Video coaxial cables

Video cabling introduction

Analog signals (audio or video) require adequate shielding to protect them from interference. They also require sufficiently low capacitance (relative to the impedance of the source and load) to avoid high-frequency losses. For carrying high frequency signals (video signals, digital audio, TV/radio antenna) you need a cable with right impedance matched for signal source and receiver impdances. All but the cheapest right type cable usally meets these requirements easily. For example any good-quality 75-ohm coaxial cable will work properly for either video or digital audio in a home system. Even in professional systems.

Generally the traditional cable for video installations has been RG-59 and RG-6. RG-59 is the default cale for many not so demanding antenna wiring applications and for baseband video. For a good quality antenna cable wiring quad-shielded all-copper center conductor RG-6 cable is recommended. outlet with two such Copper-plated steel center conductors are also available, providing additional stiffness, but are unable to handle low-frequency currents used to power some devices.

Audio and video cabling systems are generally build using four different types of cables: 75 ohm coaxial cables for video signals (both baseband video signals and RF-modulated antenna/cable-TV signals) and digital audio, shielded single wire cables (coaxial like construction) for unbalanced analogue audio signals used in consumer audio systems, shielded twisted pair wiring for balanced analogue and digital audio signals used in professional audio systems, and thick insulated wire pair for speaker wiring.

Besides that some AV-system installations use unshielded twisted pair wiring for transporting some control signals between equipments. In professional TV broadcasting environment you can also see triax cables, which are are used in TV broadcast industry for TV camera interconnections. Triax Cable is designed as a 75 ohm coaxial cable with two isolated shields to provide multiple functions through one cable to your camera such as power. There are two versions of triax cable commonly used in TV industry: RG59 (3/8") and RG11 (1/2").

Those are the basic cable types used in audio/video systems wirings. The basic wiring systems consist of those wire types. In some applications the unshielded twisted pair (UTP) used in a structured cabling systems can be used as part of audio/video wiring system. When specifying whatever cable type for AV installation, look at the specifications on attenuation, temperature ratings, voltage and current capacity, and the type of outer jacket used.

In video equipment wiring the 75 ohm coaxial cable is the standard to carry video signals. Cable selection is important to achieving high quality design. Installers need to know when to use 75-ohm baseband or broadband coaxial cable, RG-59, RG-6 or RG-11 cable. The standard medium installed in video applications is 75-ohm baseband and broadband coaxial cable. Most manufacturers publish specification sheets listing cable property characteristics. In terms of attenuation, for example, RG-59 baseband cable can be run to 600 feet (200 meters), and RG-6 and RG-11 baseband cable are effective to 850 feet (270 meters) and 1200 feet (400 meters), respectively. For some applications where a small cable is needed to carry video signal RG-179/U is used quite commonly (there are also many small non-standard ccoaxial cables used mostly as part of larger cables with multiple coaxial conductors). Those smaller cables have much higher attenuation than the larger cables, so they are usable only for shorter distances.

Broadband cable distances, however, vary with channel frequencies. For very long runs, where baseband and broadband cable would experience attenuation problems, consider using fiber optic cable. For baseband applications, the cable typically used is 62.5/125- micron multimode fiber, while broadband normally uses single-mode fiber. Baseband video signals can also be transmitted over unshielded twisted pair cable (you need suitable adapters for this). At least Category 3 UTP is recommended to provide attenuation properties similar to coaxial cable. These properties are sufficient for using UTP in such applications as video to the desktop and CCTV camera signals.

For most baseband video applivation almost any well-made 75-ohm cable will be fine. The main requirements for video cable are adequate shield, and proper (75-ohm) impedance. A normal RG-59 cable (or better) is good to carry video signals (compsite video, S-video, RGB, component video etc.) for long distances. For best baseband video performance avoid cheap TV aerial cable, use proper pro-grade video cable, which has much heavier shielding.

NOTE: For short (less than 1m) distances video connections (composite video), you can even dispense with the 75-ohm impedance. Many consumer BNC hook-up cables that come with VCRs, etc. are not 75-ohm. Because they are so short it doesn't really matter. For any longer distances the impedance matters.

To carry antenna signals TV aerial cable is best, because it has low attenuation qat high frequencies and good shielding on those RF frequenices (some properties some other video cables not always have). Same applies to cable TV systems. In cable TV systems at the central point, the 5-6 MHz baseband video signals are fed into individual broadband channel modulators. These units modulate the signals to cable television frequencies of 50 to 450 MHz and distribute them to the receivers. For fixed installations to carry antenna signals installing RG-6 coax cable is recommended. Also RG-11 broadband cable is suitable for this application.

For one reason or another, manufacturers started to make also specialty cables for video that are convenient to use, but exhibit poor performance. One example of such cable is S-video-cable, which is typically made of two strands of very small (and very lossy) 75-ohm coax terminated at either end with a connector that is anything but constant impedance. With cables that are very small and easy to bundle, we pay pay a price in signal strength for convenience.

Point-to-point wiring is most common for analog baseband video. Video surveillance, one such point-to-point application, is only one image per cable. Monitors in airports, and similar displays carry one image per cable. Classroom television is one image per cable.

Coaxial cables are also available in different RG types. RG stands for Radio Guide, or Radio Grade and is a term that is used when sending Radio Frequency (RF) signals down a coaxial cable. 75 ohm coaxial cable comes in several sizes with the most common types being RG-179, RG-59, RG-6, RG-7, and RG-11. An RG-179 & RG-59 cables are the most commonly used coax because they are small in diameter and easy to work with. The RG-11 cable is the largest in diameter and harder to work with. The RG-6 and RG-7 sizes are between the RG59 and RG11. The difference between the RG types are not just size, but also the attenuation characteristics and therefore the transmission distance. Typically, the coax cable transmission limitations for CCTV will be as follows:

The RG-179 (minimax) has the highest attenuation of all. The 25 AWG cable can expect distances up to 500 ft. colored video.

The RG-59 has the highest attenuation of the three other types and you can expect to get a distance of about 750 - 1,000 ft.

Cable data

RG-59 data

Propably most common type of cable used to carry video signals is RG-59 B/U type 75 ohm coaxial cable. The general data for this cable is:

         RG-59 B/U

        Specification :   MIL - C - 17
        Impedance     :   75  3 Ohm
        Attenuation   :     1 MHz    1,5 dB / 100m
                          100 MHz   12,0 dB / 100m
        Capacitance   :   67 pF / m
        Frequency     :   3 GHz

Here is some more detailed data on the cable (data from Belden 8263 RG-59 B/U type cable, data can vary slightly from manufacturer to manufacturer:
  Impedance: 75 ohms
  Inductance: 0.115 michohenries/feet
  Capacitance: 20.5 pF/feet 
  Progapagation velocty: 66%
  Delay: 1.54 ns/feet
  Conductor DC resistance: 49 ohms / 1000 feet
  Shield DC resistance: 2.6 ohms / 1000 feet
  Max. operating voltage: 150 VRMS (UL) / 1700 VRMS (non-UL)
  Temperature: -40 to +60 degrees Celsius
  Min. bendign radius: 2.5"
  Max. pulling tension: 78 LBS
  Nominal weight: 35 LBS / 1000 feet
  Conductor material: 23 AWG solid bare copper codered steel 0.023"
  Insulation material: Polyethylene
  Conductor insulation diameter: 0.146"
  Shield type: Bare copper braid with 95% coverage
  Jacket: Non-contaminating PVC (black color)
  Cable diameter: 0.242" 
  Use: Suitable for indoor and outdoor
  

Attenuation data for RG-59 B/U:

         MHz     dB/100 feet
        -----    -----------
           1         .6     
          10        1.1     
          50        2.4     
         100        3.4     
         200        4.9 
         400        7.0 
         700        9.7 
         900       11.1 
        1000       12.0 
  

Transfer impedance at 3 MHz 36 mohms / m.

NOTE: The original RG-59 B/U MIL-C-17 uses Copper Plated Steel center condictor, In addition to this type many cable manifacturers manufacture also RG-59 versions with Bare Copper center conductor (less resistance, more flexible cable construction).

RG-6

Another commonly used cable type is RG-6. It is a high quality 75 ohm coaxial cable used generally for antenna and CATV wiring. RG-6 is a higher-grade cable than RG-59 and therefore a better choice where performance is important. It uses a larger, higher-capacity center conductor (18 AWG) and a larger insulating dielectric. It may also have additional foil shields or braids (as in tri-shield and quad-shield types) to fight electromagnetic interference.

Cable comparision data

General data on some other 75 ohm coaxial cables compared to RG-59 (most data from http://dct.draka.com.sg/coaxial_cables.htm and http://users.viawest.net/~aloomis/coaxdat.htm and Tasker catalogue):

Cable type            RG-6        RG-59 B/U   RG-11       RG-11 A/U    RG-12 A/U  TELLU 13      Tasker RGB-75
Impedance (ohms)      75          75          75          75           75         75            75
Impedance accuracy                +-2 ohms    +-3 ohms                            +-2 ohms      +-3%
Conductor material    Bare        Copper      Bare        Tinned       Tinned     Bare          Bare
                      Copper      Planted     Copper      Copper       Copper     Copper        Copper
                                  Steel     
Conductor strands     1           1           1           7            7          1             10
Conductor strand(mm2) 0.95        0.58        1.63        0.40         0.40       1mm diameter  0.10mm diameter
Resistance (ohm/km)   44          159         21          21                      22            210
Insulation material   Foam PE     PE          Foam PE     PE           PE         Foam PE
Insulation diameter   4.6 mm      3.7 mm      7.24 mm     7.25 mm      9.25 mm    
Outer conductor       Aluminium   Bare        Aluminium   Bare         Base       Copper        Tinned
                      polyester   copper      polyester   copper       copper     foil under    copper
                      tape and    wire        tape and    wire         wire       bare copper
                      tin copper  braid       tin copper  braid        braid      braid 
                      braid                   braid
Coverage              Foil 100%   95 %        Foil 100%   95%          95%        Foil          ~95%       
                      braid 61%               Braid 61%                           Braid 66%     
Resistance (ohm/km)   6.5         8.5         4           4                       12            ~40
Outer sheath          PVC         PVC         PVC         PVC          PE         PVC (white)    PVC
Outside diameter      6.90 mm     6.15 mm     10.3 mm     10.3 mm      14.1 mm    7.0 mm        2.8 mm
Capacitance per meter 67 pF       67 pF       57 pF       67 pF        67 pF      55 pF         ~85 pF
Capacitance per feet  18.6        20.5        16.9        20.6         20.6 pF
Velocity              78%         66%         78%         66%          66%        80%           66%
Screening factor                                                                  80 dB
Typical voltage (max)             2000V                   5000V                                 1500V
Weight (g/m)          59          56          108         140          220        58
Attenuation db/100m
5 MHz                             2.5                                             1.5          
50 MHz                5.3         8           3.3         4.6          4.6        4.7           19.5
100 MHz               8.5         12          4.9         7            7          6.2           28.5
200 MHz               10          18          7.2         10           10         8.6           35.6
400 MHz               12.5        24          10.5        14           14         12.6          60.0
500 MHz               16.2        27.5        12.1        16           16         ~14           ~70 
900 MHz               21          39.5        17.1        24           24         19.2          90.0
2150 MHz                                                                          31.6
3000 MHz                                                                          37.4
NOTE: The numbers with ~ mark in front of them are approximations calculated and/or measured from cables or cable data. Those numbera are not from manufacturer literature. NOTE2: Several of cables mentioned above are available in sepcial versionswith foam insulation material. This changes the capacitances to somewhat lower value and gives higher velocity (typically around 0.80).

What cable for what use

Both broadband RF video, and baseband (lower-frequency RF) are commonly carried using 75-ohm coaxial cables. There are often differences between the construction of these cables, depending on the frequency ranges being carried.

Broadband (antenna, cable-TV, and satellite) video signals range up into the high hundreds of MHz, or even higher in the case of some satellite-dish downfeeds. Because the frequencies are so high, the "skin effect" in the cable conductors limits the current flow to a very thin layer on the surface of the conductor. To keep losses in the cable low, you need a fairly large-diameter conductor (with a relatively large surface area). Most RG-6 cable is of this type... some variants use a stranded-copper center conductor, some a solid copper, and (I believe) some use a "copperweld" (steel core, thin copper cladding) for tensile strength on long cable-TV runs. As a result of the conductor thickness and construction, RG-6 is often a fairly stiff cable.

Composite and component video are usually referred to as "baseband" signals, and their frequency content doesn't go up above a few MHz (few tens of MHz or up to something like 150-200 MHz on some very high frequency computer signals). You can get lower losses with a thinner (often stranded) conductor, and thus have a more flexible and easier-to-route cable. Hence composite-video cables are probably better suited for carrying basedban videon than RG-6 broadband coax would be. Same that applies to composite video applies to component video as well. Individual component video signals are not significantly different from composite.

Cable connectors

The most commonly used connector in video wiring is BNC connector. Because the cable used it 75 ohms cable, the right connector type for this cable is 75 ohm BNC connector. The traditional usage of 50 Ohm BNC connectors on 75 Ohm cable with analog equipment (video & telco) has little distortion effect on the signal at frequencies below 300 MHz. However, digital signals in video and telephony applications have necessitated the usage of 75 Ohm connectors.

Cable TV and antenna cabling

The earliest cable TV system and house antenna wiring, in effect, are just strategically placed antennas (or other signal sources) with very long cables connecting them to subscribers' television sets. So it just basically a very large RF signal distribution network.

Cable TV systems and other large TV antenna systems are traditionally built using RG-6 or RG-11 coaxial that is terminated with F-connector (or IEC antenna connector on European countries). RG-6 cable is the preferred cable for in-house antenna wiring. RG-6 is a rating of quality; some manufacturers use RG-59 or lower grades, but you should avoid these as your picture quality will not be as good (more loss is cable and not so good shielding at high frequencies).

In a coaxial cable like the one that brings CATV signals to your house, radio signals travel at about two-thirds the speed of light. The system bandwidth has effect on what and how many TV channels can be transported through the wiring. Early cable TV systems in USA operated at 200 MHz, allowing 33 channels. As technology progressed, the bandwidth increased to 300, 400, 500 and now 550 MHz or even up to nearly 1 GHz. Today it is estimated that there are over 60 million cable TV subscribers in the USA.

Antenna networks generally consists of signal source (antenna or modulator rack), signal amplifiers, cable, splitters and antenna outlets. The whole TV antenns system is matched for 75 ohm impedance, which means that all components are matched for this impedanse.

The cable used in TV antenna networks is 75 ohm coaxial cable. Cable runs with copper braid RG59 should not exceed 750 ft (250 meters). while RF6 copper braid is good for 1,500 ft (500 meters). The RF signal is generaly split to multiple TV antenna outlets using splitters. The loss of a 2-way splitter is approximately 4-6dB and 6-9dB for a 4-way type. Signal loss should be compensated by an amplified signal.

Splitters are used to split one incoming wire to two or more outputs. Splitters split the incoming signal power to the outputs evenly and maintain the impedance matching on the systems. Splitters provide isolation between receivers (Without isolation one receiver could degrade the quality of the signals going to other receivers). When using splitters be sure that all unused outouts are terminated with 75 ohm terminator to avoid signal reflections and degration of the splitter performance.

Add amplifiers to compensate the losses when signal needs to travel long distances of cable (add amplifiers to the signal source end of the cable). Long cable runs will weaken the signal, which means increased noise in analogue TV transmissions and increased number of transmission errors in digital TV broadcasting. Amplifiers are also needed to compensate for the loss from splitting the signal. Add attenuators when the signal is too high. Many amplifiers have a gain control that can be adjusted to prevent strong local stations from causing signal overload. Add tilt compensators on long runs (logn cables attenuate higher frequencies more than low frequencies, tilt compensator compensates for this).

The simplest antenna outlets are just F-connectors on the end of the antenna cable coming from the wall (this is commonly used in USA). The antenna outlet can also be a wall jack with F-connector. In Europe IEC/DIN45325 antenna connectors (9.5 mm coaxial connectors) are generally used. In large antenna networks special antenna outlets are used. Those outlets have the following functionality: attenuate the signal from the incoming wire to outputs and filter the different frequencies to different outlets (TV and radio). The signal attenuation is generally used to provide some isolation between the incoming antenna signal cable and the output connector so that what is connected to outlet does not disturb what is in the line (so it does not matter if there is emply outlet, jsut piece of antenna cable or TV connected). The isolation between the output and signal cable is specifically needed in antenna networks which use daisy-chained antenna outlets. Typically the systems are aiming 20 dB or better isolation between the different antenna outlets in the system.

A standard procedure for new homes is to have a junction box in the basement. The cable from antennas or cable TV source will come into the junction box, and there will be cables going to every place there might be a need for a TV. With this, you can do most any splitting you might need. Well almost everything.

The ideal video distribution system has balanced signal strengths at all termination points. In many European countries the signal level on antenna connector should be between -60..80 dBmW. Lower signal strenght causes noise to the picture and higher levels cause problems. In USA maximum signal strength to TV outlet allowed by the FCC is 15.5dBmV (systems aim to supply at least 10 dBmV, devices generally work well in 0-20 dBmV range).

Nowadays the preferred method to wire the antenna network inside s house is to use "star" form antenna wiring. This means that from each antenna outlet there is one cable from the outlet to the antenna distribution center, where a large splitter splits the signal to all those outlets. This arrangement gives best performance and is the most flexible (if you have two or more splitters in distribution center with different services comign to them, it is possible to wire different services to different antenna outlets).

Note on 300 ohm cable: Once in the history of antenna networks a flat 300 ohm "twinlead" antenna cable was popular. The use of it is not much used nowadays, because of poor performance compared to today's standards. You would almost certainly get better results with coaxial (round) antenna cable rather than 300-ohm "twinlead" (flat) cable. The flat cable is not shielded and recieves (and radiates) along its entire length. If you have some device still in use which has 300 ohm connections, then use an antenna balun for conversion between 300 ohm interface and standard 75 ohm coaxial wiring. In some special applications "twinlead" can still be useful with careful planning. The advantage of coax over flat 300-ohm cable is that longer runs can be used without signal degradation (flat ladder line is the lowest loss cable). Flat cable when twisted in the prescribed manner (6 turns per metre) is actually quite impervious to spurious signals of placed away from objects. Coax is certainly better when placed close to objects, as flat cable should be floating free of objects, especially metal. I do not recommend 300-ohm "twinlead" in any new installation.

Video coax installing tips

First and foremost, follow all electrical wiring related requirements when installing coaxial cables. In USA those regulations are in NEC and possibly additions in local building codes. Other countries have their own regulations.

Distribute the pulling tension evenly over the cable and do not exceed the minimum bend radius. Exceeding the maximum pulling tension or the minimum bend radius of a cable can cause permanent damage both mechanically and electrically to the cable.

When pulling cable through conduit, clean and debur the conduit completely and use proper lubricants in long runs.

Outdooe installations require special installation techniques that will enable the cable to withstand harsh environments. When using cable in an aerial application, lash the cable to a steel messenger wire. This will help support the cable and reduce the stress on the cable during wind, snow, and ice storms. When direct burying a cable, lay the cable without tension so it will not be stressed when earth is packed around it. When burying in rocky soil, fill the trench with sand. In cold climate areas, bury the cable below the frost line.

There are a variety of termination methods for cables. The termination method utilized depends basically on the system installed, type of cable used, and type of connector. Using the proper termination method allows for good mechanical and electrical integrity.

Solder type connector can be used on solid or stranded conductors. It perform well when properly installed.

Crimp method is probably the most popular method of terminating BNC connectors on coax cable. Like the solder method, it can be used on solid or stranded conductors and provides a good mechanical and electrical connection. A crimp type connection allows for quick and simple installation while still maintaining a mechanical and electrical connection fairly close to a solder type termination. The most common crimp method involves two crimps. One is on the insulation for a stronger mechanical connection and one is on the conductor or shield for a good electrical connection. Remember to use the proper size connector for the size coax you are using. A tight fit on the cable is important. important. When crimping the connector, use the proper tool!

Twist-on method is the quickest way of terminating a coaxial cable; however, it does have some drawbacks. In this type of connector the center conductor is cut into by the center pin on the connector, thus too much twisting can cause damage to the center conductor. Twist-on termination is less reliable than other methods. Twist-on termination is not recommended for pan and tilt installations because the constant movement may work the connector loose.

The traditional use of a 50. BNC connector with a 75. coax cable in a analog video system operating at frequencies below 300MHz will have little distortion to the signal. However, digital video applications and analog applications above 300MHz requires the use of a 75. connector with a 75. coax cable.

Testing audio/video wiring

Usually the best way to test cables is to do the test in the intended system - provide a known input from the intended source and analyze the output at the input or output of the intended load.

The most common cable fault is an open circuit, usually due to problems close to or at the ends of the cables. A simple ohm meter test generally suffices.

75 ohm coaxial cable testing includes the following tests:

Checking the coaxial cable length and impedance continuity is usually done using a time domain reflectometer (TDR), which transmits a pulse down the cable, and measures the elapsed time until it receives a reflection from the far end of the cable. Each type of cable transmits signals at something less than the speed of light.