Practical_Antenna_Handbook_0071639586

C h a p t e r 2 9 : T o w e r s 657
But the force is nothing more than the wind pressure multiplied by the total effective
projected surface area (A) of the antenna exposed to that pressure:
FANT = PWIND × A
(29.2)
ANT
The reason for the term “effective projected” is that different surface contours having
the same area develop different resistance, or drag, in response to identical wind
speeds. It should be obvious, for instance, that the force on a flat plate squarely facing a
30 mph breeze is greater than the force on a long piece of cylindrical tubing in the same
breeze. Surface areas for tower-Âmounted antennas manufactured in the United States
are usually given in equivalent or effective square feet. The “e-Âword” qualifier is included
because most antenna elements and booms are cylindrical and the force on them for a
given wind speed is less than if they were flat plates. But read the manufacturer’s literature
very carefully to be sure you understand how (or “if”) the cylindrical nature of
the antenna elements and tower members has been accounted for. Common convention
is to convert the actual area of one face of each cylinder into an equivalent flat plate
area—often by multiplying the cylindrical surface area by 0.6—to account for the reduced
drag when air flows past a cylindrical tube, as compared to a flat plate.
For most types of antenna, the equivalent area that directly faces the wind varies
with the wind direction (and with the compass orientation of the antenna). In general,
the only number of interest is the worst case, or maximum equivalent area. For a multielement
Yagi, for instance, that may occur when the beam is facing the wind or when it
is broadside to the wind; most likely, however, it will occur when it is turned at some
angle in between those two extremes.
The “raw” pressure resulting from wind is given by:
2
P = K × v
(29.3)
WIND
WIND
where (in the U.S., at least) P WIND = pounds per square foot (psf)
v WIND = miles per hour (mph)
K = constant equal to 0.00256 when using mph and psf
Of more interest to us is something called flat plate impact pressure (P FLAT ). This is the
net pressure increase experienced by a flat plate squarely facing into a steady wind at a
given speed (v). P FLAT is also proportional to the square of wind speed, but the constant
is different from the K of Eq. (29.3) and depends on a number of geometrical characteristics—not
just of the plate but of the environment around the plate. Thus, the equation
for P FLAT may vary with the underlying assumptions embedded in a particular analysis.
For some years, curves relating the flat plate impact pressure of flat microwave antenna
panels to true wind speed often used a K FLAT = 0.004. (See Fig. 29.2.) Today the determination
of the K-Âfactor to be used may be conducted on an installation-Âby-Âinstallation basis.
(See the nearby Note regarding Revision G of TIA-Â222.) Nonetheless, the single most
important fact to take away from Eq. (29.3) or Fig. 29.2 is that pressure on an antenna—
hence, horizontal force on a tower—is directly proportional to the square of the wind
speed!