Practical_Antenna_Handbook_0071639586

672 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
• Conventional concrete block guy anchor
• Earth screw-Âin anchors (called screw anchors for short)
• Special screw eyes held in boulders with equally special expanding cement
Examples of inappropriate and unacceptable guy anchors are:
• Houses, garages, sheds, or other buildings
• Utility poles
• Trees (or any other living things)
• A spike hammered into the top of a tree stump (the author actually has a photo
of a real installation where this was done!)
• A steel or iron post anchored in concrete
A special note about the last anchor is in order. Occasionally a steel post anchor appears
to be an attractive way to deal with a difficult guying situation, such as when the
lower end of a guy wire needs to cross over a driveway, parking area, small shed, or
sidewalk. Typically, the plan is to use a 10-Âft-Âlong steel tube or I-Âbeam of unknown
strength, with 3 or 4 ft of it buried in a concrete pad and 6 or 7 ft sticking up in the air.
The guy wire is then attached to a hole drilled near the top of the post. Unfortunately,
the very guy wire that is reducing the wind-Âgenerated stress on the tower is creating a
similar stress on the post. The horizontal component of a 1000-Âlb tension in the guy wire
is 707 lb. This acts with a moment arm of 6 or 7 ft around the point on the I-Âbeam where
it enters the concrete. This overturning moment of 4000 lb-Âft or greater can become a
20,000-Âlb compressive force in the post, bending it right at the top of the concrete. The
odds are very high that the post is going to bend or break during the first windstorm.
Certainly the easiest, least expensive, and fastest way to create guy anchors in most
soils is with screw anchors. Figure 29.8 shows a typical device, available from some
tower manufacturers (such as Rohn) and from utility company suppliers such as Graybar
and countless regional companies. One excellent anchor is the 6-Âft rod with 6-Âin
auger made by A. B. Chance Company. Larger capacities are also available. However,
some professional tower installers refuse to climb towers guyed with screw anchors.
Compared to concrete pedestals, earth screw anchors have at least two shortcomings:
• The holding power of the soil may be unknown or may vary with weather
conditions, flooding, etc.
• The speed and likelihood of corrosion resulting from direct contact with soil of
unknown characteristics is uncertain.
A chart relating soil type to screw anchor holding power can be found on the A. B.
Chance Web site. Some, leaving nothing to chance (bad pun), have inserted a screw
anchor in their soil and measured the tension (force) required to pull it out with a tractor
or 4WD SUV. However, holding power is not the only important soil parameter. Soil
acidity and the types of trees in the guy anchor area may have a big effect on anchor
lifetime. To combat this, some users of screw anchors coat them with a tarlike substance
before installing them. Others have recently described electrochemical techniques for
neutralizing the effect of corrosive soils. Good news: This is a technical area currently
getting a lot of attention as new research and findings are being reported.