"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