Design of compact size and tunable band pass filter for WLAN applications

Compact size and tunable band pass filter based on open-loop resonators is proposed. The proposed structure consists of dual-armed microstrip open-loop resonators. A second armed open-loop resonator, which acts as band rejection resonator, is connected directly to the input and output ports to achieve the desired band pass behaviour. The open-loop microstrip resonators are combined with four lumped capacitors to reduce the overall size of the proposed filter. The capacitors are inserted at places where maximum electric field is occurred in order to increase the effective capacitance and reduce the resonant frequency as well. The resonance frequency is translated from 6.75 to 5.3 GHz for WLAN applications. Consequently, the filter size will be more compact and the filter performance will be improved by enhancing the energy stored in the resonator. The proposed design using lumped capacitors represents a reduction in size of more than 35% when compared with conventional filter without using lumped capacitors. A good consistency is achieved between simulated and measured results.