Technically speaking Selecting the proper base station antenna–Part 2
Last month, the most important electrical specifications of base station antennas were discussed. This month we will look at some of the mechanical specifications and then get down to choosing the best antenna to suit the requirements of various situations.
Mechanical considerations There are some important mechanical considerations that should be taken into account when selecting a base station antenna. Lateral thrust, bending moment and torque are some of the more important factors.
Lateral thrust — Lateral thrust is side force exerted upon an antenna by the wind. (See Figure 1 below). Lateral thrust is dependent on the equivalent flat plate area of the antenna and upon the wind velocity. The greater the equivalent flat plate area or the greater the wind velocity, the greater the lateral thrust exerted on the antenna.
Generally, a wind velocity of 100mph is taken as the reference. At 100mph, the lateral thrust exerted on an antenna is taken as 40 pounds per square foot (40psf or 40lbs/ft^2). The formula for determining the lateral thrust is F = kAV^2 where F = force of lateral thrust in pounds per square foot k = wind correction factor (generally taken to be 0.004) A = equivalent flat plate area V = wind velocity in miles per hour. By substituting the reference wind velocity of 100mph acting upon an area of 1ft^2 into the formula, we find the lateral thrust to be 0.004 x 1 x 100^2 = 40lbs/ft^2.
Thus, if an antenna has an equivalent flat plate area of 2ft^2, then the lateral thrust is 2 x 40 = 80lbs of lateral thrust.
Bending moment — Lateral thrust produces a force, called bending moment, on the antenna support. This specification is usually given for the point at the top mounting clamp as shown in Figure 2 below.
Torque — In Figure 3 at the right, a side-mounted antenna “catches” the wind, and as a result of the lateral thrust exerted on the antenna, a twisting force, or torque, is exerted on the tower. To reduce the torque exerted on a tower, special torque resistors are used. Basically, a torque resistor is just a horizontal extension mounted to the tower to provide guy points further from the center of the tower. This allows the guys to overcome the torque produced by the wind similar to the way that a simple lever provides more leverage the further it is from the fulcrum.
Ice loading — The effects of ice loading on a tower are caused by more than just the weight of the ice. Buildup around the antenna increases the surface area of the antenna, thus increasing its wind resistance. To illustrate, a round antenna rod 2ft long with a 1″ diameter has an equivalent flat plate area of 24 x 2/3 = 16in^2. If the same antenna rod were covered by 1/2″ of ice, the diameter would become 2″, thus doubling the equivalent flat area of the antenna rod and thereby doubling the lateral thrust on the antenna. The 2/3 correction factor is used in the above calculation because round surfaces are considered to have wind resistance equivalent to 2/3 that of a flat surface.
Other considerations Service area and possible interference also must be considered in choosing the proper base station antenna.
Service area — Before choosing the antenna, carefully consider the service area to be covered. At what point within the service is the antenna to be located? What is the general shape of the coverage area? These factors play an important role in the antenna choice. In Figure 4 at the left, the area to be served by the base station is fairly round, with the antenna located near the center of the service area. The choice here is simple–an omni-directional antenna should be used with the necessary gain to adequately cover the area. In Figure 5 (above center), the service area is elongated with the antenna located at the center of the service area.
What kind of antenna? A horizontally collinear antenna may be the best choice. A horizontally collinear antenna consists of two vertical radiators spaced at a horizontal distance of [lambda]/2. This causes the radiation to cancel in the directions of undesired coverage and to reinforce in the desired directions (See Figure 6 above.) By special design, the sharp nulls can be reduced somewhat to provide some side coverage as needed. Figure 7 above shows a service area in which a high-gain directional antenna, such as a yagi, comes into play. The size of the yagi used depends on the gain required to cover the opposite end of the service area. Another antenna that can be used in this situation is the corner reflector. The corner reflector produces a cardioid, or heart-shaped, pattern as shown in Figure 8 above center.
Minimizing interference Figure 9 at the right shows a situation that requires a directional antenna that is oriented so that the nulls are positioned to minimize possible interference to other stations operating on the same frequency. Points B, C and D represent receivers and locations operating on the same frequency. The idea is to communicate from point A (center) to point D without interfering with receivers at B and C.
First, overlay several transparencies outlining the antenna radiation pattern on a map of the service area to determine if the nulls are in the proper place to minimize interference. The proper orientation of the antenna then can be determined by adjusting the overlay until the nulls are properly positioned. Once this is done, record the azimuth of the antenna orientation and install and orient the antenna in the proper azimuth.
Conclusion As you can see, there is more to selecting a base station antenna than simply gain and directivity. Care taken in the process of choosing the proper antenna can save you many headaches down the road. ‘Til next time–stay tuned!
Bibliography EIA Standard RS-329-A, Minimum Standards for Land-Mobile Communication Antennas, Part 1–Base or Fixed Station Antennas. NIJ Standard-0204.01, Fixed and Base Station Antennas. Sinclair Technologies, Technical Notes.