Enterprise Design Guide - Public - CommScope

Coaxial Cable Operating Standards and Performance
While coaxial cable was once the media of choice for data networking, its merits of fairly low
attenuation and excellent protection from RF interference have been superseded by twisted pair
and fiber. It is still the media of choice for video distribution.
Coaxial cable is in effect a waveguide; it efficiently channels radio/TV frequencies and is
capable of carrying hundreds of different frequencies simultaneously. It is an ideal media for
video transmission and for linking to satellite and microwave dishes. Coax is also used to
transport signals between the hubs and remote antenna units that power wireless networks.
Conductor materials vary per application; CCTV cables use solid bare copper while CATV
cables use copper-clad steel conductors.
Impedance
Impedance is the resistance to current flow of the cable measured in ohms (Ω). The nominal
impedance of the transmitter, receiver and cable must match precisely for a system to work at
maximum efficiency. An incorrect match will produce return loss. Video cable is 75Ω; other
values used are 50Ω and 93Ω.
Attenuation
Attenuation is the limiting factor for coax cable with higher transmission frequencies being more
affected. Specifications are usually written with the maximum available loss in decibels per
100 feet or 100 meters for a set of frequencies. Cables with larger diameter center conductors
tend to have lower attenuation.
Velocity of propagation
Another performance factor is the velocity of propagation (VP), or how close to the speed of
light the signal travels. Higher numbers are better. CommScope video coax generally uses a
foamed dielectric material that significantly increases VP.
Return loss
Return loss is the ratio of the power of the outgoing signal to the power of the reflected signal
expressed in dB. Structural return loss is the measure of power loss on a cable caused by
discontinuities in the cable conductor or dielectric. If these discontinuities are regularly spaced
along a cable, they can cause severe transmission losses for frequencies whose wavelengths
are fractional proportions (1/2, 1/3, 1/4, etc.) of the distance between these discontinuities.
Lower numbers are better in both cases.
Coax is a viable media for industrial networking, especially in areas where the electromagnetic
interference (EMI) created by electrical motors and manufacturing processes such as arc
welding would render an unshielded cable useless. Protocols like ControlNet TM and MAP
specify coaxial media.
Look for the lowest possible
attenuation for the intended
operating frequencies
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