Practical_Antenna_Handbook_0071639586

696 P a r t V I I I : M e c h a n i c a l C o n s t r u c t i o n a n d I n s t a l l a t i o n T e c h n i q u e s
• If you need to run a radial slightly “off course” for part of its run to get past a
building or other obstruction, do it.
• Don’t bother connecting radial tips with circumferential wires; all the return
currents from the antenna’s fields are radial in orientation. Similarly, adding
chicken wire mesh under the tower is problematic; in return for lots of added
conductor that may or may not be augmenting your original radials, you could
be plagued after a couple of winters by rectification from a zillion corroded
junctions where the wires of the mesh cross each other.
• Use just about any kind of copper wire you have on hand for your radials. Strip
multiconductor cables into their individual wires, if you wish. The primary
determinants of what wire to use are cost (if it’s free, use it!) and longevity on/
in the ground.
• If you have to purchase any radial wire, get ordinary insulated THHN #14
house wire from a building supply store. The insulation will prolong the life of
your radials.
• Even at kilowatt power levels, wire gauge is not critical. The primary reason for
specifying #14 wire is so that it will stand up to stresses on it, such as when it is
buried or when someone or something trips over it. Many radial fields consist
of #18 or finer wire.
• Avoid stranded wire in outdoor applications.
• Ideally, your radials should lie on top of the ground for maximum antenna
efficiency, but you can bury them an inch or two below, if necessary, to keep
people and animals from tripping on them. Many amateurs have reported that
sod staples, available at home and garden supply stores, are excellent for holding
radials down and out of the reach of lawn mower blades until they sink into the
grass and become part of the thatch.
• At least two suppliers of accessories to the amateur market (Array Solutions
and DX Engineering) sell radial attachment plates intended to simplify the
connection of large numbers of radials to the base of the antenna. Others,
including the author, have formed a grounding ring from soft copper tubing
purchased from the local hardware store and attached the radials to it with
solder joints or noncorroding fasteners.
• If you have multiple verticals (a four-Âsquare, for instance), do not connect the
radials of one vertical to those of another.
• Never directly connect copper to galvanized steel. Use a transition metal in
between the two.
Elevated Grounds
Most users of VHF equipment are familiar with the ground-Âplane antenna—a vertical
with either three or four radials, mounted atop a mast. The same technique can be used
at MF and HF, but the mechanical challenges are not trivial. At VHF, the idea behind the
ground-Âplane antenna is simple: a vertical monopole (whether l/4 or 5l/8 or any
length in between) requires a return path for the “missing” half of the charged dipole
structure that is necessary for electromagnetic radiation. Starting with a vertical dipole
(Fig. 30.6), if we split the wire that is the lower half of the radiating element into four