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.
650 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 ber can be purchased up to 20 ft, although 16-Â and 20-Âft lengths may have to be obtained at professional “contractor” lumberyards rather than local hardware stores. The lumber should be kiln-Âdried and pressure-Âtreated. Attach the 2 × 4 to the fence post with U-Âbolts. A 2 × 4 scrap is used as a wedge or shim to take up the difference between the post and the mast. In some installations the U-Âbolt will go around the perimeter of the 2 × 4, while in others a pair of holes can be drilled in the 2 × 4 to admit the U-Âbolt arms. The U-Âbolt must be at least ¼-Âin #20 thread, and either a 5 ⁄16-Âin bolt or a 3 ⁄8-Âin bolt is highly recommend if the mast supports any significant weight. Some installers place a cinder block or large brick at the base of the 2 × 4 to bear the static and dynamic loads (i.e., gravity and wind) that eventually can cause the mast to list because of eccentric loading on the footer. An added reason is to keep the butt end of the 2 × 4 off the ground and thereby prevent or slow down rot. Alternatively, a pressure-Âtreated 4 × 4 of the sort used to support decks (Fig. 28.22) can be used. 2"4"16' 2--4 Wedge Wedge U-bolt and nut U-bolt Brick Figure 28.21 Mast mounted to a fence post.
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650 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<br />
ber can be purchased up to 20 ft, although 16-Â and 20-Âft lengths may have to be obtained<br />
at professional “contractor” lumberyards rather than local hardware stores. The lumber<br />
should be kiln-Âdried and pressure-Âtreated.<br />
Attach the 2 × 4 to the fence post with U-Âbolts. A 2 × 4 scrap is used as a wedge or<br />
shim to take up the difference between the post and the mast. In some installations the<br />
U-Âbolt will go around the perimeter of the 2 × 4, while in others a pair of holes can be<br />
drilled in the 2 × 4 to admit the U-Âbolt arms. The U-Âbolt must be at least ¼-Âin #20 thread,<br />
and either a 5 ⁄16-Âin bolt or a 3 ⁄8-Âin bolt is highly recommend if the mast supports any significant<br />
weight. Some installers place a cinder block or large brick at the base of the<br />
2 × 4 to bear the static and dynamic loads (i.e., gravity and wind) that eventually can<br />
cause the mast to list because of eccentric loading on the footer. An added reason is to<br />
keep the butt end of the 2 × 4 off the ground and thereby prevent or slow down rot.<br />
Alternatively, a pressure-Âtreated 4 × 4 of the sort used to support decks (Fig. 28.22) can<br />
be used.<br />
2"4"16'<br />
2--4<br />
Wedge<br />
Wedge<br />
U-bolt<br />
and nut<br />
U-bolt<br />
Brick<br />
Figure 28.21 Mast mounted to a fence post.
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