Radio coverage of road tunnels at 900 and 1800 MHz by discrete antennas

Cellular network operators are greatly interested in radio propagation in tunnels and other wave guiding structures. Road tunnels are commonly served by leaky feeders to provide VHF radio coverage. For UHF and up, we present an attractive alternative to this solution, using discrete antennas in- or outside the tunnel, depending on the tunnel´s length. The key to coverage predictions is a “critical distance”, after which higher-order modes excited near the antenna have died out. This distance depends on tunnel cross-section dimensions and wavelength. A propagation model using this critical distance can predict power level versus length for distances larger than the critical one (e.g. 300 m for 900 MHz in a Euro-standard dual-lane tunnel). Two reference measurements are needed as input for the prediction. Validation measurements taken in a dual-lane road tunnel at 900 and 1800 MHz showed the prediction to be useful at various traffic rates. Traffic does not significantly hamper propagation, if only the vehicles leave enough space in the cross section, nor do bends with curvature radii commonly used in motorway tunnels