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Thread: coaxial cable

  1. #1
    Automated Home Lurker
    Join Date
    Nov 2005

    Default coaxial cable

    I am trying to run an extension lead from a wall aerial socket to my tv , the cable in the wall is an "old" one and the current extension is old too but its make up is copper centre, plastic core and copper only on the outside.
    I bought 2 new extension leads, one is a 5c-2v 75ohm coaxial cable it has the copper centre a plastic core and then this is wrapped in ali foil(?) and then copper on the outside. the other is a 3c-2v 75 ohm coaxial cable which has been maufactured with the male connectors on both the ends so don't know construction. I assume the same as the 5c-2v. When I use the new extension i can no longer get channel 5 on my freeview box or channel 4. Which cable do i need to use for my extension...please help


  2. #2

    Default Re: coaxial cable

    Hi Graham

    The numbers you posted dont mean anything to me but any decent ready made TV extension cable should work with no problems whatsoever.

    Does the Freeview box work OK with the old lead ?

    Does the Freeviw box have a diagnostic screen that gives you an indication of signal strength ?

    Are there any other channels that you cant receive ?

    Digital signals are broadcast on 6 MUXes which each carry several TV channels. If one of the MUXes isnt being received with a strong enough signal then all channels on that MUX would be unavailable.

    The biggest problem with Digital signals is the same thing that makes Digital good. The fact that it is binary data. A signal can only ever be one of two states "0" or "1" and as long as you can differentiate between those two levels equipment can use or regenerate that signal.

    Suppose that the signal originated as two voltages of 10V for a "one" and -10V for a "zero". If that signal gets really badly attenuated to a tenth of its original level so that you get +1 and -1 you can still identify two different levels.

    If the receiver requires a minimum level of 0.5V to detect you can reduce it further and still not get problems. However when the levels get to +0.4 and -0.4 the receiver can no longer detect the signal and it stops working.

    In short, digital means it either works or it doesnt :!:

    Actually the above statement is not totally true, there is a point just before the signal fails totally where it is so borderline that lots of errors occur resulting in blocky pictures and pops etc on the sound. Then it dies totally

    By comparison an Analogue signal will gradually degrade and although the signal is impaired it can actually get very bad before you lose everything. Whether or not you can bear to watch it in its seriously degraded state is another thing

    As you point out, the downlead is old so in all probability so is the aerial.
    As the aerial was erected before digital TV started it is probably not as efficient at receiving the signal as a modern aerial would be. Add to that the fact that Digital transmitters operate as a fraction of power that the existing analogue transmitters work on and you can see that the odds are stacked against it.

    Have you added anything to your setup at the same time eg Satellite Reciever, VCR etc as the RF output of one of these may be on the same frequency as the missing MUX.

    It could be something REALLY stupid like the old lead has bent the center contacts of the aerial sockets and the new leads arent making good contact.

    It could also be that the new leads are of poor quality.

    And finally, it could just be that you live in one of the many UK househods that will have to upgrade their existing TV aerial in order to receiver good quality Digital TV signals.

    Dont forget that in the late 60's / Early 70's as colour TV was becoming popular all households had at some point to replace their existing VHF aerials with shiney new UHF aerials, many of which are still in use 30+ years on :!:

    Hope that helps

    Keith - affordable high quality AV Distribution

  3. #3
    Automated Home Guru Anonymous's Avatar
    Join Date
    Nov 2000

    Default Re: coaxial cable

    Hi Graham and Keith

    There is some real bad coaxial cable on the market, especially with connectors manufactured at both ends (so you can not see what's inside). This kind of coaxial cable does not have the normal outher shield of alu foil nor a maze of metal to really shield the inner conductor from any interference.
    It has just a thin wire at the outside. It is not even co-axial.
    This causes all kinds of interference problems, ghost images, disturbance of reception, etc.

    One way of detecting bad cable without cutting it is resistance measurement. I measured the ohmic resistance of the shield was even higher than the inner lead, where normally the shield has a lower resistance than the inner lead.


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