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Old 12-Aug-2014, 4:46 PM   #1
Darken
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Grounding Question

Hello,

Ive been looking up on how to ground an exterior antenna and seem to be getting conflicting info. In my situation i have a winegard 7698p and it will be mounted on a tripod and 5 to 10 foot mast. However, it will not be the highest point of my house. Because of the way the house is built (cathedral style ceiling) and the direction i need to point it for best reception it will be mounted on the lower back side addition of the house. I wanted to ground the coax and mast with copper wire to a grounding rod but then i see best practices often make reference to another copper wire being grounded to the main electrical service panel. The service panel is in the front of the house and the grounding rod and antenna will be on the complete opposite side on the back of the house. Is it incorrect to simply have the grounding rod in place in the earth for the coax and mast without then linking that to anything else? Will i then need to somehow pass another copper wire through the house/foundation to get it to the front service panel? Any suggestions appreciated.

Thanks

Last edited by Darken; 12-Aug-2014 at 4:56 PM.
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Old 12-Aug-2014, 6:54 PM   #2
GroundUrMast
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The NEC deals with antenna systems in article 810. It specifically states the need to connect (bond) any added ground rod to the existing electrical service grounding system, using #6 AWG copper and approved connectors at a minimum. This will ensure that there will be very little difference in voltage levels between the new ground rod and the rest of the grounding system on the premise.

So the best practice is to use only one grounding system. (Which can include a variety of driven rods, buried pipe, metal plates, foundation steel, etc... provided that they are all boned together to ensure that no significant voltage differences can develop between parts of the grounding system.) If you build two or more grounding systems, there is very real possibility that there will be undesirable or even dangerous voltage differences between the two systems, particularly when some fault current exists.

For example, if your antenna comes in contact with power or is charged by atmospheric static, current will flow to ground through whatever path you have provided. If that path has just a few ohms of resistance, a significant voltage drop will be present so long as the fault current exists. Driven ground rods are almost always going to have significant amounts of resistance to earth. The last two I drove and checked had about 1500 ohms of resistance to the surrounding earth. The two in parallel were about 800 to 900 ohms. That means that if a 1 amp fault current were to occur during a nearby lightning event, I could expect there to be about 800 volts between the antenna and nearby soil.

As you can see, an antenna system connected only to these two isolated ground rods could be energized with a hazardous voltage relative to an electrical system elsewhere on the property. If I ran the coax to a TV that is connected to power that is protected by a separate ground, there could be a large voltage difference between the coax and TV chassis.

In my case, the two rods were to serve an outbuilding power sub-panel. The supply from the main building includes two 'hot' wires, a neutral and a grounding conductor that originates from the main electrical service panel. So I'm not depending on the ground rods at the out-building to provide the primary connection to earth... They simply add to the already existing grounding system thats present on the premise and which is also tied (bonded) to the utility companies grounding network via their service drop to my electrical service panel. The resistance of the grounding conductor between the out-building sub-panel and main service is less than 1 ohm, so even if current flows, the voltage drop is far less than if I had relied on the soil to connect the grounding system in the out-building and the main building.

So my advice would be that if you choose to add a ground rod near the antenna (or anywhere else on your property for that matter), run a #6 AWG copper 'bonding' wire to the existing electrical service ground.

You also have the option of running a mast ground all the way to the existing electrical service ground... AND, run the coax to the electrical service ground where you would install the coax grounding block. These connections can be done with #10 AWG copper (or heavier if you wish).

(I keep the coax outside the building until it has passed through the coax grounding block. I locate the coax grounding block close to a bonded ground rod so that the wire from the block to the grounding system is as short as practical.)

http://forum.tvfool.com/showthread.php?t=901 (see post #20)
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Old 12-Aug-2014, 8:54 PM   #3
stvcmty
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GroundUrMast had a very good explanation of grounding.

I only have one thing to add. It is good to have the coax ground block close to the electrical service entrance, and have it grounded as close as possible to the electrical service ground. The coax should not enter the house until it has been grounded.

In your case, that would mean having the antenna in the back, bringing the coax around the house, and bringing it into the house in the front. Without knowing the specifics of your situation, (how much signal is in the air, how your antenna affects the signal, preamp and so on), that may or may not add unacceptable loss.

My view of grounding comes from an amateur radio grounding concept best explained as “a rising tide lifts all ships”. If your electrical service, phone, cable (if you have it), antenna, and any other services all enter at close to the same point, then a surge or lightening strike on any them will get dumped to ground at the same point. An rise in potential (voltage) that gets in will be on all wires in the house equally so hopefully no voltage will develop on connections that expect to be low voltage. On the other hand, if something had a separate ground (such as your antenna), a spike on the separate ground could cause large voltage to develop between the separate ground (the antenna), and the house ground. To see how that could be bad: if your TV has a 3 pin plug then a spike on the coax shield would try to equalize its voltage through the TV’s 3rd pin to the house ground. GroundUrMast keeps that from being a problem in the DC case by bonding all grounds. The problem is lightening is not just DC. Lightening has components that are up to 3MHz. For an AC wave of 3MHz, the peak potential difference will be at ¼ of a wavelength, or 25 meters, or 82 feet. So, if two grounds are separated by 82 feet, a huge potential could develop between them from a lightning strike. On the other hand, if everything enters your house at the same point, short grounding leads clamps everything to close the same potential in the same potential even for the RF components of lightning.

I am not saying remote ground rods are bad. A ground rod near the antenna with the mast tied to it is a good thing, it gets static or lightening into the ground as fast as possible. That remote ground rod should be tied to the house electrical ground system with heavy wire run outside of the house. What I am saying is everything that enters the house should enter the house together and be grounded within a few feet of each other so any spike on any line is equal on all lines so no device in the house sees a RF potential develop across any of its paths to ground.

Grounding is a tricky subject. What works well at DC or 60Hz may not work well at the RF components of lightening. I highly recommend reading the grounding section in Practical Antenna Handbook 5/e by Joseph Carr and George Hippisley.
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Old 13-Aug-2014, 3:12 AM   #4
GroundUrMast
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@stvcmty, thanks for the kind words.

As I re-read my earlier post, I see that it would be easy to think I was favoring the addition of a ground rod. That is not the case. I was focused on addressing the question of adding an un-bonded ground rod which I believe should be avoided in virtually any circumstance.

Thank you for clearly stating 'best practice'.

As an aside, I believe it may prove to be less costly to run #10 AWG copper from the mast to the electrical service even though the coax run may be longer as well. Ground rods, bronze connectors and #6 AWG copper aren't being handed out as door prizes at my local home center.

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Old 16-Aug-2014, 7:22 PM   #5
Darken
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Thanks very much for the detailed answers. So i've been considering how to ground my antenna based on the info provided and i'm a little bit stumped. I would have to run my coax and copper wire around the side of the house to the front where my service panel is located (meaning under the front deck as well which is quite low to the ground in many parts so quite difficult). 1) is this safe? Seems scary to think i'd have a potentially charged copper wire running all long the side of the house, most likely touching the wood exterior where it meets the foundation all the way to the front of the house? and 2) ive attached two pictures. The first picture is the service panel on the front of the house from the outside. You'll see the box and below it there is nothing except the deck several feet below. The 2nd pic is what it looks like below the deck (directly below the panel). It would seem as though i dont have access to the grounding setup. The middle cable coming out from the deck runs approx 10 feet away from the house and disappears into the ground so i assume thats the grounding cable. From the inside i can see all the electric cables disappear into the wall right below the panel and are partially spray foamed over (the basement is only about 4 feet high and unfinished). Is this a typical way to ground a service panel? Would it make sense to lead a cooper wire from behind the house, all long the side of the house, under the front deck just to reach the visible grounding cable that is seen in Pic 2? if so, would i just wrap the copper wire to the grounding cable? Any ideas would be great. Thanks
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File Type: jpg Pic 2.JPG (130.0 KB, 175 views)
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Old 16-Aug-2014, 8:58 PM   #6
GroundUrMast
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The chance of the #6 bonding conductor becoming energized is much lower than that of the isolated ground rod and any parts connected to it. (Provided the connections to the existing electrical service ground are made securely, using connectors that are appropriate and the cables are prepped to remove corrosion and other sources of resistance.)

The goal is to minimize resistance/impedance between the two points being 'bonded'. If you use soil instead of copper, the resistance will be much higher, and quite variable. If you use wire that is too small, it may fail while conducting fault current. If you allow corrosion to remain in a connection, or use a connector that corrodes, you can expect the 'bonding' connection to fail... allowing a voltage difference to exist between parts that should be at the same voltage level.

I would verify that the heavy cable shown in the photo under your deck is the grounding conductor of your electrical service. If so, I believe that a split bolt connector such as this, http://www.homedepot.com/p/Blackburn...B1-5/100125660 would be a secure means to join new #6 AWG to the existing. I would not rely on twisting or wrapping.

If the underside of the deck is inaccessible, there are connectors that can securely connect to the meter-base... which is also a legitimate point of electrical service ground. I would consult with an electrician if that was the route you needed to go.
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