Multiband BPF with wide upper stop band using asymmetrically positioned stub loaded open loop resonators

Abstract

In this paper, asymmetrically positioned stub loaded open loop resonators with pseudo interdigital coupling are used to design compact multiband planar bandpass filters. The first design pertains to a dualband BPF that operates at 3.5GHz and 5.7GHz. The parameters like position of stub, which quantifies the asymmetry, and length of stub are further optimised using real coded genetic algorithm to evolve a triband BPF. The evolutionary design procedure is supported with an example of triband BPF having passband at 3.5GHz, 5.5GHz and 6.8GHz, respectively. The transmission line models for both filters are developed as well as fabricated prototypes are realised and tested. There is a good agreement between the measured and simulated results. The measured insertion loss at first and second band centred around 3.5GHz and 5.7GHz of the dual band BPF are 1.5dB and 1.25dB, respectively. For the triband BPF the values are 1.24dB, 1.6dB and 1.8dB at 3.5, 5.5 and 6.8GHz, respectively. The dualband design covers the WiMAX and IEEE 802.11a bands where as the triband design also covers the 6.8GHz RFID frequency.