Many 4 foot ground rods vs one 8 foot?
I suspect the single 8 foot rod is better. Am I right? Or would several 4 footers some close distance together be acceptable? If so, how many and how close? If we need a soil composition, assume average to above average conductivity.
If it's for your house AC power ground, If you use rods for grounding electrodes, 8 foot (at least) is what's specified by NEC.
If it's for lightning protection, I don't know. My gut feeling is that it isn't the total length of rod that matters but the volume of earth around the ground rod that can dissipate energy. And I doubt if it adds up linearly. In other words, two 4-foot rods probably won't equal one 8-foot rod. If it did, everyone would be using 4-foot rods! Easier to drive in.
If it's for RF ground for something, ground rods don't really do anything for you.
Question is why? Are you unable to drive an 8 foot rod all the way in because of bedrock? (A very common problem around here)
If that is the case, The standard 1/2" or 5/8" diameter copperclad 8 foot rods can be cut in half and more driven in. Rods are spaced about twice the distance apart as the depth. (8 foot rods are spaced about 16 feet apart, 4 foot deep rods should be spaced about 8 feet apart, etc)
It is almost always better to have fewer deeper rods than more shorter rods.
NEVER use the big box store bought 3/8" diameter 4 foot rods that are simply thin copper PLATED, Not copperclad! Those turn to rust in about one year or less.
Always use the 5/8"x8' copper clad rods buy them from your local electrical supply house not a big box store they are quite different in price the copper clad is so thin on the cheapest they aren't worth buying.
If you have trouble driving them borrow a hammer drill and a ground rod bit and use it I have one but it seems friends have it most of the time, but that's why I bought it.
73 de Fred N0AZZ
The License is Only Your Starting Point in Radio!
MVDX/CC of SW MO., DX Hogs, OARS, NARC, NCDXF
ARRL member, ARRL and W5YI VE
DX the thrill of the chase
""D-STAR making use of the 2/ 440m repeaters for real world Digital Voice usage around town and around the world""
" Not one of us can do what all of us can do " ** Max Lucado
In the commercial tower business we always prefer many, many ground rods. (lightning protection) I'm not a grounding protection authority, I'm just telling you what all the big players do. People like Motorola, Verizon, AT&T and so on.
We installed 10' rods spaced every 10' in a ring around the tower and then again around the equipment shelter. The rods were then joined by typically #2 or 4/0 copper(difference depending on customer's spec's) All connections are cad-welded (exothermic welding). They permitted no mechanical connections below ground (split bolt or clamp type).
When it came to rocky soil we just drove them in as far as possible and then moved to the next rod. (all of this is done in a 18 to 24" deep trench and then back-filled when done)
In your case I would prefer four 4' rods to one 8'. If you have no cad-welding experience (most don't) don't sweat it. It's a tad pricey and potentially dangerous without experience. Just make sure all connections are clean and shiny and use good quality clamps when joining rods and wire. (use the largest wire you can afford. Typically #2 stranded is great and easy to handle. Take one lead off each tower leg to your ground ring.
For lightning protection each tower leg should have its own ground rod with the lead from each leg going through a gentle curve. Lightning definitely does not like sharp angles in the conductor and will almost always jump from the conductor if the bend radius is too sharp. If possible, the ground rod needs to be 8 feet long.
For r.f. grounding things are somewhat different. In most soil types the maximum effective grounding takes place in the first 5 feet and extensive experiments have shown that when the rods are spaced 2.4 times the length of the ground rod apart this is maximized. Of course, the old "rule of thumb" of spacing the rods twice their length apart works fine, it is just that this spacing does not optimize the effective grounding. Using the 2.4 spacing then 2 each 5 foot ground rods spaced 12 feet apart will approximately double the effective grounding of a single 10 foot ground rod. If 4 foot rods are used, then spacing them 9.6 feet apart will increase the effective grounding by about 1.6 times that of a single 8 foot ground rod.
The calculations behind the 4 foot ground rods versus an 8 foot ground rod are as follows: Most of the effective grounding takes place in the first 5 feet and therefore, for all practical purposes, the extra 3 feet of the 8 foot ground rod is contributing very little. Comparing a 4 foot rod with the 5 feet means that the 4 foot rod will have approximately 80 percent of the effective grounding of the 5 foot rod. Then, installing 2 each 4 foot rods 9.6 feet apart will have about twice the effective grounding of a single 4 foot rod. Multiplying the 80 percent by 2 gets 160 percent or 1.6 times the effective grounding by installing 2 each 4 foot ground rods versus a single 8 foot ground rod. Of course, the actual calculations are more involved. However, the error introduced into the calculations by simplifying the parameters is only a very small percentage.
Unless ridiculously short in length, more rods spread out are always much better than one deeper rod for lighting or RF.
Originally Posted by NZ9Y
Four foot deep in medium or good soil is almost the same as eight foot deep for lightning. We are almost always better off with two properly spaced 4-foot rods than a single 8 footer if the soil is normal moisture and conductivity. This not true for power line frequency or DC ground resistance, but it is for RF and lightning.
None of my towers have deep rods. They have buried radials and many shorter rods. My house power mains ground is deep, because that is a code requirement.
For lightning protection, how you route wiring and bond grounds and entrance panels together is far more important than anything else, including grounds!!!
I still have to install ground rods for my station. My dream of dreams is to rent a TE-905 ground rod system. Take a look at it on Youtube. You attach an adjustable vise to the ground rod at a comfortable height. Then you drive that with the hammerdrill. When it reaches the surface, you slide it up the ground rod, retighten and drive some more. You finish a typical ground rod attachment.
Originally Posted by N0AZZ
And it looks like Erico has a similar muscle powered system.
If you have to worry about the cost of HF e-mail, you can't afford the boat.
CW: The mode that accomplishes the most with the least circuitry, the least spectrum, and the least power.
What hath God wrought?
He hath wrought that pounding brass still kicks .- ... ...
SO I would imagine radials plus ground rods would be ideal for lightning and RF.
The job is a basic triangular tower less than 50' high. There are many taller structures in the area. I will have a tribander on top, MAYBE a 2 meter vertical, and am planning to run a 160-40 meter sloper.
I know the sloper is looking for a good RF ground. My plan was to do three 4 foot rods for each tower leg, running #4ish copper in a gentle arc from several feet up the tower to each rod. One run of wire will run the entire length of the tower from the mast to one of the rods. Then I would bond all the rods together.
As I excavate, I will be ripping up wire from a radial field nearby. That few dozen yards of wire I will bond to the rods as well and spread out(I may as well, it cant hurt and its there). I will then do lightning arrestors on each of the 2 (maybe 3) coax runs at the bottom of the tower and run them to the closest rod on thier route to the shack.
I know lots of guys do alot less, and others do more. If I do this (and all the stuff at the house end too), can I call it good? Any MINOR changes to this plan I should consider?