Leakage-Reduced Conductor-Backed Coplanar Waveguide With Periodic Structures and its Antenna Application

A simple method is described where the need of via holes is not required to suppress the inherent leakage of a conductor-backed coplanar waveguide (CBCPW). Pairs of slots are periodically etched on the side ground planes in close proximity to the central signal line. When one period of the structure is considered as a coupled-line section, it provides an open-open boundary condition leading to the all-pass frequency response. To further reduce the possible mode conversion caused by the impedance discontinuity, pairs of small patches are loaded inside the slots. Since the self-resonant frequency of the small patches is far beyond the frequency of interest, the impedance seen from the signal line to the small patches is relatively high suppressing the leakage. The proposed leakage-reduced CBCPW transmission lines both possess good insertion/return loss responses, low loss factor, and flat group delay characteristics. The operation frequency range of the proposed structures is up to 45 GHz. Both structures are then integrated with the coplanar patch antenna. It is demonstrated that using the proposed structures as feed lines could not only improve the antenna efficiency but also lower the cross polarization levels of the coplanar patch antenna, which operates at 5.5 GHz. More importantly, the proposed via-free structures would be more suitable for use in microwave and millimeter wave integrated circuits.