Characteristics and applications of antenna cable for highway advisory radio

We developed a suitable antenna cable, for an AM advisory radio to broadcast traffic information, which can make a zonal service area along a road. This cable generates an inductive field by a current induced on an earth-return circuit from an RF energy transmitting inside of the coaxial cable, which has an open type outer conductor. The inductive field can be controlled easily by number of turns of helically formed inner and outer conductors. The antenna cable has two merits; one is that the electrical properties are little influenced when being buried either in the ground or covered by snow or water, and another is that the inductive field hardly interferes an adjacent AM radio service area thanks to its big attenuation of a field towards transverse direction of the cable axis. A transmitter of 10W, 1620kHz produces a field of not less than 70dBµV/m within an approx. 3km section through the antenna cable. Since the first system for Japan Highway Public Corp. was constructed in 1985, the antenna cable has been installed 64km in total and being in good service for traffic information.