Multistage directional filter based on band-reject filter with isolation improvement using composite right-/left-handed transmission lines

The λ strip-resonator directional filter has good balance between compactness and robustness against manufacturing error. However, there have been difficulties in generalizing the strip-resonator directional filter to realize multipole filtering responses. This paper presents the theory, experiments, and improvement of a new structure of strip-resonator directional filter based on a band-reject filter. This new structure allows the realization of multipole responses and theoretically does not limit the types of filter responses. A three-pole elliptic directional filter prototype is studied thoroughly to show that its performance can be predicted accurately by a conventional filter structure. We also propose a modification of the delay-line part using composite right-/left-handed transmission line to improve the isolation. An isolation level of 30 dB is shown to be achieved consistently.