Increase the soil conductivity?


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Old 07-06-07, 06:44 PM
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Increase the soil conductivity?

First time poster in this forum;

When my soil samples were tested, the Electrical Conductivity result was 49 micromhos/centimeter. (Taking the reciprocal means it's roughly 21,000 ohms/cm).

What should I do to the soil to increase the conductivity? Bentonite? Copper Sulfate? (and do I apply it?) I'll save the background reasoning for a later post. But suffice it to say, I need the resistance to be less than 25 ohms, preferably less than 5 ohms. (Hence, the reason this message is posted in the Electrical- A/C, D/C forum).

Suggestions?
Thanks,
Kelly
 
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Old 07-06-07, 07:02 PM
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well, the reasoning might make a difference. The 25 ohms just happens to be the NEC maximum allowed ground resistance if using 1 ground rod before needing to add another grounding electrode. Makes me wonder what this is about.

as well, what type of soil are we talking about? moisture content?
 
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Old 07-06-07, 08:21 PM
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Originally Posted by nap
well, the reasoning might make a difference. The 25 ohms just happens to be the NEC maximum allowed ground resistance if using 1 ground rod before needing to add another grounding electrode. Makes me wonder what this is about.

as well, what type of soil are we talking about? moisture content?
Sorry, didn't mean to be cryptic .
I left out the "reasoning" specifics initially so the thread woud not become about "how to install ground rods", etc. (That's not the part I'm asking, although I may come to that part later.) My focus in this thread was limited to the conductivity of the soil.
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I'll be installing a tower (Rohn 45G) and putting in ground rods; one from each leg of the tower. And those rods will be cadwelded together with 2/0 copper wire forming an earth ring. All of this will then be bonded to the Service Entrance ground, in accordance with NEC (which is indeed where the 25ohm figure comes from.) All of that is fine for 60Hz A.C. But for purposes of RF, and the RF energy components of lightning, I need closer to 5 ohms resistance. I'm having to view this as Electrical A.C., but with a 100MHz R.F. issue included).

As for the type of soil, test results show it as "Silt Loam". Moisture content is not listed.

Hope this helps.

Suggestions?
Kelly
 
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Old 07-07-07, 07:14 AM
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well, that does explain a lot. Just for reference, when installing an electrical service GE system, you need less than 25 ohms if using one rod but code allows the simple (but sometimes ineffective) installation of a second rod with no further test required. Doesn't always allow adequate low resistance but that is what it is.

So, I see that you are from Arizona. That could mean yo simply have dry soil.

At what depth was your test performed? Neither salt or copper sulfate will improve the readings unless there is moisture present to facilitate conductivity into the soil.

the only 2 solutions I see at the moment would be;

deeper

bigger

deeper often allows you to reach moist soil


bigger allows more soil contact so if the soil is less conductive than preferred, the larger contact area causes a lower resistance.

I'm from N. Indiana, S. Michigan so I do not have much experience with soil conditions such as you would have in Arid-zona. Maybe a few of the other guys that hang out here can offer some typical solutions used in that area of the country.
 
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Old 07-07-07, 09:01 AM
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Hello,
If I may ask, is this for a private tower or public? It may make a difference.
If it is private I would suggest going to some ham radio sites. I know of a few if you are interested. Some of these guys have done some serious towers and may have the experience you need.

From an electrician's standpoint, we rarely care what the resistance is. We generally sink two rods to meet code and are done with it. Ground rods are quite useless in the majority of installations, UNLESS engineered to be effective, such as in your case.

Are you a ham? I just passed my Tech test last weekend. Waiting on my call sign.
 
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Old 07-07-07, 11:22 AM
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Smile re: Improving Soil Conductivity

Thanks Nap;

I'm in central Arkansas (which is the AR you saw). It's amazing how many people get that confused for the postal abbreviation of Arizona (which is AZ). By the way, I was born in South Bend, and still have relatives in Mishawaka, and Granger, IN and in New Buffalo, MI.

The soil test samples were taken quite shallowly, now you that mention it. We're not as arid here as in Arizona. But, we're still a bit behind in our rainfall still. (Wish my friends in Dallas and San Antonio would stop hogging the rainfall<grin>).

I see your point about bigger/deeper penetration into the soil for greater earth bonding. I may have to include to Bentonite to increase the capabilty of the soil to hold water.

Thanks again to you both.
Kelly
 
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Old 07-07-07, 11:34 AM
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re: improving Soil Conductivity

Speedy Pete;

Congratulations on passing your Tech exam. Let me know when you receive your callsign.

Yes, I'm a ham (my call is KA5MGL) and the tower will be private. I sat in on some of the lightning and grounding forums at Dallas HamCom last month trying to get some understanding of how to approach the earth-bond aspect of installing my tower. One of the speakers talked about supplementing the ground rod system with perferated copper pipes hammered into the soil, which are filled with Rock Salt (from the homemade ice-cream supply), and filled with water. This intentionally leechs copper (and sodium??) into the soil and provides a chemical increase in the conductivity. Another possibility, I've since learned is to use Magnesium Sulfate (plain ol' Epsom Salts) to do something similar.

I know that at some of our transmitter sites (I'm a televison Engineer for the local PBS station), that the electricians cadwelded the ground wire and ground rods to everything......even threading the bare #6 through the weave of the metal fence and gate. That's why I decided to ask the grounding quesiton in this Electrical A.C./D.C. forum. To see if they could offer tips for how they improve conductivity on their installations.

My intent is to have a good bond to the earth (as described earlier in this thread), and to build/install a device to bleed off the static before the voltage attains strikeover potential (same idea as a Stati-Cat). Obviously, I want that stati-cat to have a very low loss direct-path to the earth so that energy can be dissipated AWAY from the radio gear (<grin> and my house in general! )

I never had any idea that grounding a tower (properly) could be SO involved! hahaha

Look forward to working you on HF (75m, 20m) ; on Echolink; or on the IRLP. Did you get to operate during Field Day? (probably need to PM me on that one, since it would off-topic for this forum).

again, thanks.
Kelly
 
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Old 07-07-07, 12:00 PM
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Wow!! It' a small world. I live in Niles.

Yes, I was mistaken with the postal code. I always seem to have problems with those.

concerning cadwelding everything. In a tower, everything is bonded to prevent a difference of potential between any given two points. With the RF, anything can act as a rectennae to some extent and this prevents undesirable voltages on equipment via the ground system.

I had heard of salting the area to be used to improve conductivity but it still requires some moisture to achieve this. Dry salt doesn't conduct any better than dry dirt. It just helps when there are lower moisture levels.

I have driven 30' or longer ground rods before to achieve acceptable readings (when spec'd) or multiple long rods even. The idea being there is more moisture deep so this merely accesses that area.
 
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Old 07-08-07, 04:48 PM
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Originally Posted by KBoswell
-------------
I'll be installing a tower (Rohn 45G) and putting in ground rods; one from each leg of the tower. And those rods will be cadwelded together with 2/0 copper wire forming an earth ring. All of this will then be bonded to the Service Entrance ground, in accordance with NEC (which is indeed where the 25ohm figure comes from.) All of that is fine for 60Hz A.C. But for purposes of RF, and the RF energy components of lightning, I need closer to 5 ohms resistance. I'm having to view this as Electrical A.C., but with a 100MHz R.F. issue included).Kelly
If you look at table 9 in the NEC you will see that 2/0 cu has an X = 0.043
ohms per 1000 ft at 60 cps. From this L = 1.14 uh for 100 ft. X at
100 MHZ = 2 x pi x f x L = 2 x 3.14 x 100 x 10^6 x 1.14 x 10^-6
X = 2 x 3.14 x 100 x 1.14 = 715 ohms for 100 ft of 2/0 cu. One ft of 2/0
cu can ground its self at 100 MHZ with a 5 ohm value.

You do not need to protect at a 5 ohm value at 100 MHZ.
you can use 10 ohms as a target value using the 60 cps values.
There are instruments to measure the value of the grounding impedance after
you have your system installed. 40 ft- 2/0 copper radials from each antenna leg
with ground rods every 20 ft will steer the ltg away from your equipment.
You will still need surge protection at the building. Do a google for Lightning Equipment MFG and they have suggestions for the installation.
 
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Old 07-09-07, 01:09 PM
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re: Improving soil conductivity.

Originally Posted by wareagle
If you look at table 9 in the NEC you will see that 2/0 cu has an X = 0.043
ohms per 1000 ft at 60 cps. From this L = 1.14 uh for 100 ft. X at
100 MHZ = 2 x pi x f x L = 2 x 3.14 x 100 x 10^6 x 1.14 x 10^-6
X = 2 x 3.14 x 100 x 1.14 = 715 ohms for 100 ft of 2/0 cu. One ft of 2/0
cu can ground its self at 100 MHZ with a 5 ohm value.

You do not need to protect at a 5 ohm value at 100 MHZ.
you can use 10 ohms as a target value using the 60 cps values.
There are instruments to measure the value of the grounding impedance after
you have your system installed. 40 ft- 2/0 copper radials from each antenna leg
with ground rods every 20 ft will steer the ltg away from your equipment.
You will still need surge protection at the building. Do a google for Lightning Equipment MFG and they have suggestions for the installation.
-------
Thanks for the information. I'm trying to apply it to the question posed in the original post---how to improve conductivity of the soil.

I think I can get a good bond to the ground rods, but with a soil admittance of only 49 micromhos/cm I'm wanting to increase the ability of the soil itself to readily dissipate the energy. Any suggestions for how to improve that?

tnx in advance,
Kelly
================
 
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Old 07-09-07, 02:42 PM
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> Any suggestions for how to improve that?

Lawn irrigation system with a 24 hour timer? :-) Keep that soil nice and wet. A more serious suggestion could be to research vegetation species that retain soil moisture and plant those around the tower.

The only method I have heard of is driving the ground rods deeper to reach wet soil or using more surface area (bigger rods, pipes, plates, etc).
 
 

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