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Old 16-Apr-2014, 1:59 PM   #1
timgr
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Chimney installation?

Reading some older threads, chimney instllation is suggested instead of drilling through the roof.

Here's my chimney -



Any comments? Looks like I could add a mast and easily get above the roof peak. My main question is - where do I run the wires, across the roof or down? I understood that going through the roof with the wire(s) would be bad, since it penetrates the shingles.

Re the grounding, when an electrician ran the service to my separate garage, he buried two ground rods in the service trench rather than driving them into the ground. The ground is very rocky here, and it's unlikely that I would be able to drive a rod into the ground fully. Maybe I could follow his example?

And here's my radar plot, if it's relevant - http://forum.tvfool.com/attachment.p...1&d=1397656293
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Old 16-Apr-2014, 11:52 PM   #2
GroundUrMast
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I try to avoid traversing the roof with coax. It provides a place for debris to get caught and then create a dam to water and ice. I prefer to run down to the edge and then proceed under the eve or soffit.

But as I look at your photo, I wonder if a set of wall brackets would be even better. I've yet to have any leak issues with a mast supported by wall mounted or gable end mounted brackets.

Re. grounding, I'd suggest you not add another ground rod, but run the mast and coax grounds to the existing electrical service ground. You want to use the same ground system any way, and an added rod would require that you tie it to the existing ground system to ensure there is minimal voltage difference between the existing and new ground rods.

http://forum.tvfool.com/showthread.php?t=901 (summary at post #20)
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Old 17-Apr-2014, 12:33 AM   #3
timgr
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This is great - I really appreciate the feedback.

I thought about burying an additional ground rod because of the drawing here - http://www.hdtvprimer.com/ANTENNAS/basics.html - that shows a ground rod plus an NEC-required #6 copper run. Service to the house is grounded to the water pipe from the street that emerges in the basement, before the water meter. Tying into that should be straightforward. However, this is roughly 30' from where the ground wire might enter the basement. Using #10 solid Cu wire as in your post #20 would be a considerable savings over the #6 wire suggested in the earlier link. I wonder if the #10 is up to a proper lightning discharge. Looking at lightning rod wire, it's a #1 open braid at about $2 per linear foot.

The roof peak is about 24' off the deck, and there is a location around the corner from the picture that would support a 30 foot-ish mast from eves at about 12 and 20 feet (picture will have to wait for daylight). But I'm unsure about how a mast like that would be erected - ie whether the antenna(s) would be assembled to the mast, or if the antennas would be assembled to the mast after it was erected.

You mention roof brackets - the roof you see in the foreground of the picture isn't that high - maybe 12'. So that's another 12' to reach the roof peak. The location around the corner seems more practical, if using brackets. Then run the mast to the ground. Steel tube is cheap compared to heavy copper wire.
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Old 17-Apr-2014, 5:01 AM   #4
GroundUrMast
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I have installed several ground rods over the years (related to antenna installations). In each case, I've taken what some may erroneously consider an extra step, tying (bonding) the new rod to the existing electrical service grounding system, using a #6 AWG copper conductor. My reasoning was similar to yours, there was a significant distance between the antenna mast and existing electrical service grounding system... I wanted to provide access to earth as soon as practical. However, I also wanted to have some assurance that the antenna system was connected to the same grounding system as the electrical service, to avoid a big voltage difference between the antenna system and any other grounded devices in the house.

The NEC calls for a minimum conductor size and choosing to use a heavier conductor is certainly a choice that errs on the side of caution. If the question is whether to build a system so strong that it can't be broken, consider... As you have already observed, a direct strike could easily cause failure of light gauge conductors. If the weakest link in the grounding path of a lighting strike is the coax, one can have a reasonable expectation that the coax would fuse open first (provided the coax is connected to a substantial grounding system, outside the building) isolating the outdoor portion of the system from the cabling inside the house. If the mast grounding conductor failed next, you could then have some hope that with the coax already open, there is a significantly lower chance of the energy entering the building (at least not through a failed coax). I propose that a #10 AWG copper wire is going to have a higher fusing point than RG-6 coax and so it's an appropriate size for protecting a receiving antenna system. (But one can still argue that bigger is better... if cost is not a factor.)

The bottom line for me is, I want to take reasonable steps to encourage fault current to stay outside my house. I also want to take reasonable steps to prevent a significant voltage difference from developing between to objects in my home if they are easy to come in contact with.

Also consider that the impedance of a driven or buried ground rod can be surprisingly high. The last two I drove were 5/8" X 8' copper clad, into sandy glacial till, common to the NW. I calculated the effective impedance of each one at approximately 1500Ω. (Based on measured AC current flow and known voltage applied. Clearly, it would take the voltage of a lighting strike to make any significant amount of current flow. I don't depend on the soil to be a conductor that's capable of carrying fault current while holding voltage differences low enough to ensure safety. That's the job of the #6 AWG copper that ties (bonds) the various parts of my grounding electrode system together. If you choose to add a rod, please be sure to bond it to the existing electrical service grounding system.


Take a look at the options you have when it comes to mounting hardware, http://www.3starinc.com/antenna_mounts_and_masts

FWIW: My current antenna installation is mounted on a 5' Ronard tripod which holds a 10' 6" chain-link top rail tube (Home Depot). The lag bolts are run into 2X4 bracing block glued and screwed to the underside of the roof sheeting, between rafters. The base clips of the tripod and the anchor bolts are sealed to the fiberglass/asphalt shingled roof with mastic tape (Scotch 2228) and roofing cement (Henrys # 209). My previous installation used a set of wall brackets to support the same type of mast, a 10' 6" chain-link top rail. In both cases, I ran #10 AWG copper from the mast to the nearest ground rod, using corrosion resistant grounding clamps to make the connection to the mast and ground rod.
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Last edited by GroundUrMast; 17-Apr-2014 at 7:27 AM. Reason: Clarification & added descriptions
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Old 17-Apr-2014, 3:59 PM   #5
timgr
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Thanks for the interesting and detailed response.

Looking at this a little more, I notice that the roof has an abandoned bathroom vent next to the chimney. I could take that over as my wire entry point (the alternative is to drop wires over the side of the house and go through the shingles and sheathing into the basement stairwell, where the small low window is). The mast ground could then follow the stack chase down to the basement floor, around the perimeter of the basement, and close to the current water pipe service ground attachment. See any issues with this?

The coax could also follow the stack to the basement, but there it must leave the stack. Whatever I end up with, the coax must diverge from the mast ground quite a ways from where the mast ground would be bonded to the service ground.

Now, your post #20 says to install a ground block and "connect it to the electrical service ground at the same point you connected the mast ground." That could be interpreted as sending the ground block wire back to the water pipe where the service ground is. I don't see any need for using a parallel conductor to the mast ground, as long as the coax ground ties into the mast ground before they diverge.

You also advocate "reasonable steps to encourage fault current to stay outside my house" - so you would take the mast ground around the perimeter of the house to the point of entry with the shortest path to the service ground? Doing that, the ground block would be way far away from the service ground... so put the ground block where they diverge, up by the mast?

Last edited by timgr; 17-Apr-2014 at 4:45 PM.
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Old 17-Apr-2014, 5:35 PM   #6
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So far as I am aware, the NEC and similar codes would allow the grounding and bonding conductors to be run in and through the building. I choose not to do that with the antenna system ground because I'm uncomfortable with the idea that the energy of a direct lighting strike would be directed into or though my house intentionally.

I prefer to to keep the mast and coax grounding connections separate until they meet at the ground rod or similar connection to the electrical service ground. The goal I have in mind is to keep the length of wire that serves the coax grounding block as short as possible. This will provide the lowest voltage drop possible if fault current flows in the coax. I don't want to share this grounding conductor with the mast in case the mast is energized... Any voltage drop in the mast conductor could then cause fault current or at least impose voltage in the coax runs if the mast and coax grounds shared a common grounding conductor. As the coax ground block is located further from the electrical service ground (along the mast grounding conductor), the resistance of the ground wire increases, as a result, the magnitude of the voltage drop due to fault current increases. Shared grounds means you will share faults.

In the case where I added a ground rod, I used #6 AWG copper to permanently join the new rod to the existing electrical service grounding system. The newly installed rod was then an integral part of the 'electrical service grounding system and located much closer to the antenna mast than any other part of the electrical service, so that was the location I chose to mount the coax grounding block.

It's entirely possible that the #6 AWG will have a significant voltage drop if current from an antenna system fault occurs, but I think it's reasonable to expect the weakest link to fuse open first in the event of a catastrophic event such as a direct lighting strike. Ie., the coax should fail first, then the #10 AWG... the #6 AWG should last the longest. If I've kept all those conductors outside, I can sleep well knowing I've taken what I think are reasonable steps to direct potentially destructive energy outside rather than into my house. I don't hope to prevent any and all damage if I do get a direct lightning strike, but rather, if my antenna system fails, it should fail 'safe'.
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Last edited by GroundUrMast; 17-Apr-2014 at 8:49 PM.
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