1. The impact of wind turbines on fixed radio links Börje Asp, Gunnar Eriksson, Peter Holm Information and Aeronautical Systems FOI, Swedish Defence Research Agency

2. Outline Introduction Measurements: observed phenomena Modeling of scattered field Simulations under normal and anomalous tropospheric conditions Simulations - With turbine Conclusions

3. Introduction Strong demands for renewable energy Conflict of interest unavoidable For example, fixed radio links (Military and civilian) Strong influence on frequency allocation/use No reliable models available to predict effects on links from wind turbines Most link operators apply widely different “exclusion zones” around their link paths

4. Characteristics of fixed microwave links Elevated antennas (line-of-sight propagation) Directional antennas (large directivity, narrow beams) Designed for very high availability Permitted outages: In the order of seconds per month Obtained by large fading margins + diversity arrangements Outages usually caused by fading under anomalous atmospheric conditions (e.g. ducting conditions)

5. Ongoing studies and measurements We study how scattering from wind turbines may affect fixed radio links under natural tropospheric fading Measurements suggest that the strength of the scattering from turbines is only slightly affected by the fading Hence, the signal to interference ratio reduces drastically during natural fading This finding is not well known

6. Measurements – 8 GHz path over water (21 km) View from one of the antennas in the direction to the other antenna Antenna location Turbine

7. Fading event 2012-01-20 Start at 20:27 (LT) Measurements – 8 GHz path over water (21 km) Zoom 1 (hp) (vp: green, hp: blue)

8. Zoom 2Zoom 3 Measurements – 8 GHz path over water (21 km) Fading event 2012-01-20

9. Measurements – 2 GHz path (60 km) Fading event 2011-04-15 Start at 03:45 (LT) Zoom

10. Modeling of the scattered field Model assumptions Turbines located between the link terminals, relatively close to the direct path between the antennas. This implies Effects dominated by forward scattering Very different from the radar back-scattering case Turbine dimensions are large Neither the transmitter, nor the receiver, is in the far-field region of the turbine Under those assumptions, the following should be adequate A wind turbine is described by it’s two-dimensional projection on an aperture plane Scattering is described by Fresnel-diffraction theory in the aperture plane

11. Modeling of scattered field Calculation of direct field (under normal and anomalous tropospheric conditions) Calculation of turbine field by means of apertures Calculation of total received field by adding turbine field using Babinet’s principle Babinet’s principle: Total field = Direct field – Aperture (turbine) field Direct field Turbine field

12. Simulation at 2 GHz - PE model, no turbines Duct 1Normal troposphereDuct 3 Modified index

13. Simulation at 2 GHz - PE model, one turbine Normal troposphere Duct 1Duct 3 Reciver height: 206-222 m above sea level Distance transmitter - turbine: 38.8 km Horizontal offset : 25 m Turbine diameter: 100 m --- Field with no turbine --- Min field with turbine --- Max turbine field

14. Simulation at 2 GHz - PE model, one turbine Duct 3 --- Field with no turbine --- Field with turbine --- Turbine field

15. Simulation at 2 GHz - PE model, one turbine (2) Normal troposphere Duct 1Duct 3 Reciver height: 214 m above sea level Distance transmitter - turbine: 38.8 km Horizontal offset : 25 m Turbine diameter: 100 m Turbine rotation: 0.2 Hz (typical for a large turbine) --- Field with no turbine --- Field with turbine --- Turbine field

16. Simulation at 2 GHz - PE model, one turbine (3) Duct 3 --- Field with no turbine --- Field with turbine --- Turbine field

17. Simulation at 2 GHz - PE model, one turbine(4) Normal troposphere Duct 1Duct 3 Reciver height: 219 m above sea level Distance transmitter - turbine: 38.8 km Horizontal offset : 25 m Turbine diameter: 100 m Turbine rotation: 0.2 Hz --- Field with no turbine --- Field with turbine --- Turbine field

18. Simulation at 2 GHz - PE model, one turbine(5) Duct 3 --- Field with no turbine --- Field with turbine --- Turbine field

19. Conclusion Measurements and simulations suggest that the strength of the turbine field is only slightly affected of natural fading Hence, the signal to interference ratio reduces drastically during natural fading Fading margin is decreased by about 10-15 dB for the investigated case As turbine fields are only slightly affected of natural fading Simpler (less complex) propagation models can be used If the strength of the turbine field is known, the fading margin can be estimated Simulations have also shown a remarkable slow decrease in the turbine field for an increasing horizontal offset Although, as the horizontal offset increases, the turbine field decreases faster for higher than for lower radio frequencies

20. Further work Target: Outage statistics as a function of distance from direct path to turbine Actions Impact of the static fields from the turbine towers (ongoing) Impact from large wind farms (more analysis, ongoing) Turbine size (more analysis, ongoing) Path length and distance to turbine from direct path (more analysis, ongoing) Simplified tool (first version running)

21. Simulation at 2 GHz - Two turbines (3) Simple free space model Reciver height: 219 m above sea level Distance transmitter - turbines: 38.8 km Horizontal offset : 50 and 100 m Turbine diameter: 100 m Turbine rotation: 0.2 and 0.21 Hz

22. Backup slides

23. Simulation at 2 GHz - Two turbines Normal troposphere Duct 1Duct 3 Reciver height: 214 m above sea level Distance transmitter - turbines: 38.8 km Horizontal offset : 50 and 100 m Turbine diameter: 100 m Turbine rotation: 0.2 and 0.21 Hz --- Field with no turbine --- Field with turbines --- Turbine field

24. Simulation at 2 GHz - Two turbines (2) Normal troposphere Duct 1Duct 3 Reciver height: 219 m above sea level Distance transmitter - turbines: 38.8 km Horizontal offset : 50 and 100 m Turbine diameter: 100 m Turbine rotation: 0.2 and 0.21 Hz --- Field with no turbine --- Field with turbines --- Turbine field

25. Modeling of the scattered field (3) Model assumptions: The turbines are located between the link terminals and relatively close to the direct path between the antennas. This implies Effects dominated by forward scattering Very different from the radar back-scattering case Turbine dimensions are large Neither the transmitter, nor the receiver, is in the far-field region of the turbine

26. Modeling of the scattered field (4) Under those assumptions, the following should be adequate A wind turbine is described by it’s two- dimensional projection on an aperture plane Scattering is described by Fresnel- diffraction theory in the aperture plane The aperture field equals the impinging field in the aperture plane Field at Rx is computed by integration over the aperture field Tx Rx Aperture plane

27. Simulated scattered field Normal atmosphere Duct 1Duct 3 Maximal turbine field: -20±5 dB in all three case Regions where scattered field is larger than direct field: Simulations of fading events support observations from measurements: Turbine field only slightly affected by natural fading