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Chiou, Bandwidth enhance- ment of inserted microstrip-line-fed equilateral-triangular mi- Ž . crostrip antenna, Electron Lett 34 1998 , 21842185. 8. C.L. Mark, K.M. Luk, and K.F. Lee, Wideband triangular patch Ž . antenna, IEE Proc Inst Elect Eng 146 1999 , 167168. 9. F. Croq and A. Papiernik, Large bandwidth aperture coupled Ž . microstrip antenna, Electron Lett 26, 1990 , 12931294. 2001 John Wiley & Sons, Inc. A LOW-PROFILE CIRCULARLY POLARIZED CURL ANTENNA OVER AN ELECTROMAGNETIC ( ) BANDGAP EBG SURFACE Fan Yang 1 and Yahya Rahmat-Samii 1 1 Department of Electrical Engineering University of California at Los Angeles Los Angeles, California 90095-1594 Recei ed 29 May 2001 ( ) ABSTRACT: An electromagnetic bandgap EBG surface is utilized as a ground plane for a curl antenna to achie e a low-profile design as well as a circularly polarized pattern. The antenna height is greatly reduced compared to a normal curl antenna oer a PEC ground plane. The frequency bandgap and ground plane size are discussed. Experimental results demonstrate the applicability of this concept. 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 31: 264267, 2001. Key words: curl antenna; EBG; low profile; CP 1. INTRODUCTION Ž . The electromagnetic bandgap EBG surface, also referred to Ž . as a photonic bandgap PBG surface or a high-impedance  surface 1 , has demonstrated a good potential to build low-  profile and high-efficiency antenna structures 2 . Its in-phase reflection feature enables one to put wire antennas very close to the surface without losing radiation efficiency. In addition, its high surface impedance helps to suppress the surface wave. Therefore, an EBG surface is desirable to be used as an antenna ground plane. A curl antenna was proposed as a  simple radiator to generate a circular polarization pattern 3 . The design has been suggested for applications in wireless  communications such as GPS, LAN, and satellite links 4. However, it cannot function well when it is placed close to the finite PEC ground plane because of its reverse image current. In this paper, the performance of a square curl antenna over the EBG ground plane is investigated in depth. Contract grant sponsor: U.S. Army Research Office Contract grant number: DAAH04-96-1-0389 Compared to the conventional curl antenna, this design has an attractive low-profile structure, as well as a circularly Ž . polarized pattern. The finite-difference time-domain FDTD  method is used for analysis 5 . A curl antenna with only 0.06 height has been simulated, and it provides a good axial ratio of less than 3 dB. The frequency bandgap and EBG ground plane size are also discussed. Finally, some experi- mental results for return loss and axial ratio are presented to demonstrate the applicability of this concept. 2. CHARACTERISTICS OF CURL ANTENNAS OVER AN EBG SURFACE The configuration of the curl antenna over an EBG surface is displayed in Figure 1. It consists of two parts: an EBG ground plane and a square curl. The electromagnetic bandgap struc- ture is achieved by periodic patches connected to the ground Ž . plane through vias mushroom-like surface . The frequency bandgap is determined by the patch size, the width of the gap between the patches, the height, and the dielectric constant  of the substrate 2 . The square curl antenna is investigated in this paper because it is easily analyzed by the finite-difference Ž . time-domain FDTD method. The parameters of the curl are curl height h, curl radius R, and extended curl length L. The FDTD method is used to design the circularly polar- ized antenna at a GPS frequency of 1.57 GHz. A finite ground plane of 1 1 is kept for practical applications. Here, is the free-space wavelength at 1.57 GHz. The parameters of the EBG surface are carefully chosen so that the bandgap will cover the GPS frequency. The EBG patch size is 0.12 0.12 , and the gap between the patches is 0.02 . The substrate thickness is 0.04, and its dielectric constant is 2.20. The curl antennas are designed to obtain good circularly polarized patterns over a PEC or EBG ground plane. For the PEC ground plane, the optimized curl parame- ters are Ž. R 0.13, L 0.11 , h 0.231 and the optimized parameters of the curl over the EBG ground plane are Ž. R 0.14, L 0.08 , h 0.14. 2 Figure 2 compares the simulated axial ratio of these two antennas. Both antennas have good axial ratios at boresight, Figure 1 Configuration of a square curl antenna over an EBG surface. The EBG surface consists of periodic patches connected to Ž . the ground plane by vias mushroom-like surface MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 31, No. 4, November 20 2001 264