1. Eskil Bendz Johan Malmström ÅF-INFRASTRUKTUR AB
Electromagnetic Interference from Wind Turbines on Onsala Space Observatory
Eskil Bendz
Johan Malmström
ÅF-INFRASTRUKTUR AB

2. Outline Introduction The Saab report The ÅF report
Methods to calculate interference levels
Main results
The ÅF report
Assessment of Saab report
Other important considerations

3. Introduction Purpose: 34 Wind Turbines focus is the 10 encircled
UMTS Radio Base Stations
Purpose:
Estimate and compare interference levels from direct signals and scattered signals [dBW/m2/Hz]
18 km
Space Observatory

4. The Saab report Approach:
Calculate direct signals from UMTS radio base stations
Calculate scattered signals from wind turbines
Compare the two levels of interference at Onsala

5. Direct Signals Method: Contribution from nearby radio base stations
Free space path loss at 2 GHz
Extra attenuation due to terrain profile (heights and vegetation)
Summarize all signals
Spectral power density: dBW/m2/Hz

6. Scattered Signals Method: Divide wind turbine into two parts:
Tower - conical cylinder
Blades – elliptical cross section
Material – perfect electric conductor

7. Scattered Signals Method: Spherical wave
Scattering cross section from each segment

8. Scattered Signals Method:
Calculate contribution from base stations close to the 10 wind turbines
10 towers - spectral power density: dBW/m2/Hz
One blade - spectral power density (maximum): dBW/m2/Hz

9. Main results Both signals above recommended -250 dBW/m2/Hz @ 2 GHz
Direct signal is 32 dB higher than scattered signals (~1500 times)
or Scattered signals increase interference level <0.1%

10. The ÅF report Assessment of the Saab report found no severe errors
Simplifications overestimated the direct signals
E.g. down-tilt reduce direct signals in the Saab report 6-8 dB

11. Adjusted signal levels
New values:
Direct signal: -142 dBW/m2/Hz (Saab: -132 dBW/m2/Hz)
Scattered signals: -159 dBW/m2/Hz (Saab: -164 dBW/m2/Hz)
Direct signal still 17 dB higher than scattered signals (~50 times)

12. Other important considerations
Direct emission from radio base stations contributes largely to the interference level
E.g. UMTS, GSM, LTE
Direction of antennas
Usually three antennas at a site, covering 120 each
Suggestions to decrease interference level:
Turn off sectors
Rearrange antenna directions to obtain a ”zero”
Rearrange antenna away from telescopes
Increase down-tilt (10 might be obtained)
In accordance with mobile telephone service providers

13. Other important considerations
Some other sources that might contribute to the interference level are:
Neighboring radar stations (high power sources)
Wireless radio links (narrow beam width)
Harmonics or intermodulation products