Practical_Antenna_Handbook_0071639586

C h a p t e r 3 0 : G r o u n d i n g f o r S a f e t y a n d P e r f o r m a n c e 685
The usual method for dissipating the energy in a lightning surge is to attempt to
spread that energy to a large area of ground surrounding the target tower(s) and antennas.
This is done most often by connecting the base of the tower to a set of lightning radials
comprised of straight runs of #6 AWG or larger copper wire or flat copper strap with
a cross-Âsectional resistance no greater than that of #6 wire. Soil conditions permitting,
each radial should be connected mechanically or with a brazed joint to multiple 8-Âft
ground rods of the type found at electrical utility suppliers such as Graybar. As shown
in Fig. 30.1, the first ground rod on each radial should be one ground rod length (or 8 ft,
in this example) from the base of the tower, and all subsequent rods should be 16 ft (i.e.,
twice the length of the rod) beyond the previous rod. The minimum practical number
of radials is probably 4, with 8 to 16 being a better choice. Two or three rods per radial
probably constitute a minimum system.
Since the objective of lightning radials is to “dump” the surge currents—whether
direct or induced—from the lightning bolt into the soil, there is no reason the radial
wires (and the tops of the ground rods) can’t be buried many inches under the surface
of the ground. However, as we discuss later, if the lightning radials are to be a part of a
ground-Âmounted vertical antenna’s radial system, they should not be located more
than a few inches below the surface, or the ground losses associated with the vertical’s
return currents will increase and overall antenna efficiency will suffer. Strange as it may
Ground
rods
16'
16'
16'
8'
8'
8'
16'
8'
8'
16'
8'
8'
16'
8'
16'
16'
Figure 30.1 Tower lightning ground system.