Satellite cable screen

[Buzz84]

Hello,

I've not had much luck on Google so far so thought I would ask on here.

Does anyone know if the screen on a sky/satelite system is used for anything other than just protection from interference?

I know some situations use the shielded wire as part of the circuit. But cannot find any reference to satellite cable pinouts or similar.

The reason I ask as I am having my house done up and I have run shotgun cable to all possible places a TV could go back to the loft (14 points in total). I also have a 8 way LNB to fit and will wire all 8 to the same place.
I have made up a satellite patch panel so that it keeps everything all neat end easy. I will just connect up what's needed when and where it's needed.



But it had occurred to me that I have used metal female to female F connectors screwed into a metal plate to do so. So all the screens will be combined electrically.

The rooms aren't finished and the LNB hasn't been wired up to the loft yet so I cannot wire up nothing to test what happens in this situation yet.

[Mr Pointy]

I'm pretty sure that electrically the screens just act as earth/0v for the LNB power. If you think about it all the cables on the back are joined together by the panel & in one big star arrangement & could be picking up intereference, even though they are nominally grounded by the connections to the receiver/distribution box. You'll probably be ok, but if it's easy to change at this point I'd do it. Having said that though, I'm having trouble finding an isolated panel or feedthough connctors so you may not have a choice.

[nyt]

I'm pretty sure that a voltage is sent up the satellite cable to switch polarisation (Horizontal/Vertical)

http://www.satcure.co.uk/tech/lnb.htm said:

A modern "Universal" LNB requires a 22 kHz signal at 0.5v p-p to switch its Local Oscillator to 10.6 GHz ("high band"). Otherwise it uses its 9.75 GHz oscillator.

Polarisation switching is controlled by DC voltage supplied by the receiver. 12.5v to 14.5v gives vertical and 15.5 to 18v gives horizontal polarisation. A higher voltage than that can damage the LNB (although I have run it up to 24 volts, briefly, without damage). A voltage that is too low will prevent the LNB from working correctly.

This switching voltage, fed up one or more cables, also powers the internal electronics of the LNB. A modern LNb requires 125mA or more. Older LNBs and LNBs with several outputs may take more current.

I'm not sure how this would affect your arrangement

[Mr Pointy]

The screen is the 0v/earth/ground & the center conducter carries the switching voltage.

[Buzz84]

Almost as per my thinking that it is mainly a screen.

So if they are linked up, it might be OK for one box, but it might become an issue when other boxes are added onto the equation, each with their own 0v reference.
(in electronics/electrics systems containing different power supplies the 0v are kept just for the items connected to it)

Might only tell when I go to do it, if I get issues with signal, I can easily remove the connector panel to see if thats what's causing it. If if is then I can address it, I could always make up form of some plastic liners to fit into the plate to isolate the barrel connectors.

[Mr Pointy]

Joining the 0v from different bits of kit isn't an issue - you'll often see multiple STBs connected to a splitter or smart switch. It's the (admitedly unlikley) interference form al the unconnected cables that could be an issue. I doub't you'll have any problems though.

[VEX]

No, no issues at all.

Industry usually joins up all the screens for earthing requirements.

Relax and crack on.

[OldGermanHeaps]

I would earth bond the plate to the cabinet, and earth the cabinet through a decent size cable.
When you are connecting aerials or satellite to earth either go all (heavy bonding) or nothing (direct to double insulated appliances)
Half arsed invites trouble, especially if there is a nearby lightning strike.

Edited by OldGermanHeaps on Monday 23 April 22:50