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ABC guide to... Step-by-step guides to understanding digital TV

June 2011

Choosing the right co-ax cable Want to see more?

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coaxial cable

Choosing the right coaxial cable Both Freeview and satellite use coaxial cable, and it’s handy for home distribution too. Follow our guide to pick the right quality cable for the job Co-ax cable is used to connect both satellite dishes and TV aerials to a receiver or TV set and also for distributing the signals around the home. To get the best out of your system it’s important to use the right sort of cable, and a high-quality product. But how do you sort the coaxial wheat from the chaff? What does coaxial cable do?

01 Both terrestrial and satellite TV systems require co-ax to carry the signal from an antenna or a distribution amplifier to one or more receivers, to successfully deliver the tiny signals to their destination with as little attenuation as possible – and to protect them from interference picked up along the way. Co-ax cable consists of a central conductor surrounded by a conducting tubular outer shield that’s connected to ground with a ‘dielectric’ insulating layer in between (and further insulation/ protection around the whole cable too). This arrangement acts as a transmission line for radio frequency electrical signals (a little like a waveguide does for radio waves). The outer shield acts to block external magnetic or radio signals from reaching the central conductor and producing interference in the signal it carries. It also prevents radio waves generated by the signal in the central conductor escaping from the cable and generating interference elsewhere. The size and distance between the central conductor and the shield

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determines the ‘characteristic impedance’ of the cable, which must be matched to the equipment at each end for optimum transmission. The shield should be continuous all around the cable, and any sharp bends, dents or kinks in the cable avoided as these can cause reflections within the cable, severely attenuating the signal. What are the different types of 02 cable available? So-called ‘Type 100’ co-ax cables for domestic satellite and terrestrial connections are 7mm in diameter (thicker 125 and 165 types are for trunk feeds in communal systems) and have a characteristic impedance of 75Ohms. You are unlikely to be offered co-ax with a different impedance, but do check. All types use a central conductor of 1mm diameter and have a plastic outer covering. It’s what in between these two that differs between the types and makes. The dielectric can either be a plastic framework that holds the central conductor and shield separate with an

airspace between them, or plastic foam. Cables with the foam dielectric have two mechanical advantages – they kink less easily and better resist water travelling along the cable. The shield is usually two layers of metal – a foil wrapped around the dielectric and a woven lattice or braid of fine wire around that. You will also see so-called ‘low loss’ cable – often sold cheaply in DIY shops – with no foil shield, just a thin, sparse braid. Avoid it.

(Above) Foam dielectric cable with copper braid and copper foil. The best quality cable for domestic connections. Should be rated with attenuation of about 0.3dB/m at satellite IF frequencies

What different materials are used?

03 In most cable the central conductor is copper (about 1mm thick). You may be offered cable with a steel wire core but this is a poorer conductor than copper and so a false economy. The shield braid is also usually made of fine copper wire (sometimes aluminium) and the foil from thin copper or aluminium sheet. Aluminium is a poorer conductor than copper and the two metals in contact can lead to accelerated corrosion if the cable gets damp inside, so copper braid and copper foil is the best choice. The denser the braid, the better the shielding. Occasionally, you may find co-ax with shield ‘foil’ made of a plastic film metallised on one side. This provides poor shielding and should be avoided.

(Below) Copper braid and copper foil shield around an air-spaced dielectric. Excellent electrical performance (as above) and usually cheaper. Can be more susceptible to crushing and kinking than the foam dielectric cable, but is more flexible (for flying leads, etc)


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Similar to the cable above but with an aluminium foil shield. This usually performs nearly as well, but the dissimilar metals in contact can corrode more easily

Does it matter what the outside is

04 like?

The outer covering is the least important component of the cable so far as signal transmission goes. It’s mainly there to insulate the conductors inside and to stop water getting in. If you are using the cable outside, make sure that it has a UVstabilised covering that will not degrade in the sunshine. Cable buried underground should be inside a conduit to prevent attack by microbes and animals. You should also make sure that the cable is fixed to walls, etc, properly so it does not whip around in the wind. This can kink the cable, break the inner wire, and abrade the outer layer to let in water. Co-ax cables do come in different colours, although black is by far the most common. This has no effect on the performance. However, look carefully at white and brown cable, as this is the most common colour for the poorest quality ‘low loss’ type. Which is the best cable?

05 A cable that works well for terrestrial TV may not be adequate as an LNB cable. Cable that performs adequately for a short run in one situation may not be suitable in a longer length

where there’s stronger interference. What’s more, the performance of all cable degrades over time. So for home installation it’s best to use the best cable that you can afford for all the jobs. The extra cost is well worth better performance all round. The ‘best’ cable is probably cable with both foil and braid screen in copper and a foam dielectric. However, an air-spaced dielectric will perform just as well provided it is handled and installed with care. Raydex CT100 cable was a favourite for years but it’s not been made since 2005. That hasn’t stopped some folk from still selling cable claimed to be CT100. A number of manufacturers produce near-identical products with similar names (WC100, WF100) and different names (H109, CB10R) – but there are also a number of poorer products with similar names to fool you. The lesson is to check carefully what it is you are offered – the nature of the shield and dielectric and the materials used. The CAI tests cables and benchmarks those that reach a strict

minimum performance. This acts as an excellent safety net for consumers; ask to see the benchmark on the cable drum. Incidentally, if you use double ‘figure-of-eight’ or ‘shotgun’ cable, the same rules apply, but be aware that the thin 4mm shotgun cable (for two cables to fit in a single-cable hole or conduit) is a much poorer performer than the standard size and shouldn’t be used for runs over 20m. Finally, avoid using the cheap ‘low loss’ cable for anything whatsoever. It may me cheap, but it is usually of such bad quality as to be nearly useless n Geoff Bains

(left) An apparently high-quality cable that uses a metalised plastic film for the foil shield and/or even a copper-plated steel central conductor. Performance is far worse than the types above

(Below) So-called ‘low loss’ aerial cable with no foil shield and thin braid. Cheap but nasty. A poor performer as an aerial cable and completely useless for satellite. Best avoided altogether

June 2011  What Satellite & Digital TV  3


ABC guide to choosing coaxial cable