This paper describes a non-contact vibration measuring system that is based on a microwave interferometer similar to those used to detect the presence of people in monitored areas. Displacement response of a target surface on the structure relative to the interferometer is measured by analysing the interference in microwaves reflected off the vibrating target surface. The system was built from commercially available off-the-shelf electronic components and, unlike most commercially available far-field non-contact vibration sensors, it does not require a mounted target when used to measure the response of metallic and concrete structures. In this study, the interferometers are coupled with homodyne detection hardware and software to make low-noise measurements during swept-sine tests. These interferometers can also be used to detect static displacements and the transient response of the structure. In addition to its non-contact feature, this system offers other advantages including low cost, quick set-up time, a wide frequency range of response and portability. An application of these sensors to the measurement of vibration response on a bridge structure in a non-contact manner is summarised. Resonant frequencies of the bridge that were identified with the microwave interferometers are compared to similar quantities measured with conventional accelerometers. In all cases, resonant frequencies identified from data obtained with the microwave interferometers were within 2.2% of the resonant frequencies determined from data obtained with the conventional accelerometers.
