Co-ax Cable

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Co-axial (co-ax or coax) cable is invariably used for TV and Satellite aerial connections.

Coax basically comprises a conductor - a copper wire - run centrally within a conductive cylinder. The name indicates that the axis of the cylinder and the axis of the central conductor are the same: hence co-axial.

Co-ax in More Detail

Since a wire run through a pipe will not stay central of its own accord some means of keeping it in place is necessary. This is accomplished by filling the space with some sort of insulator. The insulation may be solid, a foam or an extruded shape with radial ribs holding the central core in place. At the high frequencies used by UHF TV and even more so by Satellite downlinks the insulation material causes attenuation of the wanted signals, and the choice of insulation material and the way in which it is formed are inevitably a compromise between maintaining the structure of the cable and reducing its losses.

The outer, cylindrical, conductor of the cable was traditionally a braid of thin copper wires. At the frequencies used by FM Radio and the older black-and-white 405-line TV standards even a fairly sparse braid 'appeared' electrically to be a solid cylinder inasmuch as radio signals would not leak into our out of the cable. At the higher frequencies used by UHF (colour, 625 line) TV and Satellite gaps in the braid lead to more leakage of signal into and out of the cable so better quality TV cables, and all cables suitable for satellite feeds, also employ a metal (or metallised) foil layer under the braid.

Characteristic Impedance

A non-intuitive property of transmission lines such as coax is that to electrical signals they appear to have a particular resistance which is nothing to do with any power that is lost in the cable. To understand this it may help to consider what happens to an electrical impulse applied to one end of a cable: it will travel along the cable at a finite speed so there will be a few nano- or micro-seconds before it reaches the other end and finds out what is connected there. During this time the ratio of voltage to current of the impulse travelling along the cable is determined by the capacitance and inductance of the cable, which happens to 'look' like a particular value of resistance for a particular type of cable. If what is actually connected to the other end of the cable does not match this 'characteristic' resistance then, like ripples in water reflected against the concrete edge of a pond, the electrical signal will be reflected back down the cable. This usually results in loss of signal power and is therefore undesirable, so it is important to use the correct impedance cable for a particular application.

Which Impedance

Co-axial cables are commonly available in 50Ω (Ohm) and 75Ω or 80Ω impedances. 50Ω is used in old-fashioned Ethernet computer networks and baseband CCTV. 50Ω may be thinner or sometimes the same size as TV/Satellite aerial cable. 75 and 80Ω are considered the same and are used for TV and satellite.

50Ω cable is entirely unsuitable for TV and satellite use. Unfortunately some 50Ω cables look just like 75Ω.

Which Co-ax Cable?

Bill's article on co-ax cable describing the different types, the differences and which to choose:

http://www.wrightsaerials.tv/articles/coax-cable-quality-hmdi-m.html

Summary: use CAI certified cables for better results all round.

Low quality flylead: the cable is too thin

Cable Colour

It might seem odd to say that cable colour has a bearing on electrical performance, but when used outdoors it does. Black co-ax is uv-proofed, whereas white is not. Thus one should use only black outdoors. White cable would deteriorate, allowing water penetration and degradation and loss of signal.

If it is desired to use another colour, use black cable painted with household gloss topcoat. The paint bonds to the PVC much better than it does to wood, and lasts very well.

Cable Installation

  • Avoid sharp corners and kinks in the cable.
  • Avoid squashing the cable
  • Avoid long unsupported runs of cable, eg across roofs
  • Clip exterior vertical cable runs frequently. A climbing plant can put a good deal of weight & wind load on cable.
  • Fitting exterior vertical cable runs partially behind a downpipe makes them less visible and ensures load sharing of climbers, reducing strain on the cable.
  • Use the suitably-sized round mains cable clips. Avoid using clips which deform the cable, such as fencing wire clips.
  • Avoid using 50Ω cable. It looks like TV cable but is totally unsuitable.


Attenuation

Attenuation means loss of signal strength. All cables attenuate high frequency signals (eg TV aerial signals) to some extent. The amount of signal lost in a cable depends on how long the cable is, the frequency of the signal, and the type of cable.

Since satellite signals use higher frequencies than terrestial TV, a cable may be suitable for TV but not satellite. Satellite cable is suitable for TV aerial feed too. A cable's attenuation is expressed in decibels (dB) per metre (or per 100m etc) at a particular frequency.

See Also

Wikipedia article