# Radio Channel Modelling for VHF System Operating in the Offshore Wind Farms Propagation Environment
## 2 Methods
### The Measurement Campaign  
For the purposes of the research conducted by authors, a measurement campaign was organized in October 2021. From the beginning it was clear that it would be implemented on an onshore farm (this is because, as of 2022, no offshore wind farm exist yet on the Polish waters). So, one of the criteria was to look for a farm where the turbines have the following characteristics: (a) Are as high as possible; (b) Have as large of a reflecting surface as possible. The idea was to provide a good approximation of the conditions prevailing in offshore wind farms (although it is clear that, as a rule, offshore wind farms use much larger turbines than the farms located on land). In the course of the analyzes, the Jasna wind farm located in the northeastern part of Poland, near the city of Malbork, was selected for the measurement campaign. In order to try to compare the levels of the measured signals with the mathematical models, it was necessary to obtain certain physical parameters of the wind turbines used in the Jasna wind farm. Table 1 summarizes that input data. \begin{table}
\begin{tabular}{c c c} \hline Wind farm & Jasna & \\ \hline Part of the farm & North & South \\ \hline Number and types/models of turbines & 25 Vestas turbines V126, & 14 Vestas turbines V126, \\ on the farm & 3.3 MW/3.45 MW & 3.3 MW/3.45 MW \\ \hline Turbine’s tower height [m] & 117 & 137 \\ Material (tower) & Steel & \\ Length of the blades [m] & & 61.7 \\ Max. Blade width [m] & & 4 \\ Area of a single blade [m\({}^{2}\)] & & 123.4 \\ Number of the blades & & 3 \\ Total area of the blades [m\({}^{2}\)] & & 370.2 \\ Material (blades) & GFRP (Glass Fiber Reinforced Polymer) \\ Average radius of the tower & & 3.4 \\ Turbines’ layout (arrangement) & Non-uniform \\ Speed range [rpm] & & 5.9–16.3 \\ Distance between the turbines [m] & & 450–550 \\ \hline \end{tabular}
\end{table} 
Table 1: Parameters of the Jasna wind turbines for which measurements were made (based on the data from [25; 26; 27] and the farm operator’s data). Two types of measurements were carried out as part of the research on radio channel modeling for systems operating in the VHF band in the propagation environment of wind farms:  
1. Measurements of the signal levels reflected from the rotors of wind turbines using the pulse signal;
2. Doppler spectra measurements using signal carrier transmission. 3.1 Measurements of the Signal Levels Reflected from the Rotors of Wind Turbines Using the Pulse Signal  
The purpose of this part of the measurements was primarily to obtain a power delay profile, which is one of the parameters modeling the radio channel, and to verify the applicability of the ITU-R BT.1893-1 model in Annex 1 and Annex 2 versions. During the measurements, two professional, calibrated measurement sets were used: transmitting and receiving. The transmitting set consisted of the following (see Figure 6a):  
* Signal generator (250 kHz-6 GHz): Agilent N5182A MXG Vector Signal Generator with the pulse modulation capability;
* Transmitting directional antenna: SIRIO WY155-3N (155-175 MHz);
* Power amplifier: Popek Elektronik type PEA02-2-50 with reflectometer;
* Two measurement cables: 3-m cable RG-214 N-N;
* Rubidium oscillator Quartzlock E80-GPS (+GPS antenna). * The receiving set comprised the following parts (see Figure 6b):
* Spectrum analyzer: Keysight FieldFox N9914A with the appropriate options;
* Receiving antenna: Emco Model 3121C (28 MHz-1 GHz);
* Measurement cable: 3-m cable Huber+Suhner ST18A/11N468/11N468/3000 MM;
* GNSS RTK Receiver GINTEC M1G2. Measurements were made for the frequency of 161.8375 MHz. The pulse parameters and the mutual distances between the transmitter, receiver, and the wind turbine were selected in such a way that it was possible to distinguish the reflected signals from individual wind turbines using the presented above measuring equipment.