Tips for grounding an antenna in thin soils (2' depth to rock)?

[Krichards]

I am unable to drive a grounding rod for an outside celluar phone antenna more than 2' into the ground, due to shallow bedrock conditions. Does anyone have suggestions?

 

[VEBill]

Are you trying to ground it for lightning protection as per code? I assume you already know that it doesn't need to be grounded for signal performance purposes.

Can you ground it to the building's existing ground system?

As an alternate to ground rods, there are grounding plates.

My house is on rock, but close to the foundation is rock and dirt fill and I was able to find a crack to hammer the ground rods in between the buried rubble.

 

[Krichards]

Thanks for your response. I'm on pretty solid rock...the foundation hole had to be blasted out. Unfortunately, the house electric service grounding rods are a long way from where the antenna is.

I am grounding the antenna mast strictly for lightning protection, not signal performance. My antenna is mounted on a metal mast that is about 15' above the peak of my house, which is a 3-story colonial that sits up on a hill. The top of the antenna is at tree top level and probably about 50' above the ground, so I figure it's probably a pretty good target.

Since my last post, I have learned a little about concrete encased (Ufer) electrodes and am considering installing a homemade one made of a 20' length of 1/2" rebar set in a 5" x 5" concrete casing that I would pour with ready mix concrete. Any thoughts on this?

 

[VEBill]

I've heard about Ufer grounding, but don't know much about it. Maybe someone else knows. Or google it.

The excavation is probably at least several feet larger than your foundation. You should be able to hammer a ground rod into the back fill fairly close to the foundation (check the code). It might require several attempts, but it would still be cheaper and easier than anything else. Use a big wrench to twist it out if required.

You'll still want to disconnect the cell phone during storms. The grounding is simply to keep you house from catching fire, it won't really protect the sensitive electronics.

If you get most of the gnd rod in, some people might suggest that you simply cut off the 'excess' and beat the end a few more times. Not me.

 

[VEBill]

Someone that requires an outdoor cell phone antenna on a tall mast probably doesn't have city water (I'm guessing).

 

[Higgler]

Grounding may not be useful if your cable coming from your lightning rod is too thin for a fast rise time lightning strike.
Inductance in the wire can build up a megavolt on the wire and force the lightning bolt to jump thru your roof to ride the electrical wire and heating conductors path to ground. It puts a big hole in your roof too.
I believe the recommended low inductance cabling is a wide braided copper strap.

http://www.lightningsafety.com/nlsi_lhm/lpts.html

is a website with some info.

kch
PS: If lightning is imminent near you, don't have both feet on the ground at the same time. Just my own theory based on what I've read. I think a short one legged man with insulated crutches is safest.

 

[Krichards]

Folks, thanks for all the input,

VE1BLL is correct, we are on well water...I had considered grounding to our well casing, however, that is about 60' from the house and I wasn't crazy about sending a lightning surge in the direction of my well pump (rather lose the phone than my water...).

I have bought 6 gauge stranded copper to go from my mast to the grounding round and had planned to run it down the side of the house to the Ufer electrode through 1/2" PVC electrical conduit (more for cosmetic purposes). I had considered braided copper strap, but I didn't see it at Home Depot... I considered 4 gauge copper, but it was alot more expensive and the guy at the store looked at me as if I was nuts (I do tend to go overboard...) My limited understanding was that the copper strap would give me more soil contact area when buried, but I wasn't sure it would provide more benefit (over solid or stranded cable) on the run from the mast to the grounding rod. Faulty thinking?

 

[VEBill]

"...rather lose the phone than my water..."

Grounding the mast isn't going to save the phone in the event of a strike (or likely even a nearby strike). The difference is that the smoldering ruins of your cell phone might not be distributed over quite as large an area (perhaps a slight exageration).

Grounding masts is really all about keeping your house from catching fire. For protecting equipment, it is perhaps just Step 1 of a multistep process.

 

[BrianR]

The only way to protect the receiver if it is connected and there is a nearby lightning hit is by having a surge diverter in the coax,ounded very effectively. They use gas discharge tubes in a metal cylinder in-line with the coax and if properly selected, have little effect on the signal performance.

A good low inductance ground lead is easier to describe than to install because surface area is what it's all about, flat strap or braid. In fact to make it much better than a round conductor you need a length to width ratio < 5x and then it's a lot better.

The ground itself can be made quite effective by using more than one stake, but you have your rocks to consider so you'll do what you can.

The danger to structure comes from the enormous electric and magnetic fields that surround the mast when it's hit and they induce voltages into house wiring causing arcovers. I saw a technique used on an aircraft control tower which was regularly hit by lightning. They connected the lightning arrestor spikes on the roof to grounf through thick (2") coax. Grounding the shield of that will effectively suppress the electric field but I'm not sure what they did to suppress the magnetic field - it worked though.

 

[fb599]

You could try core drilling an earth stake in and pouring bentonite in to make the connection with real earth. This is what we do at electrical substations that have bedrock underneath. Hope this helps.

fb599

 

[K7FE]

UFER (cement encased) grounds can explode when hit with lightning. It is a bad engineering practice to use the rebar or a wire encased in the cement for a “lightning ground“. The moisture in the cement vaporizes instantly when superheated during a direct lightning strike causing the cement to explode. Radio towers have been known to fall when this occurs. It is a better practice to use multiple and separate lightning ground rods with no bends or minimum gentile sweeps in change of ground wire direction. Lightning will launch off of sharp bends and may go places that you won't like. UFER grounds are just fine for electric service panels as a “safety ground“.

Two ground rods four feed deep will have a lower impedance that a single rod eight feet deep. Make the rod spacing about the same distance as the length of the rods. Chemicals around the ground rods will help improve soil conductivity and salt is the least environmentally damaging. There are many ways to add chemicals, a simple way is to use a hollow pipe with holes drilled in it to allow the chemical to leach out into the soil when you add water in the top of the pipe. You may add more salt as needed.

Dissipating lightning “feelers” that begin at the ground (or on your antenna and support) is also important to the prevention of lightning damage to your antennas and equipment. This web site by Glen Zook has some excellent information on various grounding and specifically lightning grounding.

http://home.comcast.net/~k9sth/

A hammer drill from a rental shop will drive a ground rod much better than you could with a large hammer.
Good luck with your project.