Design of a Novel Low-Pass Filter Cable Utilizing a Defected Conductor Transmission Structure With Cascaded Isosceles U-Shaped Resonators

This article presents a novel low-pass filter cable with a layered structure, including an inner cylinder dielectric, a defected conductor transmission structure (DCTS), a transmission dielectric, and an outer shielding conductor layer. The DCTS, consisting of four groups of closely adjacent heterogeneous isosceles U-shaped defected structures, is wrapped around the inner cylinder dielectric. The cable's transmission performance can be adjusted by varying the geometry parameters of the DCTS. The proposed filter cable can be considered as a cascade of parallel RLC resonant circuits, as confirmed by simulated S-parameters. To fabricate the cable, a polytetrafluoroethylene (PTFE) cylinder dielectric is used, along with a copper layer featuring 12 U-shaped etchings on a polyimide substrate using a flexible printed circuit board technique. The cable also includes a PTFE transmission dielectric and a copper shielding layer. With a length of 40.87 mm and a diameter of 4.772 mm, the proposed low-pass filter cable exhibits a low insertion loss of less than 1 dB under 2.4 GHz. It also achieves a rejection level greater than 35 dB in the wide stopband from 3.5 to 8.0 GHz, as confirmed by both simulation and measurement results.