LAUSR

This paper presents a transformer-based 3-dB differential coupler, which features the suppression of both common-mode and cross-mode signals. Transformer is utilized rather than the quarter-/half-wavelength lines, and the circuit size can be significantly reduced. The bandwidth of the proposed differential coupler can be enhanced by increasing the coupling coefficient of the transformer. For the proof of concept, the proposed transformer-based 3-dB differential coupler is implemented on the substrate integrated suspended line (SISL) platform. Well agreed simulated and measured results verified the feasibility of the proposed differential coupler. From 0.4 to 0.6 GHz, the measured common-mode suppression is better than 10 dB with a maximum 30 dB, and the measured cross-mode suppression is better than 30 dB. Compared with the previous designs, the self-packaged SISL differential coupler has a much smaller size of $0.08 \lambda _{0} \times 0.08\lambda _{0}$ . The proposed design methodology has advantages of compact size, suppression of both common-mode and cross-mode signals, and controllable bandwidth.