Wireless tower structures are going through a bit of an overhaul as mobile operators move to densify their networks and keep up with subscriber demands. As coverage and capacity needs continue to soar, operators are deploying increasingly more infrastructure - specifically higher port base station antennas, small cells, fiber connections, and specialized mounts. All this equipment adds load to the towers, not only due to their size but also in regard to dynamic loading caused by wind. The result is towers are reaching their limits in terms of load capacity. Therefore, understanding the impact of wind loading is critical to your tower design choices and resultant affects to lease costs and ultimately your bottom line.
Until recently, comparing wind load values between manufacturers was not an apples to apples exercise. It could be confusing at best. Even though most manufacturers “adhered” to the EN 1991-1-4 standard, the procedures were not uniform or even conducted with the same wind speeds. In early 2016, with Kathrein’s leadership in the NGMN working group, antenna manufacturers developed a methodology for calculating wind load values consistently and accurately with the goal of implementing into future revisions of the BASTA standard. As a result, it will be easier for mobile operators to accurately compare values when evaluating antenna selection and the impact to their networks.
So what is wind load and how do wind load factors impact the densification of your network? What is the methodology for calculating wind load, and how does this impact your bottom line? We explain why and how Kathrein is reporting dramatically reduced wind load values.
What is Wind Loading?
Wind loading is a measurement of the force or drag that wind causes when blowing against a tower mounted antenna. Manufacturers report both frontal and maximum wind load values for every antenna they make. Wind load values are represented as XXXX N where the N represents a Newton (which is one kilogram meter per second squared). The U.S. measurement is listed in pounds of force (lbf). Think of an antenna on a tower as a sail on a ship. The size and shape of the antenna affects how much wind drag that antenna places on the tower structure. As with golf, a lower number is a very good thing.
In the mid 2000’s, the European Standard for Wind Load Calculations was updated with the introduction of the EN 1991-1-4 standard. Like other antenna manufacturers, Kathrein bases its wind load reporting on this standard. Until the BASTA meetings in 2016, Kathrein had been very conservative with its reported wind load values. After the meetings with other leading manufacturers concerning the testing and reporting of wind load values, Kathrein has revised wind load calculations and values based on the latest, more accurate methodology.
Antennas are not perfect rectangles, and wind doesn’t always hit a base station antenna straight on (orthogonally). Taking these facts into consideration, new calculations are being used in compliance with the standard based on an antenna body with a rectangular cross section with rounded-off corners. Wind tunnel tests have shown that the results obtained from previous calculations, based on the standard, are considerably higher than the real wind loads.
Since it’s too expensive to test every antenna in a wind tunnel, manufacturers report wind load values based on the standard formula: Fw = cf ∙ Aref ∙ qp, where the wind load, Fw, is the product of the force coefficient, cf, the projected area Aref (m2), and the dynamic pressure qp (N/m2).
While the formula remains the same, the change is in how the force coefficient, cf, is calculated. It now includes a radius reduction factor, Ψr , that accounts for antenna cross sections with rounded edges- which are more aerodynamic. The more favorable the shape for the airflow, the smaller this value. As for wind speed, manufacturers have agreed to test wind load at 93 mph (150 km/h).
The bottom line: the new method of calculating wind load has proven to more accurately reflect real world wind conditions and antenna body characteristics, resulting in trustworthy wind loading values that can be calculated without the need for further wind tunnel testing.
Kathrein’s Updated Results:
Due to the radiation-optimized shape of Kathrein’s base station antennas, the revised wind load calculations show reductions from 5% - 40% or more across the board, proving that Kathrein’s previously documented wind load values were far too conservative.
Here is a sampling of the results:
Wind load of 80010865, 6-Port Antenna (377mm wide):
- using the original method: Maximum 1210 N (272 lbf) | Frontal 1160 N (260 lbf)|
- using the improved method: Maximum 730 N (164 lbf) | Frontal 630 N (142 lbf)|
- 46% reduction in frontal, 40% reduction in Maximum Wind Load Specification
Wind load of 80010965, 8-Port Antenna (508mm wide):
- using the original method: Maximum 1650N (371 lbf) | Frontal 1270N (186 lbf) |
- using the improved method: Maximum 1140N (256 lbf) | Frontal 1130N (254 lbf)
- 11% reduction in frontal, 31% reduction in Maximum Wind Load Specification
To confirm the revised wind load calculations, third party tests were carried out in the wind tunnel at Dresden University. Not only did they confirm the new results, but showed that the frontal and maximum wind load values are the most important for describing the behavior of an antenna in the wind flow.
If you would like more information on Kathrein’s wind load results, and a complete list of wind load values for all current base station antennas, go to http://www.kathreinusa.com/wind-load/