Coaxial Cable Handbook

History   Conductors   Dielectric   Shielding   Jacket
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.
  

History
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.
 

Conductors

Depending upon the application, many different types of conductor constructions may be found in coaxial cables.

Solid Conductors
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.

Stranded Conductors
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.

Copper-Covered Steel Conductors
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.


  

Dielectric

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.

Popular Dielectric Materials include:

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.

Shielding

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 Shield
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 Shield
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.

Coax Cable

Foil/Braid Shield
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 Cable

Tri-Shield
Construction consists of a foil-braid-foil shield.

Quad-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


Jacket
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:

PVC-Polyvinyl Chloride: Very flexible thermo-plastic material commonly used on general purpose coaxial products. Temperature rating: -40 to 80ºC.

PE-Polyethylene: A thermoplastic material which exhibits excellent electrical properties. Often used in direct burial applications. Temperature rating: -55 to 85ºC.

FEP-Fluorinated Ethylene Propylene: A thermoplastic material with good insulating properties. Excellent in high temperature, plenum and chemical environments. Temperature rating: -50 to 200ºC