Measurement of the broadband complex permittivity of soils in the frequency domain with a low-cost Vector Network Analyzer and an Open-Ended coaxial probe

The performance of a handheld Vector Network Analyzer (VNA), the nanoVNA, a low-cost, open-source instrument, was evaluated. The instrument measures the complex permittivity of dielectric media from 1-port reflection parameters in the 1 – 900 MHz bandwidth. We manufactured an open-ended coaxial probe using a SMA-N coaxial adapter to perform dielectric measurements. The accuracy of the nanoVNA was comparable to that of a commercial VNA between 1 and 500 MHz according to tests in reference organic liquids, while a lack of stability was found beyond 700 MHz. The self-manufactured open-ended coaxial probe was subjected to a Finite Element Method (FEM) analysis and its electromagnetic (EM) field penetration depth was determined to be 1.5 mm at 100 MHz, being reduced to 1.3 at 900 MHz and thus demonstrating a frequency-dependent support volume. The broadband complex permittivity of three mineral soils of varied textures was obtained for a range of bulk densities and water contents from dry to water-saturated conditions. The dielectric response of the soils approximated the well-known Topp et al. (1980) equation at high frequencies. At lower frequency however, higher permittivities were exhibited due to dielectric dispersion, which emphasizes the importance of EM-based soil moisture sensor operating frequency when considering sensor calibration or comparing the response of different sensors.This research was funded by Agencia Estatal de Investigación (AEI), project numbers: AGL2016-77282-C3-3-R and PID2019-106226-C22 AEI/https:///https://doi.org/10.13039/501100011033 | Ministerio de Educación y Formación Profesional, grant numbers: FPU17/05155 and FPU19/00020. Funding for David A. Robinson was provided by a Natural Environment Research Council (NERC) award number NE/R016429/1 as part of the UK–ScaPE Programme Delivering National Capability. We also acknowledge funding from the Polish National Agency for Academic Exchange, grant number: PPI/APM/2018/1/00048/U/001. The authors wish to thank Agencia Estatal de Investigación (AEI), Ministerio de Educación y Formación Profesional, Natural Environment Research Council (NERC) and Polish National Agency for Academic Exchange (NAWA) for the funding provided. The authors also wish to thank Juan Antonio Albaladejo for his help in machining the experimental OE coaxial probe

Measurement of the broadband complex permittivity of soils in the frequency domain with a low-cost vector network analyzer and an open-ended coaxial probe

The performance of a handheld Vector Network Analyzer (VNA), the nanoVNA, a low-cost, open-source instrument, was evaluated. The instrument measures the complex permittivity of dielectric media from 1-port reflection parameters in the 1 – 900 MHz bandwidth. We manufactured an open-ended coaxial probe using a SMA-N coaxial adapter to perform dielectric measurements. The accuracy of the nanoVNA was comparable to that of a commercial VNA between 1 and 500 MHz according to tests in reference organic liquids, while a lack of stability was found beyond 700 MHz. The self-manufactured open-ended coaxial probe was subjected to a Finite Element Method (FEM) analysis and its electromagnetic (EM) field penetration depth was determined to be 1.5 mm at 100 MHz, being reduced to 1.3 at 900 MHz and thus demonstrating a frequency-dependent support volume. The broadband complex permittivity of three mineral soils of varied textures was obtained for a range of bulk densities and water contents from dry to water-saturated conditions. The dielectric response of the soils approximated the well-known Topp et al. (1980) equation at high frequencies. At lower frequency however, higher permittivities were exhibited due to dielectric dispersion, which emphasizes the importance of EM-based soil moisture sensor operating frequency when considering sensor calibration or comparing the response of different sensors

Numerical Analysis of a Conical Type Coaxial Open-Ended Probe for Dielectric Measurement

RÉSUMÉ L'objectif principal de cette recherche est de trouver un modèle précis pour le calcul de la constante diélectrique et le facteur de perte d'un matériau diélectrique homogène partir d'un coefficient de réflexion mesurée a l'aide d'une sonde coaxiale de type conique dans les fréquences radio et micro-ondes. Ce type de sonde peut être facilement inséré dans plusieurs types de tissus biologiques et dans des matériaux semi-rigides comme le caoutchouc, certains plastiques et des matières organiques (ex.: des produits laitiers comme le beurre, pour en mesurer la teneur en humidité). Cette caractéristique est très importante pour des applications biologiques et industrielles. Le principe à la base de ce type de mesure est la détection de la variation de capacité par rapport au mouvement de la masse diélectrique dans le champ de frange électrique. L'admittance d'entrée électrique, ainsi que le coefficient de réflexion, sont calculés à l'aide d'une analyse par éléments finis. Contrairement à de nombreuses autres techniques de modélisation utilisées pour l'étude des sondes coaxiales qui sont approximatif et donc limitées, le modèle par éléments finis est plus précis et est applicable aux géométries complexes. Afin de valider le modèle numérique, une étude expérimentale a été menée en parallèle en laboratoire, avec mêmes dimensions géométriques.Les résultats sont compares et une excellente correspondance est observée, ce qui démontre que la modélisation par éléments finis est une bonne approche pour la conception optimale de sondes coaxiales coniques. Une formulation éléments finis tridimensionnelle est utilisée dans la région du matériau diélectrique et dans une petite région voisine de la structure de la sonde sur laquelle elle est montrée. Le coefficient du facteur de réflexion, le champ électromagnétique à l'ouverture de la sonde et la permittivité de différents matériaux ont été analysés pour des fréquences de 300 MHz à 3 GHz. De plus, une étude comparative de trois modèles de sondes coniques à ouverture coaxiale (modèle capacitif, modèle d'antenne et modèle améliore de la ligne virtuelle) qui fait un lien entre le coefficient de réflexion à la permittivité complexe du matériau testé est présentée. L'influence de l'angle du cône et de la forme du câble coaxial (plat-conique-elliptique) est aussi étudiée en détail. La conception de sondes diélectriques ainsi que leur utilisation efficace nécessite des méthodes de calcul précises et optimales, validées de façon rigoureuse et à coûts de calculs réduits.---------- ABSTRACT The main objective of this research is to find an accurate model for computing the dielectric constant and the loss factor of a homogeneous dielectric material from a measured reflection coefficient by using conical-type coaxial probe in radio and microwave frequencies. This sort of probe can be easily inserted into a wide range of biological tissue types and semi-rigid materials like rubber, some plastics, and organic materials (e.g., dairy, butter, etc.) for measuring moisture content. This feature is very important in biological and industrial applications. The measuring principle is based on detecting capacitance change with respect to the dielectric mass movement in the fringe electrical field. The electrical input admittance as well as the reflection coefficient is found from the finite-element analysis. In contrast to many other modelling techniques used for coaxial probe which are approximate and hence limited, the finite-element model is more accurate and is applicable to complicated geometries. To demonstrate the accuracy of the numerical model, a parallel experimental study was carried out in the laboratory in the same geometric dimensions. The numerical and experimental results are compared and showed an excellent agreement, demonstrating that finite-element modelling is a good approach for optimized conical coaxial probe design. A three-dimensional finite-element formulation is employed in the dielectric material region and a small neighbouring region of the probe structure on which it is mounted. The reflection coefficient factor, electromagnetic field at the probe aperture and permittivity of different materials were analyzed in the frequency range of 300 MHz to 3 GHz. Moreover, a comparative study of three conical open-ended coaxial probe models (capacitive model, antenna model, improved virtual line model) which relate the reflection coefficient to the complex permittivity of the Material Under Test (MUT) is presented. In addition, the effects of the cone angle and shape of the coaxial cable (at-conical-elliptical) are studied and evaluated in detail in this research. It is shown that for designing an efficient and operational dielectric measurement probe, an accurate and computationally efficient method with rigorous validation and reduced computational burden is required

Fiber-optic and coaxial-cable extrinsic Fabry-Perot interferometers for sensing applications

”The fiber-optic extrinsic Fabry-Perot interferometer (EFPI) is one of the simplest sensing configurations and is widely used in various applications due to its prominent features, such as high sensitivity, immunity to electromagnetic interference, and remote operation capability. In this research, a novel one-dimensional wide-range displacement sensor and a three-dimensional displacement sensor based on fiber-optic EFPIs are demonstrated. These two robust and easy-to-manufacture sensors expand the application scope of the fiber-optic EFPI sensor devices, and have great potential in structural health monitoring, the construction industry, oil well monitoring, and geo-technology. Furthermore, inspired by the fiber-optic EFPI, a novel and universal ultra-sensitive microwave sensing platform based on an open-ended hollow coaxial cable resonator (OE-HCCR, i.e., the coaxial cable EFPI) is developed. Both the theoretical predictions and experimental results demonstrate the ultra-high sensitivity of the OE-HCCR device to variations of the gap distance between the endface of the coaxial cable and an external metal plate. Additionally, combining the chemical-specific adsorption properties of metal-organic framework (MOF) materials with the dielectric sensitivity of the OE-HCCR, a mechanically robust and portable gas sensor device (OE-HCCR-MOF) with high chemical selectivity and sensitivity is proposed and experimentally demonstrated. Due to its low cost, high sensitivity, all-metal structure, robustness, and ease of signal demodulation, it is envisioned that the proposed OE-HCCR device will advance EFPI sensing technologies, revolutionize the sensing field, and enable many important sensing applications that take place in harsh environments”--Abstract, page iv

Modelado de las propiedades dieléctricas del suelo. Aplicación en el diseño de sensores para sistemas de control en agricultura de precisión

[SPA] Esta tesis doctoral se presenta bajo la modalidad de compendio de publicaciones. El agua es una sustancia clave para el desarrollo de la vida en La Tierra. Es por ello que la búsqueda de oportunidad de vida en otros planetas y satélites se basa en la presencia de agua en los mismos. La gestión ecológica del agua es necesaria para la sostenibilidad de los ecosistemas. Uno de los ecosistemas más amplios y donde el agua juega un papel más importante es el suelo, que alberga multitud de variedades de microorganismos cuya actividad, en parte resultante en la generación de nutrientes para el desarrollo de las especies vegetales, es totalmente dependiente del contenido de agua en el suelo. En zonas áridas y semiáridas, como es el caso de la cuenca Mediterránea, la escasez de agua supone un grave problema a la hora de gestionar los pocos recursos hídricos disponibles. En este caso, donde las condiciones geográficas son idóneas para el desarrollo de la agricultura, las soluciones pasan por una optimización de las técnicas de riego y un mayor control sobre los recursos hídricos. En este sentido, las técnicas de riego deficitario controlado se han mostrado exitosas en la reducción de la dotación hídrica a los cultivos en fases no críticas. Sin embargo, para realizar una aplicación prudente y eficiente de las mismas, resulta necesario monitorizar el estado hídrico de los cultivos, con el objetivo de que éstos no alcancen situaciones de estrés irreversible en términos de producción o estado vegetativo. Los indicadores que mayor información aportan sobre el estado hídrico de la planta suelen estar relacionados con variables medibles a partir de la propia planta, pero que son difícilmente automatizables debido a las operaciones de manejo asociadas. Este es el caso del potencial hídrico de tallo a mediodía medido con cámara de presión, considerado hasta la fecha como el indicador más fiable del estado hídrico de los cultivos en general. Es por ello que, para lograr una monitorización continua de esta variable, se busquen otras variables del continuo suelo-planta-atmósfera que puedan estar relacionadas y a partir de las cuales obtener una estimación indirecta. El suelo es la matriz de donde la planta adquiere la mayor parte del agua y los nutrientes que necesita para realizar la fotosíntesis. La relación entre el estado hídrico del suelo y el estado hídrico de los cultivos está más que demostrada. Sin embargo, la precisión alcanzada en los modelos de correlación entre ambos estados requiere de una mejora considerable para hacer un uso realmente fiable de los mismos, y esta mejora no solo pasa por encontrar mejores métodos de correlación, sino también por mejorar la precisión de las medidas obtenidas del suelo. Para monitorizar el estado hídrico del suelo, existen diversas metodologías que ofrecen parámetros medibles como el contenido de agua. El método de medida más extendido para monitorizar el contenido de agua en el suelo es a través del uso de sensores dieléctricos. Sin embargo, la precisión de los mismos está sujeta a diversos factores, entre ellos las características propias del suelo donde se instalan y su coste, relativamente alto para el pequeño y mediano agricultor, condicionando una implantación extensiva de la Agricultura de Precisión y limitando a veces la aplicación de algunos desarrollos únicamente a trabajos de investigación. Esta tesis, elaborada bajo la modalidad de compendio de publicaciones, aborda a través de cuatro artículos científicos la propuesta de soluciones accesibles para la medida del estado hídrico del suelo, con especial enfoque en el contenido de agua; explora las limitaciones y retos asociados con la calibración de los sensores dieléctricos de suelo; participa en la generación de nuevos conocimientos y propuestas para un mejor entendimiento del comportamiento del agua en el suelo y de su interacción con las ondas electromagnéticas; y establece nuevos enfoques y modelos que mejoran la predicción del estado hídrico de los cultivos a partir de medidas indirectas y automatizables en suelo y atmósfera. [ENG] This doctoral dissertation has been presented in the form of thesis by publication. Water is a fundamental substance for the development of life on Earth. That is why the search for life on other planets and satellites is based on the presence of water on them. Ecological water management is necessary for the sustainability of ecosystems. One of the most extensive ecosystems where water plays a major role is soil, which hosts a large variety of micro-organisms whose activity, partly resulting in the generation of nutrients for the development of plant species, is totally dependent on the water content of the soil. In arid and semi-arid regions, as it is the case in the Mediterranean basin, water scarcity is a serious problem when it comes to managing the few water resources available. In this case, where the geographical conditions are ideal for the development of agriculture, the solutions involve optimization of irrigation techniques and greater control over water resources. In this sense, regulated deficit irrigation strategies have proven to be successful in reducing the water supply to crops in non-critical periods. However, in order to apply them prudently and efficiently, it is necessary to monitor the water status of the crops, so that they do not reach irreversible stress situations in terms of yield or vegetative state. The indicators that provide the highest amount of information on the water status of the plant are usually related to variables that can be measured from the plant itself, but which are difficult to automate due to the labor and time-consuming associated operations. This is the case of the midday stem water potential measured with a pressure chamber, considered to date to be the most reliable indicator of the crop's water status in general. In order to achieve a continuous monitoring of this variable, it is necessary to look for other variables of the soil-plant-atmosphere continuum that may be related and from which to obtain an indirect estimate. Soil is the matrix from which the plant acquires most of the water and nutrients it needs for photosynthesis. The relationship between soil water status and crop water status is well established. However, the accuracy achieved in the correlation models between the two requires considerable improvement to make a truly reliable use of them, and this improvement is not only to find better correlation methods, but also to improve the accuracy of the measurements obtained from the soil. To monitor soil water status, there are several methodologies that provide measurable parameters such as water content. The most widespread measurement method for monitoring soil water content is through the use of dielectric sensors. However, the accuracy of these sensors is subject to various factors, including the characteristics of the soil where they are installed, and their relatively high cost for small and medium-sized farmers, conditioning the extensive implementation of precision agriculture and sometimes limiting the application of some developments only to research work. This thesis, elaborated under the modality of a compendium of publications, addresses through four scientific articles the proposal of affordable solutions for the measurement of soil water status, with special focus on water content; it explores the limitations and challenges associated with the calibration of soil dielectric sensors; participates in the generation of new insights and proposals for a better understanding of the behavior of water in soil and its interaction with electromagnetic waves; and establishes new approaches and models that improve the prediction of crop water status from indirect and automatable measurements in soil and atmosphere.Esta tesis doctoral se presenta bajo la modalidad de compendio de publicaciones. Está formada por un total de cuatro artículos: Article I. González-Teruel, J.D., Torres-Sánchez, R., Blaya-Ros, P.J., Toledo-Moreo, A.B., Jiménez-Buendía, M., Soto-Valles, F., 2019. Design and Calibration of a Low-Cost SDI-12 Soil Moisture Sensor. Sensors, 19, 491. DOI: 10.3390/s19030491 - Article II. González-Teruel, J.D., Jones, S.B., Soto-Valles, F., Torres-Sánchez, R., Lebron, I., Friedman, S.P., Robinson, D.A., 2020. Dielectric Spectroscopy and Application of Mixing Models Describing Dielectric Dispersion in Clay Minerals and Clayey Soils. Sensors, 20, 6678. DOI: 10.3390/s20226678 Article III. González-Teruel, J.D., Jones, S.B., Robinson, D.A., Giménez-Gallego, J., Zornoza, R., Torres-Sánchez, R., 2022. Measurement of the broadband complex permittivity of soils in the frequency domain with a low-cost Vector Network Analyzer and an Open-Ended coaxial probe. Computers and Electronics in Agriculture, 195, 106847. DOI: 10.1016/J.COMPAG.2022.106847 Article IV. González-Teruel, J.D., Ruiz-Abellon, M.C., Blanco, V., Blaya-Ros, P.J., Domingo, R., Torres-Sánchez, R., 2022. Prediction of Water Stress Episodes in Fruit Trees Based on Soil and Weather Time Series Data. Agronomy, 12, 1422. DOI: 10.3390/agronomy12061422Escuela Internacional de Doctorado de la Universidad Politécnica de CartagenaUniversidad Politécnica de CartagenaPrograma de Doctorado en Tecnologías Industriale