Substrate Integrated Suspended Slot Line and Its Application to Differential Coupler

This article proposes a novel substrate integrated suspended slot line (SISSL) platform, which embeds the slot line inside a metallic shielding enclosure to minimize the radiation loss and solve packaging problems. The propagation characteristics of SISSL including characteristic impedance and guided wavelength are investigated. By using the transition from SISSL to shielded microstrip, a type of differential branch-line-type coupler is proposed, and good common-mode suppression is achieved over a wide frequency band. In the meantime, the SISSL coupler features filtering response in the differential mode when the impedance values of the slot line are selected properly. The proposed SISSL circuit is implemented by using printed circuit board (PCB) process, and it is self-packaged. Two design cases are fabricated and measured to demonstrate the feasibility.