Highly Sensitive Reflective-Mode Defect Detectors and Dielectric Constant Sensors Based on Open-Ended Stepped-Impedance Transmission Lines

Abstract

In this paper, reflective-mode phase-variation sensors based on open-ended stepped-impedance transmission lines with optimized sensitivity for their use as defect detectors and dielectric constant sensors are reported. The sensitive part of the sensors consists of either a 90° high-impedance or a 180° low-impedance open-ended sensing line. To optimize the sensitivity, such a sensing line is cascaded to a 90° transmission line section with either low or high characteristic impedance, resulting in a stepped-impedance transmission line configuration. For validation purposes, two different sensors are designed and fabricated. One of the sensors is implemented by means of a 90° high impedance (85 Ω) open-ended sensing line cascaded to a 90° low impedance (15 Ω) transmission line section. The other sensor consists of a 180° 15-Ω open-ended sensing line cascaded to a 90° 85-Ω line. Sensitivity optimization for the measurement of dielectric constants in the vicinity of that corresponding to the Rogers RO4003C substrate (i.e., with dielectric constant 3.55) is carried out. The functionality as a defect detector is demonstrated by measuring the phase-variation in samples consisting of the uncoated Rogers RO4003C substrate (the reference sample) with arrays of holes of different densities.

Planar Phase-Variation Microwave Sensors for Material Characterization: A Review and Comparison of Various Approaches

“…For this reason, the FoM is not as good as the one of the reflective mode sensors reported in [ 62 ]. Indeed, the main relevant and unique feature of the step-impedance transmission line based sensors of Section 5 (a subset of those reported in [ 62 , 63 , 64 ]) is the fact that the sensitivity can be enhanced at wish by merely adding high/low impedance 90° transmission line sections. Moreover, the design of these sensors is very simple, as far as they are based on a cascade of transmission line sections.…”

“…Dimensions (in mm) are: W d = 7.5, l d = 12.8, W S = 0.235 and l S = 13.5. Reprinted with permission from [ 64 ].…”

“…Additionally, the phase of the reflection coefficient has been experimentally obtained with the sensing line uncovered ( ε MUT , air = 1). Reprinted with permission from [ 64 ].…”

“…From sample ( a ) to sample ( d ), the density of holes increases. Reprinted with permission from [ 64 ].…”

“…The main objective of this review paper is to point out, discuss and compare different sensing strategies devoted to performance optimization (specifically sensitivity enhancement, the typical challenging aspect) in phase-variation microwave sensors [ 36 , 37 , 38 , 45 , 46 , 61 , 62 , 63 , 64 ]. Even though examples of planar phase-variation microwave sensors focused on sensing spatial variables have been reported [ 65 , 66 , 67 ], the sensors to be discussed in the present paper are mainly devoted to material characterization.…”