Millimeter-Wave Narrow-Width Directional Couplers on Via-Free Ridge AFSIW Technology

A novel directional coupler topology featuring a narrow width is proposed using via-free ridge air-filled substrate integrated waveguide (VF-RAFSIW) technology. The coupler exhibits a 54% reduction in width compared to other AFSIW directional couplers. This reduction enables effective integration with ridge waveguide antennas, which enhances sidelobe suppression and beamforming performance at large scanning angles. The couplers use a ridge shape to control the frequency responses while keeping the sidewalls straight. The proposed ridge shape also shortens the coupler length, reducing the circuit area by 43%. Phase constant differences between the two modes in the VF-RAFSIW are calculated using the Ritz-Galerkin method to design couplers. The proposed topology supports both hybrid and crossover coupler responses, depending on the coupler length. Two types of the proposed couplers are demonstrated, with insertion losses ranging from 0.7 to 1.6 dB and phase delays of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$90^{\circ }~\pm ~2^{\circ }$ </tex-math></inline-formula> at 60 GHz. In addition, the use of FR-4 substrates and standard printed circuit board (PCB) processes makes the VF-RAFSIW couplers cost-efficient and suitable for mass production.