A microwave technique for nondestructive evaluation of radome sandwich structures is presented. The method employs a rectangular patch sensor for detecting defects of the radome core, such as water ingression, in the form of a change in resonant frequency of the sensor due to permittivity variations in the test-piece. Using well-established models that predict the resonant frequency of a rectangular patch antenna in the presence of a dielectric superstrate, the effects of different patch sensor parameters on sensor performance are analyzed. Based on this analysis, design of a half-wave patch sensor is optimized. A prototype half-wave patch sensor is fabricated and tested. Experimental results show that the sensitivity of the sensor to permittivity changes in the core of a laminar structure is better than Δε = 0.046.