Abstract—Based on a new open-stub loaded open-loop resonator and step impedance cells, a low insertion losses dual band microstrip filter is designed to create two passbands at almost 2.4 GHz (exactly at 2.35GHz) and almost 5.7 GHz (exactly at 5.68GHz) for Multimode Wireless LANs application. The insertion losses at the 2.4GHz and 5.7GHz are 0.35 dB and 0.25 dB respectively. Also two width fractional bandwidths in its first and second bands are obtained. Several step impedance cells are used in loops and tapped line feed structures to obtain a novel dual band bandpass filter with compact size. The input impedance of the proposed resonator is calculated. The effect of the microstrip stubs and their impedances and impedances on the resonance frequencies is obtained using the input impedance of the proposed resonator. Index Terms—Dual band bandpass filter, step impedance cells, compact size, low insertion loss. I. INTRODUCTION High performance dual band bandpass filters that operate at IEEE802.11a and IEEE 802.11g Wireless LAN bands play an important role in modern wireless communication systems such as RF transmitters and receivers. For example by using of the microstrip open loop resonators two different dual band bandpass filters operated at 2.4 GHz and 5.7 GHz are designed in [1], [2]. In [3] a compact dual band bandpass filter with small fractional bandwidth using open loop resonators is proposed. In this filter an undesired return loss is seen, where the stopband between two passband is not enough wide. In [4] a dual band band pass filter with very large size and large insertion losses is offered to operate at 1GHz and 2GHz. In [5] the step impedance coupled lines, short circuited and open circuited stubs are used to design a dual band bandpass filter with asymmetric structure which has high insertion losses. In [6] using the step impedance resonators with very large sizes a high losses dual band bandpass filter is designed for ultra wideband applications. In [7] using the open loop resonators and half wave step impedance resonators a large insertion losses dual band bandpass filter is proposed. In [8] a large size step impedance resonator is used to design a dual band bandpass filter with four transmission zeroes. But it has large insertion loss while its large size problem is not solved. In [9] the asymmetric SIRs and open stubs are used to control the transmission zeros of a proposed dual band bandpass filter. Manuscript received October 22, 2013; revised March 5, 2014. The authors are with the Department of Electrical and Electronics Engineering, Shiraz University of Technology, Shiraz, Iran (e-mail: salehi@sutech.ac.ir, abiri@ sutech.ac.ir, Leila_noori62@yahoo.com). Another problem of this filter is its small rejection-band between the two passbands. In [10] a compact dual band bandpass filter is designed by combining bandpass and band-stop filters to operate at 2GHz and 2.2GHz. In this structure low insertion losses are obtained in the both passbands. But the passbands have the small fractional bandwidths. In [11] a double-slot-loaded resonator is used for exploration of a dual band bandpass filter with two closely spaced passbands operated at 2.1GHz and 2.6GHz. In this structure a large insertion loss at second passband is obtained. In [12] a small fractional bandwidth dual band bandpass filter is designed and fabricated using open loop resonators for multimode WLANs. This filter has two undesired return loss in the both passbands. In this paper a compact microstrip filter is designed using two open loop resonators that connected together with mixed coupling. The loops are loaded by simple open stubs. Also in the proposed structure two new tapped lines feed structures are added at the input and output ports to improve impedance matching and achieve dual band bandpass response with good performance in terms of the low insertion losses and the wide fractional bandwidth. This paper is organized as follows: Section II describes design and structure, Section III introduces the simulation results, and Section IV presents the conclusion. II. DESIGN AND STRUCTURE The open loop resonator loaded by three open end stubs is shown in Fig. 1. A novel step impedance taped line feed structure is connected to the open loop resonator to control the resonance frequencies. This taped line feed structure is added to achieve a good impedance matching too. The proposed resonator is divided to nine paths with different impedances and admittances. Fig. 1. Proposed microstrip resonator. Design of a Microstrip Dual-Band Bandpass Filter with Compact Size and Tunable Resonance Frequencies for WLAN Applications M. R. Salehi, E. Abiri, and L. Noori International Journal of Computer and Electrical Engineering, Vol. 6, No. 3, June 2014 248 DOI: 10.7763/IJCEE.2014.V6.832