International Journal of Antennas and Propagation

Volume 2017 (2017), Article ID 5619728, 13 pages

https://doi.org/10.1155/2017/5619728

## A Review of Sensing Strategies for Microwave Sensors Based on Metamaterial-Inspired Resonators: Dielectric Characterization, Displacement, and Angular Velocity Measurements for Health Diagnosis, Telecommunication, and Space Applications

CIMITEC, Departament d’Enginyeria Electrònica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain

Correspondence should be addressed to Lijuan Su

Received 17 January 2017; Accepted 9 March 2017; Published 14 May 2017

Academic Editor: Mirko Barbuto

Copyright © 2017 Lijuan Su et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Four sensing approaches for the implementation of microwave sensors based on transmission lines loaded with metamaterial-inspired resonators are considered in this review paper, and examples of applications are pointed out. In all the cases, sensing is based on the effects that the magnitude under measurement causes in the transmission properties of the resonator-loaded line. Such four strategies are (i) resonance frequency variation, (ii) coupling modulation through symmetry disruption (causing variation of the notch depth), (iii) frequency splitting (also exploiting symmetry properties), and (iv) amplitude modulation of a harmonic signal. Such sensors are useful in various scenarios, of interest in fields as diverse as characterization of dielectric materials for communication circuits, medical diagnosis and treatment with microwave technologies, and sensors for space applications, among others.

#### 1. Introduction

Metamaterial-inspired resonators are electrically small resonant particles useful for the implementation of one-dimensional (e.g., metamaterial transmission lines [1]), two-dimensional (e.g., metasurfaces [2]), and three-dimensional (e.g., lenses for MRI [3]) metamaterials. Such resonant elements are “atoms” (sometimes called “meta-atoms”), which can be structured (or engineered) to form periodic artificial materials with unusual electromagnetic properties (negative refraction [4], backward wave propagation [5] and radiation [6], slow and fast waves [4–7], cloaking [8], etc.). Such properties, in general, arise as long as the composite acts as an effective medium for the electromagnetic field with which it interacts. In an effective medium, the properties can be tailored to some extent and are different from those of the constitutive elements, typically conventional metals and dielectrics in most metamaterials (obviously, this does not exclude the use of advanced materials, such as ferroelectrics [9, 10], liquid crystals [11, 12], and graphene [13], or components, such as microelectromechanical systems—MEMS [14, 15]). A well-known example of unusual (effective medium) property is the negative refractive index achievable in composites made of split ring resonators (SRRs) and metallic strips (or posts), related to the simultaneous negative effective permeability (due to the SRRs) and permittivity (related to the metallic strips) of the structure [7]. Key to achieve such effective medium properties is the characteristic dimension (period) of the composite, which must be much smaller than the wavelength of the illuminating radiation. In this regard, metamaterial-inspired resonators are semilumped elements with electrical size significantly smaller than the wavelength at their fundamental resonance frequency, and hence they are useful particles (“atoms”) for the implementation of metamaterials. Examples of such resonant elements are the SRR [16], the complementary split ring resonator (CSRR) [17], the broadside coupled SRR (BC-SRR) [18], the electric LC (ELC) resonator [19], the S-shaped SRR (S-SRR) [20], the folded SIR (F-SIR) [21], and many others (the authors recommend the book [22] for an exhaustive list, analysis, and applications of such resonant particles).

Besides these effective medium properties, which arise in periodic (or quasi-periodic) structures made of the previous (or other) metamaterial resonators and are useful for the implementation of microwave components with small size or superior performance or based on novel functionalities [1, 22], it is possible to use the resonance, electrical size, shape, and specific properties of some metamaterial resonators in other applications, including sensing (the purpose of this review article). Metamaterial-inspired resonators are very useful particles for the implementation of compact, high-sensitivity, and robust sensors on the basis of different strategies or approaches, for applications as diverse as characterization of dielectric materials for communication circuits, medical diagnosis and treatment with microwave technologies, and sensors for space applications, among others.

In this paper, four sensing approaches for the implementation of microwave sensors based on metamaterial resonators are reviewed, and examples of applications are pointed out. In all the cases, the sensing strategies are based on transmission lines loaded with such resonant elements. Such lines resemble metamaterial transmission lines, but the resonance phenomenon, rather than effective medium properties, is exploited. Such sensing strategies are resonance frequency variation, coupling modulation through symmetry disruption (causing variation of the notch depth), frequency splitting (also exploiting symmetry properties), and amplitude modulation of a harmonic signal.

#### 2. Sensing Strategies

In this section, the four sensing strategies (or principles) are reviewed, whereas some applications of them are included in the next section.

##### 2.1. Sensors Based on Frequency Variation

A transmission line loaded with a resonant element (either coupled to it or in contact with it) exhibits a set of transmission zeros (notches) in the frequency response. These transmission zeros occur at those frequencies where the resonant element produces an open or a virtual ground to the line, and the injected power is completely reflected back at these frequencies (excluding the effects of losses). Typically, the frequency of interest for microwave circuit and sensor design is the first (fundamental) resonance frequency, where metamaterial resonators can be used in order to achieve compact dimensions. This frequency (and higher order harmonic frequencies) may be altered by the effects of external stimulus or perturbations (e.g., moisture, temperature), by the relative position or orientation between the line and the resonant element (distance, lateral displacement, etc.), or by the presence of substances/materials surrounding the resonant element. Therefore, it follows that resonance frequency variation can be used for sensing many different variables, including position, velocity, material characteristics (e.g., permittivity), and moisture. These sensors are in general very simple but may suffer from cross-sensitivities, defined as the sensitivity of the sensors to other variables different from the one of interest (measurand). For example, since permittivity depends on environmental conditions (e.g., temperature), the resonance frequency can be unintentionally shifted by spurious effects in permittivity sensors. Nevertheless, in many applications, external factors such as temperature or humidity do not experience significant variations. Moreover, these frequency variation based sensors are typically calibrated for accurate measurements. Therefore, these sensors are useful in many applications where design simplicity and low cost are key aspects.

##### 2.2. Coupling Modulation Based Sensors

This sensing approach belongs to the so-called symmetry-based sensing [22–24], where symmetry properties are exploited for the implementation of sensors. In these sensors, a transmission line is loaded with a single symmetric resonator (electromagnetically coupled to the line), and the sensing principle is the control of the level of coupling between the line and the resonator, caused by the measurand and related to disruption of symmetry. These sensors are particularly useful for the measurement of spatial variables (e.g., alignment, displacement, and velocity) [24–27], and in this case the resonator is etched on a substrate (or object) different from that of the transmission line, in order to allow for a relative motion between the line and the resonator. However, symmetry can also be disrupted by asymmetric dielectric loading of the resonant element.

In these sensors, the symmetry plane of the transmission line is aligned with the symmetry plane of the resonant element, and both symmetry planes must be of different electromagnetic nature (one an electric wall and the other one a magnetic wall). Under these conditions, line-to-resonator coupling is prevented, and the structure exhibits total transmission. Conversely, by breaking symmetry through the effects of the variable under measurement (e.g., a spatial variable), a transmission zero, or notch, appears, and the notch depth depends on the level of asymmetry, since such level determines the magnitude of electromagnetic coupling between the line and the resonant element. Figure 1 illustrates this sensing principle, where symmetry disruption is caused by lateral displacement of the resonant element, a SRR, and the considered transmission line is a CPW. It is interesting to mention that, for the parasitic slot mode (odd mode) of the CPW transmission line, where the axial plane is an electric wall, rather than a magnetic wall, the structure exhibits a notch when the SRR is symmetrically loaded.