A Lumped-Element Directional Coupler for Bandwidth Enhancement, Impedance Matching, and Harmonic Suppressions

Abstract

This paper presents a lumped-element wideband directional coupler that enables arbitrary impedance matching and harmonic suppressions from $2f_0$. The coupler consists of four filtering networks (FNs) and an asymmetric branch-line hybrid serving as the matching network. The use of lumped-element topology helps reduce circuit size and improve harmonic suppressions. Each FN, composed of two lumped-element resonators, contributes two additional transmission poles, enabling bandwidth scaling, bandwidth enhancement, and harmonic suppressions. To demonstrate the effectiveness of the proposed design, a wideband coupler with various terminal impedances operating at 0.5 GHz is designed, simulated, and fabricated. The measured size is $0.039\times 0.039{{\lambda }_{\text{g}}}^2$, the fractional bandwidth for –14.3 dB return loss is 46%, and the harmonic suppression is more than –30 dB from 0.69 to 4.5 GHz.