Air-Gap Detection in Dielectric Materials by a Step-Frequency Microwave Technique

  • John M. Liu


Most microwave NDE has been performed using continuous wave excitation and reception, due to the general availability of such equipment and the acceptable procedure of extracting information from the amplitude and phase of such signals. With the availability of sources that can be swept over a band of frequencies, the amplitude and phase information can be converted via the Fourier Transformation to the impulse response in the time domain. Instead of searching for changes in the amplitude and phase of microwave reflected from boundaries, interfaces, and defects, this time domain approach concentrates on the recognition of “echoes”. This approach is implicit in some of the more complex approaches in microwave imaging [1–3]. It is the purpose of this paper to demonstrate that this approach facilitates the detection of internal defects using microwave, in a manner similar to the practice of pulse-echo ultrasound. The time delay for a microwave “echo” is related to the location, and the Fourier transformed amplitude is related to some characteristics of the defect inside the material.


Reflection Coefficient Impulse Response Relative Permittivity Dielectric Slab Time Domain Approach 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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Copyright information

© Plenum Press, New York 1996

Authors and Affiliations

  • John M. Liu
    • 1
  1. 1.Code 684, Carderock Division, White Oak Det.Naval Surface Warfare CenterSilver SpringUSA

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