ ൌ ߝ ௥ , צ െ ߝ ௥ , . (1) where ߝ ௥ ,  and ߝ ௥ , צ are respectively the perpendicular and parallel relative permittivity of the liquid crystal in the nematic phase. In practical designs, thin layers of rubbed polyimide coating (3-4 μm) are used to enclose the liquid crystal and define the molecule alignment in the unbiased state [7], [8]. III. ELC RESONATOR DESIGN The original ELC resonator consists of inductive loops and a capacitive gap (Fig. 1a). To design an ELC tunable array an original tunable design [9], including varactors and bias lines is modified to incorporate a 0.2 mm deep microfluidic channel inside the substrate between the capacitive plates (g = 0.15 mm). The channel is extended to above the central copper plates , as shown in Fig. 1. (a) (b) (c) Fig. 1. ELC resonator (a) Original cell, (b) Modified cell with liquid crystal in a micro-fluidic channel (shown in light blue). The dotted lines indicate the boundaries of the unit cell, (c) Side view - with liquid crystal (shown in light blue) and sealing Pyrex top cover. The two strips on both sides are for applying an external bias voltage. As in [9], additional thin conductor strips continuously connect every unit cell for applying an external bias voltage. A tapered strip width b is used in the central capacitance [10], which contributes to miniaturization of the design. A Pyrex cover is placed on top of the ELC resonator array ( ߝ ௥ = 4.82, Duroid 6002 Top cover