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In this paper, a compact and broadband conductor-backed coplanar waveguide (CB-CPW) to substrate integrated waveguide (SIW) transition using a stepped-impedance resonator (SIR) with a 90°-bent slot is proposed. The proposed transition can achieve a 36.8% 15-dB fractional bandwidth, which almost covers the S-band (2.6-3.95 GHz). Compared to the CB-CPW-to-SIW transition using the single-section quarter-wavelength transformer, transition using the SIR with the 90°-bent slot can increase the 15-dB fractional bandwidth from 23.44% to 36.8% and reduce the physical length from 14 to 7.2 mm. In order to verify the simulation results, back-to-back transitions are fabricated and measured, and the measurement results are in good agreement with those of simulation.