The AM band is broadcast at frequencies too low to support propagation in the tunnel. The AM band is therefore rebroadcast by an electric or voltage field and not by conventional transmitting methods used in open space.

For optimum efficiency, the AM field must be supported between two conductors located above and below the vehicles. This system requires a radiator along, under or in the roadway and a pair of AM return wires mounted along a wall or on the ceiling.

The radiators are installed with the lowest possible coupling capacity to the tunnel ground.

The uniformity of the signal through the length of the tunnel is dependent on the isolation of the AM radiators and return wires from the tunnel ground system. The twin-lead radiators can be located above or below the roadway, and can be located in the traffic area when an AM return wires is accommodated on the Twin Lead. The FM signals will propagate effectively in the tunnel; therefore, the FM Radiators must be terminated within the tunnel to control the signal within the tunnel structure per FCC requirements. In these cases, the AM band radiators are passed through the FM termination and continue as independent radiators the full length of the tunnel. The AM will run as a single radiator in the wider segments of the tunnel, where the two-radiator configuration will not adequately cover the roadway. AM band radiation out of the tunnel is not a concern.