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Concrete is a porous, heterogeneous material whose abundant use in numerous applications demands a detailed understanding of its electrical properties. Besides experimental measurements, material theoretical models can be useful to investigate its behaviour with respect to frequency, moisture content or other factors. These models can be used in electromagnetic compatibility (EMC) to predict the shielding effectiveness of a concrete structure against external electromagnetic waves. This paper presents the development of a dispersive material model for concrete out of experimental measurement data to take account of the frequency dependence of concrete's electrical properties. The model is implemented into a numerical simulator and compared with the classical transmission-line approach in shielding effectiveness calculations of simple concrete walls of different moisture content. The comparative results show good agreement in all cases; a possible relation between shielding effectiveness and the electrical properties of concrete and the limits of the proposed model are discussed.

This paper presents an investigation of the transmitted power in a wireless power transfer system that employs a metamaterial. Metamaterials are a good means to transfer power wirelessly, as they are composed of multiple inductively-coupled resonators. The system can be designed and matched simply through magneto-inductive wave theory, particularly when the receiver inductor is located at the end of the metamaterial line. However, the power distribution changes significantly in terms of transmitted power, efficiency and frequency if the receiver inductor slides along the line. In this paper, the power distribution and transfer efficiency are analysed, studying the effects of a termination impedance in the last cell of the metamaterial and improving the system performance for the resonant frequency and for any position of the receiver inductor. Furthermore, a numerical characterisation is presented in order to support experimental tests and to predict the performance of a metamaterial composed of spiral inductor cells with very good accuracy.

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Leakage currents from a dc electrified railway requires careful consideration throughout the life time of the railway. The design objective is to minimise the current leakage from the railway return path (the running rails for a dc electrified railway, with possibly additional conductors in parallel for ac electrified systems). For this objective to be met, the design needs to account for foreseeable misuse during the construction phase (i.e., the lack of independency between the earthing system and stray current collection system due space and construction exigencies in a viaduct) and degradation of insulation due to ageing and poor maintenance. Perhaps more importantly, other influences such as the train characteristics, time tabling, headway, multiple train movement, etc., need consideration to determine the worst case leakage current and thus the ability to define appropriate mitigations with respect to stray current management. In this paper, we describe the implementation of an integrated model for the assessment of stray current for a dc electrified railway, in which these factors are considered. We provide an analysis of the stray current magnitude under worst case train operating conditions (i.e., multiple trains with a 90s headway accelerating) as well as highlight factors that determine the efficiency of the stray current collection system. The integrated model presented enables a detailed assessment of all factors impacting the stray current magnitude as well as an assessment of the overall performance of stray current collection system.

This paper provides the accurate characterization of a wireless power transfer system consisting of two resonant air-core coils mutually coupled in free space. The lumped-circuit parameters of the equivalent circuit are determined with analytical formulas taken from the literature and validated by comparison with numerical simulations with a finite-element computer code and with experiments. The parameters are determined taking as input only the geometry of the system (coil size and mutual distance, conductor radius, and turn distance) and the frequency. Once the lumped-circuit parameters are known with good accuracy, the assessment of the power transfer system can be carried out by evaluating the current and voltage gains and efficiency for different system geometries, operating frequencies and load conditions. The Scilab programming environment was used to perform all the calculations. The characterization presented in this paper can then be considered as an effective tool in designing an efficient wireless power system. identification systems [8], or mobile appliances such as portable computers or mobile phones [9]. In general, the applications can be divided into two main areas: direct wireless powering of stationary or dynamic devices and automatic wireless charging of portable/movable devices. In the former applications, power is supplied directly to the electrical devices, whereas in the latter, a battery storing energy is necessary. Wireless transmission of electrical energy can be achieved with techniques that can be broadly classified as far field (radiative) and near field (nonradiative). The former class is particularly suitable for transmitting information at low power. High power radiative transfer is in fact undermined by the waste of energy in free space and thus by the low efficiency when omnidirectional antennas are used and by safety issues and the need for sophisticated tracking systems when directional antennas are used to supply power to mobile objects. The technique that has been recognized so far as the most promising for transmitting power belongs to the class of nonradiative techniques and is based on the magnetic coupling in a resonant system. Several applications of this technique have been proposed, which show that a high efficiency in transmission can be achieved; however, the distance of transmission and amount of power are still limited, thus, restricting the practical applicability of this technique. It has been shown that a higher efficiency can be obtained at a distance of transmission a few times larger than the largest dimension of both objects involved and can be reached with two resonant coils of the same resonant frequency magnetically coupled [10,11]. The resonance condition in the coils is essential as the magnetic coupling between two air-core coils is intrinsically weak [12]; it can be obtained by arranging capacitance in series or in parallel with the coil inductance. The exchange of energy (and thus the efficiency) between the coils can in this way be raised, whereas ...

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