A Low-Loss Millimeter-Wave Fully Differential Coupler Using Dual Patch on SISL Platform

This letter proposes a low-loss fully differential patch coupler using dual square patch based on the double inner board substrate-integrated suspended line (SISL) platform. The conductor loss, dielectric loss, and radiation loss of the coupler are reduced by using patch elements, cutting the undesired dielectric, and embedding the core circuit into the SISL air cavity. A fully differential patch coupler operating at the center frequency of 26 GHz is designed, fabricated, and measured. The measured insertion loss is only 0.37 dB at the center frequency. From 24 to 28 GHz, the measured return loss is better than 16 dB and the phase imbalance is 90 <tex-math notation="LaTeX">∘</tex-math> <tex-math notation="LaTeX">±</tex-math> 2 <tex-math notation="LaTeX">∘</tex-math> .