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

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

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

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

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: -132 dBW/m2/Hz

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

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

Scattered Signals Method: Calculate contribution from base stations close to the 10 wind turbines 10 towers - spectral power density: -164 dBW/m2/Hz One blade - spectral power density (maximum): -164 dBW/m2/Hz

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%

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

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)

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

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