Permittivity of materials is of utmost importance for microwave applicators’ design and to predict high-frequency dielectric heating of materials. In the case of aromatic plant biomass, however, there are few data that help researchers design microwave applicators for microwave-assisted extraction. In this work, the permittivity of cypress and rockrose biomass samples were measured versus temperature, density, and moisture content. A resonant technique based on a coaxial bi-reentrant microwave cavity was employed to obtain the complex permittivity of biomass samples as a function of those magnitudes around the 2.45 GHz ISM frequency. The obtained measurements show that large variations for permittivity values can be found with moisture content and density changes for both cypress and rockrose biomass. Temperature also has effects in a lesser degree, although it has an important influence on the cypress biomass loss factor. Polynomial expressions fitting the experimental data were provided in order to facilitate the estimation of intermediate values, which were not explicitly arranged in this work. As a general trend, the permittivity of cypress and rockrose biomass increases with increasing values of moisture content and density, whereas the biomass loss factor increases when temperature rises.

Quinoa is a nutrient-rich pseudocereal that is gaining popularity in European countries since it is gluten-free and an interesting source of fat, proteins, minerals, and amino acids. However, up to date, the electric permittivity of quinoa seeds has not been measured and, therefore, this hampers the possibility of designing optimized recipes for its microwave processing. In this work, the permittivity of both raw and boiled quinoa seeds is measured around 2.45 GHz at several conditions for temperature, moisture content, and bulk density. The grain kernel permittivity is also estimated from the Complex Refractive Index (CRI) mixture equation and different bulk density measurements. The obtained results show different temperature behaviours for raw and boiled seeds, whereas the permittivity of quinoa seeds versus moisture content and bulk density was as expected: the permittivity (both the dielectric constant and loss factor) levels increased as the observed variables did. From the measured data, it can be concluded that both raw and boiled quinoa can be processed with microwave technology, although care must be taken with raw quinoa grain kernels since the permittivity increases significantly with temperature, and a thermal runaway may occur.

Fabric permittivity is critical for the manufacturing of wearable sensors and antennas as well as predicting how fabrics interact with electromagnetic fields. Engineers should also understand how permittivity changes under different temperatures, densities, and moisture content values, or when several fabrics are mixed in aggregates, when designing future applications such as microwave dryers. The permittivity of cotton, polyester, and polyamide fabric aggregates is investigated in this paper for a wide range of compositions, moisture content levels, density values, and temperature conditions around the 2.45 GHz ISM band using a bi-reentrant resonant cavity. The obtained results show extremely comparable responses for all characteristics investigated for single and binary fabric aggregates. Permittivity always increases as temperature, density, or moisture content levels rise. Moisture content is the most influential characteristic, causing enormous variations in the permittivity of aggregates. Fitting equations are supplied for all data, with exponential functions used to accurately model variation in temperature and polynomial functions employed to precisely model density and moisture content variations with low error levels. The temperature permittivity dependence of single fabrics without the influence of air gaps is also extracted from fabric and air aggregates by using complex refractive index equations for two-phase mixtures.

Electromagnetic shielding of metallic enclosures with an aperture are simulated and measured in the frequency and time domain in this paper. Recently, several new figures of shielding effectiveness (SE) in the time domain have been proposed. Much work has been done regarding numerical simulations; however, little work related to measurements has been carried out. In this paper, we obtain the simulated and measured results for these SE definitions when an incident plane wave, with a determined bandwidth, excites the enclosure. The plane wave can be treated as a reference interference to compare with other cases. Measurements and simulations are in good agreement. This study evaluates the new definitions and compares them with the classical definition in the frequency domain. The effect of the probes, a dipole or a loop, has also been analyzed. Finally, the SE values are obtained for a sweep of the size of the aperture providing a unique value of equivalent SE for the most critical parameter and for a determined bandwidth, and allowing direct comparison with other enclosures.