Installation guide

Planning and Designing RIO Cable System
44
890 USE 101 00 October 2006
Coaxial Cable Characteristics
Cable Bend
Radius
All cables have a minimum allowable bend radius—i.e., a certain degree beyond
which it cannot be bent—and a minimum support requirement. If the cable is bent
more than the allowable bend radius or if the installation is not adequately
supported, you can easily damage the center conductor, the dielectric, and the cable
shield.
This damage can cause signal waveform reflections back into the cable system and
distortions due to cable impedance alterations away from 75 Ω. The end result will
be a series of transmission errors or a nonfunctioning cable system. The situation
creates a high voltage standing wave ratio (VSWR) on the system—high VSWR
causes the transmitted signal to reflect back to the source.
When designing the cable system, consult the manufacturer’s specifications on the
cable bend radius. Design the routing of the cable so that when rounding corners
with cable, the cable is not bent more than the specification and put this specification
on the design drawings.
Cable Support Most cable manufacturers recommended that RG-11 and RG-6 cable be supported
at least every 50 ft (15 m). Consult the cable manufacturer for more detail about
minimum support requirements for other types of cables.
Cable Pull
Strength
Every cable has a maximum allowable pull strength. Any cable that must be pulled
through wiring ducts or conduit should have its pull strength labeled on the design
drawings. If cable is pulled beyond the maximum allowable limits, the cable will
stretch or break causing an impedance mismatch. The stretch or break may not be
apparent in a visual inspection—e.g., the dielectric inside the cable could become
damaged or the center conductor could break. Cable pull strength ratings can be
obtained from the cable manufacturer—they are also listed in the cable
specifications in RG-6 Cable, p. 68, RG-11 Cable, p. 69, and Semirigid Cable
Connections, p. 113.
Environmental
Considerations
Cable components will degrade if subjected to extremes of temperature and
humidity. Consult the manufacturer specifications on the cable components used in
the RIO network to assure that they meet the requirements of the application.
Provide excess cable in each cable segment of your cable run to allow for
temperature changes. Cable system components will expand and contract as a
result of temperature variations. Several inches of excess cable should be provided
to ensure that the cable will not be damaged by temperature changes. Consult the
cable manufacturer for the expansion and contraction specifications.
This document provided by Barr-Thorp Electric Co., Inc. 800-473-9123 www.barr-thorp.com