Underground use of a coaxial cable with leaky sections

The optimum efficiency of a leaky cable as a support for radio communications in tunnels involves a compromise between high leakage fields and a low increase of the coaxial mode attenuation. The latter is an important disadvantage of continuous leaky feeders. To obviate this, short leaky sections can be inserted in an otherwise well-shielded coaxial cable. These sections act as mode converters or radiators. A theoretical analysis based on coupled line theory enables us to determine the optimum characteristics of the leaky sections. Numerous experiments were carried out in a tunnel at various frequencies, using different lengths and characteristics of the leaky sections.