Transition from Waveguide to Two Microstrip Lines with Slot Radiators in the Millimeter-Wave Band

Many kinds of microstrip array antennas have been developed in the millimeter-wave band. In order to avoid feeding loss and the decrease of antenna gain by beam shift due to frequency changes, center-fed array antennas are advantageous. In this case, the element spacing around the feeding circuit of the transition from the waveguide to two microstrip lines is larger than one wavelength. Therefore, the sidelobe level grows significantly. In order to suppress the sidelobe level, we propose transitions with slot radiators. Moreover, any polarization angles can be achieved by changing the slot angle. A wide variety from 1.5% to 70% of slot radiator coupling powers can be achieved. To investigate the performance of the proposed transition, 10, 22 and 30-element center-fed microstrip combline antennas with the proposed transition were developed at 76.5 GHz, and measured performance was evaluated in the millimeter-wave band. key words: microstrip transition, millimeter-wave circuit, waveguide transition, comb-line antenna