As the need for NDE in our technology-based society grows, inspection requirements become ever more demanding and it is essential that we have as many NDE tools at our disposal as possible. Although not as widely known or understood as the NDE techniques discussed in earlier chapters, microwave NDE has proven to be very useful in certain applications. Microwave NDE may be defined as the inspection and characterization of materials and structures using high-frequency electromagnetic energy. For example, police routinely use radar "guns" to "nondestructively inspect" our moving automobile to determine its speed without having to be in physical contact (ride in the car with us). In addition, if the body of the car were made of a low-loss dielectric material, the microwaves would penetrate the car and could provide information on the

contents of the car as well. Noncontact inspection and the ability to penetrate dielectric materials are two of the most important attributes of microwave NDE.