In this paper a method is presented to examine the suitability of planar structures to work as a resonator for IMPATT diodes in a frequency range above 70 GHz. The IMPATT diode is replaced by a Schottky diode previously characterized by impedance. It is suggested to test the suitability of the resonator for an IMPATT diode by employing the radiation characteristics of the structure. This set-up allows the determination of the detector's sensitivity depending on the frequency. The sensitivity corresponds to the matching of the resonator and the Schottky diode. Thus, for maximum sensitivity the equation Z/sub Schottky//spl ap/-Z*/sub IMPATT/ allows to assess the suitability of the planar structure to function as a resonator for an IMPATT diode.<<ETX>>