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Leaky Coax Cable

A leaky coax cable is very similar to normal coaxial cable in its construction. The major difference is in the cable's outer conductor. Normal coaxial cables use outer conductor shields that are designed to minimize RF leakage. The outer conductor of a typical leaky coax cable has holes or openings in the outer conductor to allow a controlled amount of RF signal to leak out into the surrounding environment. Although most of the signal still travels through the cable, these openings allow the signal to radiate out from the cable or currents to travel on the outer conductor surface, creating an RF field around the cable.

Leaky coaxial cables have been used for many years as wireless feeders in coalmines and vehicular tunnels. More recently, leaky coax has been used as a base-station antenna for in-building wireless systems for hard-to-cover areas such as elevator areas, and for building-to-building tunnels.

Figure C-1 shows the two major kinds of radiating coax (more commonly known to as leaky coax). The type of leaky coax known to many, called coupled mode, has closely spaced slots milled in a corrugated outer conductor of a rigid type of coax (often referred to as hardline). One such brand name is Radiax, manufactured by Andrew Corporation. More recently, a new type of leaky coax has been developed. This newer kind, called radiating mode, typically has a foil outer conductor with nonuniformly spaced slots arranged in a periodic pattern. In free space, its external fields are closely bound to the cable and do not radiate, except for minor end effects. Radiation depends on scattering of the local fields by nearby random objects.

Radiating-mode cable with a nonuniform periodic slot pattern will radiate in free space. However, radiating-mode cable also has strong local fields, and it is probable that in any environment where coupled-mode cable radiates effectively by random scattering, the principal mechanism by which radiating-mode cable works is also random scattering.

It is unpromising to look for a detailed model of scattering, because directional or point-source antennas, by design, have strong signal levels near the antenna. Signal levels get weaker as you move away from the antenna. They also tend to yield more line-of-sight radiation characteristics. This effect creates shadowing and uneven signal distribution. Leaky coax cable emits a low-level signal and actually provides a uniform distribution of signal around the cable. Point-source antennas such as omni or patch and panel antennas are very practical and recommended for distribution of RF signals in large open areas. For more congested areas such as narrow aisles, tunnels, and confined environments, however, leaky coax can resolve some difficult coverage problems.

Leaky coax comes in various diameters, and the major difference from one to another has to do with the insertion loss of the cable. Similar to coaxial cables, as the leaky coax cable length or operating frequency increases, so does the insertion loss for that cable. To minimize losses or their impact on the overall system performance, you can use a larger-diameter cable for longer lengths or higher frequencies. Another difference in leaky coax is the outside jacket on the cable. Many applications require the cable to have a fire-retardant jacket (outside covering) to meet building fire codes.

The performance of leaky coax depends on the environment it is installed in. In general, specifications provided in manufacturer catalogs apply to in-building applications and environments. If you install the cable outdoors, you might experience fewer coupling effects and slightly lower insertion loss (because outdoors is similar to free space). If the cable is installed in a tunnel, the different types of tunnels (concrete versus steel) have different impacts on the cable's performance. Some tunnels enhance coupling effects, whereas others dramatically increase insertion loss.