Determination of scattering parameters of junctions associated with electromagnetic topology

This paper is dedicated to the determination of topological S parameters in the transmission line networks. We are especially interested in ideal junctions which consist of short circuit connections. For this type of junctions, it is impossible to deduce the S parameters from the usual impedance or admittance matrices which are not defined in this case. We suggest a method allowing a direct calculation of S. This procedure is described and then carried out for a two wire fork. At last, an experimental application is given for this model.     

Electromagnetic Subsurace Radar Using the Transient Field Radiated by a Wire Antenna

A radar based on the propagation of a short pulse of current along a wire parallel to the ground surface is described. The wire acts both as a traveling-wave transmitting antenna and as a receiving antenna. For applications to civil engineering, the depth of exploration may be less than 1 m, the purpose being, for example, to detect discontinuities situated near the ground surface, to measure the thickness of a thin layer (10-50 cm), or to characterize a change in its reflection coefficient. Therefore, the width of the pulse of current is about 1 ns. The theoretical approach shows that the wire must be situated at about 10 cm above ground and that there is an optimum position for the current probe that detects the induced current due to a reflecting layer. Several experiments will also be described showing the feasibility of the method.     

Modélisation de l’impédance de transfert des câbles coaxiaux à blindages multiples

The shielding effectiveness of multishielded coaxial cables is determinated through the concept of the equivalent transfer impedance. The transfer impedance is computed from the main parameters of the coaxial structure. We describe in this paper the theoretical formulation to evaluate the amplitude of the disturbing voltage at the end of the cable flowed by the disturbing current. This result is used for the computation of the equivalent transfer impedance when the cable is made of various shields for exemple: homogeneous screens or braids. A comparison with the experimental results is also described.     

Natural wave propagation in mine environments

Theoretical and experimental aspects of the natural propagation of high-frequency waves in mines are presented. A narrow-band and a wide-band analysis have been carried out to determine the most important characteristics of the channel such as the longitudinal attenuation, the coherence bandwidth, and the delay spread of the impulse response in the various areas of an underground mine. The basic principle of a localization of a mobile has also been checked by determining the direction of arrival of the waves     

Wireless propagation in tunnels

In this article, we give a presentation of wireless electromagnetic propagation in tunnels. We discuss in depth the characteristics of multimodal propagation. We show analytical models whereby we are able to study electromagnetic fields in tunnels. To complement the analysis, we give an account of experiments performed in the Channel Tunnel between France and England, and the Massif Central in south-central France. We conclude with a discussion of the results, followed by some comments about bandwidth and the possibility of using multiple-input multiple-output (MIMO) systems to improve communication-channel capacity.     

Simulation of the High-Frequency Response of a Heterogeneous Ground through the Finite Difference Technique

To simulate the propagation of an electromagnetic plane wave in an inhomogeneous ground, the finite difference approach can be used. One of the main problems in using this method is imposing the boundary conditions near the ground surface, especially at high frequency. Indeed, for the E polarization, the upper top of the numerical grid must be sufficiently far away from the air-ground interface in order to neglect the field due to the heterogeneities and the discretization of the atmosphere is necessary. For magneto-telluric modeling, improved boundary conditions have already been proposed. This paper deals with a new condition, valid everywhere in air and which can be applied for E and H polarization. Thus even at high frequency, as for radar applications, only one line is added to the grid discretizing the ground.     

Remarks on the transmission-line approach to determining thecurrent induced on above-ground cables

Transmission-line theory is often used to predict the current distribution along long cables parallel to the ground surface. The difficulty in predicting the resonances of the structure is one of the main weaknesses of the transmission-line approach. It is shown that if the boundary conditions are properly written, taking the effect of the vertical risers into account, the current distribution can be determined easily with a rather good accuracy     

Contribution of the discrete modes to the current propagating along a wire parallel to a moderate conducting earth

Previous theoretical work on the propagation of electromagnetic waves along a thin wire parallel to the air-ground interface is applied to determine the current propagating along the line when a moderate conducting earth is assumed. We only consider the discrete modes due to residue contributions of the modal equation. The results show the limit of validity of the quasi-TEM approximation and that the fast-wave mode excited by a delta-function voltage generator at the centre of an antenna above a moderate conducting earth cannot be a priori neglected.     

Approche expérimentale de l’efficacité de blindage des bâtiments de télécommunications

Methods for deriving the shielding efficiency of buildings from the electric- field and magnetic-field attenuations field trial results, are presented. All measurements are made on incident signals under sinusoidal conditions in the frequency range 100 kHz to 30 GHz.     

Propagation in road tunnels: a statistical analysis of the field distribution and impact of the traffic

This paper presents a narrowband statistical analysis of the field distribution inside a road tunnel. Both fixed and mobile antennas are situated inside the tunnel and the transmitting frequency is 2.1 GHz. A theoretical propagation model based on the image theory is first used and the results are then compared to those obtained from experiments carried out either in an empty tunnel or in presence of traffic.