# 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  
During the measurement campaign, three measurements were carried out (on 26, 27, and 28 October 2021), the purpose of which was to determine the level of the signal reflected from the rotors of wind turbines at the receiving point--thanks to which it was possible to obtain the power delay profile needed to model the radio channel. The measurements concerned the frequency of 161.8375 MHz, which is related to the fact that it is a frequency in the range of the so-called the VHF marine band (156-174 MHz), to which this article applies. On 26 and 27 October, the measurements were carried out on the northern part of the Jasna wind farm (Figure 7a), whereas on 28 October research was conducted on the southern part (Figure 7b). The general methodology of measuring the level of the reflected signals realized with the use of the impulse signal was described in the previous section. At this point, it should only be remembered that its essence was the emission of a pulse with a duration of 250 ns, directed towards a specific turbine. Of course, the impulse reflected not only from the target turbine but from other turbines as well. The signals reflected from the farm were visualized on the spectrum analyzer screen (in time domain using zero span function) in the form of successive peaks with increasing delays in relation to the moment of receiving the direct signal. Knowing the exact locations of each wind turbine within the farm and the exact location of the transmitter, receiver, and the corresponding distances, we are able to calculate (using the formula for the speed of an electromagnetic wave) the exact delays after which the signal reflected from a specific turbine should reach the receiver. Consequently, by analyzing the delay values of successive peaks on the spectrum analyzer, we can determine with high probability from which specific turbine the given component of the received signal came from. With this knowledge, this scenario--for a specific turbine--was \"re-enacted\" in the simulation tool, in which the theoretical reflected signal level (UPSR) was determined for the ITU-R BT.1893-1 model in the Annex 1 and Annex 2 versions [11], and then these theoretical levels were compared with the reflected signal values that were actually measured. Figure 7 shows the location of the transmitter and receiver for each measurement scenario against the background of the turbines of the north and south Jasna farms. For the first scenario, the location of the transmitter and receiver was marked with Tx1 and Rx1, respectively. Analogous notation was used for the subsequent measurement scenarios. Wind turbines are marked with red dots. Arrows mark the direction in which the impulse was pointed at the turbine. Table 2 shows all distances at which measurements were made for the given scenarios. It includes the distances between the transmitter and receiver and their distances between the wind turbine at which the signal was aimed. The maximum theoretical UPSR was calculated for each case. The reflectance from Annex 1 is approx. 10 dB higher than the Annex 2 reflectance. It should be noted, though, the blades are made of GFRP material, which has been shown to reduce the reflection by approx. 9 dB. All this considered, the maximum theoretical UPSR levels are very similar for both Annexes. Table 3 shows the results of the time domain measurement, for which the maximum measured level of the UPSR reflected signal was obtained in comparison with the calculated theoretical power delay profile. In the case of the first presented scenario, it can be determined that the signal reflection could have occurred from at least six wind turbine rotors (peaks marked in the screenshot with numbers 1, 2, 3, 4, 6, and 7; the first peak marked No. 5 represents the direct signal, the level of which is denoted by the acronym FSR). In that scenario, the transmitter and the receiver were in line with the turbine No. 8.