Can unscreened coax cause lcd tv splutter ?

[DIYspanner]

I have digital tv but the coax cable for the main lcd tv is old fashioned un-screened cable.

The cable, chased into the wall runs from the attic down the g. floor and in the 1st floor room, there's a pc tower and screen where the coax runs down the wall. All the mains cable for the pc are tucked in behind it against the wall.

Would it improve if I got some rg6 cable and run it down the outside wall and into the tv socket ?

The tv transmitter is 323 degrees clockwise from north from my house. I used a compass to align the aerial.

 

[Lucid]

Short answer: Yes.

All coax - even the really old brown stuff from decades ago - will be screened with a single layer of braid. The stuff we use now is double screened. It has a foil layer covered by a braided layer. The foil helps block higher frequency interference. The braid blocks lower frequency interference and provides the earthing for that blocked and trapped energy to be drained away. Any coax (even double screened) with an inadequate braid layer is ineffective at draining the interference to earth so it breaks through to interfere with the signal carrying centre core wire.

What's probably happening is that the coax is picking up the signal from the PC power supply along with other sources of interference. RG6 has better shielding but it's not great cable. If you're going to go to the trouble of installing new wire then put in something that is much better. Go for Webro WF100. It's only pennies more per metre and will do a better job as well as lasting much longer.

 

[DIYspanner]

Many thanks.

I'll re-cable with WF100.

 

[Gees]

Lucid.. can you explain this draining of energy you refer to? I have never heard of it. Blocked and trapped would appear to be totally opposite ?

 

[Lucid]

Sure. Energy transfer from interference sources to cables happens by induction. It's the same for how aerials work, but they are tuned to specific frequency ranges. Electromagnetic induction is well understood too in transformers and in circuit cables. This harks back to the 1800's and Faraday's discoveries that lead to the development of the whole field of electronics. So the basic principle is if a wire moves in a magnetic field then a current is induced. It follows then that if a static wire is subject to a fluctuating magnetic field then a current is also induced. This is the situation with signal cables and why cables carrying small amplitude signals need to be protected from electromagnetic interference. Leaving the current in the shield wire simply defeats the effectiveness of the shielding. The shield needs a ground connection.

Does that help?

 

[bernardgreen]

The shield needs a ground connection.

If only it were that simple.

The screen on a coax cable does NOT prevent electro-magnetic induction of current in the inner conductor, current is induced in both inner conductor and screen. A magnetic screen would be needed to prevent this. A screen of Mu-metal or ferrite loaded tape would reduce induced currents or use steel conduit.

Grounding the screen other than at the receiver input would remove the current in the screen leaving the current in the inner core to enter the receiver. The induced currents cancel each other out if the screen is connected only to the outer connector of receiver's aerial input socket.

The screen on a coax cable DOES reduce prevent electro-static induction of voltage in the inner conductor, a voltage is induced in the screen.

In practise the current induced in the screen will be partially disipated by capactive coupling to items close to the cable.

If currents induced in the cable by electro-magnetic action are too high for the receiver to filter out then fitting an RF transformer between the coax cable and the receiver's input is one option. This works well when only one frequency is involved and a narrow band ( or tuned ) transformer can be used. It does not work as well when many frequencies are involved and a wide bad tranformer has to be use.

Another option is to amplify the signal at the aerial with a mast head amplifier and then attenuate ( reduce ) both signal and induced currents at the receiver's input socket.

 

[Gees]

The screen on a coax cable does NOT prevent electro-magnetic induction of current in the inner conductor, current is induced in both inner conductor and screen. A magnetic screen would be needed to prevent this. A screen of Mu-metal or ferrite loaded tape would reduce induced currents or use steel conduit.

Thats more like it..

 

[DIYspanner]

Very informative folks, many thanks.

Would an old Grundig Skydigibox work with the digital signal from an aerial ?

 

[Lucid]

If you mean will the Grundig receive satellite signals from a TV aerial, then the answer is no. You need a satellite dish with the LNB and then a single feed to one of the SAT inputs on the box. Have you got a satellite dish on the house?

 

[DIYspanner]

Yes, I have a dish on the house buy I cancelled sky many moons ago.

The sky box feeds a single tv upstairs but since its now only freeview channels, I thought perhaps I could feed the box from the aerial.

May as well a tv signal booster multiplug from argos and feed both TVs from the aerial and remove the dish.

A pity discovery and history are not freeview !

 

[Sam Gangee]

A Sky Digibox can't receive terrestrial freeview signals. It's a satellite TV receiver. Without a card it will receive most of the Freesat programmes. If they include programmes that you want to watch then carry on using the Digibox or buy a Freesat receiver.

I've never heard of a "tv signal booster multiplug". Maybe you mean a multi-output masthead amplifier?
http://www.satcure.co.uk/accs/booster_index.htm

Discovery and History Channel are both subscription channels. Freeview terrestrial is a free service and therefore doesn't include them. Likewise Freesat.

 

[DIYspanner]

I've got some wf100 on order so will get re-cabled in due course.

Are one of the wf100 stripping tools worth getting or can I, with care, do terminations with cutters ?

No F connectors on the job, the aerial has screw terminals and the rest is screw cap coax connectors. TV aerial socket is standard.


This is the type of signal booster I have -

http://www.satgear.co.uk/ta33

 

[Sam Gangee]

Impossible to say whether that "signal booster" will be useful because there are no specifications. No gain figure; no noise figure. Also, it requires a power point nearby - do you have a power point near your aerial? How much gain do you need?

WF100 'F' plug fitting instructions here:
http://www.satcure.co.uk/tech/fconn.htm

Information about boosters here:
http://www.satcure.co.uk/tech/amps.htm

 

[DIYspanner]

Here the booster spec and its location -

........ and this is the loft mounted aerial -

 

[Sam Gangee]

Ah, you have a low gain aerial in the loft pointing through roof tiles and you are asking about intermittent reception?
I'd recommend a decent aerial, preferably above the roof.