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Analogue TV has got by since 1950 with so called TV low loss coax. Typically brown, cheap and nasty, it has served us well, but no longer for the digital age. All you need to know for DTT is

use WF100 or CT100 satellite cable. Don't EVER user brown unspecified TV Low loss coax

You naturally ask yourself why I am recommending you should spend double on satellite cable? Impulse interference. It comes from the neighbour's central heating thermostat, the kid next door's moped, your own light switches. On analogue, all these little sparks of interference gave you a short line of black or white flecks for a few lines or so on one frame. With DTT you get a freeze frame of half a second or more. and that hacks you off far more.

Good cable is properly screened. With DTT it is not just the loss from aerial to set that matters, how well the cable keeps out interference is key to your viewing experience.

In a test1 comparing a standard (unbalanced) contract aerial, normal low-loss TV coax and a freebie flylead from an unscreened wall plate to the DTT box with the equivalent balanced aerial, CT100 grade coax, screened outlet and CT100 fly lead, an improvement of 22dB in interference reduction was observed. If the original system just suffered interference from a source 50m away, the use of correct installation would mean the same interference would have to come to about 15 m of the installation before causing interference. It's cheaper to buy good cable to keep interference out than it is to buy up all your neighbours' houses to shut their electrics down and dig up the road for 50m either side of your house.

Okay, so how do I know the good from the bad?

With a sharp Stanley Knife, skin about 3cm of the insulation from the end, taking care not to cut through to the metal screens. Remove insulation and it should look like this

WF100 good quality coax (CT100 is also good and looks pretty much the same)

Good - WF100 good quality coax

CT100 is also good and looks pretty much the same at this point. For DTT runs to external aerials, prefer WF100 to CT100 if you have the choice. WF100 has a foam inner insulation whereas CT100 has airspaced inner insulation. If water gets into CT100 it runs into the insulation and the cable can fill with water. WF100 won't do that, and is also slightly more resistant to kinking. Both are good with good workmanship.

If, however, you skin the insulation and are faced with this

BAD - old-style low loss TV coax

BAD - old-style low loss TV coax

you have a SERIOUS problem if you are thinking of running DTT. Observe the knitted copper strands - with massive gaps which interference will seek out and winkle its way onto the inner conductor. Resulting in those half-second picture freezes which always happen at critical places in the dialogue. Just say no to this. If it's in the walls of your house the best use for it is to tape to the end of some WF100 or CT100 which you pull through with the old brown stuff. Job done.

WF100 or CT100?

The satellite industry in the 1980s made demands of standard domestic TV coaxial cable which it couldn't meet, so they defined some cable benchmarks, of which the most widely used is CT100, which specifies a tape and braid construction, and tends to be semi-airspaced. Satellite IF cable uses frequencies up to 2.5GHz which is more than twice the highest UHF frequency of 860MHz, so keeping high-frequency loss down is a key requirement that your DTT aerial lead doesn't have.

CT100 airspaced dielectric left, WF100 foam dielectric right

CT100 airspaced dielectric left, WF100 foam dielectric right

Air is a better insulator that plastic, so CT100 has as much of the plastic removed from the insulator as can be practically achieved. As can be seen from the photo above, however, this has one disadvantage - there are now tubes running down the insulator. Which can fill with water in outside applications, and the cable kinks more easily. So for the lower frequency requirements of DTT, or short cable runs, a foam insulator like WF100 stops water running down the cable like a hose, and give better mechanical resistance to kinking. The foam still incorporates a lot of air for low loss.

Overview of the cable construction

Overview of the cable construction

Now that you've got your cable sorted, you need to make your Belling-Lee aerial plugs up right.

Selecting and siting your TV aerial

Aerial system components

Why DTT is different from an analogue install

 

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1 CAI publ "Feedback" V7 # 9 p20 Autumn 2003


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Text and photographs RM 2003 unless otherwise credited