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HOME REVIEWS CONTACT ABOUT December
23, 2009
Coaxial Cable for
Critical Communications
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Cables and Wiring are not
glamorous. It's one of those details
that is
so frequently overlooked, even by professionals, and the
result
can be poor system performance.
Have you
ever heard someone say, this "worked a lot better
in the
store"? There are usually reasons.
Coaxial
cable is used for transmitting high frequency signals
for
communications (i.e. video or satellite signals). It's one
of the
more important cabling jobs to get right.
A
coaxial cable has two conductors, which share a single common axis.
A
coaxial cable has a solid copper (SC) or copper plated conductor (CCS)
surrounded
by dielectric insulating material. The dielectric is surrounded
by foil
and/or braid shielding which forms the outer conductor. This outer
conductor also
shields against electromagnetic interference (EMI) from
external
noise sources.
Some
coaxial cables are identified with an "RG" designation. RG stands
for
"Radio Government". The number
following is a specification
identification. The number value is arbitrary.
High
quality coax cable is used for High definition TV (HDTV), Satellite TV (SATV),
Broadband, Cable TV (CATV), VSAT (very small aperture terminal, satellite
communications including broadband), TV Antenna, and Satellite Master Antenna
Television (SMATV). The three most
common coax cable types used for these applications are RG59 (low grade), RG6
(medium grade), and RG11 (high grade).
Coaxial cable for these applications has an impedance of 75 ohms.
RG59
cables use a 20 or 22 AWG center conductor, RG6 cables have an 18 AWG center
conductor, and RG11 cables use a 14 AWG center conductor (the smaller the AWG number, the larger the
diameter of the center conductor). There
is much variance in the cable specification within each class. An economy RG6 might have a thin aluminum
braid and a copper plated steel center conductor, as opposed to a high
performance RG6 cable with quad-shield shielding and a sophisticated
dielectric.
Video/HDTV
Most
analog video cables are coaxial. For
example, an "S Video" cable is two mini-coaxial 75 ohm cables
combined in a common outer jacket. S
video keeps the luminance and chrominance signals separated. One line carries
the luminance signals, one carries the chrominance signals, and the other two
lines are ground wires.
Component
Video Cables use three separate 75 ohm coaxial cables with connectors at each
end. The three cables are in a single jacket or three separate cables. This allows for separate transmission for the
red, green, and blue signals (RGB).
Serial
Digital Interface (SDI) is the standard for digital video transmission over
coaxial cable. The SMPTE 295M standard
provides a maximum distance of 300 meters (about 1000 feet) for standard
definition TV and 140 meters (about 500 feet) for HDTV. SDI provides a method for transmitting
uncompressed digital video, audio and other data between video devices. SDI is currently only available in professional
video equipment. Licensing agreements, restricting the use of unencrypted
digital interfaces, prohibits its use in consumer equipment.
Signal
Loss (Attenuation)
One of
the main factors when choosing a cable is a calculation of signal loss (attenuation). Attenuation is often expressed as in decibels
(db) per distance. This ratio is
expressed as log ratio of input: output.
A high
performance RG6 cable at 100 megahertz could have a signal loss of 6.4 db per
hundred meters. Since the decibel scale
is logarithmic, this means that the signal in this cable will have been reduced
is signal strength by about 75% over a distance of 100 feet.
If the
run is short, this may be a minor consideration. Often, however, signal loss will be of
paramount importance. RG11 cable will
typically exhibit a signal loss of about 4.5 db per hundred meters at 100
megahertz (loss of about 65%). RG59
cable will typically exhibit a signal loss of about 7.5 db per hundred meters
at 100 megahertz (loss of about 82%).
Signal
"leakage" occurs when the coaxial cable allows some of the signal to
be radiated. All coaxial cables have a
certain amount of dielectric and resistance loss. Resistance loss is the
largest contributor to signal loss in coaxial cable. Losses caused by the
resistance of the inner conductor vary with the diameter of the conductor.
The more
significant loss is frequency related. As the frequency of the signal
increases, the signal is carried through the conductor closer to the perimeter
of the cable. This is called "skin effect". The same RG6 cable that has an attenuation of
6.4 db per hundred meters at 100 megahertz might have an attenuation of 23 db
per hundred meters at 1000 megahertz (loss of more than 99%).
Cable
Sub-Classifications for Application and Safety
CATVX is
the lowest grade of cable. It is suitable only for limited use in residential
buildings.
CATV has
a higher-grade jacket, but this cable should not be used in risers or air
handling ducts. Riser spaces are cavities or openings that penetrate more than
two floors. Commercial buildings often
use the same space to install cables as air handling ducts.
CATVR
(Riser cable) has a slow vertical burn rate and is suitable for any application
other than air handling (plenum) ducts.
CATVP
(Plenum cable) is the highest rated cable jacket type. Plenum cables can be
used anywhere within a building. It has a slow burn rate and emits lower toxic
fumes when burning. Plenum cable is
typically color coded white, and costs about 75% more than standard cable for
similar electrical performance. Plenum
cable will often not withstand outdoor conditions as well as standard cable.
"Flooded
cable" is designed for burial underground.
Flooded cable has a more robust jacket to withstand the compression of
being buried, and also contains a gel substance within the outer most braided
shield. The gel compound prevents water
migration along the braid when the jacket is damaged. This cable is unsuitable for above ground
applications.
Coaxial
Cable Connectors
Coaxial
connectors are available for communication applications such as audio, video,
HDTV, digital applications, and satellite communications. Impedance, frequency range, power
capabilities, and physical size are important considerations when selecting a
coaxial connector.
BNC
connectors are bayonet type connectors, commonly used in CCTV systems. They are
the most suitable connector for use with RG59 cable. The BNC connector has
gained wide acceptance in video and RF applications for frequencies up to 2
GHz. The BNC connector uses a plastic dielectric. This dielectric causes
increasing losses at higher frequencies. BNC connectors are specified by IEC
standard IEC60169-8.
F-Type connectors
are used for Cable TV, Satellite TV, and Digital TV (HDTV) in conjunction with
either RG6 or RG11 cables. Usually the inner conductor of the cable forms the
inner "pin" of the connector. Although "twist-on" type
connectors are available, they do not produce a reliable connection in
comparison to a connector that has been terminated with a ratchet-crimping
tool. F-type connectors are specified by
IEC standard IEC60169-24.
N-Type: A 75 ohm version of the N Type connector is
widely used by the CATV industry. The
N-type connector has an air gap between center and outer conductor. Better N-type connectors can be used at
frequencies up to about 18 GHz.
UHF-Type:
UHF connectors have an impedance which tend to vary,
and are unsuitable for use at frequencies above 300 MHz.
Summary
High
performance communications requires sophisticated technology. As the environment continues to evolve, the
system may need to support voice, data, video, and more recently, HDTV. The
growing size of networks and the introduction of high-speed access create a
critical need for reliable, high performance cabling systems.