Design of a Flexible BPF Using Coaxial Open-Loop Dumbbell Branch Defected Conductor Structures

This letter introduces a flexible bandpass filter (BPF) that employs a coaxial structure, consisting of an inner substrate, an inner conductor featuring open-loop dumbbell-shaped defects, an outer conductor with a loop-shaped defect gap, and a transmission dielectric situated between the two conductors. For fabrication, the inner and outer defective conductor structures are manufactured using flexible printed circuit board (FPCB) technology and assembled to form the BPF with a diameter of 3.42 mm and a length of 80 mm for three cascaded resonators. Simulation and measurement results indicate that the insertion loss within the passband of 2.4–4.0 GHz is less than 1 dB, with a rejection of more than 20 dB below 1.2 GHz and above 4.6GHz.