Size reduction and harmonic suppression in branch line couplers implemented by means of capacitively loaded slow-wave transmission lines

© 2017 Wiley Periodicals, Inc. A systematic approach for the design of compact branch-line hybrid couplers with harmonic suppression is proposed in this article. The couplers are implemented by means of slow-wave artificial transmission lines based on patch capacitors. It is shown in this article that the number of unit cells of the artificial lines should not be arbitrary. Specifically, it is demonstrated that to efficiently suppress the harmonic bands of the coupler and simultaneously keep the response of the coupler unaltered in the region of interest, two unit cells are required. Based on the proposed design methodology, a compact branch line coupler with 46% size, as compared to the ordinary counterpart, and efficient rejection of the first and second harmonic bands, is reported in this article. The designed coupler exhibits good performance in the region of interest, very similar to one of the conventional coupler.