A Compact DBR Filter Using $\Pi$-Network and Dual-Line Equivalent Circuit

This letter presents a new design structure for compact Dual Behavior Resonator (DBR) filter using
$\Pi$
-Network and dual-line equivalent circuits. The proposed
$\Pi$
-Network replaces the J-inverter in DBR filter, which consists of a high impedance transmission line and parallel capacitances loaded on both sides, where the parallel capacitances can be integrated into neighboring resonators. The dual-line structure is used to replace open-circuited transmission line segment. The relevant equations of the proposed filter are derived. The whole structures have the advantages of better out-of-band rejection and easy fabrication. A 3-order DBR filter is designed and measured. Its size is only 28% of the classical one, and experiment results are in good agreement with simulation results.