Practical_Antenna_Handbook_0071639586

242 P a r t I I I : H i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s
190
Field intensity (mV/m)
170
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0 10 20 30 40 50 60 70 80 90 100
Number of radials
Figure 9.9 Method for connecting many radials together.
a variety of stresses, whether from being stepped on or tripped over, or after easing
their way into the sod. Some terrains do not lend themselves to burial techniques, and
highly visible insulation may be important to prevent unsuspecting visitors from tripping
over them.
Caution If you decide to use an aboveground radial system, be sure to include measures to
prevent people from tripping over it or (if it is an elevated system) running into it. Depending
on the law where you live, there can be liability implications when a visitor gets injured,
even if that person is an intruder or trespasser.
The return currents in radials result from displacement currents flowing in the capacitor
formed by the vertical element and the ground system. Displacement currents
(see Chap. 2) are RF alternating currents that do not require a conducting medium for
their existence. Consequently, radials can be either insulated or bare, although bare
metals used outdoors will develop an oxide insulating layer on them sooner than insulated
conductors. Radials can be copper or aluminum. Many amateurs have formed
their radial fields out of scrap wiring from multiple sources, while others have managed
to create additional wire by separating multiconductor cable into individual
strands. If buying wire new, it’s hard to beat the price of 500- or 1000-ft spools of interior
house wiring at the large discount home supply stores.
Because of the orientation of the electromagnetic fields around the vertical, only the
ground resistance along radial lines from the base of the vertical is important. That is to
say, there is little merit to connecting the tips of the radials together or adding other