A microstrip-to-waveguide transition has been realized for radio astronomy applications, designed to operate in the Q-band (33–50 GHz). As part of an array radio frequency (RF) receiver, the main requirement of such a transition is the reduction of transverse space occupation for the integration in the entire receiver chain, so an in-line configuration has been developed. Moreover, the high frequency band implies that an easy fabrication is a critical requirement if a good match between the two guiding structures is to be obtained in the desired band (with a relative bandwidth of 40%). The combination of CAD software and an optimization tool allows the device to achieve a good return loss over the entire band.