Hi, JC;
I will answer you question more directly than in my first response. I
apologize for missing your direct question.
I recommend branching from the tower legs, to ground each leg
independently. Those tower legs may also be tied to the Ufer ground
terminals and to any other ground rods you choose to place near the
tower base. However, each radial from a tower leg should go straight to
a ground rod, with no unnecessary bends or turns for minimum inductance.
I also thought of another analogy for the ground rods in soil: the
capacitance represented by each ground rod is like a filter capacitor in
a power supply. The conductance into the earth is like the load and a
bleed resistor. If the capacitance and conductance are great enough, the
voltage never gets very high. Even if the conductance is poor, the
lightning event is so short it doesn't charge to a very high voltage if
the capacitance is large. The capacitors are a low pass filter that
grounds the ripple (lightning surge). The conductance, even if poor,
bleeds off the accumulated charge shortly after the event.
73 de Red
JC Smith wrote:
>Hi Red,
>
>Just a quick question about those six ground rods. I'm told that the NEC
>requires a minimum of two ground rods located within two feet of the tower.
>This prevents having them spaced twice their depth. Are you talking about
>branching out from the rods at the base of the tower or are you just
>installing six rods equally spaced around and away from the tower? (Which
>would mean no rods within 2' of the base.)
>
>Thanks,
>JC
>
>
>-----Original Message-----
>From: towertalk-bounces@contesting.com
>[mailto:towertalk-bounces@contesting.com]On Behalf Of Red
>Sent: Sunday, July 09, 2006 7:42 AM
>To: TowerTalk
>Subject: Re: [TowerTalk] Heavy Guage Copper Wire Source??
>
>The optimum pattern of radials for lightning dissipation from a tower or
>vertical antenna is a pattern of 6, equally spaced around the center.
>More won't hurt, but don't help a whole lot for dissipating lightning,
>because 6 will utilize the available capacity of the soil. This assumes
>that each radial terminates in a ground rod to a depth at least twice
>the length of the radial. Each of these radials may be extended with
>one or two more, also terminated with a ground rod as described.
>Extending each to two more radials creates a pattern in which all
>grounds are spaced optimally, that is, to a depth equal to half the
>distance between them. This pattern may be extended, but the
>effectiveness of the additional radials is less than that of the first
>two rings because of the inductance of the greater length. That
>effectiveness doesn't become zero, it is simply less.
>
>Think of the earth as a lossy capacitor. The earth surrounding each
>ground rod will quickly charge to high voltage as it absorbs the current
>of a lightning stroke. Following that, it will discharge through its
>conductance to the surrounding earth. That is the reason for spacing
>the ground rods apart. It permits each ground rod to be modeled as
>another capacitor, in parallel with its companions. The total capacity
>of the group is the capacity of one times the number of rods, if they
>are spaced this way. The impedance of the pattern is the impedance of
>one (capacitor and associated resistance and inductance of the radial)
>divided by the number of rods. Two or more ground rods close together
>are very slightly more effective than one, for lightning dissipation.
>
>The current will not necessarily be equal in all these radials. It will
>divide proportionally to the admittance (reciprocal of impedance) of the
>radials, which will vary because of varying soil characteristics,
>spacing, etc.
>
>The current capacity of these radials is much greater than the steady
>state capacity of the same wires encased in insulation due to the short
>time duration of a lightning strike. The peak current of lightning
>strikes is, according to Uman, 150,000 A. Half exhibit a peak of 30,000
>A. The peak is usually on the first stroke. Subsequent strokes are
>typically less than half the peak current of the first stroke.
>
>Six number 12 copper wires will carry the current, but number 6 is
>recommended for mechanical endurance. The lightning dissipating radials
>should be bare and buried, to take advantage of contact with the soil.
>That advantage is, in typical installations, small, but it's free.
>
>73 de WOØW
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