In the 2007 ARRL Handbook page 3.6, there is a discussion of building a
ground ring around the base of the tower, connected to three ground rods
each connected to each leg of the tower.

In the narrative it says "Space rods at least 6 feet apart" but then in
the caption for the accompanying drawing it says Locate ground rods on the
ring as close as possible to their respective tower legs".

On a Rohn 25G, the rods can't be "as close as possible" to their
respective tower legs and still be anywhere near 6 feet apart.

So, which is it ... close to their respective tower legs, or 6 feet apart?

What is the reason for the 6-foot separation, anyway? The rods all go
into the ground. Why would it make a difference if they're closer
together than 6 feet?

"Rick (W-A-one-R-K-T)" wrote in message
news

In the 2007 ARRL Handbook page 3.6, there is a discussion of building a
ground ring around the base of the tower, connected to three ground rods
each connected to each leg of the tower.

In the narrative it says "Space rods at least 6 feet apart" but then in
the caption for the accompanying drawing it says Locate ground rods on the
ring as close as possible to their respective tower legs".

On a Rohn 25G, the rods can't be "as close as possible" to their
respective tower legs and still be anywhere near 6 feet apart.

So, which is it ... close to their respective tower legs, or 6 feet apart?

What is the reason for the 6-foot separation, anyway? The rods all go
into the ground. Why would it make a difference if they're closer
together than 6 feet?

Rick

General guidance is to space the rods a minimum of their length from each
other. So 8 foot rods should be at least 8 feet apart for optimum results.
Its all to do with voltage gradients through the ground when a fault current
is flowing through the electrode. Wider spacing tends to reduce the
steepness of the voltage gradient and is considered to reduce the risks to
people or animals in the vicinity of the earth electrode in event of a fault
or lightning strike. Ground voltage gradients of only 50 volts can be lethal
to cattle or other quadrupeds.

Placing electrodes close in to tower legs will reduce the path for current
to flow if the Rohn 25G tower is struck by lightning and represents a
compromise between optimum earthing practice and discharging excess voltage
on the tower by the most direct path possible. Precautions are normally
taken to keep people and animals away from the immediate vicinity of the
base of an antenna tower, so this method of earthing would tend to reduce
the hazards associated with this particular design of antenna tower. The
requirements for other types of antenna tower installation can be very
different depending on the width of the base of the tower, number of support
legs and method of anchoring into the ground.

Mike G0ULI

Mike G0ULI


Excellent posting Mike.
Very impressive, very well thought out and so well written.
Thank you very much.


Rick K2XT

Rick (W-A-one-R-K-T) wrote:
In the 2007 ARRL Handbook page 3.6, there is a discussion of building a
ground ring around the base of the tower, connected to three ground rods
each connected to each leg of the tower.

In the narrative it says "Space rods at least 6 feet apart" but then in
the caption for the accompanying drawing it says Locate ground rods on the
ring as close as possible to their respective tower legs".

On a Rohn 25G, the rods can't be "as close as possible" to their
respective tower legs and still be anywhere near 6 feet apart.

So, which is it ... close to their respective tower legs, or 6 feet apart?
probably the 6 feet apart, but it doesn't make a heck of a lot of
difference..
one approach gives you a lower ground resistance
the other approach gives you shorter wires from tower to ground rod,
which has less inductance.

And, if your tower is going to be sitting on a big block of concrete and
rebar, and you put a suitable copper wire in the concrete, that is
probably a better ground than any set of rods, because the surface area
is huge. (unlike a rod, where you've got all of 1/2 a square foot or so
surface area)


What is the reason for the 6-foot separation, anyway? The rods all go
into the ground. Why would it make a difference if they're closer
together than 6 feet?


Consider if you drove two rods two inches apart.. it would have the
ground resistance of one rod (or very close to it). If you separate the
rods by their length (or thereabouts), the effect is to almost halve the
ground resistance. Separataing by much more than a rod length doesn't
change things much.

Think of a rod as not just a rod, but a sort of squashed hemisphere.


if you want equations for all sorts of configurations:
http://home.earthlink.net/~jimlux/hv/grounds.htm

mostly taken from IEEE Std 142

Mike Kaliski wrote:
"Rick (W-A-one-R-K-T)" wrote in message
news
In the 2007 ARRL Handbook page 3.6, there is a discussion of building a
ground ring around the base of the tower, connected to three ground rods
each connected to each leg of the tower.

In the narrative it says "Space rods at least 6 feet apart" but then in
the caption for the accompanying drawing it says Locate ground rods on the
ring as close as possible to their respective tower legs".

On a Rohn 25G, the rods can't be "as close as possible" to their
respective tower legs and still be anywhere near 6 feet apart.

So, which is it ... close to their respective tower legs, or 6 feet apart?

What is the reason for the 6-foot separation, anyway? The rods all go
into the ground. Why would it make a difference if they're closer
together than 6 feet?


Rick

General guidance is to space the rods a minimum of their length from each
other. So 8 foot rods should be at least 8 feet apart for optimum results.
Its all to do with voltage gradients through the ground when a fault current
is flowing through the electrode. Wider spacing tends to reduce the
steepness of the voltage gradient and is considered to reduce the risks to
people or animals in the vicinity of the earth electrode in event of a fault
or lightning strike. Ground voltage gradients of only 50 volts can be lethal
to cattle or other quadrupeds.

Also to reduce the grounding resistance. Close rods don't reduce the
resistance as much as farther rods. This is probably a more common
reason for multiple rods, rather than reducing step potential. Places
with step potential hazards (e.g. electrical substations) tend to use
ground grids, since, as long as you're buying all that heavy copper
cable to connect the rods (not to mention the above ground
infrastructure), you might as well just bury it.

"Rick (W-A-one-R-K-T)" wrote in
news

In the 2007 ARRL Handbook page 3.6, there is a discussion of building
a ground ring around the base of the tower, connected to three ground
rods each connected to each leg of the tower.

In the narrative it says "Space rods at least 6 feet apart" but then
in the caption for the accompanying drawing it says Locate ground rods
on the ring as close as possible to their respective tower legs".

On a Rohn 25G, the rods can't be "as close as possible" to their
respective tower legs and still be anywhere near 6 feet apart.

So, which is it ... close to their respective tower legs, or 6 feet
apart?

What is the reason for the 6-foot separation, anyway? The rods all go
into the ground. Why would it make a difference if they're closer
together than 6 feet?

The electrodes are placed in a quite resistive medium.

Increasing the diameter of an electrode does reduce the resistance of the
earth connection but by very little. Fig 1 at
http://www.vk1od.net/post/earthing.htm shows the effect of different
diameters on electrode resistance.

If you place a second electrode right next to an existing one, you will
obtain almost no reduction in resistance to earth, it is not a lot
different to increasing the diameter of the electrode. At increased
spacing, the improvement is better and at large spacing aproaches halving
the resistance of the single electrode, but offset to some extent by the
inductance and resistance of the connecting conductor.

In homogenous ground (and that is some assumption), it turns out that
increasing the spacing of driven vertical electrods beyond their own
length is of quickly diminishing benefit, so a common ROT is to drive
electrodes in a matrix of cell size about the length of the rods.

Someone else raised the issue of step voltage (the voltage between your
feet when you walk on ground carrying a large current). That is usually
mitigated by a (amongst other things) a conductive earth mat layed on the
surface and bonded to the electrode system. Nevertheless, it is not a
good idea for two people to carry a length of pipe (or a beam) over
ground subject to fault currents or likely lightning discharge.

So back to your specific question, if you place the three electrodes
close to each other (as a consequency of placing them at the tower legs),
the earth resistance is higher than it should be with greater spacing. If
you space them too far, the added inductance of the conductor connecting
them offsets to some extent the reduction in electrode resistance. I
haven't read the article to which you refer, but I wouldn't waste time
driving an electrode less than 2.4m, and I would not bother driving a
2.4m electrode electrode within 2.4m of another.

Electrodes are real cheap, and excluding rock, you can drive them with an
electric hammer and an adapter in minutes. You are trying to shed
discharge current to earth, so don't be shy about it.

Owen