Antonio Martínez-González

The interaction processes of electromagnetic fields with biological tissues have been extensively studied over the last decades. However, while the exposure of humans and animals to electromagnetic radiation has been the subject of numerous research papers, it is not so common to find studies in the scientific literature that address the effects of plant exposure to radioelectric emissions. In this paper, 24 different seed sets were exposed to an electromagnetic field continuous wave at 2.45 GHz of low and medium power levels in a controlled environment. Radiated power densities at seed location for low levels range from 3.5 to 3820 μW/cm ² in free space inside an anechoic chamber. For medium power levels a waveguide was used, which produced power density levels at seed location from 502.7 to 2097.9 mW/cm ² . Exposition time was also controlled from periods of just 4.7 seconds to 16 full days. Subsequently, the samples were transferred to a culture chamber for germination keeping growth conditions constant for all samples. Great care was taken in the strict protocols of sample preparation, radiation, and growth tests. Finally, different growth parameters were analyzed. The results show that, under certain exposure conditions, the germination process is affected. Generally, most of the seeds exposed to medium power, completed their germination earlier than the untreated control. In the other hand, seeds treated at low power and high exposure times generally slowed down their germination.

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.

Metallic stirrers are commonly used in mode-stirred reverberation chambers but are less common in microwave ovens, because the use of a metallic axis creates a coaxial port in the cavity wall where the axis is introduced, which may result in significant microwave leakage levels. However, due to electromagnetic, mechanical, temperature, or chemical requirements, metallic axes must be used in some cases. In this paper, we present a high power coaxial filter that enables the use of metallic stirrers in microwave applications. The filter employs two-port coaxial cavities that are analyzed to determine their design parameters. These cavities are coupled via coaxial lines to achieve proper attenuation levels. The cavities were designed using commercial electromagnetic (EM) software, and a filter prototype was validated using vector network analyzer (VNA) measurements. The simulated and measured results show that a three-stage configuration can achieve attenuations greater than 70 dB at the 2.45 GHz ISM band, allowing the use and external handling of the metallic axis inserted into microwave ovens with negligible microwave radiation leakage.

Distributed MIMO (D-MIMO) systems are expected to play a key role in deployments for future mobile communications. Together with massive MIMO technology, D-MIMO aims to maximize the spectral efficiency and data rate in mobile networks. This paper proposes a deep study on the spectral efficiency of D-MIMO systems for essential channel parameters, such as the channel power balance or the correlation between propagation channels. For that purpose, several propagation channels were acquired in both anechoic and reverberation chambers and were emulated using channel simulators. In addition, several frequency bands were studied, both the sub–6 GHz band and mmWave band. The results of this study revealed the high influence of channel correlation and power balance on the physical channel performance. Low-correlated and high-power balance propagation channels show better performances than high correlated and power unbalance channels in terms of spectral efficiency. Given these results, it will be fundamental to take into account the spectral efficiency of D-MIMO systems when designing criteria to establish multi-connectivity in future mobile network deployments.

In this article, a multimode characterization of coaxial-to-waveguide transition is carried out by means of inverse techniques. The multimode coaxial-to-waveguide transition is characterized by the generalized scattering matrix (GSM). A number of measured standards considering shorts are used to extract the matrices. By using optimizing techniques, such as genetic algorithms (GAs) and a gradient descent (GD)-based method, the multimodal scattering matrix is calculated with the help of a fitness function that compares the standards with analytical calculations. The same measurement setup using uniquely shorts has been simulated, and the GSM results have been retrieved, thus validating the proposed technique. This method easily allows working in the multimode range of waveguides, thus increasing considerably the bandwidth of classical applications, such as permittivity characterization. The benefits and drawbacks of the implementation are also discussed.

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.

Over-the-air characterization of wireless devices is performed using specific channel models. Channel models define the distribution of the incident signals over angle and polarization, but always assuming a large number of signal paths. However, in some real cases, the number of signal paths can be limited. This letter studies the effect of the number of signal paths on the high-speed downlink packet access (HSDPA) throughput, using a commercial phone. Differences in performance are found, depending on the number of signal paths. This highlights the importance of the number of signal paths in the characterization of wireless devices.