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Proper repeater site grounding

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SSMHAZMAT

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I know this has been tackled many times so I will try to be brief. I am a fire Chief for a semi-rural FD. We operate on a 100w Motorola moto turbo vhf repeater system. After having some crippling issues with noise discovered and repaired on our system (loose heliax connector) I have taken a special interest into how our station is grounded. and before I continue, yes there is still a small amount of noise on our system.

Currently all110v a/c is grounded per code except for that ground rod only being driven down about 6’. There is another exterior ground rod (very corroded and old) driven on the opposite end (radio install end of the shack). This ground has antenna shielding for the other vhf/uhf connected at the rod then a #2 copper wire running into the shack where the chassis ground for our radio’s single point ground and duplexer ground are tied into, as well as the chassis grounds for the other vhf and uhf radios that we share the shack and 120’ antenna mast with. The lightening arrester for our duplexed antenna coax is also bonded to this ground... it should be noted that this ground system is not tied to the shack’s main a/c ground.

Then the steel 120’ antenna mast is also seemingly grounded at all 3 points where bolted to the cement base. I say seemingly because I found the copper ground wire from one leg only going into the ground about a foot and not mechanically connected to a rod. This third ground system is also not tied to either of the other 2 ground systems - a/c ground and radio chassis grounds.

I know I should drive the shallow a/c ground down all of the way, replace the radio chassis ground rod with a properly installed rod, and also make sure the antenna mast ground is also properly grounded with a new rod. But are any-or all of these separate grounds supposed to be tied together? Is It ok for the chassis grounds to be separate from the a/c power ground?

I know about the NEC and Motorola’s document but I wanted to get your opinion on my questions posed above.
 

prcguy

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Jun 30, 2006
Messages
11,131
Location
So Cal - Richardson, TX - Tewksbury, MA
I would hire a professional grounding company to assess the situation and make recommendations, its probably too far beyond a lay person to get it right. Sounds like the original design may have been very lacking in proper grounding, compounded by problems over the years.

A typical radio shelter has a ring of very large dia (up to 500mcm) bare wire buried around the entire perimeter bonded to 10ft ground rods at roughly 20ft intervals. A tower would have large dia ground wires bonded to each leg going into the earth with no bends and sometimes to very long buried lines heading outward from the tower for upwards of 60ft or more bonded to 10ft ground rods about every 20ft. The tower ground would then be tied into the shelter ground with only one point where all power and antenna feeds are bonded as they enter the building. This is just the start and even though I'm giving examples please don't take this advise as your goal and seek a grounding professional.

When grounded properly the electronics inside the shelter can easily withstand direct lightning hits, although the antennas can be destroyed in the process.


I know this has been tackled many times so I will try to be brief. I am a fire Chief for a semi-rural FD. We operate on a 100w Motorola moto turbo vhf repeater system. After having some crippling issues with noise discovered and repaired on our system (loose heliax connector) I have taken a special interest into how our station is grounded. and before I continue, yes there is still a small amount of noise on our system.

Currently all110v a/c is grounded per code except for that ground rod only being driven down about 6’. There is another exterior ground rod (very corroded and old) driven on the opposite end (radio install end of the shack). This ground has antenna shielding for the other vhf/uhf connected at the rod then a #2 copper wire running into the shack where the chassis ground for our radio’s single point ground and duplexer ground are tied into, as well as the chassis grounds for the other vhf and uhf radios that we share the shack and 120’ antenna mast with. The lightening arrester for our duplexed antenna coax is also bonded to this ground... it should be noted that this ground system is not tied to the shack’s main a/c ground.

Then the steel 120’ antenna mast is also seemingly grounded at all 3 points where bolted to the cement base. I say seemingly because I found the copper ground wire from one leg only going into the ground about a foot and not mechanically connected to a rod. This third ground system is also not tied to either of the other 2 ground systems - a/c ground and radio chassis grounds.

I know I should drive the shallow a/c ground down all of the way, replace the radio chassis ground rod with a properly installed rod, and also make sure the antenna mast ground is also properly grounded with a new rod. But are any-or all of these separate grounds supposed to be tied together? Is It ok for the chassis grounds to be separate from the a/c power ground?

I know about the NEC and Motorola’s document but I wanted to get your opinion on my questions posed above.
 

K4EET

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Feb 18, 2015
Messages
1,685
Location
Severn, Maryland, USA
@SSMHAZMAT, for anybody's reading enjoyment, this "white paper" by Reeves is good for folks that might find NEC Article 810 hard to understand/visualize. While it does not replace NEC Article 810, if you have some firefighters on your staff that do not need to read the NEC but would benefit from an understanding of its concepts, this is an excellent document to pass out. Just an FYI that its available and well written. Dave

P.S. While mainly written for the home installation, many folks have antennas at home with inadequate grounding. This article is clearly written for them! Better safe than sorry...

P.S.S.: Motorola's R56 is a great document. I say this as an ex-Motorola engineer.
 
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K8AI

Member
Joined
Jul 30, 2001
Messages
9
Location
Salt Lake City, UT
Sounds like the grounding and bonding needs remediation. I would either hire someone like VFC that will follow either R56 or L3Harris guidelines. Everything is supposed to be bonded and it sounds like you're not.
 
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