Material influence on the resonant frequency of cavity and transmission line resonators is well known and documented. However, the use of resonant structures for nondestructive testing has been relatively limited. Part of the reason is the lack of suitable models for analysis and part is due to the need for specialized equipment, poor signal to noise conditions, and low resolution. With all that, microwave methods, and resonant microwave methods in particular, are very useful and deserve much more attention. The hardware problems have been largely solved and newer millimeter wave setups promise better resolution. Models for the testing environment are currently being developed both for closed and open resonant structures. This paper describes a number of applications of testing in closed microwave cavities and the associated models, as well as testing with open transmission line resonant structures. The latter are particularly challenging from a modeling point of view because of coupling with materials outside the structure. The applications discussed relate to testing of lossy dielectrics, monitoring of drying processes, and detection of lossy materials in hazardous environments. The models are general and apply equally well to other microwave applications although the question of identification of resonant modes is a rather unique and difficult feature in these models. Test and computation results for controlled conditions are given to show the accuracy and capabilities of the models presented.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 1, pages 259 - 264 Publisher:Ashgate Publishing Company
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