Design of a Microstrip Dual-Band Bandpass Filter with Compact Size and Tunable Resonance Frequencies for WLAN Applications

Abstract

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. 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 STRUCTUREThe 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. International Journal of Computer and Electrical Engineering, Vol. 6, No. 3, June 2014 248 DOI: 10.7763/IJCEE.2014.V6.832The resonance frequencies of the proposed resonator as shown in Fig. 1 can be controlled by adjusting its input im...

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