José Manuel Catalá-Civera

Accurate determination of the complex permittivity of materials with transmission reflection measurements in partially filled rectangular waveguides

An enhanced transmission reflection technique for the precise determination of the complex permittivity of dielectric materials partially filling the cross section of a rectangular waveguide is described. Dielectric properties are determined by an iterative procedure from two-port S-parameter measurements and a numerically generated propagation constant obtained from the analysis of a partially filled waveguide. Convergence of the solution is ensured from perturbational approximations. Unlike previous approaches, an uncertainty investigation is performed, taking into account all the parameters involved in the dielectric characterization. Permittivity accuracy values are presented and, hence, an optimum measurement setup can be established. Measurements of reference materials have been carried out to validate the method.

Effect of mode-stirrer configurations on dielectric heating performance in multimode microwave applicators

In this paper, several mode-stirrer configurations are compared in order to establish their influence on the electric-field uniformity within an irradiated dielectric sample inserted in a microwave-heating applicator. Two different scenarios are evaluated with metallic sheets moving inside the multimode applicator. The different stirrer configurations are tested and compared for low-, medium-, and high-loss dielectric sample materials. Additionally, a straightforward procedure based on a generalized plane-wave approach is proposed and evaluated as a computationally efficient alternative for calculating the electric-field distribution inside materials processed in these microwave applicators with mode stirrers. Although very different electric patterns are achieved depending on stirrer geometry and sample permittivity, the plane-wave approach has been shown to provide a very good approximation for medium and high lossy dielectric materials.

A novel technique for deembedding the unloaded resonance frequency from measurements of microwave cavities

A novel technique to extract the influence of coupling networks on the resonant frequency of cavities in one-port measurements is presented. The determination of the unloaded resonant frequency is performed directly from measurements without either the need to obtain the electromagnetic fields in the resonator or to deembed the delay of transmission lines from the measuring equipment to the resonator. The importance of the Fosters form on the modeling of the frequency detuning of the resonators is also discussed and a criterion for the choice of the appropriate Fosters form is suggested. The procedure is validated with simulations and experimental measurements of manufactured cavities

Analysis, design, and experimental verification of microwave filters for safety issues in open-ended waveguide systems

Safety issues must be seriously considered in the practical implementation of microwave industrial systems with open ports. To preserve the radiation of these open-ended waveguide systems into permissible levels, bandstop microwave filters are widely used. In this paper, the accurate analysis, design, and experimental verification procedure of such filters are extensively studied. A singly and a doubly corrugated filter for continuous flow microwave industrial systems are designed. Two prototypes of such devices have been manufactured and experimentally verified.

Determination of the permittivity and permeability for waveguides partially loaded with isotropic samples

A method to measure the complex permittivity and permeability of a sample of isotropic material partially filling the rectangular waveguide cross section is presented. The method is based on a multimodal analysis of the waveguide and it does not imply any restriction on the general use of this technique for different widths, lengths or location of the samples. Electromagnetic properties of the material are determined by matching the S-parameters measured with a vector network analyser and the S-parameters calculated theoretically. Convergence to the correct solution is ensured using an optimization technique based on nonlinear minimization. A comparison between theoretical and experimental measurements is realized, which shows the feasibility of the implemented software. Besides, the accurate determination of the properties of the well-known materials validates the performance of the measurement method. Also, an error analysis is performed which confirms the reliability and robustness of our method.

Dynamic Measurement of Dielectric Properties of Materials at High Temperature During Microwave Heating in a Dual Mode Cylindrical Cavity

A microwave cavity and heating system for microwave processing and in situ dynamic measurements of the complex permittivity of dielectric materials at high temperatures ( ~ 1000 °C) has been developed. The method is based on a dual-mode cylindrical cavity where heating and testing are performed by two different swept frequency microwave sources. A cross-coupling filter isolates the signals coming from both sources. By adjusting the frequency bandwidth of the heating source and the level of coupling to the cavity, an automatic procedure allows for the establishment of a desirable level of heating rate to the dielectric sample to reach high temperatures in short cycles. Dielectric properties of materials as a function of temperature are calculated by an improved cavity perturbation method during heating. Accuracy of complex permittivity results has been evaluated and an error lower than 5% with respect to a rigorous analysis of the cavity has been achieved. The functionality of the microwave dielectric measurement system has been demonstrated by heating and measuring glass and ceramic samples up to 1000 °C. The correlation of the complex permittivity with the heating rate, temperature, absorbed power, and other processing parameters can help to better understand the interactions that take place during microwave heating of materials at high temperatures compared to conventional heating.

Enhancement of Sensitivity of Microwave Planar Sensors With EBG Structures

A strategy to improve microstrip sensor performance for monitoring dielectric properties of materials is proposed. The method relies on the reduction of the wave group velocity to induce higher interaction between the sensor and the material. This is achieved by the design of periodic patterns in the sensor ground plane, which exhibit electromagnetic band gap (EBG) effects. The presence of these EBG structures turns out to be highly effective, inducing a noticeable decrease of the wave velocity. The sensitivity is defined and measured for different sensor configurations in order to quantify the improvements obtained. It is observed that, with the EBG structures, the residence time of the wave in the material under test is longer, and a substantial increase of the sensor sensitivity is obtained. EDICS Category-MICR

Design rules for the optimization of the sensitivity of open-ended coaxial microwave sensors for monitoring changes in dielectric materials

Open-ended coaxial probes are widely used for non-destructive measurement of dielectric properties of materials, and also as microwave sensors for industrial processes and quality control applications. The main design parameters of these sensors are the coaxial radii and working frequency. In this paper, the influence of these variables on the final sensitivity of the coaxial sensor when monitoring dielectric materials is analysed, and a novel expression for this parameter selection is proposed. Moreover, a method to select the optimum parameters of experimental configurations is provided. Measurements demonstrate that high discrimination can be achieved with this method when monitoring changes in the dielectric properties of materials.

Full-Wave Analysis of Dielectric-Loaded Cylindrical Waveguides and Cavities Using a New Four-Port Ring Network

In this paper, a full-wave method for the electromagnetic analysis of dielectric-loaded cylindrical and coaxial waveguides and cavities is developed. For this purpose, a new four-port ring network is proposed, and the mode-matching method is applied to calculate the generalized admittance matrix of this new structure. A number of analyses on dielectric-loaded waveguide structures and cavities have been conducted in order to validate and to assess the accuracy of the new approach. The results have been compared with theoretical values, numerical modeling from the literature, and data from commercial electromagnetic simulators. The method has been also applied to the accurate determination of dielectric properties, and we provide an example of these measurements as another way to validate this new method.

Simulation of temperature distributions in pressure-aided microwave rubber vulcanization processes

In this paper microwave heat generation in materials surrounded by dielectric molds is studied. Particularly, the pressure-aided microwave rubber vulcanization process, where the use of dielectric molds that exert the proper pressure over the rubber samples is essential for this application. Temperature distributions of rubber and mold materials have been simulated using a discretization strategy based on the FDTD and FEM techniques for one and two-dimensional cases, respectively. Simulation results show that the dielectric and thermal properties of the pressing mold are of the utmost importance in order to obtain the desired temperature profiles throughout the volume of the rubber sample and therefore to achieve a uniform degree of vulcanization in the rubber Experimental measurements in a monomode cylindrical cavity with several configurations of mold materials, pressing low thermal conductivity rubber samples have been performed and used to validate the simulation results.