# 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
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**f [Hz]** & \(-\)**30** & \(-\)**20** & \(-\)**10** & \(-\)**5** & \(-\)**3** & \(-\)**1** & **1** & **3** & **5** & **10** & **20** & **30** \\ \hline Low & \(-\) & \(-\)24.7 & \(-\)22.7 & \(-\)17.8 & \(-\)13.6 & \(-\)6.8 & \(-\)6.6 & \(-\)13.4 & \(-\)17.7 & \(-\)22.7 & \(-\)24.8 & \(-\) \\ Medium & \(-\)28.4 & \(-\)26 & \(-\)20.5 & \(-\)15.7 & \(-\)13.1 & \(-\)10.1 & \(-\)7.1 & \(-\)11.7 & \(-\)15.3 & \(-\)21.5 & \(-\)27 & \(-\)28.7 \\ High & \(-\)34.7 & \(-\)32 & \(-\)27.1 & \(-\)23.3 & \(-\)21.5 & \(-\)19.4 & \(-\)18 & \(-\)20.4 & \(-\)22.5 & \(-\)26.7 & \(-\)32.2 & \(-\)34.9 \\ Measurement & \(-\)42 & \(-\)33.8 & \(-\)29.2 & \(-\)24.9 & \(-\)8.9 & \(-\)8.5 & \(-\)12.1 & \(-\)8 & \(-\)20.9 & \(-\)26.7 & \(-\)30.6 & \(-\)34.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Doppler power spectral density [dB/Hz]—scenario 2. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**f [Hz]** & \(-\)**30** & \(-\)**20** & \(-\)**10** & \(-\)**5** & \(-\)**3** & \(-\)**1** & **1** & **3** & **5** & **10** & **20** & **30** \\ \hline Low & \(-\) & \(-\)24.5 & \(-\)21.9 & \(-\)16.6 & \(-\)12.4 & \(-\)6.2 & \(-\)6 & \(-\)12.2 & \(-\)16.4 & \(-\)21.8 & \(-\)24.6 & \(-\) \\ Medium & \(-\)27.6 & \(-\)24.9 & \(-\)19.4 & \(-\)14.8 & \(-\)12.5 & \(-\)9.9 & \(-\)6.7 & \(-\)10.8 & \(-\)14.1 & \(-\)20.1 & \(-\)26 & \(-\)28.3 \\ High & \(-\)33.7 & \(-\)30.9 & \(-\)26.1 & \(-\)22.7 & \(-\)21.1 & \(-\)19.3 & \(-\)17.8 & \(-\)19.9 & \(-\)21.8 & \(-\)25.7 & \(-\)30.9 & \(-\)33.9 \\ Measurement & \(-\)42.6 & \(-\)41.2 & \(-\)29.2 & \(-\)24.9 & \(-\)8.9 & \(-\)8.5 & \(-\)12.1 & \(-\)8 & \(-\)20.9 & \(-\)28 & \(-\)33.8 & \(-\)36.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Doppler power spectral density [dB/Hz]—scenario 2. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Limit Frequency from ITU-R** & **Stenario 1** & **Scenario 2** & **Scenario 3** \\ \hline
0.9 \(f_{\text{B}}\) max (High variability) & 68.39 & 79.68 & 47.68 \\
0.7 \(f_{\text{B}}\) max (Medium variability) & 53.19 & 61.97 & 37.09 \\
0.3 \(f_{\text{B}}\) max (Low variability) & 22.8 & 26.56 & 15.89 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Limit frequencies from ITU-R BT.1893-1 model of the Doppler power spectral density for the measurement scenarios. spectrum. As a result, the Doppler spectrum measurements were conducted for the entire wind farm. Based on the obtained results and observations, appropriate conclusions were drawn and included in the article. It should be noted that the central part of the spectrum of the received signal, around the carrier frequency, corresponded to the direct reflection from the turbine rotor used as the main source of the reflected signal (towards which the transmitting antenna was directed) and also to the \"static\" part of the reflected signal, which may come from, e.g., the nacelle or the tower. The rest of the measurement graphs (above 10 Hz/below \(-10\) Hz) are similar to the theoretical graphs for high variability, the spectrum of which extends farthest from the carrier frequency and at the same time is the most suppressed in relation to it. This situation is most likely also due to the fact that the signals reflected from neighboring wind turbines are also received. It should be mentioned that for the scenario 1 and 2, different rotational speeds of the rotors of neighboring wind turbines were observed in relation to the nearest turbine, which is the main source of the reflected signal. On the basis of the collected and analyzed measurement data, it should be stated that recommendation ITU-R BT.1893-1 illustrates the shape of the Doppler power spectral density very well. For the existing measurement conditions, it is stated that for frequencies from about \(-10\) to \(+10\) Hz the low variability version of the Doppler spectrum is the best representation, while outside this range the best approximation is the high variability version.