Radiating cable
Radiating cable/coaxial antenna, also called a leaky feeder, is a coaxial cable with openings in the outer conductor. A regular coaxial antenna is designed to trap a signal within the cable and prevent unwanted signals from penetrating it. In contrast, a radiating cable purposefully leaks signals to establish a connection, thus establishing a two-way radio signal that ensures reliable communication. Radiating cables are therefore well-suited for use in tunnels and other areas where signals struggle to permeate.
There are many variants of two types of cables: coupled mode cables with a corrugated and milled outer conductor, and radiating mode cables with a periodically slotted and overlapped copper tape outer conductor, optimised for specific frequency bands.