Multiple rg-6 cable ground?

alan_s_thefirstSeptember 22, 2012

On this installation I'm working on, homeowner pre-purchased his media stuff, Hubbell. I'm not really familiar with it, I tend to buy Leviton.

He's got RG-6 video/sat/cable splitters, (not power injectors) although he doesn't know what's going to be hooked up - most likely satellite, slight possibility of cable if it's at that location (small town, limited cable runs)

Looking under the cosmetic plastic mount, these splitters have provision for a grounding cable, but it's concealed by the plastic trim. Presumably just like with electric, we don't want multiple ground connections, so I assume they're ignored.

If it's cable, the demarc should have a grounding block that ties the braid of the cable to ground, if satellite, a ground cable runs to dish, also grounding braid. (here in BC few people seem to ground their dishes) Presumably in both cases, tying these splitters to ground is unnecessary and undesirable, since they're already grounded?

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petey_racer

Splitters whenever possible should be grounded (bonded) to the house's electrical system. Of course in wall plates this is kind of unrealistic.

"Presumably just like with electric, we don't want multiple ground connections,"

What do you mean by this?

    Bookmark   September 23, 2012 at 8:51AM
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brickeyee

"Presumably just like with electric, we don't want multiple ground connections, so I assume they're ignored."

Fort RF installations the first cut is normally grund early and ground often.

The wire is rarely a low impedance ground at RF anyway, and absent time varying magnetic fields you will have minimal pickup anyway.

60 Hz pickup is so ubiquitous that just about any RF equipment already guards against it.

    Bookmark   September 23, 2012 at 9:13AM
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westom

No multiple ground connections for a long list of reasons. A typical example is ground loops created by interconnected stereo equpiment with multiple grounds.

All RG-6 must share a common 'earth' ground. Not AC electric (receptacle) safety ground. Earth ground and safety ground are electrically different even if a wire interconnects both. RG-6 must be earthed at the service entrance - if used for cable TV, antenna, or satellite dish.

Serious and technical electrical reasons say why. But one example that everyone should be familar with is hum created by ground loops between stereo components.

    Bookmark   September 23, 2012 at 11:33AM
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brickeyee

"No multiple ground connections for a long list of reasons. A typical example is ground loops created by interconnected stereo equpiment with multiple grounds. "

Wrong.

RF is not audio.

RF equipment is NOT affected by 60 Hz hum or none of it would ever work.

Add to that the >60 db rejection of good quality coax to outside interference.

    Bookmark   September 23, 2012 at 5:11PM
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yosemitebill

A rooftop antenna mast or satellite dish mast require direct earth grounding, bonded to the structure's electrical earth ground, for protection against lightning strikes. And also for safety reasons when located near pool/hot tub areas.

Ideally, a coaxial static-discharge block is used to dissipate static build-up charges for antenna/dish applications and also ties the coax shield to ground.

A simple grounding block used in CATV is used primarily in an attempt to equalize the ground difference potentials between the CATV line and the structure's ground.

For whole-house signal distribution, it is best to tie the coax into a single bonded grounded point at the service entrance. Directly after, use a passive splitter or distribution amplifier to feed each location.

    Bookmark   September 23, 2012 at 10:13PM
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brickeyee

"Directly after, use a passive splitter or distribution amplifier to feed each location. "

And make sure you have ALL the shields in multi-shielded coax grounded.

I have seen numerous double shielded cables that only carry the inner signal shield on the RF connectors.

That leaves the outer shield hanging in space and not doing a while lot of anything.

    Bookmark   September 24, 2012 at 12:41PM
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yosemitebill

"I have seen numerous double shielded cables that only carry the inner signal shield on the RF connectors."

People will also use RG6 Quad because it "sounds better" but don't understand that it is specs the same as it's RG6 counterpart - it is simply meant to be used in electrically noisy environments. Then they improperly terminate the shields and use RG6 F connectors instead of RG6 Quad F connectors.

    Bookmark   September 24, 2012 at 9:32PM
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brickeyee

Without a separate grounding path for the outer shield is does virtually nothing.

The signal travels between the center conductor and the inner shield in the cable dielectric.

The outer shield is intended as a Faraday cage NOT connected to the signal path (inner shield) bit carried separately.

    Bookmark   September 26, 2012 at 11:58AM
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yosemitebill

Brickeye, in regards to your last post it appears you are confusing RG6 Quad with Triaxial cable.

In RG6 Quad all wraps and shields are conductive to one another throughout the cable and all are terminated together at each end.

In Triaxial cable the two shields are insulated from one another creating three conductors including the center conductor. It's most common usage is in television broadcast from studio cameras to the CCU (camera control unit).

It is also used occasionally as you suggested as as two independent shields, one for signal and the other for earth grounding, in RF applications requiring additional noise immunity.

However, in RG6 Quad - all foil wraps and braids are common.

    Bookmark   September 26, 2012 at 7:00PM
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brickeyee

"all foil wraps and braids are common. "

Then it is a cheap attempt to get 100% shield coverage.

Braid is often less than 100% optical coverage, and adding foil is normally adequate to get up to 100% without another layer of braid.

Two layers of braid that are connected with foil present are just a gimmick.

The foil should be more than adequate for 100%$ shield coverage.

    Bookmark   September 27, 2012 at 3:53PM
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yosemitebill

Shields and foils in coax are selected and based upon the signal frequency spectrum carried in the cable and the frequency spectrum you are attempting to prevent from entering the cable.

The aluminum braid and shield work better at the frequency range RG6 was designed for at CATV RF frequencies than RG59's copper braid that works better at lower frequencies.

Two layers are not a gimmick - it simply provides the best shielding within the frequency range of it's intended application.

    Bookmark   September 28, 2012 at 8:47PM
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