Presentazioni e Articoli Tecnici

In the recent years, Rai Research Centre (CRITS) has been involved in investigating solutions to guarantee a satisfactory coverage of Digital Audio Broadcasting (DAB) service, in the 200 MHz frequency band, for challenging scenarios like tunnels. Preliminary field tests receiving the RF signal from nearby broadcast transmitters, have confirmed the known difficulty of assuring a fair quality of service in tunnels. Typically, radio service inside galleries is provided by radiating cables (“leaky-feeders”) installed along the tunnel ceiling. However, the installation of new radiating cables is expensive: even in tunnels already installed with radiating cables and in use for other radio systems (e.g. FM radio, mobile telephony…), the introduction of additional services / frequencies requires expensive adaptation. As an alternative and cheaper approach, we are currently investigating the “direct RF radiation” by means of antennas positioned near gallery’s entrance, either internally or just outside it. To evaluate the feasibility of this solution, test campaigns have been performed measuring the field strength obtained at various distances inside the tunnels. As expected, the result strongly depends on the gallery’s geometry, the transmitter’s antenna position, its aiming and polarization. Therefore, a simulative approach is desirable, to investigate the impact of the design parameters, but also to serve as a tool for estimation of the achievable service coverage. By means of COMSOL Multiphysics®, we have modelled a segment of gallery, parametrized in terms of section geometry, length, curvature radius, and surface’s electromagnetic parameters. However, for the FEM simulation the maximum length of the segment is limited by the available computer memory. Therefore, employing the LiveLink™ for MATLAB® product, the full tunnel has been simulated as a sequence of short segments. At every loop iteration, the new curvature radius is set, the excitation field is applied to the near-end, the field distribution at far-end is then simulated, stored, and used as input excitation field for the next segment. For each loop, the E-field components in the (curved) path are also saved. In post processing, the results are joined together to get the complete behaviour of the propagation along the tunnel. The first excitation is obtained from a separate model, including the transmitter antenna, positioned either outside the gallery, or at the ceiling of the first segment (Fig.1). Simulations have been performed using gallery’s geometry of the motorway tunnels considered for field tests, to compare measurements and simulations results. It was found that the simulated field values are in good agreement with the measured ones, although the latter seem more affected by fast fading. The reason seems to be traceable to the peculiar radiation pattern of the whip antenna mounted on the car laying on the partially reflecting road surface. A very good agreement was obtained between the attenuation rate (dB/km) of straight gallery tracts and the values obtained with mode analysis in COMSOL Multiphysics® performed on the tunnel’s transverse section. Although the study is still ongoing, interesting results have emerged from this preliminary analysis, suggesting useful advice for the design of direct RF radiation installations for DAB/DAB+ services.