Proper location for ground block on amplified TV antenna feed vs power inserter.

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Hi Guys,

In another forum a co-member posted a statement as to the proper location of power inserter and a grounding block on an amplified TV antenna system.:blink:

We are in a bit of disagreement on it and I'm looking for a code interpretation. :(

In my opinion, a coaxial power inserter, one that puts DC on the coaxial line should be placed on the inside of the grounding block. :happyyes:

He said that in his experience it should be reversed with the power inserter between the grounding block and the antenna. Otherwise it dragged down the power supply to a lever of 3 or 4 volts. Removing the ground from the grounding block fixed the voltage drop. Placing the power inserter beyond the grounding block also corrected the problem. :happysad:

Technically / electronically, it would make no difference as the inserter has a capacitor to prevent DC from going downstream but allows it to go upstream to the antenna amplifier. ( Or so me speaking from 54 years or so experience in electronics. ) It also has a choke coil, on the lead connected to the coax center, inside it to prevent attenuation of the RF by the power supply circuitry. :huh:

He claimed to have installed hundreds of antennas his way. Granted I've installed less than 20 home TV antenna systems and had never encountered this problem.

What say you? :?

What does the code say? :?

Thanks for your time and help. :)

Some references I made are also in the thread here: Handyman Club

Cordially,
Gerry
 

hurk27

Senior Member
Welcome to the forum. but keep in mind of our rules of How to type questions as we are only allowed to help those who have a working relation in the field.

Code only says that the shield of the coax is to be bonded at the point of entrance to the building and article 800 give you a few ways of doing it, but the NEC is not a design manual and how it gets done is up to the installer.

With that said there are grounding blocks that are only a bond for the shield and maybe a spark gap to shunt the voltage by lightning, but it does not have any filtering or DC block so these will work fine for installing a DC power supply ahead of them, but there are grounding blocks that also filter and block any DC and will block the DC power impose upon the cable supplying the line amp, these have to be avoided, cable TV uses a DC block to prevent the 60 volts DC from their power supplies from coming into a building but most of these are blocked out on the pole at the tap not at the house, any grounding block that has a choke between the center and the shield will block DC as it will shunt it (short it out) RF can pass right through such coil, so it is all going to depend upon which type of ground block gets installed.
 
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hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
Considering that all a ground block does is ground the braid or shield of the coax, why does he think it matters which side of it you locate the power inserter?

The NEC says that the ground block should be located as close as possible to where the cable enters the building, and that can be either on the inside or outside. It couldn't care less where a power inserter is located.


The only thing I can think of is that he is not talking about a ground block at all. A ground block is just an F-81 barrel "splice" on some sort of metal bracket so it can be connected to a ground wire and attached to the wall. He may be talking about a lightning arrester or surge suppressor that has elements between the center conductor and ground that may clamp at a low voltage. Again, I'm only guessing because I can't think of ever seeing one and it doesn't completely fit with his scenario. If he could give you the manufacturer and some part numbers maybe I could chase it down and give a definite answer.

-Hal
 
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egnlsn

Senior Member
Location
Herriman, UT
Occupation
A/V/Security Technician
A standard groundblock for TV is nothing more than a barrel connector with mounting feet. There is no shunt or any type of path from center conductor to ground. There's no reason for the voltage to be reduced by any amount at all.

As hurk27 wrote, the shield needs to be bonded to ground prior to entering the building. Personally, I would rather have the power supply inside the building than outside being part of the grounding system.
 
Thanks Guys.

Thanks Guys.

Thanks Guys,

I thought ground blocks incorporated spark gaps inside the barrel.

I was not aware of the "special" ground blocks used by cable companies to isolate voltages from consumers.

It seems it can be done either way, but up till today, I did not know of any that were weatherproof and could be installed outdoors. That being said, I suppose the type that can be installed outdoors would incorporate some method of delivering the DC to an inserter that would allow it to have the power supply indoors. Then I would imagine it would not matter if it was installed on the antenna side of a ground block or on the inside of the structure.

Personally I agree with Egnlsn and would install the inserter inside the structure if at least for the reason of less number of connectors exposed to the weather.

Thanks again.

Cordially,
Gerry
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
It seems it can be done either way, but up till today, I did not know of any [power inserters] that were weatherproof and could be installed outdoors.

Depends on what kind of equipment you are talking about. If it's a VHF or UHF TV antenna preamp like from BT or Channel Master then the power inserters are normally an indoor only design. There are however power inserters that look like a 2-way splitter. The power to it is supplied with a piece of coax to a wall-wart that has an "F" connector for it's power out. This type is certainly weather proof. The wall-wart power supply is the only part inside the building. I see these used with CATV and satellite amps and LNBs.

-Hal
 

egnlsn

Senior Member
Location
Herriman, UT
Occupation
A/V/Security Technician
Thanks Guys,
I was not aware of the "special" ground blocks used by cable companies to isolate voltages from consumers.

Thanks again.

Cordially,
Gerry

The voltage blocking is done at the tap utilizing a capacitor (part of the design of the tap), not at the groundblock. The groundblocks used by the cable companies are merely F-81 barrel connectors with mounting feet.
 
Thanks again.
Apparently the ground block was not the problem.
He related in another post that it may have been the FM trap / switch that may have been a contributing factor.

Very Strange. There was some kind of ground loop going on and I suspect the particular model of a Radio Shack amplifier was the culprit.

Thanks again, case closed,
it should not matter what side of the ground block the power injector is installed on, except if the injector is weatherproof or not. :thumbsup:

Which brings to mind a final question; if it is indeed a coaxial type power feed to the injector, the power injector is also weather proof, shouldn't a dual dual ground block be utilized, one connection for the RF and one for the DC before it enters the structure? If not, then with just the power coax entering the structure and because it is connected on the antenna side of the ground block, it may be in violation of code.

Thanks again guys.

Cordially,
Gerry
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
if it is indeed a coaxial type power feed to the injector, the power injector is also weather proof, shouldn't a dual dual ground block be utilized, one connection for the RF and one for the DC before it enters the structure?

That would not be necessary. A single ground block would suffice by grounding both cables through the power inserter housing. It wouldn't be any different than a splitter located outside in the drop after the ground block with a number of cables entering the building.

-Hal
 
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