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.
Highlights
Many approaches for the measurement of the dielectric constant of materials and other variables related to it, e.g., material composition, or for the detection of defects in samples, using microwaves have been reported
The working principle is the variation of the transmission coefficient of a transmission line at the frequency of operation caused by a variation in the coupling level between such a line and a sensing resonant element
With the previous expressions, a mathematical model linking the input and output variable is provided. For both designed and fabricated sensors, we obtained the phase variation in the reflection coefficient as a function of the dielectric constant of the material (or sample) under test (MUT), taking as reference the phase of the reflection coefficient when the sensing line is covered by the reference substrate
Summary
Many approaches for the measurement of the dielectric constant of materials and other variables related to it, e.g., material composition, or for the detection of defects in samples, using microwaves have been reported. The reason is the requirement of wideband voltage controlled oscillators (VCOs) To alleviate this limitation, single-frequency sensors constitute a good alternative. Examples of single-frequency sensors based on coupling modulation have been reported [28,29,30,31,32,33,34,35,36,37,38,39,40,41,42] In such sensors, the working principle is the variation of the transmission coefficient of a transmission line at the frequency of operation caused by a variation in the coupling level between such a line and a sensing resonant element (or set of resonant elements). Such sensors have been mainly devoted to the measurement of linear and angular displacements and velocities, as far as a relative displacement between the line and the resonant element modifies the coupling level between both elements
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