Theoretical and experimental foundation of dual-loaded dipole scatterer as an embedded sensor

The modulated scatterer technique (MST) has shown promise as an embedded sensor technique for materials characterization and flaw detection. MST is based on illuminating a loaded scatterer/probe, usually a dipole antenna loaded with a PIN diode, with an electromagnetic wave. By detecting the backscattered (reflected) wave, information about the material properties in the vicinity of the probe can be obtained. Modulating the PIN diode changes the overall dipole impedance in a specific manner. Hence, two different states of the backscattered signal can be detected, the complex ratio of which provides measurement independence from a number of parameters. However, the modulated backscattered signal must still be separated from other (static) signals present at the receiving antenna. This can be accomplished a number of ways, but with the addition of measurement complexity, increased data processing, or potential decrease in accuracy. To this end, the basics of a new probe design were proposed that utilizes two PIN diodes simultaneously. Using the four possible combinations of the backscattered field from such a probe, in conjunction with a differential measurement scheme and subsequent ratio calculation, independence from several measurement parameters as well as other static terms may be achieved. This paper outlines the complete theoretical derivation for the backscattered fields, along with experimental verification of the methodology.