# Radio Channel Modelling for VHF System Operating in the Offshore Wind Farms Propagation Environment
## 1 Introduction  
Renewable energy sources are developing much faster around the world than conventional energy sources, and among them the highest growth rate is wind energy. Currently, it provides approx. 7% of global electricity consumption, which makes it a world leader in green technologies. The latest statistics from the GWEC Global Wind Report for 2022 [1] show that the total power of all wind turbines in the world has now exceeded 837 GW. This corresponds to a growth rate of wind power production of 13% compared to 2020 (despite the overall economic disruption caused by the COVID-19 pandemic) and is a continuous upward trend compared to previous years. Such records in energy production by wind farms show a general development trend in the direction that ensures energy independence of countries and, thus, energy security. The current development of wind energy in the world takes place in two basic segments: onshore wind power and offshore wind power, where the latter direction has now become a sector with great development potential. The biggest advantages of the intensive development of offshore wind turbines include on the use of sea areas with higher wind speeds, resulting in higher efficiency; wind conditions at sea are more stable compared to onshore; the possibility of using turbines with higher power and, hence, sizes; and no controversy related to the installation of a power plant in the vicinity of inhabited areas. Turbines are being built further and further from the coast and they are getting bigger--both in terms of a single turbine, which has a higher mast, a larger blade span, a larger generator and, consequently, more power, and in terms of the size of wind farms with dozens or even hundreds of turbines. Offshore wind farms adopt larger and more clustered structures, which worsen the propagation conditions in suchenvironment. This creates the need to analyze and evaluate the impact of the emerging offshore turbines on critical radiocommunication systems. In the case of offshore wind farms, the most important frequency range in the context of such analysis is definitely the so-called maritime VHF band (156-174 MHz), because it is utilized by most of the safety- and navigation-related systems (including AIS [2; 3], GMDSS, VDES [4; 5; 6], and many other systems ensuring safety, security, and monitoring of shipping) operated by maritime administrations, navy, border guard, coastal guard, etc. (to a lesser extent, the medium frequency is also relevant to such analysis). For example, in Poland, any investor intending to build and operate an offshore wind farm on the Polish exclusive economic zone (EEZ) is obligated by law to submit an expert opinion discussing the possible impact of the planned farm on the critical maritime systems, and if any negative impact is anticipated, appropriate mitigation measures must be proposed. Such expert opinions must be submitted before any construction works even start and failure to do so means that the investor's application will be rejected. This obviously only applies to specific Polish conditions, but at the same time it also amplifies the significance of this issue. That being said, we have to make an observation that offshore wind farms' impact on the VHF systems is not an easy matter to analyze. Very few sources discuss it let alone provide some theoretical/mathematical background for such a task. Obviously there exist numerous papers regarding channel modeling in the VHF band (see, e.g., [3; 7; 8; 9; 10]), but they usually concentrate on the typical applications of the VHF band, including maritime, military, or aeronautical communication, but hardly ever address the issue of VHF in the context of the wind farms. Consequently, their results cannot be easily \"translated\" (if at all) into the scenario that will be discussed in this article.