A parallel plate transmission line (20) is formed by conformal application of one or more dielectric and conductive layers to the surface under test (70). A signal generator (42) is employed to transmit variable frequency signals (60) along the transmission line (20). Discontinuities in the transmission line (20) cause reflections (62) which are received by a receiver (48). A processor (44) coupled to the signal generator (42) and receiver (48) detects and locates the discontinuities.

System and method for detecting and locating flaws on or beneath a surface

Microwave measuring methods can be applied to determine the properties of materials and hence it is possible to develop microwave sensors for processing industry. The first applications were in moisture measurement but lately many new sensor applications have appeared. Microwave sensors especially suitable in forest industry (wood, paper), in chemical industry (plastics, chemicals), in food industry (tobacco, butter) etc. In developing sensors several problems must be solved. The dielectric properties of the material in question must be known. They can be measured or in some cases determined theoretically by applying mixing theories. The sensor is often some kind of a resonator. Its structure must be such that the electric field penetrates the material as required, and allows free flow of material. Displacements occurring normally in the material flow or dirt in the sensor must not cause measuring errors. Several examples of lately developed sensors are given.

Microwave Sensor Applications in Industry

Microwave sensor applications in industry

The objectives of this paper are to provide a historical perspective of microwave NDE and to review the associated experimental techniques by discussing them in the context of specific applications. The application examples are drawn from the published literature and the author’s own experience. The applications discussed include measurements of layer thicknesses and material parameters, detection and quantification of internal flaws inside dielectric objects, detection and quantification of surface flaws on conductors, and imaging of flaws inside material media. The discussion will seek to clarify the potential advantages and disadvantages of various microwave NDE techniques.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=2094&context=qnde

Experimental Techniques in Microwave NDE

Microstrip resonant probes have been developed which can measure dielectric constant uniformity with spatial resolution of 0.030 in.*0.010 in. and accuracy better than 2% at 10 GHz. The accuracy of the measurement can be improved to be better than 1% by using a closed-loop frequency discriminator to measure the frequency shift automatically. The measurement principle, microstrip resonator design and fabrication, microwave measurement setup, and dielectric constant uniformity measurement are discussed. >

High spatial resolution dielectric constant uniformity measurements using microstrip resonant probes

A microwave instrument for non-destructive testing of the homogeneity of supercalendar rollers used in paper mills is described. The sensor consists of a flat coaxial transmission line a half wavelength long. A sensitivity of more than 0,1% is achieved in measuring small changes in the dielectric constant. Measurement of absolute values of the dielectric constant is also possible.

M. Tiuri

Papers

A Microwave instrument for Measurement of Small Inbomogeneties in Supercalendar Rollers and Other Dielectric Cylinders