In recent years, coaxial
cables have become an essential component of our information superhighway.
They are found in a wide variety of residential, commercial and industrial
installations. From broadcast, community antenna television (CATV), local
area network (LAN), closed circuit television (CCTV) to many other
applications, coax has laid the foundation for a simple, cost effective
communications infrastructure.
Government and military
applications summoned in the initial development of coaxial cables. Built
to military specifications and classified according to Radio Guide Utility
numbers, these products were developed to help support high frequency
radio transmissions. After some time, these “RG/U” numbers fell into
classifications according to impedance characteristics. The steady growth
of the computer industry further developed the need for coaxial cables for
commercial use. Manufacturers of “proprietary” system demanded a variety
of unique cable designs.
Depending upon the application, many different types of conductor constructions may
be found in coaxial cables.
Solid copper conductors are popular on many CCTV
installations. Solid conductors provide less chance for distortion and
line loss than copper covered steel. However, copper is a soft material
and will break if repeatedly flexed. It is best used on permanent
installation.
Many small strands of copper can be used to make up any
gauge size. Stranded conductors provide increased flexibility over solid
conductors. This design has become popular for use on pan and tilt
cameras, robotics and other applications that require repeated
flexing.
In some applications strength is a key requirement.
Steel conductors covered with copper may be needed to prevent breakage in
an active environment. Copper covered steel conductors provide added
strength and RF support and are often used in CATV and MATV
applications.
The inner conductor of a coaxial cable is separated by an insulating material from
the surrounding shield(s). This “dielectric” material is often chosen in
order to maintain consistent electrical properties and minimize signal
loss. The result is a clear, trouble free
transmission.
Coaxial Cable
Note: foamed dielectric materials
provide a lightweight composition suitable for many coaxial constructions.
Foaming the dielectric also contributes to increased clarity in high speed
transmissions.
PE-Solid
Polyethylene: good for low temperature
applications. FPE-Foamed Polyethylene: provides lower
attenuation and capacitance than solid PE. Air Spaced:
supports a lower dielectric constant than Polyethylene while allowing for
a small diameter cable size. FEP-Fluorinated Ethylene
Propylene: excellent for high temperature and plenum applications. Low
dielectric constant for clear signal transmission. FFEP-Foamed
Fluorinated Ethylene Propylene: extremely low dielectric
constant.
In coaxial
applications, shielding is an important part of the overall composition of
the cable. Shielding not only protects the loss of signal in high
frequency application, but also helps to prevent EMI (electromagnetic
interference) and RFI (radio frequency interference) in the circuit. There
are three popular types of shielding: overall foil, braid and
foil/braid.
Foil shields are commonly used as an effective, low cost
application to prevent EMI/RFI interference. The construction consists of
a layer of aluminum with a polyester backing that is overlapped to provide
100% coverage.
Braid shields are constructed of thin strands of aluminum,
tinned copper or bare copper that are interwoven. Many different coverages
are available with 40-67% aluminum and 95% copper being the most popular.
High percentage braided shields offer an excellent protection from EMI and
RFI and are popular in CCTV applications.
Coaxial Cable
Foil/braid shielding is extremely popular with CATV
and MATV applications. The 100% aluminum foil is accompanied by a braided
shield to provide improved protection from EMI and
RFI.
Dual Shield Coax
Construction consists of a foil-braid-foil
shield.
Construction consists of a
foil-braid-foil-braid shield. Very popular for CATV and MATV applications
in high EMI/RFI locations such as urban areas.
Quad Shield Coax Cable
The outer sheath found on
most coaxial cables is called the jacket. The main function of the jacket
is for protection from the environment and as an additional form of
insulation. The compounds used to make the jacket may have different
temperature ratings. The temperature rating of a cable, along with the
location rating (i.e. plenum, wet, sunlight resistant etc.) will determine
the minimum or maximum operating temperature of the cable. In today’s
multi-application world, many jacketing choices exist:
Very flexible thermo-plastic material commonly used on general
purpose coaxial products. Temperature rating: -40 to
80ºC.
A thermoplastic material which exhibits excellent electrical
properties. Often used in direct burial applications. Temperature rating:
-55 to 85ºC.
A thermoplastic material with good
insulating properties. Excellent in high temperature, plenum and chemical
environments. Temperature rating: -50 to
200ºC