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Author Topic: RF Ground Rods... Do 2 - 4s = 1 - 8????  (Read 680 times)
JOHNNE507
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Posts: 37




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« on: July 21, 2006, 11:38:51 AM »

If I use 2 4' grounds wired in parallel will they be less, more or just as effective as 1 8 foot ground rod?

I've done a fair amount of reading on this topic, but I have not come up with a direct answer to this question.  I know it is a fuzzy and debatable topic!  hihi

Thanks!
73 DE W1YB
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KZ1X
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Posts: 3228




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« Reply #1 on: July 21, 2006, 11:43:19 AM »

well...

as effective, for what purpose?

ground rods, made of what?

spaced, how far apart?

in what sort of medium?

connected together, how?

... too many variables to answer this question ...
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WB2WIK
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Posts: 20611




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« Reply #2 on: July 21, 2006, 11:45:32 AM »

I agree with Steve KZ1X.

Per NEC, if it's an electrical service ground or SPG for multipurpose, 4' is too short and violates code, which requires 8 feet.  100 four foot rods in parallel would still be a code violation.

If it's an "R.F. ground" for your ham station only, then you can do whatever you want, and see what works.  For an R.F. ground, a bunch of wires laying on the ground and not buried at all will usually work just fine.

WB2WIK/6
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KB9CRY
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« Reply #3 on: July 21, 2006, 12:14:49 PM »

NEC is concerned with electrical shorts / shocks safety grounding.  Their spec for 8 ft minimum allows for some variability in soil conditions, etc. and better ensures that the bottom of the ground rod will normally be in moist soil.

From what I've learned from my lightning grounding readings and discussions with Polyphaser/ICE folks is that the effectiveness of a rod to dissipate energy surge during an event is to think of the charge dissipating into the ground in an upside down cone.  At the surface, the cone is a diameter equal to the length of the rod and at the bottom of the rod, there is no diameter, just the point of the rod.  This is why for example with 8 foot rods, you'd space them at 16 foot intervals.  Each rod contributes an 8 foot radii and two together gives you the 16 ft.  

If you're using these for RF or lightning grounding, then I'd space 4 footers at 8 ft. intervals.  But,  here's the but, you may not be ensured that the bottom is in moist soil being so near the surface and a lightning strike when the ground is very dry like in the summer, you may not have the protection.  Best bet is to use the std. 8 footers.

Phil  KB9CRY
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K1CJS
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« Reply #4 on: July 21, 2006, 12:19:34 PM »

Two 4 foot rods are not as good as an 8 foot rod generally speaking.  One of the keys to a good ground is to get the rod (conductor) near or into moist ground, the closer to the water table the better.  That is why the electrical codes specify an eight foot rod driven down into the ground.

Of course, that is an electrical ground.....or a lightning ground, an RF ground doesn't actually require it.  If you really need to use four foot ground rods because of rocky soil or a ledge in the way, you need to drive at least one eight footer into the soil somehow--at an angle if necessary--keeping in mind that it will not be as effective as an eight footer driven straight down.  This way, the national electrical code is satisfied.

One guy I know drove the eight footer down near his septic tank--the only way he could get it driven down into the ground.  The ground rod was driven in about twelve feet away from the foundation, and eighteen feet away from the electrical service entry point.  He was required to show the installation to the inspector, but on explanation was complemented and had the certificate issued with no further questions.
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W8JI
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« Reply #5 on: July 21, 2006, 12:40:55 PM »

You asked about RF, not safety, so I will answer that way.

It depends on the skin depth of your soil at whatever frequency you are operating on.

If the skin depth is two feet, which it can be, an eight foot rod is a total waste.

The real question is why you think you need an RF ground, and why you might think a rod makes a good RF ground.

Generally an RF ground is only needed if you end feed an antenna or have antenna or feedline problems. If you do need a good RF ground for some reason, a rod generally makes a very poor RF ground.
 
Safety grounds are another issue.

73 Tom

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W8JI
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« Reply #6 on: July 21, 2006, 12:40:56 PM »

You asked about RF, not safety, so I will answer that way.

It depends on the skin depth of your soil at whatever frequency you are operating on.

If the skin depth is two feet, which it can be, an eight foot rod is a total waste.

The real question is why you think you need an RF ground, and why you might think a rod makes a good RF ground.

Generally an RF ground is only needed if you end feed an antenna or have antenna or feedline problems. If you do need a good RF ground for some reason, a rod generally makes a very poor RF ground.
 
Safety grounds are another issue.

73 Tom

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N6AJR
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« Reply #7 on: July 21, 2006, 12:42:16 PM »

what happens if you dig a small trench, say a foot deep and lay the rod in sideways??
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KI4FH
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Posts: 4




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« Reply #8 on: July 21, 2006, 12:51:34 PM »

The deeper the better.  The more the merrier. (is that how to spell merrier?) No, two 4's will not be one 8 but it is possible that they can be a more effective ground. The more surface area that comprises your ground system, the more effective it will be.  RF travels on the skins, on the outside of the rod.  That is why using copper strip as opposed to braid will provide a better path to ground.  Not only is it less impedance, but it is a larger surface area.  You should go deep, more than one rod if possible and use strap instead of braid if you want the most effective ground.

That is a good question for the group.  Which is more effective, 4 feet deep of wide area ground like buried sheets of copper or a few 10 foot rods fed by strap?
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WB2WIK
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« Reply #9 on: July 21, 2006, 03:27:16 PM »

Depends on what you're trying to do.

All the copper sheet in the world won't meet code unless it's connected to an 8' long (and deep) ground rod.

But if you're looking for an R.F. ground, you could use copper sheet laying on the surface of the ground, without burying it at all, and that would work fine as long as you use enough of it.

WB2WIK/6
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K6AER
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« Reply #10 on: July 21, 2006, 09:20:20 PM »

Ground rods have what is known as a sphere of influence. This is the conductive coupling into the soil for lightning conduction. This is the radius squared of the ground rod depth times Pi, times twice the depth. For a 4 foot rod this would be about  403 cu ft. times two rods separated by twice their depth. For 806 cu ft. sphere of influence.

For an 8 foot ground rod the sphere of influence would be 8x8x3.14x16= 3215 cu ft. sphere of influence. As you can see a single deep rod is much better than a bunch of shallow ones.

As Tom had mentioned the grounding resistance of a lightning ground system has very little to nothing to do in the way of a RF counter poise.
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K1CJS
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« Reply #11 on: July 22, 2006, 08:25:35 AM »

"what happens if you dig a small trench, say a foot deep and lay the rod in sideways??"

Then Tom, you'll have a ground rod laying down on the job!!  ;-)
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N6AJR
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Posts: 9910




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« Reply #12 on: July 22, 2006, 09:14:20 AM »

OW, that was bad.. I bought a T-post driver. for my ground rods.  $15 at home depot, it is like a piece of pipe with a cap on it and 2 handles on the side. slide it over the top of the ground rod and slam it down. it works well. also drives in pipe for "sockets" for verticle antennas with out buggering up the top of the pipe.  Mine is purple.. find them in the fence section, ask for a T-post driver.


good tool.
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JOHNNE507
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Posts: 37




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« Reply #13 on: July 22, 2006, 09:54:29 AM »

Thanks to all who replied.  I really appreciate the math that  K6AER  provided.  I've been QRT for a l-o-n-g time and  K6AER's info was EXACTLY what I was looking for.

I guess I better buy that post driver that someone was kind enough to mention.  I think I'll put down 2 8 footers.

73 DE W1YB
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KB4QAA
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Posts: 2407




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« Reply #14 on: July 22, 2006, 10:05:27 AM »

-Do what you can.

-Anything is better than nothing.

If you are using short rods, or have very dry soil conditions you can modify the soil to improve conductivity by digging a 2 foot radius hole, 1 foot deep around the rod.  Place a generous portion of Epsom Salt or rock salt, and add some clumping type cat litter (Bentonite clay).  Mix thoroughly and soak with water.  Replace top soil.

The Bentonite clay will almost never completely dry, and the salts of course improve conductivity.

73,  Bill
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