Life Science Journal 2012;9(4) http://www.lifesciencesite.com http://www.lifesciencesite.com lifesciencej@gmail.com 4196 A Novel Miniaturized Microstrip Low-pass Filter with Wide stopband using a Modified Hairpin Resonator Hossein Shahbazitabar 1 , Mohsen Hayati* 2 , Fardad Farokhi 1 1 Electrical Engineering Department, Faculty of Engineering, Islamic Azad University Central Tehran Branch, Tehran, Iran 2 Electrical Engineering Department, Faculty of Engineering, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran *Corresponding Author: mohsen_hayati@yahoo.com, Tel +98-910-2911161 Abstract: In this paper, a novel miniaturized low-pass filter (LPF) with wide stopband and low insertion loss using a modified hairpin resonator is presented. By optimizing the dimensions of the modified hairpin resonator and addition of a symmetric semi-circular open-end unit as a suppressing cell, the undesirable response and unwanted harmonics are suppressed; hence, a wide stopband from 5.94 GHz to 32.27 GHz is achieved. The transition band is 0.4 GHz from 5.54 GHz to 5.94 GHz with the attenuation level of -3 dB and -20 dB, respectively. The proposed LPF with -3 dB cut-off frequency of 5.54 GHz is designed, fabricated and measured. The size of the fabricated low- pass filter is only 73.84 mm2. The simulated results are compared with the measured results and good agreement between them is obtained. [Hossein Shahbazitabar, Mohsen Hayati, Fardad Farokhi A Novel Miniaturized Microstrip Low-pass Filter with Wide stopband using a Modified Hairpin Resonator. [Life Sci J 2012;9(4):4196-4203]. (ISSN: 1097-8135). http://www.lifesciencesite.com . 626 Keywords: Hairpin resonator, Low-pass filter, Wide stopband 1. Introduction Microstrip low-pass filters have been widely used to suppress harmonics and spurious signals in communication systems. Compact size, low insertion loss, sharp roll off, and wide stopband are highly desirable factors to design the LPFs. One of the main microstrip structures to design a LPF is the hairpin structures, which has benefits such as easy fabrication [1]. A LPF with wide stopband using coupled line hairpin unit has been presented in [2]. In this filter, the insertion loss and the return loss in the passband and the attenuation level in the stopband are not adequate. In [3], a LPF with the shunt open stubs at the feed points of a center feed coupled line hairpin resonator has been proposed with very wide stopband and sharp response, but the rejection level and the S 11 parameters in the stopband are not satisfactory. The conventional stepped-impedance filter, with hairpin resonator, in [4] has a simple structure, but it provides a gradual response in the transition band. In [5] a low-pass filter using stepped impedance hairpin units has been proposed, which has a low insertion loss, but it is not compact and has a gradual transition band, also it does not have a wide stopband. In [6], a LPF based on microstrip coupled-line hairpin unit, spiral slot and open stubs has been presented. It has a compact size with an approximate wide stopband, but its frequency response is gradual in the transition band. In this paper, a hairpin resonator is modified by open-end step impedance stubs to have a sharp response with -3 dB cut-off frequency of 5.54 GHz. To suppress the harmonics in higher frequencies a symmetric semi-circular unit is added to the proposed resonator. The result is a compact wide stopband LPF with optimum specifications such as low insertion loss less than 0.13 dB and good return loss better than 15.8dB, in the passband. 2. Filter design In the first step, an appropriate prototype elliptic function hairpin low-pass resonator is selected as shown in Figure 1(a).The simulated S-parameters of the proposed hairpin resonator, which is designed to