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Abstract: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.

Estimating the characteristics of soil surface represents a significant area in applications such as hydrology, climatology, and agriculture. Signals transmitted from Global Navigation Satellite Systems (GNSSs) can be used for soil monitoring after reflection from the Earth's surface. In this paper, the feasibility of obtaining surface characteristics from the power ratio of lefthand (LH) reflected signal-to-noise ratio (SNR) over direct righthand (RH) is investigated. The analysis was done regardless of the surface roughness and the incoherent components of the reflected power. First, the analysis was carried out on data collected during several in situ measurements in controlled environments with known characteristics. Then, further data were collected by a GNSS receiver prototype installed on a small aircraft and analyzed. This system was calibrated on the basis of signals reflected from water. The reflectivity and the estimated permittivity showed good correlation with the types of underlying terrain.

Abstract-In the ongoing search for new materials for microwave absorption applications, Carbon Nanotubes deserve a special consideration due to their outstanding properties. In this paper, microwave absorbing properties of epoxy resin based composites containing commercial Multi Walled Carbon Nanotubes used as fillers have been analyzed. The complex permittivity of the composites was measured in a wide frequency band (3-18 GHz). The absorbing properties of a single-layer absorber backed by a metallic plate considering several concentration of CNTs was simulated taking into account the measured permittivity.

In recent years there has been a great interest in the development of high-altitude platforms, which are low cost stratospheric aircraft carrying payloads tailored for a wide range of applications in telecommunications and remote sensing. These platforms are capable of flying at altitudes ranging between 17 and 30 Km, with a potential endurance of weeks to months, features that make them attractive for the provision of future personal communication services. This paper deals with the theoretical derivation of a channel model for the communication link between the platform and terrestrial mobile users or stations. In particular, we address the problem of modeling the small-scale fading effects. It is shown that the particular geometry of the propagation scenario leads to a specific model applicable to the stratospheric channel.

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This paper 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.