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Coaxial cables Construction |
Coaxial cable (or "coax) is the most common
cable used for transmitting video signals. The name
"coaxial" refers to the
common axis of the two conductors.
Construction
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| A coaxial cable has a solid copper or
copper-clad-steel centre conductor surrounded by a non-conductive dielectric insulating
material. The dielectric is surrounded by foil shield/s and/or copper braid/s which form
the outer conductor and also shield against electromagnetic interference (EMI). The outer
conductor/shield is encased in a PVC jacket. |
Most coaxial cables for video applications have
a nominal impedance of 75 ohms. Their differing
electrical and physical characteristics
make it important to select the correct type of cable to suit the application.
Types of coax
Coaxial cables that conform to U.S. Government specifications are
identified with an RG designation.
The meaning of the individual components of the designation are:
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Radio
Frequency |
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Government |
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Government-assigned approval number |
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Universal
specification |
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R |
G |
8 |
/U |
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If the letters A, B or
C appear
before the slash (/) it indicates a specification-modification or revision.
As an example, RG 8/U is superseded by RG 8A/U.
The three most commonly used coaxial cable types for video applications are RG59/U,
RG6/U
and RG11/U.
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RG59/U/strong>
is available with either solid copper or copper-clad-steel centre conductor. It's suitable
for basic analog TV antenna feeds in residential applications and for basic CCTV systems
over short cable runs. The copper-clad-steel type has high tensile strength and should be
used when terminating the cable with F-Type connectors. |
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RG6/U
Quad-shield/strong> is the minimum requirement under the latest Australian Standard
for digital TV antenna cabling and for all TV antenna cabling for
apartments/units (MATV). It is also used for the distribution of Cable TV (CATV) and
Satellite TV (SATV) in residential or commercial premises. It features a copper-clad-steel
inner conductor. Single-shield, dual-shield and tri-shield versions of RG6/U are available
but do not provide adequate EMI shielding. |
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RG11/U Quad-shield is used for
the same applications as RG6/U for either backbone cabling or for long distribution runs.
It features a copper-clad-steel inner conductor. |
Choosing the correct cable
Use the table below to determine which cable should be used for your application.
| Analog TV |
RG59/U |
Acceptable performance on cable
runs <225 metres |
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RG6/U |
Gives superior performance on
cable runs <225 metres.
Used for cable runs >225 metres but <545 metres. |
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RG11/U |
For cable runs greater than 545
metres. |
| CCTV |
RG59/U |
Acceptable performance on cable
runs <225 metres |
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RG6/U |
Gives for superior performance on
cable runs <225 metres.
Used for cable runs >225 metres but <545 metres. |
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RG11/U |
For cable runs greater than 545
metres. |
| DTV, CATV, SATV, MATV |
RG6/U |
Standard cable for these
applications |
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RG11/U |
Recommended for long cable runs
and for backbone cabling. |
Coaxial Connectors
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BNC
connectors are bayonet type connectors, commonly used in CCTV
systems. They are the most suitable connector for use with RG59/U cable.
BNC connectors are specified by IEC standard IEC60169-8.
The argument, over what the
"BNC" in "BNC connector" means, will go on forever. It has been
variously defined as: British Navy Connector, Bayonet Node Connector,
Bayonet Nut Coupling, Baby Neil Connector,
etc. The
two Amphenol engineers who invented the BNC connector were named Paul
Neil and Carl Concelman. It therefore seems logical that the "true" meaning
of the "BNC" acronym is
perhaps "Bayonet
Neil-Concelman".
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F-Type
connectors are used for CATV, SATV and Digital TV in conjunction with either
RG6 or RG11 cables. The copper-clad-steel 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 crimp-type
connector that has been terminated with a good-quality ratchet crimping tool.
F-type connectors are also known as F-81 connectors and are specified by
IEC standard IEC60169-24.
F-type connectors are
named according to the type of cable or the application that they have
been designed for as shown in the table below.
| Connector Name |
Application /
Description |
| F-59 |
A male F-connector that seizes the outer
braid and jacket of an RG-59, RG-6 (RG-56) or RG-11 coaxial cable.
The cable's centre conductor extends through the connector to form
the centre contact. |
| F-6 (F-56) |
| F-11 |
| F-61 |
An equipment or
panel-mounted F-connector (usually female) with soldered cable
connections. A 3/8" ( 32 pitch) thread is provided to accept
the connector nut of the male connector. |
| F-71 |
A male/male F-connector. |
| F-81 |
A female/female
F-connector used to couple two male-ended cables together for
in-line or wall-plate applications. |
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PAL (Belling Lee)
connectors are a push-on connector that have been traditionally used for TV
antenna wall plates and connections. With the exception of TV/VCR hook-ups, PAL connectors
are being replaced by F-Type connectors as required for CATV, SATV and DTV. PAL
connectors are specified by IEC standard IEC60169-2. |
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Adaptors.
Where BNC connectors are required with RG6/U coaxial cable, it is recommended that an
F-Type plug be crimped to the RG6/U and an F-Type to BNC adaptor used. |
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