Balanced-to-Balanced SIW Filtering 180° Coupler with High Selectivity and Unequal Power Division

A balanced-to-balanced multilayer 180° coupler based on the substrate-integrated waveguide cavities (SICs) is proposed to realize a high-selective filtering response and unequal power division at 28 GHz. To fulfill high isolation between Σ and ∆ ports as well as anti-phase relation between the differential I/O ports, two stacked SICs operating at the degenerate modes, i.e., TE102and TE201, were implemented. The power-dividing ratio is engineerable by appropriately placing the sidewall aperture between cavities. Two prototypes, one equal and the other unequal power division, were experimentally developed. At the center frequency, the measured minimum insertion loss, isolation, 30-dB shape factor, magnitude and phase imbalances are 2 dB (excluding division loss), > 30.2 dB,30.6 dB.