Global navigation satellite system (GNSS)-reflectometry is a type of remote sensing technology and can be applied to soil moisture retrieval. Until now, various GNSS-R soil moisture retrieval methods have been reported. However, there still exist some problems due to the complexity of modeling and retrieval process, as well as the extreme uncertainty of the experimental environment and equipment. To investigate the behavior of bistatic GNSS-R soil moisture retrieval process, two ground-truth measurements with different soil conditions were carried out and the performance of the input variables was analyzed from the mathematical statistical aspect. Moreover, the feature of XGBoost method was utilized as well. As a recently developed ensemble machine learning method, the XGBoost method just emerged for the classification of remote sensing and geographic data, to investigate the characterization of the input variables in the GNSS-R soil moisture retrieval. It showed a good correlation with the statistical analysis of ground-truth measurements. The variable contributions for the input data can also be seen and evaluated. The study of the paper provides some experimental insights into the behavior of the GNSS-R soil moisture retrieval. It is worthwhile before establishing models and can also help with understanding the underlying GNSS-R phenomena and interpreting data.

A method for the synthesis of optical filters consisting of a cascade of N-coupled rings is presented. The procedure is based on the definition of a polynomial whose roots are the zeros of the channel-dropping transmittance characteristic and provides directly the ring electrical lengths and the mutual coupling coefficients. A design example of a Chebyshev-type 6-ring bandpass filter is presented.

In recent years, low-cost carbons derived from recycled materials have been gaining attention for their potentials as filler in composites and in other applications. The electrical and mechanical properties of polymer composites can be tuned using different percentages and different kind of fillers: either low-cost (e.g., carbon black), ecofriendly (e.g., biochar), or sophisticated (e.g., carbon nanotubes). In this work, the mechanical and electrical behavior of composites with biochar and multiwall carbon nanotubes dispersed in epoxy resin are compared. Superior mechanical properties (ultimate tensile strength, strain at break) were noticed at low heat-treated biochar (concentrations 2–4 wt %). Furthermore, dielectric properties in the microwave range comparable to low carbon nanotubes loadings can be achieved by employing larger but manageable amounts of biochar (20 wt %), rending the production of composites for structural and functional application cost-effective.

This letter presents a voltage-controlled tunable planar antenna based on few-layer graphene flakes. The antenna consists of a rectangular patch with a shorted microstrip stub connected to the radiating edge, and a graphene pad located at the input of the stub. The proposed design exploits the variation of the graphene resistance by an applied bias voltage. Without any bias voltage, the graphene pad behaves almost as an open circuit, not allowing any current passing and, thus, voiding the impact of the stub. Increasing the bias voltage reduces the graphene resistance, thus increasing the current passing through the pad into the stub. This results in the patch antenna radiating at a different frequency. A prototype operating at the frequency of 5 GHz has been designed and tested, demonstrating a frequency tunability larger than 10% with a limited gain degradation.