NEW MICROSTRIP FRACTAL-SHAPED SQUARE PATCH RESONATORS FILTERS Dong-sheng La, Ying-hua Lu, Ning Liu, Dong-nuan Cui, and Jin-ling Zhang School of Electronic Engineering, Beijing University of Posts and Telecommunications, Beijing 100876, China; Corresponding author: ladongsheng@163.com Received 24 November 2010 ABSTRACT: This article proposes and optimizes two types of band- pass filters using fractal-shaped square resonators. The filters have many remarkable advantages by using etched fractal-shaped defected structures in the square patch resonators, such as compact size, high selectivity, and simple structure. The resonant characteristics of the filters are reported and can be adjusted by changing the parameters of the fractal-shaped structures. Good agreement between the simulation results and the measurement results is achieved. V C 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:1950–1952, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26212 Key words: microstrip band-pass filter; fractal-shaped; square patch resonator 1. INTRODUCTION A compact and high performance microwave band-pass filter is highly desirable in wireless communications. Wolff has proposed the planar dual-mode resonator for the first time in 1972 [1]. The dual-mode concept is based on the physical coupling of two degen- erate modes in a geometrically symmetrical resonator. Because each of the dual-mode resonators can be used as a doubly tuned res- onant circuit, it leads to a compact filter configuration [2, 3]. Dual- mode microstrip resonators and filters have many advantages, such as low cost, easy fabrication, compact size, and high selectivity. Some slots have been proposed to reduce the circuit size [4–7]. Now, some reports have proposed that the fractal-based geometries can enhance the transmission energies [8, 9] and thus could be used to design antennas or filters. A microstrip coupled-line band-pass filter is designed using a Koch fractal shape in Ref. 10. Microstrip filters with fractal-shaped triangular patch resonators and sierpin- ski-based fractal resonators are proposed in Refs. 6 and 7. In this article, we propose a new class of band-pass filters using fractal-shaped square patch resonators. There are no cou- pling gaps in these filters structures. The I/O feed lines are cho- sen for the characteristic impedance of 50 X for a typical micro- strip line. Compared with the filters in Ref. 6, the structure is simple and can reduce the uncertainty in production. Two types of compact band-pass filters by using different fractal-shaped square patches are designed, fabricated, and measured. The mea- surement results agree well with the simulation results. 2. FILTER DESIGN The geometry of the new band-pass filter is illustrated in Figure 1. An identical smaller square defected structure is etched in metal square patch. The permittivity of the substrate for simulation is 10.2 with thickness of 0.635 mm. The line width w ¼ 0.55 mm is chosen for the characteristic impedance of 50 X for a typical microstrip line. The parameters of the structure are a ¼ 16 mm, b ¼ 7.725 mm, c ¼ 7 mm, d ¼ 1 mm, and a1 ¼ 6 mm. The simulation results are realized by finite element method based on the software HFSS 10.0. The reso- nant characteristics of the filter are shown in Figure 2. To investigate the filter deeply, the effect of structural pa- rameter a1 is analyzed in detail. By adjusting parameter a1, we get the resonant frequency in three cases. Figure 3 shows the Figure 1 Layout of the filter using one fractal-shaped defected struc- ture (metal regions are in gray) Figure 2 The simulation resonant frequency of the filter Figure 3 The simulation resonant frequency for various a1 1950 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 9, September 2011 DOI 10.1002/mop