MECHANICAL RATING OF COAXIAL CABLES BASIC INFORMATION

Corrugated copper cables are designed to withstand bending with no change in properties. Low-density foam- and air-dielectric cables generally have a minimum bending radius of ten times the cable diameter. Super flexible versions provide a smaller allowable bending radius.


Rigid transmission lines will not tolerate bending. Instead, transition elements (elbows) of various sizes are used. Individual sections of rigid line are secured bymultiple bolts around the circumference of a coupling flange.

When a large cable must be used to meet attenuation requirements, short lengths of a smaller cable (jumpers or pigtails) can be used on either end for ease of installation in low-power systems. The trade-off is slightly higher attenuation and some additional cost.

The tensile strength of a cable is defined as the axial load that may be applied to the line with no more than 0.2% permanent deformation after the load is released. When divided by the weight per foot of cable, this gives an indication of the maximum length of cable that is self-supporting and therefore can be readily installed on a tower with a single hoisting grip.

This consideration usually applies only to long runs of corrugated line; rigid line is installed one section at a time.

The crush strength of a cable is defined as the maximum force per linear inch that can be applied by a flat plate without causing more than a 5% deformation of the cable diameter. Crush strength is a good indicator of the ruggedness of a cable and its ability to withstand rough handling during installation.

Cable jacketing affords mechanical protection during installation and service. Semiflexible cables typically are supplied with a jacket consisting of low-density polyethylene blended with 3% carbon black for protection from the sun’s ultraviolet rays, which can degrade plastics over time. This approach has proved to be effective, yielding a life expectancy of more than 20 years. Rigid transmission line has no covering over the outer conductor.

For indoor applications, where fire-retardant properties are required, cables can be supplied with a fire retardant jacket, usually listed by Underwriters Laboratories. Note that under the provisions of the National Electrical Code, outside plant cables such as standard black polyethylene-jacketed coaxial line may be run as far as 50 ft inside a building with no additional protection. The line can also be placed in conduit for longer runs.

Low-density foam cable is designed to prevent water from traveling along its length, should it enter through damage to the connector or the cable sheath. This is accomplished by mechanically locking the outer conductor to the foam dielectric by annular corrugations.Annular or ring corrugations, unlike helical or screw-thread-type corrugations, provide a water block at each corrugation. Closed-cell polyethylene dielectric foam is bonded to the inner conductor, completing the moisture seal.

A coaxial cable line is only as good as the connectors used to tie it together. The connector interface must\ provide a weatherproof bond with the cable to prevent water from penetrating the connection. This is ensured by the use of O-ring seals.

The cable connector interfacemust also provide a good electrical bond that does not introduce a mismatch and increase VSWR. Good electrical contact between the connector and the cable ensures that proper RF shielding is maintained.