Microwave sensors based on resonant elements

This paper highlights interest in the implementation of microwave sensors based on resonant elements, the subject of a special issue in the journal. A classification of these sensors on the basis of the operating principle is presented, and the advantages and limitations of the different sensor types are pointed out. Finally, the paper summarizes the different contributions to the special issue

Analysis and investigation of a novel microwave sensor with high Q-factor for liquid characterization

In this paper, a new design of microwave sensor with high Q-factor for liquid characterization is analyzed and investigated. The new microwave sensor is based on a gap waveguide cavity resonator (GWCR). The GWCR consists of upper plate, lower plate and array of pins on the lower plate. The liquid under test (LUT) is characterized by placing it inside the GWCR where the electric field concentrates using a quartz capillary that is passing through microfluidic channels. The results show that the proposed sensor has a high Q-factor of 4832. Moreover, the proposed sensor has the ability to characterize different types of liquids such as oils, ethanol, methanol and distilled water. The polynomial fitting method is used to extract the equation of the unknown permittivity of the LUT. The results show that the evaluated permittivity using the proposed sensor has a good agreement with the reference permittivity. Therefore, the proposed sensor is a good candidate for food and pharmaceutical applications

Analysis And Investigation Of A Novel Microwave Sensor With High Q-Factor For Liquid Characterization

In this paper, a new design of microwave sensor with high Q-factor for liquid characterization is analyzed and investigated. The new microwave sensor is based on a gap waveguide cavity resonator (GWCR). The GWCR consists of upper plate, lower plate and array of pins on the lower plate. The liquid under test (LUT) is characterized by placing it inside the GWCR where the electric field concentrates using a quartz capillary that is passing through microfluidic channels. The results show that the proposed sensor has a high Q-factor of 4832. Moreover, the proposed sensor has the ability to characterize different typesof liquids such as oils, ethanol, methanol and distilled water. The polynomial fitting method is used to extract the equation of the unknown permittivity of the LUT. The results show that the evaluated permittivity using the proposed sensor has a good agreement with the reference permittivity. Therefore, the proposed sensor is a good candidate for food and pharmaceutical application

Substrate-integrated waveguide (SIW) microwave sensor theory and model in characterising dielectric material : A review

Microwave sensors offer appealing features such as susceptibility, quick response, and non-invasiveness, making them valuable tools for highly accurate measurements of material characterisation. A wide range of techniques, including cavity waveguide, planar transmission line, cavity waveguide perturbation, open-ended coaxial probe, and free-space transmission, have been employed to characterise materials that are essential for their cost-effectiveness, ease of manufacturing, high sensitivity, good quality factor (Q-factor), and compact size, allowing them to be applied to different material types. Among the microwave sensor types, the substrate-integrated waveguide (SIW) has emerged as a promising technology in order to characterise materials in an efficient manner. This paper presents a review of the current state and potential opportunities of SIW microwave sensors in the characterisation of dielectric materials. It provides insights into various design principles, techniques, and applications of SIW microwave sensors across different sectors, highlighting their advantages and limitations compared to conventional waveguide-based sensors. Furthermore, the paper summarises several fabrication methods that can be implemented for SIW microwave sensors to enable the production of efficient and reliable sensors. Additionally, the future directions provided in this paper aim to contribute to the ongoing development and optimisation of SIW-based microwave sensors for accurate and efficient dielectric material characterisation. Overall, this review article serves as a beneficial resource for new researchers seeking to understand the role of SIW microwave sensors in material characterisation. It outlines the current status, opportunities, and potential advancements of SIW sensors, shedding light on their significance and potential impact in the field of material characterisatio

Substrate-integrated waveguide (SIW) microwave sensor theory and model in characterising dielectric material: A review

Microwave sensors offer appealing features such as susceptibility, quick response, and non-invasiveness, making them valuable tools for highly accurate measurements of material characterisation. A wide range of techniques, including cavity waveguide, planar transmission line, cavity waveguide perturbation, open-ended coaxial probe, and free-space transmission, have been employed to characterise materials that are essential for their costeffectiveness, ease of manufacturing, high sensitivity, good quality factor (Q-factor), and compact size, allowing them to be applied to different material types. Among the microwave sensor types, the substrate-integrated waveguide (SIW) has emerged as a promising technology in order to characterise materials in an efficient manner. This paper presents a review of the current state and potential opportunities of SIW microwave sensors in the characterisation of dielectric materials. It provides insights into various design principles, techniques, and applications of SIW microwave sensors across different sectors, highlighting their advantages and limitations compared to conventional waveguide-based sensors. Furthermore, the paper summarises several fabrication methods that can be implemented for SIW microwave sensors to enable the production of efficient and reliable sensors. Additionally, the future directions provided in this paper aim to contribute to the ongoing development and optimisation of SIW-based microwave sensors for accurate and efficient dielectric material characterisation. Overall, this review article serves as a beneficial resource for new researchers seeking to understand the role of SIW microwave sensors in material characterisation. It outlines the current status, opportunities, and potential advancements of SIW sensors, shedding light on their significance and potential impact in the field of material characterisation

Review of recent microwave planar resonator-based sensors: Techniques of complex permittivity extraction, applications, open challenges and future research directions

Recent developments in the field of microwave planar sensors have led to a renewed interest in industrial, chemical, biological and medical applications that are capable of performing real-time and non-invasive measurement of material properties. Among the plausible advantages of microwave planar sensors is that they have a compact size, a low cost and the ease of fabrication and integration compared to prevailing sensors. However, some of their main drawbacks can be considered that restrict their usage and limit the range of applications such as their sensitivity and selectivity. The development of high-sensitivity microwave planar sensors is required for highly accurate complex permittivity measurements to monitor the small variations among different material samples. Therefore, the purpose of this paper is to review recent research on the development of microwave planar sensors and further challenges of their sensitivity and selectivity. Furthermore, the techniques of the complex permittivity extraction (real and imaginary parts) are discussed based on the different approaches of mathematical models. The outcomes of this review may facilitate improvements of and an alternative solution for the enhancement of microwave planar sensors’ normalized sensitivity for material characterization, especially in biochemical and beverage industry applications

Microwave sensing techniques for materials characterisation

Microwave cavity resonators are of major interest for characterisation of dielectric materials in many applications such as petroleum and chemical, biomedical and pharmaceutical, food and agriculture industries. In this thesis new and improved techniques for implementing microwave measurement techniques for dielectric characterisation of both liquid and solid materials have been developed. This has been actualised through several original contributions: a new high-Q and sensitive re-entrant cavity resonator structure; novel techniques of materials measurements for liquids enabled by a coaxial cavity resonator using a simplified analytical formula; a new concept of sample placement and measurements of substrate materials of varying thickness using resonant method; higher order mode re-entrant cavity resonator for dielectric property measurements at millimetre-wave frequencies; insight into capabilities of 3D printing in making effective and low-cost sensing device at both microwave and millimetre-wave frequencies; simplified evaluation method of materials effective conductivity by resonant technique; and the development of new unified analytical model for the extraction of dielectric properties of materials under test. The performance of these improved and new sensors, novel measurement techniques and the new derived analytical models of extracting the materials properties were verified through a large number of measurements on various materials. Dielectric characterisation of crude oils and their derivatives at 2 GHz using re-entrant cavity and concentration measurements of both binary and ternary mixtures at discrete resonance frequencies of 2 GHz and 6 GHz were reported. Measurements of low and medium dielectric loss liquids over a broadband using the coaxial cavity resonator at four different resonant modes from 2 GHz to 8 GHz were presented. Furthermore, measurement of liquid materials using a 60 GHz cavity resonator, dielectric constant measurement of substrates materials using modified re-entrant cavity at 2 GHz and estimation of the effective conductivity of the metals, metal coated polymers and alloys made using different fabrication technology were carried out to validate the devices and measurement techniques. Experimental results are in good agreement with both simulation and reported data in the literature

A Miniaturized and Highly Sensitive Microwave Sensor Based on CSRR for Characterization of Liquid Materials

In this work, a miniaturized and highly sensitive microwave sensor based on a complementary split-ring resonator (CSRR) is proposed for the detection of liquid materials. The modeled sensor was designed based on the CSRR structure with triple rings (TRs) and a curve feed for improved measurement sensitivity. The designed sensor oscillates at a single frequency of 2.5 GHz, which is simulated using an Ansys HFSS simulator. The electromagnetic simulation explains the basis of the mode resonance of all two-port resonators. Five variations of the liquid media under tests (MUTs) are simulated and measured. These liquid MUTs are as follows: without a sample (without a tube), air (empty tube), ethanol, methanol, and distilled water (DI). A detailed sensitivity calculation is performed for the resonance band at 2.5 GHz. The MUTs mechanism is performed with a polypropylene tube (PP). The samples of dielectric material are filled into PP tube channels and loaded into the CSRR center hole; the E-fields around the sensor affect the relationship with the liquid MUTs, resulting in a high Q-factor value. The final sensor has a Q-factor value and sensitivity of 520 and 7.032 (MHz)/Er) at 2.5 GHz, respectively. Due to the high sensitivity of the presented sensor for characterizing various liquid penetrations, the sensor is also of interest for accurate estimations of solute concentrations in liquid media. Finally, the relationship between the permittivity and Q-factor value at the resonant frequency is derived and investigated. These given results make the presented resonator ideal for the characterization of liquid materials.Publicad

Radio-Frequency Sensors for High Performance Liquid Chromatography Applications

As a fast-developing analytical technique for separation, purification, identification and quantification of components in a mixture, high performance liquid chromatography (HPLC) has been widely used in various fields including biology, food, environment, pharmacy and so on. As a critical part in the HPLC system, the detector with the feature of high sensitivity, universal detection and gradient-elution compatibility is highly desired. In this dissertation, two types of radio-frequency (RF) sensors for HPLC gradient applications are presented: a tunable interferometer (TIM) and a modified square ring loaded resonator (SRLR). For the TIM-based sensor, the sensitivity is evaluated by measuring a few common chemicals in DI water at multiple frequencies from 0.98 GHz to 7.09 GHz. Less than 84 ppm limit of detection (LOD) is demonstrated. An algorithm is provided and used to obtain sample dielectric permittivity at each frequency point. When connected to a commercial HPLC system and injected with a 10 μL aliquot of 10000 ppm caffeine DI-water solution, the sensor yields a signal-to-noise ratio (SNR) up to 10 under isocratic and gradient elution operations. Furthermore, the sensor demonstrates a capability to quantify co-eluted vitamin E succinate (VES) and vitamin D3 (VD3). For the SRLR-based sensor, where a transmission line and a ring are electrically shorted with a center gap, the detection linearity is characterized by measuring water-caffeine samples from 0.77 ppm to 1000 ppm when connected to the HPLC system. A 0.231 ppm limit of detection (LOD) is achieved, revealing a comparable sensitivity with commercial ultraviolet (UV) detectors. The compatibility of the proposed sensor to gradient elution is also demonstrated. Besides, this work presents a method for the measurement of liquid permittivity without using liquid reference materials or calibration standards. The method uses a single transmission line and a single microfluidic channel which intercepts the line twice. As a result, two transmission line segments are formed with channel sections to measure liquid samples. By choosing a 2:1 ratio for the two line segment lengths, closed-form formulas are provided to calculate line propagation constants directly from measured S-parameters. Then, sample permittivity values are obtained. A coplanar waveguide is built and tested with de-ionized water, methanol, ethanol and 2-propanol from 0.1 GHz to 9 GHz. The obtained performance agrees with simulation results. The obtained sample permittivity values agree with commonly accepted values. Radiofrequency (RF) non-thermal (NT) bio-effects have been a subject of debate and attracted significant interests due to the potential health risks or beneficial applications. A miniature transverse electro-magnetic (TEM) device is designed for broadband investigation of RF NT effects on Saccharomyces cerevisiae growth, a common yeast species. The frequency-dependent yeast permittivity, obtained by measuring the difference between the medium and yeast in the medium, was used to select the applied RF frequencies, i.e., 1.0 MHz, 3.162 MHz, 10 MHz and 905 MHz. The results showed that the RF field at 3.162 MHz reduced yeast growth rates by 11.7%; however, the RF fields at 1.0 MHz and 10 MHz enhanced cell growth rates by 16.2% and 4.3%, respectively. In contrast, the RF field at 905 MHz had no effect on the growth rates

Soil moisture remote sensing using SIW cavity based metamaterial perfect absorber

Continuous and accurate sensing of water content in soil is an essential and useful measure in the agriculture industry. Traditional sensors developed to perform this task suffer from limited lifetime and also need to be calibrated regularly. Further, maintenance, support, and deployment of these sensors in remote environments provide additional challenges to the use of conventional soil moisture sensors. In this paper, a metamaterial perfect absorber (MPA) based soil moisture sensor is introduced. The ability of MPAs to absorb electromagnetic signals with near 100% efficiency facilitates the design of highly accurate and low-profile radio frequency passive sensors. MPA based sensor can be fabricated from highly durable materials and can therefore be made more resilient than traditional sensors. High resolution sensing is achieved through the creation of physical channels in the substrate integrated waveguide (SIW) cavity. The proposed sensor does not require connection for both electromagnetic signals or for adding a testing sample. Importantly, an external power supply is not needed, making the MPA based sensor the perfect solution for remote and passive sensing in modern agriculture. The proposed MPA based sensor has three absorption bands due to the various resonance modes of the SIW cavity. By changing the soil moisture level, the absorption peak shifts by 10 MHz, 23.3 MHz, and 60 MHz, which is correlated with the water content percentage at the first, second and third absorption bands, respectively. Finally, a 6×6 cell array with a total size of 312mm×312mm has been fabricated and tested. A strong correlation between measurement and simulation results validates the design procedure