# 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  & \multicolumn{3}{c}{**Measurement**} & \multicolumn{3}{c}{**Measured Coherence**} & \multicolumn{3}{c}{**Measured Coherence Bandwidth**} \\  & **Turbine** & **Considering** & **Bandwidth** [kHz] & **Hard** & **Hard** & **Hard** & **Hard** \\ \multicolumn{1}{c}{**The Peak**} & **Number from** & **the** & **Theory—** & **Theory—** & **Measurement—** & **Measurement—** \\ \multicolumn{1}{c}{**in Table 3**} & **Which the** & **Transmitting** & **Annex 1** & **Annex 2** & **Theory Annex 1** & **Theory Annex 2** & **Theory Annex 2** \\ \multicolumn{1}{c}{**in Table 3**} & **Signal Was** & **Antenna** & **[dBm]** & **[dBm]** & **Difference [dB]** & **Difference [dB]** & **Difference [dB]** \\  & **Reflected** & **Characteristics** & & & & & \\  & & & & Direct Component (FSR) & & & \\  & & & & & & & The theoretical value of the direct component \\ \multicolumn{1}{c}{7} & N/A (direct signal) & \(-\)49 & \(-\)15.3 & \(-\)15.3 & \(-\)33.7 & \(-\)33.7 & FSR does not depend on the adopted model \\  & & & & & & & (Annex1/ \\  & & & & & & & Annex2) \\ \multicolumn{1}{c}{} & \multicolumn{3}{c}{Reflected Components (UPSR)} & & \\ \hline
1 & 8 & \(-\)54.48 & \(-\)73.35 & \(-\)48.73 & \(+\)18.87 & \(-\)5.75 & \\ \hline
2 & 3 & \(-\)59.34 & \(-\)126.63 & \(-\)51.32 & \(+\)67.29 & \(-\)8.02 & * \\
2 & 9 & \(-\)61.24 & \(-\)108.12 & \(-\)52.59 & \(+\)46.88 & \(-\)8.65 & * \\ \hline
3 & 7 & \(-\)66.74 & \(-\)49.27 & \(-\)55.52 & \(-\)17.47 & \(-\)11.22 & \\
4 & 10 & \(-\)64.99 & \(-\)157.23 & \(-\)56.28 & \(+\)92.24 & \(-\)8.71 & \\
5 & 6 & \(-\)76.31 & \(-\)59.03 & \(-\)60.11 & \(-\)17.28 & \(-\)16.20 & \\
6 & 1 & \(-\)71.31 & \(-\)132.26 & \(-\)60.81 & \(+\)60.95 & \(-\)10.50 & \\ \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 3 and 9 are only 200 ns apart, hence less than the pulse duration (250 ns). Peak 2, therefore, represents the probable situation where the signals reflected from these two turbines overlapped.  
\end{table} 
Table 7: Comparison of theoretical and measured coherence bandwidths for all measurement scenarios. \begin{table}
\begin{tabular}{c c c c} \hline  & **Theoretical Coherence** & **Measured Coherence** & **Measured Coherence Bandwidth** **after Substituting the Theoretical** \\  & **Bandwidth** **[kHz]** & **Bandwidth** **[kHz]** & **FSR Value [kHz]** \\ \hline Scenario 1 & 632.452 & 105.356 & 2390.196 \\ Scenario 2 & 608.537 & 108.365 & 2760.748 \\ Scenario 3 & 1017.126 & 122.867 & 5083.339 \\ \hline \end{tabular}
\end{table} 
Table 6: Comparison of the measured and theoretical values of the power of the direct signal (FSR) and the reflected signal (UPSR)—scenario 3.  
scenarios are similar: the theoretical coherence bandwidth is approx. 600 kHz, while the value calculated on the basis of the measurements is approx. 100 kHz. However, as already mentioned above, the measurement of the transmitted signal component arriving via the direct path to the receiver was disrupted. For this reason, further calculations were made based on the theoretical FSR value and the obtained values were approx. 2500 kHz in this case. Thus, it can be concluded that the actual coherence bandwidth, calculated on the basis of the measurement data, is clearly wider than the theoretical value calculated on the basis of the ITU-R BT.1893-1 model, which means that the model is pessimistic in this respect. The same applies to the results obtained for scenario 3, which refers to the measurements made in the southern part of the Jasna wind farm consisting of 14 wind turbines with a height of 137 m. For this scenario, all values of the coherence bandwidth were higher than those in scenarios 1 and 2, but the relations between them confirm the previously formulated observation.