Coaxial Cables
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 of a Coaxial Cable
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 sit the application.
Types of Coaxial Cables
The three most commonly used coaxial cable types for video applications are
RG59/U and RG11/U. The "RG" number originated from the "registration
number" allocated by the US military to each type of cable. The "/U"
indicates that the dielectric is unbonded to the centre conductor.
RG59U is available with either solid copper or copper
clad steel centre conductor. It is suitable for basic analogue 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.
RG6/U Dual shield is used for security systems, for
analogue TV antenna cabling and for CCTV. It features a copper clad steel inner
conductor. Tri-shield and quad-shield versions of RG6/U provide additional EMI
shielding for digital applications.
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.
Coaxial Connectors
| BNC Connector: |
This refers to the physical form of the connector.
Various suggestions are put forward as to what it stands for, including British
Naval Connection. However, it is a sturdy way to connect and involves a push and
twist due to it bayonet type connectors. They are most suitable for use with RG59/U
cable. |
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F-Type Connector: are used for CATV, SATV and Digital TV in conjunction
with either RG6/U or RG11/U cables. The copper clad steel inner conductor of the
cable forms the inner "pin" of the conductor. 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.
PAL (belling Lee) Connectors are a push-on connector that has been traditionally
used for TV antenna wall plates and connectors. With the exception of TV/VCR hook-ups,
PAL connectors are being replaced by F type connectors as required for CATV, SATV
and DTV.
Adapters. 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 adapter used.
Coaxial Cabling Glossary and Terms
Coaxial Cabling is a two conductor closed transmission medium that is often
used for the transmission of RF energy. The system offers tight control over electrical
impedance. This yields excellent performance at high frequencies and superior
EMI control/shielding when compared to other types of copper cabling. Coaxial
cabling is commonly found in test environments as well as in broadcast and networking
systems. Listed below are some common terms and definitions that are related to
coaxial cabling:
Attenuation (Insertion Loss): Loss of power. Attenuation is usually
measured in dB loss per length of cable (ex. 31.0 dB/100Ft.). Attenuation increases
as frequency increases.
BALUN: An acronym for Balanced/UNbalanced. A device commonly used to
change one cabling media to another (ex. coaxial to twisted pair balun).
Centre Conductor: The solid or stranded wire in the middle of the coaxial
cable. The conductor diameter is measured by the American Wire Gauge (AWG).
Coaxial Adapter: A device used to change one connector type to another
or one gender to another (ex. BNC to SMA Adapter).
Coaxial Cable: A two conductor cylindrical transmission line typically
comprised of a centre conductor, an insulating dielectric material and an outer
conductor (shielding). Coaxial cable can be flexible (typical to the assemblies
found in this catalogue), semi-rigid or rigid in nature.
Coaxial Connector: The interconnection device found at each end of a coaxial
cable assembly. There are many common types of coaxial connectors such as: BNC,
SMA, SMB, F, etc.
Dielectric: The insulating material that separates the centre conductor
and the shielding.
Electromagnetic Interference (EMI): Electrical or electromagnetic energy
that disrupts electrical signals.
Frequency: The number of times a periodic action occurs in one second.
Measured in Hertz.
Impedance: The opposition to the flow of alternating or varying current.
Measured in Ohms.
Jack: The female connector usually containing a centre socket.
Plug: The male connector usually containing a centre pin.
RG/U: Symbols used to represent coaxial cable that is built to US government
specifications (R=Radio Frequency, G=Government, U=Universal Specification)
Shielding: Conductive envelope made of wires or metal foil that covers
the dielectric and the centre conductor
Twinaxial: An offshoot from coaxial cabling. Two centre conductors with
one dielectric and braided shielding.
VSWR (Voltage Standing Wave Ratio): Amount of reflected power expressed
as a ratio (Ex. 1.25:1) VSWR increases as frequency increases.
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