Miniaturisation and harmonic suppression of the branch-line hybrid coupler

Abstract

In this article, two compact structures of the branch-line hybrid coupler (BLC) which can suppress upper order harmonics of the conventional BLC are proposed. By replacing the microstrip transmission lines at any electrical length of the conventional BLC with their equivalent new T-type transmission lines, two compact BLCs that can suppress upper order harmonics are achieved. Design equations of the proposed T-type line are achieved by ABCD matrices. To validate the design concept, first, a compact BLC which utilises a proposed T-type line in each of its branch is designed and fabricated to suppress the second to the eighth harmonics. Furthermore, the first BLC is analysed by even- and odd-mode analyses and the scattering parameter matrix of the first BLC is calculated at the first to eighth harmonics. There is an excellent agreement between simulated and measured results. Then, a compact BLC which uses two proposed T-type lines in each of its branch is designed at 1 GHz to suppress the second to the fifth harmonics and the second to sixth harmonics, simultaneously. The first- and second-proposed structures are 50% and 46% of the conventional BLC, respectively, while maintaining the characteristics of the conventional BLC at the fundamental frequency.