Design and Optimization of Reconfigurable Inset-Fed Microstrip Patch Antennas with High Gain for Wireless Sensor Networks Duy-Thach Phan, Gwiy-Sang Chung School of Electrical Engineering, University of Ulsan San 29, Mugerdong, Namgu, Ulsan 680-749, Republic of Korea E-mail: duythachdtvt@yahoo.com, gschung@ulsan.ac.kr Abstract— In this paper, a tunable microstrip patch antenna designed using RF MEMS switches is reported. The design and simulation antenna were performed using high frequency structure simulator (HFSS). The antenna was designed in ISM Band and operates simultaneously at 2.4 Ghz and 5.7 Ghz with a -10 dB return-loss bandwidth of 20 Mhz and 180 Mhz, respect- tively. To obtain high efficiency and improve integrated ability, the High Resistivity Silicon (HRS) wafer was used for the antenna. The antenna achieved high gain with 8 dB at 5.7 Ghz and 1.5 dB at 2.4 Ghz. The RF MEMS DC contact switches was simulated and analysis by ANSYS software. Keywords-Reconfigurable antenna; RF MEMS switches; wireless sensor network; inset-fed patch antenna. I. INTRODUCTION With fast growth of wireless sensor network, many special features of antennas are demanded for new applications. To be effective and have the ability to adapt itself to complex environment, sensors should have some ability self-powered, integrated antenna or reconfigurable antenna. Application of sensor will be highly facilitated if cheap and easy-to-use ‘on- chip’ or ‘in-package’ solutions, so many small antenna types have been proposed for wireless sensor node [1-2]. Because the gain of antenna proportion to size substrate so these small antenna have low gain, which were reported -8 dB at 2.4 Ghz and -2 dB at 5.7 Ghz [1] and leading to short-range wireless communication. In another way with this trend, the antenna in this paper was designed in high gain owing to synthesizing multi-substrate and micromachining technology. The high gain antenna with large size can apply to reader system, which will collect data from sensors via wireless communication. Micro- electromechanical system (MEMS) is a new technology, having the advantages of small volume, light weight, wide frequency-band, low insertion loss, excellent reliability, easy integrated with micro wave circuits, and so on. MEMS switch able design known as a new technology is applied in antennas. With MEMS switches the parameters of an antenna can be controlled as one wanted. The tunable frequency range is about 360 Mhz with required DC voltage in range 0-116 V in [3] or 0.8 to 15 percent of nominal operating frequency in [4]. These antennas tune operating frequency based on change value MEMS varactor located at one of the radiating edges in [3-4] so tuning value is often small. The second type of antenna can tune clearly between two frequencies owing to changing geometry, which is turned ON or OFF by MEMS switches, were reported in [5-6]. In this paper, the second type was chosen. II. ANTENNA DESIGN AND MODELING Microstrip antenna are one of the most widely used antenna types having many advantages are low cost, light-weight, small size, integrability and simple feeding network by microstrip line. The microstrip antenna was designed on silicon substrate to improve integrated ability sensor in same substrate. High index substrate materials such as Si ( ε =11.9) allow smaller overall circuit and antenna size but it is easy excite surface wave and results reduce bandwidth and radiation efficiency. This problem was solved by micromachining technology, which will synthesize silicon substrate with low index substrate or air cavity [7]. The structure of antenna in simulation was shown in Fig. 1. The relative dielectric constant of multilayer can be written as follow [7] : 1 1 1 ( ) .( ) N N n r n n n n h h ε ε - = = = ∑ ∑ (1) Where h n , ε n are thickness and relative dielectric constant of n th dielectric layer, respectively. With silicon substrate h 1 =0.5 mm, ε 1 =11.9 and glass substrate h 2 =1 mm, ε 2 =4.6, the equivalent permittivity silicon/ glass substrate was estimated ε r =5.8 in Eq. (1). Another drawback of silicon substrate in microstrip antenna is loss on substrate caused by silicon conductivity which can be reduced by using High Resistivity Silicon (HRS) or thin insulator material SiO 2 in [8]. According to above mentioned reasons, in this paper the microstrip patch antenna was design on HRS/glass substrate. The operating frequency is depended mainly on patch geometry and dielectric constant of substrate. The back air cavity has been etched on glass substrate to enhance performance of antenna and also control slightly resonance frequency in some circumstances. The size of patch in this paper can be determined by Eq (1) and following Eqs. (2)-(5) in [9] and marked in Fig .2. This work was supported by the grant No. B0009720 from the Regional Technology Innovation Program of the Ministry of Knowledge Economy (MKE). Corresponding author: G. S. Chung, e-mail: gschung@ulsan.ac.kr U.S. Government work not protected by U.S. copyright 1