Practical_Antenna_Handbook_0071639586

C h a p t e r 2 9 : T o w e r s 661
Since our objective in putting an antenna on top of a tower is to get the antenna up
above surrounding objects, we must resign ourselves to connecting the far ends of guy
wires to attachment points substantially lower than the antenna itself. In fact, in most
(but not all) cases, the best we can do is to attach the guy wires to restraints, known as
anchors, at ground level.
Figure 29.3 is a schematic of a very simple guyed tower. As before, a 6-Âft 2 antenna
sits atop an 80-Âft tower, which is guyed at two levels (40 and 80 ft), but this analysis is
going to discuss the effect of only the top set of guys on stresses in the tower members.
Again, let us assume that the tower members are extremely skinny and thus generate
no wind loading of their own. Assume also that the guy wires or guy lines are made of
material that does not stretch, or elongate, under tensions encountered here, and that
they are attached to earth anchors 60 ft out from the base of the tower. From the trigonometry
of 3-Â4-Â5 right triangles (see App. A), we know that each top guy wire is 100 ft
long.
F WIND = 240 LBS
T H = F GUYWIRE = 240 LBS
Wind
37˚
T H = 240
H
53˚
T = 400LBS
37˚
T V = 320
53˚
60'
Figure 29.3 Forces on a guyed tower.