Practical_Antenna_Handbook_0071639586

C h a p t e r 2 8 : S u p p o r t s f o r W i r e s a n d V e r t i c a l s 649
Backfill
Concrete
plug
H
Ground level
D
its entire length (especially the underground portion) with a rust-Âinhibiting paint such
as Rustoleum. If your intention is to directly mount the vertical to the metal fence post,
make sure the pipe you choose is a smaller outside diameter than the maximum mast
size accommodated by the base bracket and U-Âbolts of the antenna you’re putting up.
(The maximum for the AV-Â14-ÂAVQ is 1 5 ⁄8 in, for example.) Otherwise, fabricating a suitable
adapter can turn out to be the most time-Âconsuming part of the entire project! On
the other hand, if the fence post is supporting an intermediate mast of some height, the
larger diameters are more appropriate.
The beauty of the fence post as an antenna support is that it is relatively easy to install,
and supplies are obtained easily from hardware stores. The post is mounted in a
concrete plug set at depth D (in Fig. 28.20) in the ground. To avoid snapped ropes or
other damage from frost heaves, D should be a function of local climate and local building
codes and determined by the 100-Âyear depth of the frost line in your area, a figure
familiar to local contractors and codes enforcement officials. Keep in mind, however, that
shallower depths may be legal for fence posts, but installation of a vertical antenna (or
mast) on top of the fence post may change the mechanical situation considerably.
Dig the hole with a post-Âhole digger or an
earth-Âauger-Âbit tool. The latter often can be rented
in gasoline engine, electric, or manual versions
from tool rental stores.
Once the hole is dug, place about 4 in (or local
requirement) of clean gravel (available in bags
from hardware stores) at the bottom of the hole.
Insert the post and plumb it (i.e., make it vertical in
the hole) by adjusting a single set of guy ropes. Fill
with your favorite brand of ready-Âmix concrete to
a level just below the average surface around the
hole. Cover with straw or burlap and keep the top
of the concrete moist for a few days. Do not subject
the post to side loads for at least four days or
whatever interval the concrete manufacturer recommends.
Concrete needs time to cure; its strength
increases exponentially, reaching 99 percent of its
ultimate capability in 28 days, but a 7-Âday hiatus
should be adequate before completing the antenna
installation. Once the concrete has cured, top it off
with soil and sod or garden chips.
Regardless of the height of its base, never attach
the antenna base bracket to the mast or other
support with just a single mounting point or U-Â
bolt. Always use at least two-Âpoint mounting to
prevent the antenna from pivoting and shearing
off the mounting hardware.
Figure 28.21 shows how to mount a 2 × 4 mast
to a fence post. Standard lengths of 2-Â × 4-Âin lum-
Gravel
Figure 28.20 Fence post mounting.