Grounding a Radio Tower???

[W2XR]

I'm pretty sure I may have seen a post here some time ago about properly grounding a radio tower or antenna supporting mast, possibly from Frank, WA1GFZ, but I could well be mistaken.

My question is this; how far from the outside of the concrete base of the tower should the protective ground rod be located, and should more than one ground rod be used for the most effective ground?

Thanks & 73,

Bruce

[KA2DZT]

I usually would put them about a foot from the base.  You can use one or one off each leg if you can.  Probably would bond all the rods together with heavy copper wire.

Fred

[K1JJ]

My question is this; how far from the outside of the concrete base of the tower should the protective ground rod be located, and should more than one ground rods be used for the most effective ground?

Hi Bruce,

It's not all that critical as long as the conductor(s) are heavy and as short as possible and do not come in contact with the internal concrete rebar.  Normally all three tower legs get tied to their own ground. (Overkill, but often done)

When I put up my two 190' self supporters, I was coached by Big Al / K1JCL, a tower pro.

Here's what we did:  

The main 18' square, 4' deep tower base was lined with steel rebar in a gridwork as engineered. Before concrete was poured, we added three 0000 copper cables (1/2" dia?) that went down to the bottom of the hole and DIRECTLY came up to the top, one to each tower leg. They ran thru PVC pipe.  At each solid steel tower leg they were brazed.  Thus, we have three 0000 cables going DIRECTLY down to the earth 4' underground -  and then attached to an additional screen of copper 0000 cables lying on the bottom and terminated with ten 8' ground rods. All connections brazed using a custom cable mold and magneseum charge. Can't remmeber what they are called.   Lasts forever.

If you have normal, hollow tubing tower legs, then use heavy copper eye lugs attached to each tower leg bolt.

Make sure none of the ground cables touch the steel rebar.  The bottom line is you do not want a lightning strike to get into your concrete base rebar and blow it apart.

Each tower also has a set of longer surface ground radials that help to dissipate and BLEED-OFF the tower charge. Over the years I cannot remember these towers being hit. I think the bleed-off keeps them neutral.

The idea is to think like the lightning is 100 Mhz RF. You want a fat and direct path to ground.

I realize some ham tower installations get away with just a single #6 wire bolted to a tower leg and terminated with a ground rod. But knowing you, you will probably do it right. (overkill)  

73,

Tom, K1JJ


Here's one of the towers with this grounding system:

[W1RKW]

T,
Are the ground rods directly underneath the concrete or off to the sides 4' below?  If you had to repair those ground connections could you do that?

[KB2WIG]

"Can't remmeber what they are called"


Cadweld??

klc

[K1JJ]

Yes, that's it, Cadweld!  They burn as bright as the sun and are 5,000 degrees F for a second.  Using a custom mold we can get a really professional looking and strong connection.

Bob -  Yes the 0000 ground cables go directly down from the tower leg -  right through the tower concrete base, isolated away from the rebar by at least a foot or so.  Once the concrete is poured, they are there forever. But the cadweld is forever too, just like bronze.

The idea is to get a direct, short connection directly to Earth. Bear in mind with these large concrete bases designed for self-supporters - that the tower itself is far away from the edge of the concrete. To run a cable to the outside would be too long for minimal inductance.  

It's still a good idea to do with a guyed Rohn 45 style, but then, the edge of the concrete is close enuff for a short run with a small base.  If the concrete is already poured, then by all means a short set of cables to the edge is fine.  The longer the path, the fatter the cable needs to be for min L.

Hope your new biking adventure is going well, OM!

T

[K1JJ]

BTW, I just remembered the reasoning for the ground cables going through the concrete base....

Let's say a tower has no grounding at all. Then a strike would come down the tower legs and then try to find a path through the concrete base. It would probably arc its way through the steel rebar, which itself is wire wrapped to each other and mostly floating. This would result in big arcing and sparking, possibly enough to blow apart or crack the concrete. This could bring down a self supporter, though not as deadly to a guyed tower.

So, if we were to attach thick copper cables to the tower legs and give the lightning a direct, low inductance path THROUGH the concrete to Earth, this is a better situation. Yes, SOME residual current will still flow through the rebar, but it will be greatly reduced to hopefully a safer level.

Again, with self supporters requiring a large concrete base, the most direct path to ground is straight down. 

T

[Bill, KD0HG]

I don't agree with running any grounding conductors through the concrete.
Theoretically, no matter how large, they could go red hot or even fuse inside the concrete causing it to explode or crack on a direct hit. Same as for rebar. I have recently seem 3/4" copper pipe on a broadcast tower vaporized after a direct hit (Between the tower and the tower's tuning unit) . The idea is to use copper strap or serious cables *around* the concrete. Yes, running lightning conductors through the concrete is better than not, but there is no reason not to run the down conductors around the concrete instead of through it.

I have never seen a broadcast tower grounded with conductors going through the concrete, always around it. FWIW.

I also feel that ground rods are rather worthless for lightning protection.

Know what they're for? In the event your pole transformer suffers a primary to secondary short, they will keep your property's wiring from being elevated to the power line primary voltage, 4,800V-14 KV or whatever. Ground rods are not rated by UL for lightning protection.

As suggested, it is best to line the hole with stout conductors, copper strap or big-ass copper cable, then pour the concrete over it. Then connect the underground strap around the concrete to the tower with big cable or more strap.

Here's the resistance to ground of ground rods at 60 HZ. The resistance of an 8 foot rod is about 10 ohms in "average" ground. A direct lightning hit is going to range from 10,000 to 100,000 amps for a few microseconds. Do the math..E=IxR. So a tower grounded with a 10 ohm resistance, a typical 8 foot rod in average soil, is going to be elevated to 100,000 to 1,000,000 volts, along with every antenna on it. You have to get your ground resistance down to maybe 1 ohm for effective lightning protection on a direct hit. A ground rod ain't going to do it. Maybe 10 of them will, if spaced at least 6 feet apart. Or you can line the tower hole with strap or cable.

BURY all of your coax between the tower and shack.


Bill

[Bill, KD0HG]

More stuff:

Remember, lightning is like a pulse of MF RF. It's NOT DC or 60 HZ. If you get a direct strike on a tower, the same will be induced on the cables running down from the tower, but in opposite polarity but a similar current. The tower and cables along it act as a linear transformer. So it's important, regardless of tower base grounding, to bond the shields of all coax cables running down the the tower to the base of the tower. Otherwise, the tower strike energy will be conducted along the coax to wherever its destination is, only of opposite DC polarity. Bonding the shield of coax at the bottom of a stick to the tower itself is proper practice with everyone from the cellular folks to FM broadcasters. Everyone does it.

Only maybe two months ago I witnessed a direct strike on my stick while sitting on the porch some 50 feet away. First time ever. It sounded like putting a screwdriver across a big high voltage capacitor. SNAP! POW! Then I could hear the reflected thunder off the nearby hills.

Yes, the feeders running down from the tower were vaporized into pieces. As expected with a couple hundred thousand induced amps of current flowing through RG-213 and other conductors, but in the opposite polarity from the current on the tower itself. The cables were buried for a 40' run to the house from the tower. Absolutely *nothing* in the ham shack or house was harmed.

Other than I have a Motorola 5.8 GHz microwave system on the stick for internet access, which feeds a router in the house via buried CAT5. The router got fried. So did the TV preamp on the tower. But absolutely nothing else was harmed and the house didn't burn down. The R-390A wasn't bothered at all.

So *bond* everything together at the tower base. *Bury* your coax lines and/or completely disconnect them. Don't rely on a single ground rod at the base of the tower; strap the bottom of the hole or install multiple ground rods at least 6' apart and bond them to the tower. Nothing smaller than #4 cable.

And, good luck. It means a lot- LOL.

Bill

[W2XR]

More stuff:

So *bond* everything together at the tower base. *Bury* your coax lines and/or completely disconnect them. Don't rely on a single ground rod at the base of the tower; strap the bottom of the hole or install multiple ground rods at least 6' apart and bond them to the tower. Nothing smaller than #4 cable.

Bill

Many thanks to Tom, Bill, and everyone who replied to my question.

Bill & Tom: As the concrete base was poured quite some time ago, my only alternative is to use ground rods.

Bill, you suggested "multiple ground rods at least 6' apart and bond them to the tower. Nothing smaller than #4 cable".  Can you describe the layout or configuration for this, in terms of the suggested number of ground rods and their location relative to the concrete base? Although it is a simple concept no doubt, I can't quite visualize what you are suggesting.

Thanks & 73,

Bruce

[K1JJ]

I don't agree with running any grounding conductors through the concrete.
Bill

Bill,

We can agree to disagree.

Cables on top can be added at any time and probably a good idea too. The more the better.  Now that you mention it, I should add these too.

But the entire internal concrete base of a large self supporter is laced with disjointed rebar and is more vulnerable to severe damage when a portion of the lightning takes its path that way. SOME of the current will always take that path. I'd rather have a clean, low inductance path through the concrete to the Earth below, than a high resistive path that can equally fuse and explode.

With a large 18' square concrete base, the distance from the tower legs to the edge of the base is a good distance. Without a clean path thru the concrete, having just ground cables running on top could cause arcs straight down thru the concrete to the rebar. There is probably 1000' of 5/8" rebar down there in the concrete with most of it electrically floating, thus having arcing potential.

I will see what Al's current thinking is, as we did the installations about 15 years ago. He has put up and continues to install large commercial towers and tells me this is how it's done.  Out of curiosity, I'll take a look at some of the cell towers on the nearby hill. You have my curiosity up...

BTW, as I mentioned, I have a matrix of 0000 copper cable (about 100' worth) lying in the dirt under the concrete - cadwelded to ten ground rods and connected to these three tower leg cables.

T

[W2XR]

Gentlemen,

Thanks to all for continuing to provide good information here. I really appreciate the input.

Pardon my ignorance, and my question is serious, but what happens when a metallic tower is not grounded or is poorly grounded, and lightning strikes the structure or strikes in the near-field of it? Does the strike hit the tower and then propagate to something located adjacent to the tower, etc., as there is no ground path?

73,

Bruce

[ke7trp]

My Good friend ran the cable down the concrete and then to coiled cable at the bottom of the pit before the concrete was poured.  5 years later he got a direct hit to the tower.  The base split right at the cable.  He had to get a crane out 911 style to support the tower before it fell on the house.  He spent thousands to dig up 7 yards of concrete.  The crack was not there before the hit.  I had a hard time believing this but he will tell the story anytime grounding is brought up.


I put Put 6 rods around the base of the tower,  Use heavy cable from rod to tower. and heavy cable around to each rod.   You can use CADWELD to weld the cable to rods.  This is called a HALO.  It distributes the strike round the tower to many rods and is standard practice.

I dug bowls around each ground rod,  Then filled with Copper sulfate, then white stone.  Now I can fill the bowls with water.  If I dont water, The SWR on the vertical goes slowly up to 2 to 1.  Water, 1.0

My 2 cents.
c

[K1JJ]

The unique problem with a self-supporter using a large concrete pad is that the tower bolts go from the tower all the way down to the bottom of the concrete and end about 3" above the dirt. (nine 3.5' X 1" bolts floating) The rebar is also connected to these bolts so that there is a complete floating steel grid in the concrete.  Essentially there is a steel tower lightning path thru the concrete that stops 3" above the dirt and electrically floats.  Current could discharge not only to the bottom, but around the sides of the hole where the rebar sits just 3" away. Just like a big capacitor plate waiting to discharge/arc to earth.

The reason for the ground cables is to run parallel with this poor steel floating path and take the current off the grid by using  SHORT, direct, low inductance cables.

Certainly cables can be added on top to Earth and would be rather long due to the large concrete pad, but I feel it is also important to cover this tower/bolt/rebar path that is essentially a waiting spark gap to ground = explosions.  So in reality, there is already a lightning path through the concrete, whether we like it or not. The choice is whether it remains high resistance and prone to arcs or it is bypassed by a low inductance parallel cable to ground. I choose the bypass.

For a standard guyed tower that has no tower bolts into the concrete (except for a small pier pin) there is no need to have this additional bypass path since there are no LONG tower bolts running thru the concrete floating above ground.


T

[W1RKW]

Hope your new biking adventure is going well, OM!

T

Yep, it's going well.  See PM.

[WBear2GCR]

Not sure, but I seem to think I just read that the *direct path through the concrete* was NOT in contact with the concrete, but via heavy copper cable and through a PVC pipe??

Will this make enough difference to keep the concrete from being blown apart??
I think this is the main question...

                  _-_bear

                       

[WBear2GCR]

Thinking more, what about substituting a length of nice copper water pipe for a straight run of copper cable (which is more expensive I expect) - like that straight run direct down through the PVC rabbit hole in the tower base?

                    _-_-bear

[Bill, KD0HG]

I just purchased a 60' roll of 3/4" copper water pipe for a project. The Home Depot price? Only $250. Yikes.

Instead of ground rods, bury any sort of old heavy metal as deep as possible. Steel car wheels from a boneyard are good and cheap and have a lot of surface area. Attach a heavy cable with a lug and bolt it to one of the center holes and clamp cable to tower. You can use those bronze water pipe grounding clamps to clamp cable to tower, Don't use the small hole designed for a #4 wire; use the whole clamp to mash the cable onto the tower.

My HS science teacher, W9LBJ (SK) buried an old cast iron bathtub next to his tower. Prolly not too common an item these days..

Bill

[K1JJ]

I like the idea of using old car tire rims and scrap yard metal all connected together - to line the ground hole. Get that Earth resistance down. Maybe a good coat of paint on them will let them last 15-20 years or more underground.

After all, if the ground cable is of large diameter (<.01 ohms) but the Earth- to- cable interface is 10 ohms, where does that leave us?  It leaves us with essentially a semi-floating ground system during a strike. So lots of radials and metal in the ground helps.  Ground rods are not enuff.

I talked to Big Al yesterday about the cable thru the concrete question when bypassing a self supporter. I asked him if it was still being done. He said "absolutely! - IF the installer knows what he's doing."       He mentioned a few commercial cell tower sites in the area that used this technique. I may stop by and take a few pictures to ease my own mind.    

However, MOST self-supporters use single pylons- cheaper - one for each leg, so have no use for thru-concrete cabling. But when the hole is shallow due to rock ledge, the only solution is a large pad, like what I have here and the long tower bolts must be bypassed.

Bear, Big Al also mentioned the PVC isolation from the concrete.  Though, much depends on the diameter of the ground cable. If the wire were say, a hambone #10, then a heavy strike could vaporize it and the PVC would melt and who knows what else.  But if the cable were 2" diameter, or some ridiculously large copper pipe, then there is little danger of it getting warm and doing damage.   So that's another reason we use 1/2" copper cable.

Thinking more, I plan to add some above ground cables to my towers soon. Why not bypass as much current on top and let whatever remains go thru the bypassed concrete cables?   So this thread has helped me to improve my situation too.

BTW, this BB seems awfully quiet lately. Very few new posts.  I've been away for awhile myself but where is everybody?

T

[KB2WIG]

" I've been away for awhile myself but where is everybody?"

The weather is too nice to sit indoors playing with the radio. Maybee people are getting ready for antener weather - be ready for the first snow storm.  For one, i'm spray painting a pipe wrench orange, so I can see it when it gets stuck in a tree. My aluminum bat is still swaying in an apple tree, but I think it will come down soon. A usefull tool.


klc

[W1RKW]

BTW, this BB seems awfully quiet lately. Very few new posts.  I've been away for awhile myself but where is everybody?

Not to redirect the subject matter here but I noticed the same. I've been in lurk mode.  For me and I'll confess, I lost interest.  I'm having a tough time lately going into the shack. Doing other things like bike riding. It's an addictive health thing. Bought my first gun so I've been practicing with that. Still in cleanup mode at my parents house after the OM died.   Also been contemplating a major clean-up here and selling and dumping a bunch of stuff.  I've been checking in here on occasions but not posting until lately.  Just checking in once a week so my log in ID doesn't expire.

OK back to grounding tower bases.