Coaxial Cable For Critical Communications (Satellite, HDTV, Broadband Internet)

Cables and Wiring are not glamorous. It's one of those details that is 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 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 in signal strength by about 75% over a distance of 100 meters.

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 interconnect 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.