You misunderstand what Jim Lux and I have been saying. Accepted good
engineering practice for grounding a tower is 2-3 rods extending
radially outward from each leg of the tower, bonded to the tower, with a
Ufer ground built as a Ufer and also bonded to the tower.
73, Jim K9YC
On Mon,1/19/2015 5:19 PM, Chuck Dietz wrote:
>From what you have said, I take it that putting a single (or even 2 or 3)
ground rods on a tower base that is in a good bit of concrete is
wasted effort? The tower base and concrete should dissipate most of a
lightning strike?
Chuck W5PR
On Mon, Jan 19, 2015 at 12:59 PM, Jim Brown <jim@audiosystemsgroup.com>
wrote:
The real issue is that the concept of "RF Ground" is a myth and the result
of fuzzy thinking. Part of the reason is what Jim has addressed below. The
other reason is simply that a connection to earth does NOT make TX antennas
work better, and is NOT part of a solution to hum, buzz, or RFI. The earth
is NOT a sink into which noise and RF is dumped. The ONLY reasons for an
earth connection are to sink lightning current and other equipment-related
surge currents on the AC line.
My late colleague, Neil Muncy, ex-W3WJE, taught classes on power and
grounding for many years to audio professionals, and I took over those
classes when he no longer had the health to do them. He is also the guy who
alerted the world to "The Pin One Problem" back in 1994. He gave one of my
favorite teaching examples. He would say to a class, "park yourself at the
end of the runway of the nearest major airport with a good pair of
binoculars, and call me collect when you see an aircraft take off trailing
a ground wire."
73, Jim K9YC
On Mon,1/19/2015 9:15 AM, Jim Lux wrote:
Are there different answers depending on why we have the ground rod? (RF
ground, power line ground, or lightning protection)
Yes..
ground rods make terrible RF grounds, in general (where RF is HF and up):
skin effect means that wires and rods have high ac resistance. (skin depth
in copper at 10 MHz is about 0.8 mils/0.02 mm.)
They also have significant series L (1 microhenry/meter for a wire.. so a
30 foot run to the rod is a 10 uH inductor, that's 600 ohms reactive
impedance.
Rods are really for electrical safety ground and/or lightning ground. And
they don't work all that well for that, unless deployed in large numbers.
The advantage of a rod is that it's easy to install by driving, but as an
electrical connection to the earth, it's just not that wonderful: the
surface area is quite small (8 foot rod, 1" in diameter is only 300 square
inches. You could probably do better, electrically, by burying a 1 foot
square plate (288 square inches).
Rods are also used in phone and power line applications.. you drive a rod
at every pole (or wrap the ground wire around the foot of the pole when
planting it). Even if any one rod has crummy characteristics, there's lots
of other rods in the circuit to help establish the common voltage reference
and provide a fault current return. I've had telco installers drive a new
rod next to the existing rods on the general principle that at least they
knew the new rod was in good condition: faster to just do a new rod than to
test the existing one
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