# 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
Figure 7: Location of the transmitter (Tx) and receiver (Rx) on a given measurement scenarios (1, 2 and 3) in the back-ground of the north (**a**) and south (**b**) Jasna farms. The orange points indicate wind turbines, while arrows indicate the direction of signal transmission. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multirow{2}{*}{**Tx-Rx [m]**} & \multirow{2}{*}{**Wind**} & \multirow{2}{*}{**Wind**} & **FSR** & **Max. Theoretical** & **Max. Theoretical** \\  & & **Turbine-Tx [m]** & **Turbine-Rx [m]** & **(Theoretical** & **UFSR (Annex 1)** & **UFSR (Annex 2)** \\  & & **Value [dBm]** & **[dBm]** & **[dBm]** \\ \hline Scenario 1 & 650 & 550 & 1150 & \(-19.5\) & \(-50.38\) & \(-51.15\) \\ Scenario 2 & 650 & 750 & 1400 & \(-19.5\) & \(-54.78\) & \(-55.55\) \\ Scenario 3 & 400 & 1050 & 1450 & \(-15.3\) & \(-58.01\) & \(-58.78\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Distances between transmitter–receiver–wind turbine and theoretical signal strength parameters for all measurement scenarios (see Figure 7). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multirow{2}{*}{**Measurement**} & \multicolumn{2}{c}{**Model**} \\ \hline  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Pulse signal in the time domain in the 161 MHz band (north Jasna farm, 26 October 2021 (Scenario 1); north Jasna farm, 27 October 2021 (Scenario 2); and south Jasna, 26 October 2021 (Scenario 3)). Generally, the first \"peak\" in the diagram on the left is a direct signal and, in order to determine the delay, it is necessary to calculate the difference between the reflected signal and the first pulse and then compare this value with the second diagram (the direct signal is at point 0). For example, the first reflected signal in scenario 1 is delayed by \(7.05-5.8=1.25\) us, which is in line with the theory. As can be seen, for other scenarios, the measured delays are also consistent with theoretical values. Table 4 compares the measured results with the theoretical values derived from the ITU-R BT.1893-1 (Annex 1 and Annex 2 versions). In the second measurement scenario, the turbine at which the impulse was aimed was the turbine numbered 11, while in the third scenario of measurements there was the turbine numbered 7. The transmitter and the receiver were in line with the turbines. The measurement results compared with the theoretical values are shown successively for these scenarios in Tables 5 and 6. When analyzing the results of the individual measurement scenarios (Tables 4-6), it is worth noting, first of all, that on each of them the measured value of the direct signal was lower than it would appear from the models. More precisely, the difference between the measured value and the theoretical value of the FSR parameter for each of the three scenarios were \(-42.24\) dB, \(-36.1\) dB, and \(-33.7\) dB, respectively. The observation of the direct component (but not its power) on the spectrum analyzer screen served only to determine the time reference point against which the delays of individual reflected signals were measured and then assigned to specific turbines. Moreover, the terrain profile between the transmitter and receiver during the selection of the measurement scenarios was not optimized; on the transmitter \(>\) receiver route, there often were obstacles, which, combined with the low height of the antenna, resulted in a strong propagation attenuation. In the