A Fully Differential Millimeter-Wave Low-Loss 4 × 4 Butler Matrix With High Common-Mode Rejection Using Dual Patch Based on the SISL Platform

This article proposes a fully differential or balanced-to-balanced <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4\times 4$</tex-math> </inline-formula> Butler matrix (BM) with high common-mode (CM) rejection using dual patches based on the double inner board substrate integrated suspended line (SISL) platform. The fully differential BM consists of couplers, crossovers, and phase shifters. For ease of fabrication and low conductor loss in millimeter-wave (mm-wave) frequencies, all these components are designed using dual patch elements. The two patches are placed on the double inner boards, and differential circuits are designed vertically on the patches. The radiation loss and dielectric loss of the BM are minimized by embedding the core circuit within the SISL air cavity, utilizing sidewall copper for connection, and cutting the undesired dielectric. Grounded slots (GSs) are introduced on the grounded plane between the dual patch to reject the CM signals. A fully differential <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4\times 4$</tex-math> </inline-formula> BM is implemented by cascading these components in a multicavity structure, and excellent performance is obtained. From 24 to 28GHz, the measured CM rejection is over 50 dB, and the measured cross-mode rejection is over 20 dB. The measured insertion loss is 0.8 dB, and the measured phase error is within 5° at the center frequency of 26 GHz. Additionally, the implemented BM also features the advantages of self-packaging and high integration.