Practical_Antenna_Handbook_0071639586

638 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
Twin-lead
Clamping
Fixture
Chain Link
End
Insulator
Support
Rope
Strip insulation back 1/2”,
tie ends of both
conductors together,
and solder the junction.
Figure 28.11 Twin-Âlead folded dipole end insulator.
Supporting the Wire Antenna
Figure 28.6 shows the dipole installed. The ends of the two wire elements are connected
to end insulators, which are in turn supported by rope to a vertical support (e.g., mast,
tree, roofline of the house). Although shown perfectly horizontal here, the actual installation
will droop in the center due to the weights of the center insulator or balun, the
coaxial cable, and the wire itself.
If either end of the dipole is supported by a tree limb or anything else that can move
in the wind, add a door spring—just beyond the end insulator—in one support line.
The strength of the spring should be chosen such that the normal tension in the support
lines on a calm day causes the spring to be less than halfway extended. For an 80-Âm
dipole with substantial feedline weight, a residential garage door spring is most likely
to be appropriate. Most local hardware stores carry an assortment of door springs having
various tension ratings.
Trees, Masts, and Other Supports
The end supports of the antenna can be anything that supplies height and is strong
enough: a mast, a tree, or the roofline of a building. Figure 28.12 shows the use of a
mast, but other forms of support can be used as well. To facilitate easy raising and lowering
of the antenna for maintenance and tuning, the support rope should not be tied
off at the top, as is true in all too many installations; instead, it should be brought down
to ground level, just like the halyard of a flagpole. Be sure to provide enough “dead
slack” to make lowering the antenna feasible, although once a halyard is created, it can
be lengthened temporarily, as needed, to allow the antenna to be pulled sideways (i.e.,
along its axis) during the raising or lowering process.
The ideal support is arguably a climbable tower (or perhaps a very tall building
with an elevator to the roof), but most dipoles and other wire antennas are strung from
tall trees. How do we get the antenna up high in the trees? Here are some thoughts
about a variety of approaches: