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In this paper, a compact microstrip-to-waveguide transition using the quarter-wavelength broadside-coupled microstrip line (BCML) is proposed. The 15-dB fractional bandwidth of this transition is 20.95%, which could only cover part of the X-band (8.2-12.4 GHz). In order to enhance the bandwidth, the capacitance-compensated BCML is used to replace the quarter-wavelength BCML. The microstrip-to-waveguide transition using the capacitance-compensated BCML, which is as compact as the transition using the quarter-wavelength BCML, has 37.33% 15-dB fractional bandwidth, almost covering the whole X-band. To verify the simulation results, back-to-back microstrip-to-waveguide transitions using the quarter-wavelength BCML or the capacitance-compensated BCML are fabricated and measured where the measurement results are in good agreement with the simulation results for both designs.