A Simple Asymmetrical Step-Impedance Resonator Dual-Band Bandpass Filter

Designing of dual-band bandpass filters has become wide spreading to support wireless communication system standards including WLANs [1-6]. Although they have presented new structures for reducing size by using a quarter wavelength structure in [1-3], meandered resonator to increase coupling value for high selectivity [4-5], and interdigital capacitors between resonators for high selectivity, their performances of harmonics suppression for those proposed filters are worse. Besides small size, ease of fabricate, low return loss, and high insertion loss, unwanted signals rejection is still the important quantity for modern filters [7]. There are some papers that use the technique of multimode of resonators to cascade together but employing this method the harmonics are still too large because of a large number of resonators required [8]. This paper proposed an asymmetrical resonator concept as discussed in [9-10] to generate dual-band frequencies for act as a dual-band bandpass filter with superior harmonics suppression. The details of design, implementation, and results will be given. Section 2 explains the characteristics of asymmetrical structure based on the conventional step-impedance resonator (SIR) and then the proposed dual-band bandpass filter. Section 3 shows the optimized design structure and comparison of its simulation and measurement results. Then the conclusion is given in section 4.