One problem with big antennas has to do with what we call their ‘K-factor’, which is an important concept apparently often overlooked in determining the real antenna load on a tower.
In general terminology, a K-factor is essentially the amount of special twisting torque that an antenna or array of antennas can exert on a tower. Like most vertically slender structures, antenna towers are especially susceptible to this rotational, ‘cork-screw’ type of torque. When a heavy and/or large-spanned antenna is twisted rotationally around the vertical axis of the tower, such a force translates in to overpowering horizontal load vectors which can buckle the lattice elements of the tower.
Antennas, with what the antenna industry calls large ‘mean (average) turning radius’ are prone to being twisted by not very high or less than maximum winds. Because wind and air pressure is often apparently not consistent in its vectors of direction and volume, these physically long antennae may be more exposed to ‘off-center’ air currents than other more compact arrays. If the antenna is twisted with these winds, the resulting loads may be similar to extreme maximum high winds hitting the antenna’s sail area straight on and not creating a twisting motion.
Of course, the end-result can easily damage an antenna and the structure (tower) holding it, if not prepared or fortified against.
The simple way to get an approximate idea of your antenna’s K-factor is to multiply the antenna’s overall weight by its mean turning radius. It is incumbent upon the antenna manufacturer to provide its customers with these accurate specifications on their antennas. Weight (lbs.) x avg. Turning Radius of antenna (ft.) = K-factor (ft.lbs.) load.
The K-factor load should then be compared or checked against the allowable Shear value of the tower sections. If the K-factor exceeds the allowable Shear value (also in ft.-lbs.) of the tower sections, then the tower may be collapsed by twisting action. Unfortunately, high winds are not required to achieve the movement inertia in the antenna, which translates into K-factor load on the tower. Any wind-speed, even 20 or 30 mph can create the twist in the antenna if the atmospheric conditions are volatile and erratic.
Customers should be careful when selecting very large multi-band super-antennas or heavy masts and other components for their tower antenna arrays. A K-factor should always be calculated for each individual antenna, and also the whole antenna array, including the mast weight and any additional booms or equipment. Even antenna rotator weights should be added into the K-factor equation, despite their typical placement along the vertical axis of the tower.
Should you have any questions on K-factors, windloading or other antenna load calculations, Heights Tower's sales or technical staff will be glad to discuss and provide information, to the best of our knowledge. Nevertheless, it is the customer’s ultimate responsibility to accurately calculate his/her antenna loads. Antenna manufacturers should also be able to provide reliable specs about the antennas they make and sell.