# Radio Channel Modelling for VHF System Operating in the Offshore Wind Farms Propagation Environment
## 1 Introduction
On the other hand, a radar-based theory regarding the so-called radar cross section (RCS) of a wind turbine is known from many years of research and can provide some insight into the analysis of the wind farm impact on radiocommunication systems. In its base form, though, it applies to much higher frequency ranges than the maritime VHF Band. In 2011, the ITU-R issued the first version of the BT.1893 recommendation [11] which provides the mathematical background for the assessment of impairments caused by land wind farms, but it is originally dedicated to the digital television systems operating in the UHF Band (300-3000 MHz). This recommendation was expanded in the following years (see below). To sum up, no official standard exists which would serve as a background for the analysis of the offshore wind farms on the systems operating in the maritime VHF. On the basis of the previous experience in the field, the authors of this paper formulated a hypothesis that possibly the above-mentioned ITU-R recommendation BT.1893 (theoretically dedicated to the UHF digital TV)--in connection with the RCS theory--could be \"extended\" to cover the maritime VHF systems as well. In order to verify this concept, we carried out dedicated measurement campaigns which will be discussed in the following paper. It should be noted that this \"hypothesis\" was based on substantial grounds rather than on a \"blind guess\". In both the ITU-R recommendation and RCS theory, the key phenomenon is the signal reflection from the turbines (its blades, mast, etc.). Our understanding is that even though the wavelength in the maritime VHF is greater in comparison to UHF, and particularly SHF (radars)--which theoretically should reduce the impact of the reflection and other applicable phenomena such as absorption or scattering--this is not really the case. The offshore wind farms are generally much bigger constructions that land-based counterparts--that applies to all their components: masts, blade lengths, or blade widths. To put it in the simplest terms, offshore farms represent a much greater reflecting \"surface\". Consequently, as far as the maritime VHF is concerned, not only do all the propagation effects relevant to land farms also occur on the offshore farms, they can be even more significant at sea. That was the main rationale behind our attempt to extend the theoretical models, originally created for land turbines and for higher frequencies, on the maritime VHF band. In the following sections, we will demonstrate our assumption was indeed right. At the same time, it should also be noted that if the frequency in question gets very low this observation will not hold any more. For example, in the medium frequency band the wavelength becomes comparable with the physical size of the turbine, which definitely makes the reflecting effect practically negligible in this case. The second part of the paper will be dedicated to yet another issue relevant to the offshore wind farm analysis. In order to more effectively plan and design critical VHF radio systems in the wind farm environment, there is a need to model a radio channel that will ensure the mapping of the impact of physical phenomena occurring in the real propagation environment. The authors of the article, on the basis of the measurement campaign results, proposed the parameters of the maritime radio channel model for wind turbines propagation environment, which aimed to 'faithfully reproduce' the impact of the phenomena relevant to the offshore wind farm areas. Moreover, the article describes the methodology adopted from the recommendation ITU-R BT.1893-1, which has been compared with the results obtained during the measurements. The realized measurement campaign has been discussed and the obtained results were presented in the context of modeling the radio channel for marine VHF systems operating in an offshore wind farm propagation environment.