Dual-Loaded Modulated Dipole Scatterer as an Embedded Sensor

Abstract

The modulated scatterer technique (MST) has shown great promise as an embedded sensor for material characterization and flaw detection in nondestructive testing and evaluation (NDT&E) applications. In particular, embedded MST probes facilitate real-time measurements of the dielectric properties of a host structure in the vicinity of the probe, i.e., localized measurements. MST is based on illuminating a loaded scatterer/probe, which is usually a dipole antenna loaded with a p-i-n diode, with an electromagnetic wave, and measuring the backscattered signal while the diode is modulated. Modulating the p-i-n diode by turning it “off” and “on” changes the dipole input impedance, which, in turn, results in two different backscattered signals. These signals are functions of the dielectric properties of interest, i.e., dielectric constant, and other measurement parameters, e.g., scatterer-to-antenna distance, incident power level, and incidence angle. Using the complex ratio of these signals renders measurement independence with respect to many measurement parameters and results in a measurement that is sensitive primarily to the parameter of interest (i.e., the dielectric constant of the material surrounding the probe). However, the modulated backscattered signal must be separated from static or unmodulated signals present at the receiving antenna before the ratio can be computed. This can be accomplished in a number of ways; each has its own advantages and drawbacks with respect to system complexity, signal processing requirements, and accuracy. To improve on the previous designs, in this paper, we introduce a new probe that simultaneously utilizes two p-i-n diodes. Using the four possible combinations of the measured backscattered signals from such a probe, in conjunction with a differential measurement scheme and subsequent ratio calculation, simultaneously results in the desired independence from several measurement parameters and the removal of the static terms. The operation of the proposed dual-loaded probe is described and analyzed in this paper. Experimental verification of the probe performance is also reported herein.