A novel input coupler, 4 coaxial sector waveguides excited via a rectangular TE10, was simulated and verified by millimeter wave measurement. Simulations predicts that the average transmission was -2dB with a 3dB bandwidth of 15GHz (90-105GHz) and port reflection was less than -10dB. Excellent performance was observed, which was in good agreement with the simulation results. This input coupler has many available applications such as gyro-devices, high power transmission line, microwave and antenna system.