Modified Quasi-Yagi Antenna for Airborne Radar J. R. Brianeze, Arismar Cerqueira S. Jr., H. E. Hernández-Figueroa Microwaves and Optics Department, School of Electrical and Computer Engineering, State University of Campinas (UNICAMP). Albert Einstein Av., 400, 13083-852, Campinas – SP – Brazil Abstract  Synthetic Aperture Radar (SAR) systems, either airborne or in orbital platforms, are important tools to observe and monitor Earth surface. This paper presents an efficient solution based on a modified Quasi-Yagi (QY) antenna for airborne SAR systems operating at P-band. We have used a commercial electromagnetic simulator to fulfill the specifications of SAR systems. Simulations and experimental results have shown that the proposed antenna provides a maximum gain G = 7.5dB at 400 MHz. Key words  Antenna, Quasi-Yagi, radar, airborne. I. INTRODUCTION During the last years, embedded systems using SAR, either airborne or in orbital platforms, have been increasingly used to observe and to monitor Earth’s surface [1]. Operating at microwave frequencies, SAR systems provide images that show electrical and geometrical characteristics of a surface in virtually any climate condition, during the day or the night. A conventional SAR only measures the target localization in a bi-dimensional coordinate system. The application of interferometry techniques to these systems (InSAR) has allowed three dimensions measurements [1]. InSAR systems operating in different frequency bands can detect different and complementary characteristics from Earth surface. In vegetal covering and biomass studies, signals with frequency over 1 GHz does not efficiently penetrate the vegetation. On the other hand, signals in P band (0.3 to 1 GHz) allow a larger penetration, and consequently the detection of the terrain surface [2]. The correct choice of the antenna to be used in these systems is a very important point, because several of its parameters (such as polarization, gain, return loss and radiation pattern) are directly linked to the system performance. Microstrip antennas have been extensively used in SAR systems [3]-[4]. Despite they allow to work in two polarizations, they require many elements, as antenna arrays, in order to obtain a high directive radiation pattern. This strategy may demand a large physical space at lower frequencies. The Quasi-Yagi (QY) antenna was firstly presented in [5]. Nowadays it is very popular for combining the best characteristics from planar antennas, without compromising any important parameter [6]. This paper presents the design of a modified QY antenna operating in P-band, applied to airborne radars. The commercial electromagnetic simulator CST Microwave Studio has been used to evaluate the performance of our proposed prototype. Furthermore, we have fabricated and experimentally characterized it. II. DESIGN AND SIMULATIONS A QY antenna has the same basic structure of a Yagi- Uda antenna: an array of dipoles with a driver, directors and reflectors. The main difference between them is that the reflector is replaced by a truncated ground plane, under the substrate. Driver length is the main responsible for centering the antenna bandwidth at a desired frequency. Director and reflector have both the functions of steering the radiation pattern and refining antenna impedance match (the first at high frequencies and the second at low frequencies [7]). This contributes to the broadband characteristics of the antenna. A printed balun (a broadband microstrip – coplanar stripline (CPS) transition [8]) balances the antenna feed to its driver, without injury to antenna wide bandwidth. Finally, a CPS connects balun outputs to the driver. The radiated field of a QY antenna has a linear polarization, with a horizontally polarized electric field (in the substrate plane). A. Modified QY Antenna Our modified QY antenna is shown in Fig. 1. Some modifications were done to the original QY antenna model to use it in airborne radar, positioned externally to the aircraft. The microstrip section was bent in 90º, to remain pasted to the aircraft fuselage. The substrate around the CPS was reduced in order to make it lighter. Two directors were used to increase the antenna gain. For a better steering of the radiation patter in elevation, they were tilted around the driver. All this remodeling resulted in a 3D antenna. An J. R. Brianeze, jurbrian@dmo.fee.unicamp.br, Tel. +55-19-3521-5175; Arismar Cerqueira S. Jr., arismar@dmo.fee.unicamp.br, Tel. +55-19-3521- 5432; H. H. Hernández-Figueroa, hugo@dmo.fee.unicamp.br, Tel. +55-19- 3521-3735. This work was partially financed by CAPES and FAPESP. Fig. 1. Modified QY antenna, with main components.