Aerial Bonding

[Christopher Key]

Hello,

I'm about to start installing a loftbox aerial distribution system.

comes with a fairly heavy duty earth bonding terminal; should this be connected back to the main equipotential bonding connector? Instinctively, I would have assumed yes, in order to provide some protection in the event of a lightning strike, and to keep any large potentials from developing between exposed metalwork, e.g. radiators and the aerial system.

However, I understand that some AV equipment is designed to leak to earth slightly, and am concerned about this causing ground loops and hum.

The electrical installation is PME, with bonds to the gas and water main.

The outlet plates will all be non isolated to at least some of their connectors in order to power and control the LNBs, and to send remote control commands back to the Sky+ box.

Any advice much appreciated.

Many thanks,

Chris Key

[Andy Wade]

[Loftbox]

If all the outlet points are serving a single household then the bonding is optional. The main purpose of earth bonding an aerial system is to ensure safety in a distribution system serving more than one household - e.g. in case your neighbour decides to connect the coax to the mains (this used to happen frequently in the days of live-chassis TV sets with faulty aerial isolators). It is not a lightning protection measure - that needs to be done outside the house, bonding the antenna system and mast using much larger conductors. System earth bonding has superseded the former practice of using isolated output plates, which are a disaster EMC-wise, and don't pass LNB power/control, etc.

The earth bonding terminal should be connected to the main earth terminal of your electrical installation with a min. conductor size of

4mm^2 (or 4mm^2 copper equivalent, if not copper). (The relevant standard is BS EN 50083-1.)

That can happen, although doesn't seem to cause problems too often. Most A/V equipment is Class 2 and doesn't have its own earth connection to create a loop. Even if you don't earth-bond you'll still be connecting the 'grounds' of various bits of equipment together via the coax, into one floating mass, so to speak. If you do have a hum loop problem the best solution is to fit a 'galvanic isolator' (aka braid-breaker filter) on the antenna input of the affected equipment. The isolator is available from companies like Teldis (but sit comfortably when you ask the price!).

HTH, HNY

[Lobster]

So if you've got neighbouring homes, with their TV signals coming from separate aerials, but attached to the same metal mast up on the roof, then earth bonding would be compulsory? That's never occurred to me... what sort of bonding would be needed - what, where and how would it be fitted?

(Sounds like an easier option might be a second mast!)

David

[Lurch]

On Thu, 30 Dec 2004 14:46:44 +0000, Andy Wade strung together this:

Had to think about that for a minute.

[Mike]

English Heritage are "encouraging" groups of listed buildings to have a single aerial. And no satellite TV.

[Andy Wade]

Ooh, that's one you could debate until the cows come home. What you describe though is surely an unusual situation. The aerials for neighbouring homes would normally be on separate masts & bracketry, even if lashed to opposite sides of the same chimney.

Bond the cable outers at the points of entry. You could use US CATV-style 'grounding blocks' (basically just back-to-back female 'F' couplers with fixing screw holes and an earth terminal) or one of the types of screened outlet plate which provide an earth terminal. If the electrical earthing arrangements in the two properties are known to be different (i.e. not both PME) then use of the aforementioned galvanic isolator is advisable.

That is the usual way :-)

[Christopher Key]

connected back to the main

to provide some

optional. The main purpose of

more than one

(this used to happen

is not a lightning

system and mast using much

using isolated output plates,

electrical installation

(The relevant standard is

equipment is Class 2 and

earth-bond you'll still be

one floating mass, so to

isolator' (aka

is available from

Thanks for that. The aerial / mast etc is only serving one property, but I think it is probably worth going ahead and earthing it. The equipment is all double insulated, and their chassis are all at 75V relative to the house earth (which is pretty close to local earth), and able to supply around 2ma or so. Enough to tingle quite a bit, and certainly not the sort of surprise one wants when 25ft up a ladder fixing an aerial.

Elsewhere, you mentioned US Style grounding blocks. I've only been able to find them in the UK from CPC (CN04175, CN04176), but would rather not pay £5 handling charge on a £1 order! Can you recommend anywhere else what might carry them?

Once again, many thanks,

Chris Key

[Andy Wade]

them in the UK from CPC

you recommend anywhere

I don't see why you'll need these. Just earth the system at the earth terminal on the loft box.

[Christopher Key]

them in the UK from CPC

Can you recommend anywhere

the loft box.

Sorry, should have been more clear. As we have SKY+, i'll be taking a 2nd feed from the LNB down to the lounge, which won't be connected directly to the loft box. It seemed a little untidy having this earthed 'via' the SKY box.

Many thanks,

Chris Key

[Andy Wade]

Still no need, the 2nd IF feed will be earthed via the body of the LNB and the 1st IF cable to the loft box.

Moreover inserting low-grade connectors & adaptors (such as a grounding block or ordinary F back-to-backs) in a digital satellite IF line is not recommended since some of these devices exhibit very poor impedance match (poor return loss or high VSWR) toward the top end of the IF band at ~2 GHz. The reflections caused in the cable lead to signal degradation (inter-symbol interference) which can sometimes cause missing channels etc. If you need an in-line F coupler for satellite, use the Cabelcon F-81HQ type which provides a really good 75 ohm match from DC to 2 GHz+.