A Radio Propagation Modeling for a Cost-Effective DAB $+$ Service Coverage in Tunnels

Providing a satisfactory coverage of Digital Audio Broadcasting (DAB
$+$
) service inside tunnels, in the VHF band, represents a very challenging task. The classic -but expensive -solution adopted so far is the use of radiating cables (“leaky feeders”) installed on the tunnel’s ceiling over its entire length. An alternative and cheaper solution, investigated in the present paper, is the so-called “direct RF radiation” approach, consisting of antennas placed inside the tunnel or just outside its entrance. A simulative analysis has been carried out in order to evaluate the impact of the design parameters, but also to serve as a tool for the estimation of the achievable service coverage. In addition, assuming a gallery to behave like a lossy waveguide, a
mode analysis
has been performed on the tunnel cross section, providing a fairly good estimation of the wave propagation attenuation. Interesting outcomes have been obtained from this simulative study: for instance, the behavior of the electric field as a function of distance suggests that, in the absence of geometric perturbations, the slope in the far zone is in good agreement with the attenuation value per unit distance of the main propagation mode. Curved sections cause a further attenuation which depends on the radius of curvature and the geometric dimensions of the tunnel section have a very strong impact on attenuation. Furthermore, more interesting outcomes show that, for arched tunnel sections, the fundamental propagation mode is horizontally polarized. As a result, the typical “whip” vehicular receiving antenna is not adequate: a horizontally polarized antenna would provide a much better service inside the tunnels. The investigation of the above findings have led to the set-up of a tool that can be applicable to every type of tunnel’s configuration for the verification and optimization of direct RF radiation installations for DAB/DAB
$+$
services.