# Radio Channel Modelling for VHF System Operating in the Offshore Wind Farms Propagation Environment
## 3 Results and Discussion
### The Measurement Results
#### 3.1.1 Measurements of the Level of Signals Reflected from the Rotors of Wind Turbines
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Measurement**} & \multicolumn{3}{c}{} \\ \multirow{2}{*}{\begin{tabular}{c} **The Peak** \\ **in Table 3** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **The Peak** \\ **Wibich the** \\ **Signal Was** \\ **Reflected** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **The Peak** \\ **Transmitting** \\ **Signal Was** \\ **Reflected** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Theoy—** \\ **Annex 1** \\ **Antenna** \\ **Characteristics** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Measurement—** \\ **Annex 2** \\ **Difference [dB]** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Measurement—** \\ **Theory Annex 2** \\ **Difference [dB]** \\ \end{tabular} } & \multicolumn{2}{c}{\
\begin{tabular}{c} **Measurement—** \\ **Theory Annex 2** \\ **Difference [dB]** \\ \end{tabular} } & \multicolumn{2}{c}{
\begin{tabular}{c} **Comments** \\ **Difference [dB]** \\ \end{tabular} } \\ \hline \hline  & & & Direct Component (FSR) & & & & The theoretical value of the direct component \\
5 &
\begin{tabular}{c} N/A (direct \\ signal) \\ \end{tabular} & \(-61.74\) & \(-19.5\) & \(-19.5\) & \(-42.24\) & \(-42.24\) & \(-42.24\) & FSR does not depend on the adopted model (Annex1/ \\  & & & & & Annex2) \\ \hline \hline  & & & & & & \\ \hline \hline  & & & & & & & \\ \hline
1 & 4 & \(-57.13\) & \(-69.53\) & \(-44.14\) & \(+12.40\) & \(-12.99\) & \\
2 & 8 & \(-54.15\) & \(-43.51\) & \(-51.86\) & \(-10.64\) & \(-2.29\) & \\
3 & 7 & \(-76.21\) & \(-98.54\) & \(-57.55\) & \(+22.33\) & \(-18.66\) & \\
4 & 11 & \(-64.83\) & \(-51.49\) & \(-58.24\) & \(-13.34\) & \(-6.59\) & \\ \hline
6 & 13 & \(-73.57\) & \(-61.13\) & \(-63.15\) & \(-12.44\) & \(-10.42\) & \\
6 & 1 & \(-69.79\) & \(-123.16\) & \(-63.18\) & \(+53.37\) & \(-6.61\) & \\
6 & 5 & \(-71.46\) & \(-91.03\) & \(-63.28\) & \(+19.57\) & \(-8.18\) & \\ \hline
7 & 2 & \(-74.13\) & \(-107.71\) & \(-59.6\) & \(+33.58\) & \(-14.53\) & \\ \hline \hline  & & & & & & \\ \end{tabular} NOTE: the sign (—) in the columns “Measurement-theory difference...” means that the model shows a higher signal level than the measured value (i.e., the model is pessimistic); the sign (+) means the opposite situation (optimistic model). * The signals reflected from the turbines 1, 5, and 13 are only 120 ns apart, i.e., less than half the pulse duration (250 ns). Thus, peak 6 probably represents a situation where the signals reflected from these three turbines overlapped.  
\end{table} 
Table 4: Comparison of the measured and theoretical values of the power of the direct signal (FSR) and the reflected signal (UFSR)—scenario 1.  
course of the simulation, the simplest propagation model (free space loss) was used to determine the FSR parameter, so the calculated attenuation value is much lower than it would appear in real conditions. The above conclusion, combined with the lack of precise information on the transmitting antenna's back radiation suppression, mean that the simulated values of the FSR parameter are roughly approximate, at best. Analyzing the results from Tables 4-6, we can see that the most pessimistic results (i.e., those where the theoretical level of the reflected signal was most overestimated in relation to the actually measured one) in the case of Annex 1 were obtained:  
In Scenario 1--for reflection from turbines 8, 11, and 13 (the calculated bias level of the model is equal to \(-10.64\) dB, \(-13.34\) dB, and \(-12.44\) dB, respectively);  
In Scenario 2--for reflection from turbines 11, 13, and 12 (the calculated bias level of the model is equal to \(-14.91\) dB, \(-13.59\) dB, and \(-20.86\) dB, respectively);  
In Scenario 3--for reflection from turbines 7 and 6 (the calculated bias level of the model is equal to \(-17.47\) dB and \(-17.28\) dB, respectively).