Coaxial Cable For Non-Geeks

Coaxial cable is used for transmitting high frequency signals for communications (i.e. video or satellite signals). Electrical interconnect is not glamorous, but it's an important detail that is often overlooked. Have you ever heard someone say, this "worked a lot better in the store"? There are reasons.
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. This 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 cabling network solutions 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,cabling network solutions 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%).


      

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