Design and Installation Tips for High Quality Video Cabling
See also:
Introduction
Case Study
Glossary
High Quality Design
Cable selection is important to achieving high quality design.
Installers need to know when to use 75-ohm baseband or broadband
coaxial cable, RG-59, RG-6 or RG-11 cable, fiber optic or unshielded
twisted pair cable.
The standard medium installed in video applications is 75-ohm
baseband and broadband coaxial cable, whereas for data
communications, 50-ohm coaxial cable is commonly used. If you use
50-ohm coaxial cable rather than 75-ohm, an impedance imbalance
occurs, resulting in an attenuation problem as video signals start to
weaken.
Selecting cable that is frequency-swept will ensure attenuation
properties at listed frequencies. Most manufacturers publish
specification sheets listing cable property characteristics. In terms
of attenuation, for example, RG-59 baseband cable can be run to 600
feet, and RG-6 and RG-11 baseband cable are effective to 850 and
1200 feet, respectively. Broadband cable distances, however, vary
with channel frequencies.
Cable shielding is also critical to prevent noise problems caused by
electromagnetic or radio frequency interference (EMI/RFI). Baseband
and broadband video cabling shield cover should be a minimum of 95
percent. Before purchasing cable, you should review specifications
on shielding characteristics and cable composition of different
cable types.
For longer runs, where baseband and broadband cable would
experience attenuation problems, consider using fiber optic cable.
For baseband applications, the cable typically used is 62.5/125-
micron multimode fiber, while broadband normally uses single-mode
fiber. Fiber optic cable provides an additional benefit; it is
impervious to EMI/RFI.
Baseband video signals can also be transmitted over unshielded
twisted pair cable. At least Category 3 UTP is recommended to
provide attenuation properties similar to coaxial cable. These
properties are sufficient for using UTP in such applications as video
to the desktop. The video can be transmitted over the extra twisted
pair normally included in structured cabling systems.
Installing Video Cabling
In addition to high quality design, installing video cabling correctly
is crucial to its successful operation. The following tips address key
installation areas:
Central Power
Video systems with multiple cameras should always be powered
from a central location. Most large buildings are equipped with
multiphasic power systems. If cameras are plugged into existing
room receptacles, phase differentials can cause transmission
problems. To eliminate these problems, hook up high quality power
cabling to a centrally located power transformer and pull this cable
with the video cabling to each camera. This centralized
configuration eliminates not only phase problems but also power
surges that might damage sensitive camera electronics.
Central Grounding
Video systems should also have a central ground source, because any
type of ground loop or differential between a central power point
and video camera sites will cause transmission difficulties. Instead,
using three-conductor cable, carry the ground from the central
power source to the cameras. Use 16 AWG cable because it will
minimize resistance to ground. In addition, tape and wrap all
connections to ensure no physical grounding exists along the route;
this can also cause ground-loop problems.
Cable Pulling
Do not stretch coaxial cable when it is being pulled. Stretching
changes the distance between the cable's central conductor and
shielding, which causes a change in impedance, resulting in
attenuation. To prevent this problem, cut cable to manageable
lengths.
Connections
For baseband cable, you should use BNC connectors. For broadband
cable, use F-type connectors.
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Copyright Anixter Inc., 1995