Miniaturisation of branch line couplers with a compact transmission line topology based on coupled line section

Abstract

This study presents a compact transmission line topology to reduce the circuit size of single- and two-section branch line couplers (BLCs). The proposed transmission line is composed of a high impedance series line and one coupled line section. The coupled line section consists of two high impedance series lines and one coupled line. The proposed line is analysed with even-odd mode analysis to derive closed-form design equations. To validate the equations, a compact single- and two-section BLCs are designed at 0.85 GHz based on the proposed line. The physical size occupied by the proposed single- and two-section BLCs is 23 and 19.5%, respectively, compared to conventional BLCs. Moreover, the proposed transmission line demonstrates the harmonic suppression characteristic due to the coupled line section. As a result of which, the second- and third-order harmonic of the proposed BLCs are suppressed. The proposed BLCs are designed, fabricated, and tested.