Ken,

The reason ground rods are 8' long has to do with electrical codes,
which are strictly for AC power safety. The rest of the advice you
have gotten will get you a good rf ground, which is what matters.


--
Alan
WA4SCA

"Jimmie D" wrote in message
...



One way to determine the low frequency effectiveness of a ground is to
connect the hot side of the power mains to the ground through a fuse and
see
how big of a fuse you can blow. There are special devices built for doing
this but it can easily and safely be done with less than professional
equipment. Just get all the pets and kids out of the way first.

Well, yeah! :-)

But there is a standard test for ground conductivity. Some pros told me how
they do it.

Drive three ground rods, A, B and C along a line. Pass a known current
between A and C. Measure the voltage drop between A and B. Ohm's Law will
give you the resistance at that place. I don't personally know what a
"good" number is for that test -- however,
http://mrtmag.com/techspeak/radio_we...ed_principles/ suggests 10 ohms.
These guys http://www.rainmaster.com/pdf/TB025C.pdf say the NEC wants no
more than 25 ohms.

I suppose if the soil conductivity is poor, multiple interconnected rods are
advisable/required.

"Sal M. Onella" wrote in
:


"Jimmie D" wrote in message
...



One way to determine the low frequency effectiveness of a ground is
to connect the hot side of the power mains to the ground through a
fuse and
see
how big of a fuse you can blow. There are special devices built for
doing this but it can easily and safely be done with less than
professional equipment. Just get all the pets and kids out of the way
first.

Well, yeah! :-)

Well, dangerous and imprecise.


But there is a standard test for ground conductivity. Some pros told
me how they do it.

Drive three ground rods, A, B and C along a line. Pass a known
current between A and C. Measure the voltage drop between A and B.
Ohm's Law will give you the resistance at that place. I don't

You are measuring the resistance of the A electrode in this case. A could
be a ground system (ie more than one electrode).

There are some rules to follow wrt spacing of B and C from A. Worth a
read of how it is done before making dodgy measurements.

Commercial instruments using this principle may use an internal AC
generator somewhere around 1kHz with a synchronous detector, and chosen
to be in between the harmonics of 50Hz and 60Hz, so they can make
measurements that are less disturbed than measurements that are
susceptible to 50Hz or 60Hz ground currents from other things. Yes, you
can make measurements of a ground electrode connected to an working AC
installation... so long as the electrode voltage due to the 50Hz or 60Hz
current is too great.

personally know what a "good" number is for that test -- however,
http://mrtmag.com/techspeak/radio_we...ed_principles/ suggests 10
ohms. These guys http://www.rainmaster.com/pdf/TB025C.pdf say the NEC
wants no more than 25 ohms.

In this part of the world, the relevant standard requires that the entire
lightning protection earth system has a resistance of less than 10 ohms
before equipotential bonding to other services.

Owen

On Wed, 3 Oct 2007 19:26:59 -0400, "Jimmie D"
wrote:


"Owen Duffy" wrote in message
...
"Kenneth Grimm, K4XL" wrote in
:

I'm installing a new tower and bought, as part of the package, the
grounding kit. This consists of four 10 ft copper clad ground rods,
plus all the accessories needed to connect them to the tower.

My problem is that after about 5 feet of good old red clay I hit solid
rock. I'm tempted to cut the 10 ft rods in half and simply have eight
5 ft rods. Since this is the only practical alternative that I can
see.

Will the eight 5 ft rods give me the same "ground" as the four 10 ft
rods? Or, do I need to add even more 5 ft rods to achieve the same
ground? ( I knew I should have bought that Megger at the hamfest. )

Ken,

Some thoughts...

The main reason for using driven electrodes at the tower base is for
lightning protection. I assume that is your purpose.

To predict the effect of driving a number of electrodes, and the depth of
driving, you have to consider the effect of the soil. It has high
resistivity, and resistivity often changes with depth. Clay soils are
likely to be drier at the surface, and higher in resistivity.

So, longer electrodes are often more effective because they reach down to
wetter layers, and it is not unusual that an 8' electrode has a quarter
the resistance of a 4' electrode for that reason.

Hitting rock isn't necessarily all bad, it sometimes happens that the
rock causes wetter clay above it, and shorter electrodes may be quite
effective.

You can assess the likely outcome by driving two separated electrodes and
measuring the resistance. You can estimate the benefit of driving further
adequately spaced electrodes (assuming uniform soil).

Dont overlook using the reinforced concrete base of the antenna as part
of your ground system.

A megger! No, if your earth system is any good, a megger isn't going to
be useful. There are a number of techniques for measuring the effective
low frequency resistance of an earth electrode or earth system. Google
for things like the "three wire fall of potential method".

If you can't get sufficiently good earth from driving the electrodes as
is, choices include drilling (more effective in soft porous wet rock) or
strip electrodes laid in trenches.

As for the 30deg sledge hammer, I put some notes and a pic together on
tools for driving earth electrodes, see
http://www.vk1od.net/post/driver.htm . The smallest tool was fabricated
after recent discussion on using an SDS Rotary Hammer to drive earth
electrodes.

Owen

One way to determine the low frequency effectiveness of a ground is to
connect the hot side of the power mains to the ground through a fuse and see
how big of a fuse you can blow. There are special devices built for doing
this but it can easily and safely be done with less than professional
equipment. Just get all the pets and kids out of the way first.


Jimmie


Thanks, Jimmie, but I'm not *that* interested. If anyone was doing
this, I would want to be with the pets and kids!! 8*)

Ken K4XL


*** BoatAnchor Manual Archive ***
On the web at http://bama.sbc.edu and http://bama.edebris.com
FTP site info: bama.sbc.edu login: anonymous p/w: youremailadr