# Radio Channel Modelling for VHF System Operating in the Offshore Wind Farms Propagation Environment
## 4 Conclusions  
This article addresses the topic of radio channel modeling for VHF systems in the offshore wind farm propagation environment. Observing the continuous growth of interest in renewable energy sources such as wind, this issue will probably gain more and more importance in the design of onshore and offshore wind farms. The authors, through a measurement campaign in the wind farm environment, confirmed the applicability of the ITU-R BT.1893-1 model (both Annex 1 and Annex 2) in the marine VHF band (160 MHz) at sea. First, as part of the radio channel modeling, we performed the analysis of the signals level reflected from the turbines. In the model included in Annex 1, the basic mechanism of reflection from wind turbines is the reflection from their blades, while in the model from Annex 2 the main mechanism is the reflection from the turbine's mast. Additionally, the main area of the Annex 2 validity is the scenario when the transmitter and turbine are close to one another. In the case of the application of Annex 1, the theoretical level of the UFSR (reflected signal) resulting from the model was on average 15.1 dB higher than it was shown in the measurements. In turn, the model from Annex 2 overstated the level of the reflected signal power by an average of 14.3 dB in relation to the measurement data. As we can see, regardless of the Annex applied, the ITU-R BT.1893-1 recommendation is definitely pessimistic with respect to the VHF maritime band, which means the real level of the unwanted signal reflection from the turbines is less severe than the theoretical values. The pessimistic character of the model is mainly due to following factors:  
* The ITU-R recommendation was developed under the assumption that the propagation paths are model using the simplest possible way, i.e., the free-space loss model, while the authors of this paper employed much more sophisticated model ITU-R P.2001-4 in their simulations. The ITU-R P.2001 is a universal, complicated, general-purposepropagation model valid for radio systems operating in the wide frequency range from 30 MHz to 50 GHz. The approximation of the propagation path conditions obtained using that model are definitely more reliable and closer to reality than those obtained under the free-space loss scenario. The additional diffraction attenuation calculated for the spherical Earth and for the turbines located close to Tx and Rx does not exceed 5 dB, and for the turbines far away from Tx/Rx it can be more than 10 dB. In the future campaigns, we plan to employ high antenna mast at the transmitter and receiver sides (about 10 m), to reduce the impact of diffraction (which is otherwise difficult to estimate during the measurements). * The BT.1893-1 recommendation was developed under the assumption that crucial components of the turbines will be modelled as triangle (blade--Annex 1) and cylinder (mast--Annex 2). This is merely an approximation, as the actual components on the existing wind farms are built in a slightly different way (the mast is not an ideal cylinder--it is usually composed of cylindrical sections characterized by varying radii and the blade is not an ideal triangle). Such simplification definitely affects the results provided by the model when compared with real measurement data. * During actual measurements, it is extremely difficult to emulate the maximum reflection case (i.e., the worst-case scenario), as it requires a perfectly perpendicular orientation of the transmitter with respect to the rotor's plane. It is not easy to achieve that, especially due to variable weather conditions (changing direction of wind which moves the rotors, etc.). This issue may clearly affect the results, as even a slight deviation from the 90 degrees requirement (i.e., a few degrees off) noticeably reduces the unwanted reflection level. All these three factors combined explain why the model may give overly pessimistic data.