This paper presents a dual-band band-pass filter using modified cross-coupled step-impedance and capacitively loaded hairpin resonators for WLAN systems. The proposed filter has been designed to operate at a fundamental frequency of 2.4？GHz and the first harmonics frequency of 5.2？GHz. The techniques of step impedance and load capacitor are combined in the design of the proposed filter. In particular, the techniques of modified cross-coupling and overlap resonators are applied to improve the response of insertion losses at the first harmonic frequency of 5.2？GHz. The simulated and experimental results of insertion losses and return losses are better than 3？dB and 20？dB, respectively, at the operating frequencies. 1. Introduction Wireless communication systems have been growing rapidly in many applications. Wireless Local Area Networks (WLAN) are widely used in many countries. The most widely used standard WLAN on IEEE 802.11？b/g/a is provided at the operating frequencies of 2.4？GHz, 5.2？GHz, and 5.8？GHz, respectively. A band-pass filter is an important device which is composed in the front end of WLAN systems. Band-pass filters have been researched and developed with many materials. There has been keen interest in microstrip band-pass filters also, due to ease design, compact size, and low cost. Currently, several techniques such as a step-impedance technique [1–8], a capacitive load technique [9–13], and a dual-mode technique [14–19] have been applied to design the resonators for band-pass filters. The step-impedance resonator (SIR) [1–4] can shift the first harmonic of resonant frequency to any frequency, due to the different ratio between the impedances of resonator. Also, a step-impedance resonator can be redesigned as a hairpin line resonator in order to reduce size to the electrical length of approximately . Additionally, the capacitive load technique [9–12] can be applied to design the resonators for reduced size and shifting of the first harmonic of resonant frequency to any frequency, the effects of which are achieved by changing the capacitive load. However, it is important to design the coupling between resonators of band-pass filters. The most typical coupling between resonators include parallel, hairpin line resonator and cross-couplings [14–16]. In particular, the cross-coupling can improve the characteristics of band-pass filters, such as shape of insertion loss and transmission zero. This paper presents a dual-band band-pass filter using modified cross-coupled step-impedance and capacitively loaded hairpin resonators for WLAN system,
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