# Radio Channel Modelling for VHF System Operating in the Offshore Wind Farms Propagation Environment
## 3 Results and Discussion
### The Measurement Results
#### 3.1.2 Measurements of the Doppler Power Spectra Density
In this case, the Annex 1 model becomes unreliable, as we deviate significantly from the scenario for which the model was originally defined. In practice, however, such large, positive values of errors, should not come as a surprise. If the signal is reflected from the turbine at a shallow angle (several dozen degrees away from the direction of the maximum reflection), then the reflection takes place from the side-inverted rotor, i.e., representing an incomparably smaller reflecting surface compared to the rotor perpendicular to the transmitter. The values resulting from the Annex 1 model may, therefore, give very low values of the UFSR reflection level. In addition, since the measured value is so significantly higher than the theoretical value, it allows us to conclude that some reflection mechanism--other than the reflection from the blades--is more prevalent in this case (e.g., reflection from the mast). Consequently, for turbines that are far (on an angular scale) from the line of maximum reflection, the Annex 1 model should not be used: the Annex 2 model is the most appropriate in this case. In Annex 2 the dominant reflection mechanism from the turbine is the reflection from its mast. The main distinction is that for rotors the size of the reflecting surface depends on their temporal orientation, but in the case of the mast such relation obviously no longer exists. The second scenario in which the Annex 2 model should be used is when the transmitter is in the vicinity of the turbine (it is worth noting that in this case the angle of incidence of the signal on the rotor will very often be away from an angle of 90 degrees). According to the ITU-R BT.1893-1 model, this distance should be a maximum of 14.765 km (for a 117 m high turbine) and 20.245 km (for a 137 m high turbine). In addition, at the end of these analyzes, the mean value of the UFSR error for the Annex 1 and Annex 2 models was calculated; however, while doing so, we only considered those values that are consistent with the applicability of the specific model, i.e., in the case of Annex 1 only the values that result from reflections from turbines approximately in line with transmitter and receiver were included, and for the Annex 2 calculation the remaining values were taken into account. The final results are presented in Table 13.  
For Annex 1, the theoretical level of UFSR resulting from the model was on average 15 dB higher than it was obtained from the measurements. In turn, the model from Annex 2 overstated the level of the reflected signal power by an average of 9 dB in relation to the measurement data. These values should be treated as a correction factor in the application \begin{table}
\begin{tabular}{c c} \hline \hline
**Model** & **Mean Difference (Error) [dB]** \\ \hline ITU-R BT.1893-1 Annex 1 & \(-\)15.07 \\ ITU-R BT.1893-1 Annex 2 & \(-\)9.16 \\ \hline \hline \end{tabular}
\end{table} 
Table 13: The mean difference in UFSR determination with the use of both analyzed models. of the ITU-R BT.1893-1 recommendation in measuring the impact of wind farms on systems operating in the VHF band. This means that both models are sufficient to assess the impact of wind turbines on communication systems operating in the 160 MHz band. It is worth mentioning here that in 2013 [12], while researching the impact of wind turbines on radio/radar systems, a similar analysis was performed on the basis of the measurement data obtained at that time. In 2013, recommendation ITU-R BT.1893-0 contained only the Annex 1 model, but the results obtained at that time--with regard to Annex 1 only--are in line with the present ones. 2.2 Dependence on the Frequency Range in the Model of Determining the Doppler Power Spectral Density  
The next criterion for modeling the radio channel for VHF systems was the frequency range in the model for determining the Doppler power spectral density. Higher values of Doppler shifts occur when the signal direction is orientated parallelly with respect to the turbines rotors.