# Radio Channel Modelling for VHF System Operating in the Offshore Wind Farms Propagation Environment
## 2 Methods
### Itu-R BT.1893-1 Model
#### 2.1.1 Wind Turbine Impact Studies on Radiocommunication Systems--Model ITU-R BT.1893-1 Annex 1  
This recommendation generally concerns the impact of turbines on DVB-T digital television systems; however, as was proven through a measurement campaign organized back in 2015, this model is also applicable for radio communication systems operating in the VHF band. This model is facilitated by a mathematical description of the primary propagation mechanism at wind farm locations, i.e., the reflection of the radio signal from the wind turbine blades. The most important elements of the Annex 1 model are presented below. Please note that the verification measurement campaign results can be found in article [12].  
Let us now assume the arrangement of the transmitter, receiver, and wind turbine as depicted in Figure 1.  
Let us assume the distance between the receiver and the wind turbine is \(r\) (see Figure 1), the scattering coefficient, \(\rho\), which includes the free-space loss for the path between the turbine; and the receiver can be expressed as follows [11]:  
\[\rho=\frac{A}{\lambda r}g(\theta) \tag{1}\]  
where  
\[g(\theta)=sinc^{2}\bigg{[}\frac{S}{\lambda}(cos\theta-cos\theta_{0})\bigg{]} sin\theta \tag{2}\]  
and  
\[sinc(x)=\frac{sin(\pi\cdot x)}{\pi\cdot x}.\]  
\(A\)--total area of the turbine blades [m\({}^{2}\)];  
\(S\)--mean width of the blade [m];  
\(\lambda\)--radio signal wavelength [m];  
\(r\)--the distance between the wind turbine and the receiver [m];  
\(\theta\)--the angle between the receive direction and the plane of the rotor, i.e., the angle of the signal reflected (scattered) from the blades [\({}^{\circ}\)];  
\(\theta_{0}\)--the angle between the transmit direction and the plane of the rotor, i.e., the angle of the incident signal at the blades [\({}^{\circ}\)].  
The \(g(\theta)\) function has values in the range of [-1, 1].  
It might be stated that the coefficient \(\rho\) (1) represents the amount of the incident signal that will be reflected from the blades towards the receiver.  
The value of the \(\rho\) coefficient is at its maximum when the transmitter, receiver, and wind turbine are all in the same line and when, additionally, this line is perpendicular (normal) to the rotor's plane. In such a case [11]  
\[\rho=\rho_{max}=\frac{A}{\lambda r}. \tag{3}\]  
The FSWT (Field Strength at the Wind Turbine) parameter can be defined as the strength of the signal directly at the wind turbine location [11]:  
\[FSWT=EIRP-L_{l}. \tag{4}\]  
_EIRP_--equivalent isotropical radiated power of the transmitter [dBm];  
\(L_{l}\)--propagation loss (attenuation) on the path between the transmitter and wind turbine (length i) [dB].  
Figure 1: The arrangement of the transmitter, receiver, and wind turbine.  
If the length of the path between the receiver and wind turbine is \(r\), then the unwanted signal power (i.e., the power of the signal that propagates from the transmitter to the receiver due to reflection from the turbine blades) can be calculated as [11]  
\[UIFSR=FSWT+20\text{log}(\rho). \tag{5}\]  
The UFSR (Unwanted Field StRength) is the parameter which helps analyze the wind farm as the source of a secondary radiation.