Offshore Wind Farm Impact Assessment on Radio Systems Operating in the MF Band

Abstract

This paper discusses the impact of wind farms on systems operating in the MF band. The presented material has been gathered by the authors during several measurement campaigns they conducted in recent years on the premises of various land wind farms in northern Poland. The results of this research, however, are particularly important and relevant in the context of offshore wind farms, since several radio systems crucial for safety of navigation and life at sea operate in the MF band, most notably components of the GMDSS. At the time of writing, however, no OWFs existed yet at the Baltic Sea in the Polish Exclusive Economic Zone; therefore, it was not possible to conduct such measurements in the target area under maritime conditions. This article presents the theoretical backgrounds of the topic, and then introduces the results obtained for the lower part of the MF band (DGPS) and the upper part of this band. The results obtained in the latter case can be directly applied to the higher frequencies of the LF band and the lower frequencies of the HF band as well. The results show that the impact of wind farms (in the context of EMF, radio shadowing, and interference) on those bands is minimal or even negligible, which is an important and positive conclusion in the context of extensive development of OWFs in Polish waters scheduled for the next few years.

1. Introduction

In the era of climate change and inevitable energy transformation, wind energy is becoming one of the most important components of the global energy market due to its renewable nature and minimal environmental impact. The global installed capacity of wind energy has seen substantial growth. According to the Global Wind Energy Council (GWEC) 2024 report, global wind power capacity reached 1 terawatt (TW) in June 2023, with expectations to reach 2 TW in less than 7 years [1]. This growth is due to the expansion of both onshore and offshore wind farms, with the offshore playing an increasingly important role. Offshore wind energy has gained considerable momentum as a result of its higher capacity factors and the availability of vast, untapped wind resources in coastal areas. Offshore wind farms have been developed at an unprecedented rate, particularly in Europe and Asia. The European Union, for instance, has set ambitious targets for offshore wind capacity as part of its Green Deal, aiming to reach 60 GW by 2030 and 300 GW by 2050 [2].

Offshore Wind Farms (OWFs) have a huge potential, and intensive research is currently underway to launch such structures in the Baltic Sea. Since 2013, the National Institute of Telecommunications (NIT), the institution represented by the authors of this paper, has been carrying out expert opinions on the impact of OWFs on radio communication systems and radar systems. The NIT is the only entity in Poland that has taken or is actively involved in the implementation of these expert opinions for the investors preparing to build OWFs in Poland’s Exclusive Economic Zone (EEZ). The authors of this paper have already published several articles on this topic regarding the impact of offshore wind farms on radio systems operating in the VHF (Very High Frequency) band and on radar systems [3,4,5]. However, some radio systems utilized by Polish maritime administration and the Polish Navy operate in the MF (Medium Frequency) band as well, and future wind farms may disrupt their proper functioning. In particular, the MF systems included in the GMDSS are of key importance from a safety point of view [6,7]. Therefore, it is so important to learn about the mechanisms and scale of the mutual interaction between the OWFs and such systems. In particular, there is a lack of publications on measurement studies of radio systems in the wind farm environment. Due to the physical dimensions of the turbines and the importance of the MF band for ensuring safe navigation at sea, it was important to empirically verify whether wind farms affect MF radio systems. Modern turbines feature longer blades and higher hubs, allowing them to capture more wind energy at greater altitudes. Such structures, due to enormous dimensions of the turbines, their number, and the total area they occupy, may impact the radiocommunication in the MF band and consequently jeopardize the overall maritime safety. Turbine dimensions become comparable to the length of the radio waves for the high MF band. Wind farms are a source of many phenomena affecting radio systems, including secondary interference, radio shadowing, the Doppler effect, and EMF (electromagnetic field) radiation. It should be emphasized that our analyses focus on the effects in the far field, i.e., at distances above a few meters from the station for the MF band.

The structure of the article is as follows: the second section describes the methodology for calculating propagation loss in the MF band as well as the assumptions for the radio measurements carried out by the authors of this paper. The third section includes the description and results of the measurement campaigns conducted along with their analysis. The last part of the article contains a summary of the research carried out and plans to continue this research in the near future.

2. Methods

The MF frequency range, according to the division made by the ITU-R Radio Regulations [8], is between 300 kHz and 3 MHz. However, it should be noted here that the boundaries between the MF band and the adjacent LF (Low Frequency) and HF (High Frequency) bands are contractual. For example, some DGPS (Differential Global Positioning System) stations operate at frequencies above 300 kHz, and some operate below that limit. The radio wavelength for the MF band is between 100 m and 1 km. The frequency of radio signals directly affects the nature of its propagation in real conditions, especially in a multipath environment. As the size of obstacles increases in relation to the radio wavelength, the effects of reflection, diffraction, and scattering will be amplified, reducing the range of systems operating at higher frequencies [9]. Frequencies in the HF range can penetrate buildings, trees, and other similar obstacles relatively easily [10], whereas the VHF and UHF (Ultra High Frequency) bands have a greater tendency to diffract or reflect/scatter radio waves on such objects. The wavelengths in the MF and HF bands are comparable to or greater than wind turbines; therefore, the mechanism of reflection and deflection of radio waves as a result of interaction with the wind farm is much weaker than in the case of waves in the VHF and UHF range. Additionally, it should be noted that in the case of communication in the HF and MF bands, besides ground-wave propagation, we are also dealing with ionospheric (sky-wave) propagation [11,12], which is the basis of almost all HF communication beyond the horizon [10], as well as the possibility of formations of tropospheric ducts [11].

In the case of systems operating at higher ranges of the MF band (e.g., 2 MHz), signal reflection from the farm may theoretically occur, because in this case the length of the radio wave is 150 m, so it is comparable to the physical dimensions of the turbine. On the other hand, it should be remembered that the systems operating in that range are typically so narrowband that their bandwidth is much smaller than the coherence band of the channel. As a result, even if there were to be some fading caused by signal reflection (and interference), it will not be a frequency-selective phenomenon and will not significantly affect the operation of these systems.

The ITU-R P.368 model [13] can be used to determine the field strength of ground-based radio waves propagating in the frequency range of 10 kHz–30 MHz. In this range, the influence of terrain obstacles is negligible. However, the electrical parameters of radio paths are important in the propagation process. It should be emphasized that the ITU-R P.368 model should be used to determine the field strength only when it is known that the amplitudes of ionospheric reflections are negligible. This assumption is correct in the case of relatively small (for MF propagation) distances from the transmitter, which we maintained during the measurements described in the next section of the article. Moreover, the sky-wave effect mostly occurs at night, whereas our measurements took place during the day. Therefore, we can consider only the ground wave propagation.

Calculations of the field strength are complicated and require advanced mathematical apparatus. For this reason, reference [13] provides families of propagation curves that allow calculations of field intensity at a distance from the transmitting antenna, which will be valid for given electrical parameters of the radio path, specific frequency value, and for a nominal transmitter power of 1 kW where the radiating element is a short vertical monopole on the surface of a perfectly conducting ground. An example of a family of such field strength curves as a function of distance with frequency as an input parameter is shown in Figure 1. These curves can be used for calculations on a homogeneous radio path. If the path is electrically non-homogeneous, its homogeneous segments should be determined, as in Figure 2.

For the given frequency, the propagation curve corresponding to the distance section S₁ is selected and the field strength value E₁(d₁) in dB (μV/m) is read from the curve. The propagation curve for the distance section S₂ is then used to obtain the field strength values E₂(d₁) and E₂(d₁ + d₂) and, similarly, using the curve relevant to the section S₃, the values E₃(d₁ + d₂) and E₃(d₁ + d₂ + d₃) are read, etc. The field strength at the receiving point E_R is defined as follows:

$$E_R=E_1\left(d_1\right)\hspace{0.33em}-\hspace{0.33em}{E}_2\left(d_1\right)\hspace{0.33em}+\hspace{0.33em}{E}_2\left(d_1\hspace{0.33em}+\hspace{0.33em}{d}_2\right)\hspace{0.33em}-\hspace{0.33em}{E}_3\left(d_1\hspace{0.33em}+\hspace{0.33em}{d}_2\right)+E_3\left(d_1\hspace{0.33em}+\hspace{0.33em}{d}_2\hspace{0.33em}+\hspace{0.33em}{d}_3\right).$$

(1)

The above procedure is then reversed, with the transmitter being placed at the receiver’s location and vice versa. For such a scenario, the field strength at the receiver point E_T is defined as follows:

$$E_T=E_3\left(d_3\right)\hspace{0.33em}-\hspace{0.33em}{E}_2\left(d_3\right)\hspace{0.33em}+\hspace{0.33em}{E}_2\left(d_3\hspace{0.33em}+\hspace{0.33em}{d}_2\right)\hspace{0.33em}-\hspace{0.33em}{E}_1\left(d_3\hspace{0.33em}+\hspace{0.33em}{d}_2\right)+E_1\left(d_3\hspace{0.33em}+\hspace{0.33em}{d}_2\hspace{0.33em}+\hspace{0.33em}{d}_1\right).$$

(2)

The resultant field strength at the end of a radio path of length d = d₁ + d₂ + d₃ is equal to the following:

$$E=0.5\left(E_R+E_T\right).$$

(3)

Likewise, the field strength is determined for radio paths consisting of a different number of sections with different electrical parameters. The resultant field intensity can be easily converted to radio path loss using the following formula (taking into account the directivity of the typical reference antennas and relations between them contained in [14]):

$$L_b=139.3-E+20\mathit{log}f.$$

(4)

where

L_b—basic path loss [dB];

E—electric field strength [dB (μV/m)] for 1 kW ERP;

f—frequency [MHz].

The results of simulation calculations performed using the ITU-R P.368 model were compared with the measurement results, which are presented in the next section. The measurement methodology involved the following:

• Measurement of the signal level at a point near the wind farm in a location where it does not obscure the incoming MF signal.
• Measurement of the signal level at a point near the wind farm where the incoming signal is obscured by the wind farm.
• Calculation of propagation loss between these points.

The increased value of signal attenuation behind the wind farm, assuming a relatively small change in the distance to the MF signal transmitter, will result mainly from the influence of the wind farm. Figure 3 shows a diagram of the measurement set consisting of a spectrum analyzer and an antenna appropriate for the measured frequency range.

Essentially, our main goal was to determine if the turbines’ presence in the propagation path will result in an increased attenuation level (if this was the case, we could consider reflections or other negative effects caused by the turbines). For this purpose, we compared and analyzed differences between signal levels measured in front of and behind the wind farm with the values calculated using the ITU-R P368-10 model.

Furthermore, radio measurements in the MF band are costly and logistically challenging, as they require access to specialized infrastructure as well as formal permits to use particular frequencies for signal transmission. As a result, such measurements are not taken often in practice. Naturally, propagation measurements in the MF band have been performed by various researchers [15,16], but never in the wind farm environment. We had a rare opportunity to conduct this campaign with actual MF transmitters and antenna systems, but this would have been impossible without our cooperation with the Maritime Office in Gdynia who operates that equipment.

3. Results and Discussion

3.1. DGPS System Measurements

The DGPS system is one of the key systems utilized for maritime navigation. Additionally, it was one of the MF transmitters that was made available to us, for the purpose of measurements, by the Maritime Office in Gdynia. The DGPS system in Poland consists of two reference stations (RS) covering the entire coast: at Dziwnów and Rozewie (see Figure 4). The range of each of them is about 150 km.

Table 1. Locations and parameters of DGPS-PL reference stations.

Name	RS Dziwnów	RS Rozewie
Coordinates	54°01′20.56″ N	54°49′50.99″ N
	14°43′50″ E	18°20′05.2″ E
Frequency	283.5 kHz	301 kHz
Bandwidth	250 Hz	250 Hz
Transmitted power	23 dBW	23 dBW
Antenna height	24.3 m a.g.l.	27 m a.g.l.
Antenna gain	−7.83 dBi	−8.44 dBi

presents the coordinates, antenna heights, and the radio parameters of the DGPS-PL reference stations, which operate in the lower segment of the MF band (around 300 kHz). It can be assumed that the conclusions will also be valid for the higher frequencies of the LF band.

On 26–28 October 2021, at the Jasna onshore wind farm (near Malbork), authors of this paper carried out measurements of the DGPS signal level. The selected wind farm used the largest turbines in Poland at that time (both in terms of tower height and blade length), so by making such a selection, we “emulated” conditions close to those experienced at OWFs. The main purpose was to assess how the wind farm affects the signals of systems operating in the MF band.

The Jasna wind farm consists of two “sub-farms”:

• “North”—25 turbines with a tower height of 117 m,
• “South”—14 turbines with a tower height of 137 m.

All the turbines are Vestas V126, 3.3 MW/3.45 MW with a blade length of 61.7 m. The measurement set consisted of a Keysight FieldFox N9914A spectrum analyzer (Keysight, Santa Rosa, CA, USA) and an A.H. Systems AK-HFR-2 receiving antenna operating in the 20 kHz to 2 MHz band. Figure 5 shows the locations of all the points (designated R1–R7) where the DGPS signal level measurements were taken (for the transmitting station in Rozewie—also shown in the picture).

Figure 6 shows an example of DGPS signal level measurement for the Jasna wind farm. The RS Rozewie station transmits DGPS corrections (center of the graph) and CW (continuous wave) signals for the R-Mode (Ranging Mode) location system [12,17,18,19]. This result was obtained at the point north of the farm, where it did not obscure the station in Rozewie. Figure 7 shows the measurement of the DGPS signal taken in a location where the wind farm obscured the DGPS signal, and therefore could be the source of radio shadowing effects. The graphs show the spectra of the measured signals as well as the channel power measured in the 500 Hz band.

In order to determine the cause of the attenuation loss occurring on the propagation path with the wind turbine, simulations of DGPS signal attenuation at the Rozewie station were carried out. For this purpose, the GRWAVE tool [20] was used, available on the ITU (International Telecommunication Union) website, which is compliant with the ITU-R P.368 recommendation [13]. The input parameters for the GRWAVE simulator were determined based on the results of the range studies of the DGPS station in Rozewie conducted by the NIT and ITU-R Recommendation [21]:

• Frequency: 301 kHz,
• Transmitting antenna height: 23 m a.g.l.,
• Receiving antenna height: 2 m a.g.l.,
• Polarization: vertical,
• Soil conductivity: 0.01 S/m,
• Relative electrical permittivity of the soil: 30,
• Refractive index: 315 (N-Units).

Simulations were performed for all points where DGPS signal level measurements were taken. In order to obtain the most reliable attenuation results on the signal propagation path for the GRWAVE tool, the heights of the terrain at the antennas’ locations were appropriately corrected using the method explained below (Figure 8 and Equations (5)–(10)). H_tx/H_rx represents the height of the transmitter/receiver, while H_{t_tx}/H_{t_rx} represents the height of the terrain at the transmitter’s/receiver’s location.

In case of the following:

H_{t_tx} < H_{t_rx}

(5)

the height of the transmitting antenna, taking into account the height of the terrain, is as follows:

H_t = H_tx,

(6)

while the height of the receiving antenna, taking into account the height of the terrain, is as follows:

H_r = (H_rx + H_{t_rx}) − H_{t_tx},

(7)

In the case of the following:

H_{t_tx} > H_{t_rx}.

(8)

the height of the transmitting antenna, taking into account the height of the terrain, is as follows:

H_t = (H_tx + H_{t_tx}) − H_{t_rx},

(9)

while the height of the receiving antenna, taking into account the height of the terrain, is as follows:

H_r = H_rx.

(10)

The analysis of the data in

Table 2. Comparison of DGPS signal measurements and GRWAVE simulations.

Wind Farm	Measuring Point	Antenna	Antenna Height [m]	Terrain Height [m a.s.l.]	Antenna Height Including Terrain Height After Correction [m]	Distance of the Receiver from the Transmitting Station in Rozewie [km]	Signal Level from the Spectrum Analyzer [dBm]	GRWAVE Path Loss [dB]
Jasna—“North”	R1	Tx	23	49	Hn = 53.1	114	−89.46	66.23
		Rx	2	18.9	Ho = 2
	R2	Tx	23	49	Hn = 36.6	117.5	−84.47	66.6
		Rx	2	35.4	Ho = 2
	R3	Tx	23	49	Hn = 42.9	112.2	−88.78	66.06
		Rx	2	29.1	Ho = 2
	R4	Tx	23	49	Hn = 35.7	112.7	−87.33	66.13
		Rx	2	36.3	Ho = 2
Jasna—“South”	R5	Tx	23	49	Hn = 23	126.4	−92.14	67.48
		Rx	2	102	Ho = 55
	R6	Tx	23	49	Hn = 23	126.7	−92.96	67.53
		Rx	2	93.3	Ho = 46.3
	R7	Tx	23	49	Hn = 23	127.9	−92.54	67.61
		Rx	2	103.7	Ho = 56.2

shows that at the location where the wind farm obscured the signal from the station in Rozewie (i.e., point R2), a decrease in attenuation was recorded—i.e., the de facto signal level increased (by 2.86 dB) compared to the level at the unobstructed point (R4). On the other hand, using the GRWAVE tool, the theoretical attenuation at the obscured location should be 0.47 dB higher than at the unobstructed one. Therefore, it might appear that the wind farm does not suppress the DGPS signal but rather “amplifies” it. This observation can be interpreted that such a farm does not significantly affect the received DGPS signal level and does not cause radio shadowing effects.

It is worth noting that similar conclusions were made during the earlier measurement campaign carried out by the authors in 2013—on different Polish wind farms: at Lisewo and Gnieżdżewo [3].

To conclude, several additional explanations are required in relation to Table 2:

• The reduction in DGPS signal attenuation with distance from the DGPS transmitter did not occur in case of the Jasna “South” wind farm, because there was only a small increase in the distance between the transmitter and the measurement points. In addition, momentary propagation conditions and measurement error (the confidence interval for the measurements taken is 1 dB with a probability of 90% [22] in each and every location that the measurements were made) could have caused an apparent increase in the level of the measured signal.
• The simulated increase in attenuation was determined based on a certain mathematical model [13], which does not perfectly reflect real conditions in every situation—the apparent “amplification” of the signal by the wind farm is, therefore, probably caused by the imperfection of the model, on top of measurement and statistical error.
• The soil conductivity and relative electrical permittivity are not homogenous and may be slightly different for various paths. According to [13]—in many cases the difference in levels of a signal at nearby locations follows a log-normal distribution with a standard deviation within ±3–4 dB, averaging approximately 3.5 dB.

The observed discrepancies of the attenuation between simulations and measurements are very close to the acceptable error margin (±2 dB potential differences between the results from two different locations). Taking into account the standard deviation mentioned in the ITU-R P.368-10 model (±3–4 dB), our results are actually well within the acceptable error margin.

3.2. Measurements in the Upper MF Band

In May 2022, a measurement campaign was carried out by the authors on the farms near Ustka to examine the impact of wind farms on systems operating in the upper MF band. The measurements were made for signal frequencies below 3 MHz, but since this is the upper limit of the MF band, it can be assumed that the conclusions will be valid for the lower frequencies of the HF band as well.

Table 3. Parameters of the Ustka MF station.

Coordinates	54°34′42.39″ N
	16°48′27.60″ E
Antenna type	32 m Ground Plane type Double Vee
Frequencies	2714 kHz, 2182 kHz, 2187.5 kHz, 2174.5 kHz
Transmitter	SAIT CST3001
Transmitted power	35 dBW
Emission class	J3E, F1B

contains the parameters of the measured station, and Figure 9 presents an MF transmitting antenna. It should be emphasized that the presented measurement campaign would not have been possible without cooperation with the Maritime Office in Gdynia, which made available its MF radio infrastructure for the purpose of this research.

Due to the specific emission class with a suppressed carrier, only a 1 kHz signal was transmitted instead of the standard voice transmission, and the receiving side was offset by 1 kHz in relation to the carrier frequency from Ustka. The measurement set consisted of a Keysight FieldFox N9914A spectrum analyzer and an A.H. Systems AK-HFR-3 receiving antenna intended for the 1 MHz to 10 MHz band. The analysis of the obtained results is presented below.

The first measurement scenario was carried out in the Wrzeście wind farm, which is part of the Potęgowo Wschód wind farm. This farm consists of six GE 2.75–120 2.75 MW turbines with a height of 110 m and a blade length of 57.7 m. Figure 10 shows the locations of all the points where signal level measurements were taken (the transmitting station of this system is located in Ustka). The symbols P3 and P4 mark the places where measurements were taken in front of and behind the farm.

Figure 11 shows the signal level measurement for the Wrzeście wind farm. This result was obtained at the P3 measurement point in front of the farm, where it did not obscure the station in Ustka.

On the other hand, Figure 12 shows the signal level measurement at the P4 point, where the signal from the station in Ustka was obscured by the farm.

The second measurement scenario was conducted within the Wojciechowo wind farm, which consists of 30 Alstom ECO 110 3.15 MW turbines with a height of 90 m and a blade length of 53.2 m. Figure 13 shows points P1 and P2, in front of and behind the farm, where signal level measurements were performed (the transmitting station of this system is also located in Ustka).

Figure 14 shows the signal level measurement for the Wojciechowo wind farm. This result was obtained at the P1 measurement point in front of the farm, i.e., in a situation where it did not obscure the station in Ustka.

Figure 15 shows the signal level measurement in a situation where the wind farm obscured the signal from the station in Ustka—at the P2 measurement point.

In order to determine the cause of the decrease in attenuation occurring on the propagation route with the wind turbine in the first scenario, simulations of the attenuation of the signal from station in Ustka were carried out. Again, the GRWAVE tool [20] was used for this purpose. Simulations were performed for points P1–P4 where signal level measurements were taken. The required input parameters for the GRWAVE simulator for the Ustka MF station are as follows [21]:

• Frequency: 2715 kHz,
• Transmitting antenna height: 32 m a.g.l.,
• Receiving antenna height: 2 m a.g.l.,
• Polarization: vertical,
• Soil conductivity: 0.01 S/m,
• Relative electrical permittivity of the soil: 30,
• Refractive index: 315 (N-Units).

Table 4. Comparison of upper MF signal measurements and GRWAVE simulations.

Wind Farm	Measuring Point	Antenna	Antenna Height [m]	Terrain Height [m a.s.l.]	Antenna Height Including Terrain Height After Correction [m]	Distance of the Receiver from the Transmitting Station in Ustka [km]	Signal Level from the Spectrum Analyzer [dBm]	GRWAVE Path Loss [dB]
Wojciechowo	P1	Tx	32	9	Hn = 32	59.5	−107.2	107.6
		Rx	2	78	Ho = 71
	P2	Tx	32	9	Hn = 32	62.3	−107.6	108.3
		Rx	2	98	Ho = 91
Wrzeście	P3	Tx	32	9	Hn = 32	18.8	−92.9	83.9
		Rx	2	61.8	Ho = 54.8
	P4	Tx	32	9	Hn = 32	20.3	−89.2	85.6
		Rx	2	71	Ho = 64

summarizes the measurement and simulation results and compares them. In order to obtain the most reliable attenuation results on the signal propagation path for the GRWAVE tool, the heights of the terrain on which the antennas were located were corrected. The heights of the transmitting and receiving antennas using the terrain height corrections were calculated in the same way as in the previous paragraph.

The above calculations show that for the measurement scenario at the Wojciechowo wind farm, an increase in attenuation of 0.4 dB between points P2 and P1 was recorded due to the farm obscuring the signal from the station in Ustka. On the other hand, for the simulation in the GRWAVE tool, an increase in attenuation of 0.7 dB can be observed for these points.

In the case of the measurement scenario for the wind farm in Wrzeście, a signal attenuation of 3.7 dB was recorded between points P4 and P3 due to the presence of the farm on the propagation route from the Ustka MF station. On the other hand, for the simulation in the GRWAVE tool, an increase in attenuation of 1.7 dB can be observed due to a longer propagation route. The above results would seem to indicate that the wind farm does not suppress the upper MF signal, but rather amplifies it. This is fully consistent with the conclusions made earlier in Section 3 of this paper (GMDSS—see Table 2) and with the analogous measurement campaign conducted by the authors in 2013 on the premises of other Polish wind farms in Lisewo and Gnieżdżewo [3]. In reality, the results presented in Table 4 can be interpreted that a wind farm does not significantly affect the received upper MF signal level and does not cause radio shadowing effects. There are a few additional conclusions to be made regarding that table:

• The apparent increase in the signal level measured at the Wrzeście wind farm could be caused by a momentary signal enhancement, which may be caused by the formation of ducts for small percentages of time [23] and measurement error (the confidence interval for the measurements taken is 1 dB with a probability of 90% [22] in each and every location the measurements were made).
• The difference in soil conductivity values at different signal level measurement locations could have influenced the apparent increase in the signal level behind the wind farm. According to [13]—in many cases the difference in levels of a signal at nearby locations follows a log-normal distribution with a standard deviation within ±3–4 dB, averaging approximately 3.5 dB.
• The simulated increase in attenuation was determined based on a mathematical model [13], which does not reflect real conditions perfectly in every situation—the apparent signal amplification by the wind farm is, therefore, probably caused by the imperfection of the model and measurement and statistical error.

The observed discrepancies of the attenuation between simulations and measurements are well within the acceptable error margin (±2 dB potential differences between the results from two different locations).

4. Conclusions

Measurements, simulations, and analytical studies conducted at the National Institute of Telecommunications for over 10 years indicate that wind farms do not significantly affect systems operating in the MF band. This is confirmed by many factors. The first one is the fact that radio wavelengths are considerable in relation to the size of the wind turbines; therefore, the mechanisms of reflection and deflection of radio waves due to interaction with a wind farm are much weaker than in the VHF and UHF bands [3,4]; in fact—as shown in this article—they are mostly negligible. There are a few important things to mention here:

• in the case of communications in the HF and MF bands, besides ground waves, we are also dealing with ionospheric propagation, which is the basis of almost all HF communication beyond the horizon, as well as the possibility of the formation of tropospheric ducts.
• the narrow band of the radio systems discussed in this article is much smaller than the coherence bandwidth of the channel. As a result, even if there were to be fading due to signal reflection (and interference), this fading would not be frequency selective and would not significantly affect the performance of these systems.

Additionally, in parallel with the research described in this article, we conducted measurements of the electromagnetic field strength in the vicinity of wind turbines and the transformer station in the frequency range from 20 kHz to 250 MHz. To achieve this, Narda SRM-3006 EMF (Narda, Hanover, Germany) measuring equipment was employed. The results of that campaign did not show any increased levels of generated EMFs. All measured values were several orders of magnitude below the relevant safety limits contained in ICNIRP guidelines [24]. For example, the highest recorded measurement for a frequency of 13.557 MHz was 104.37 mV/m, while the reference level for exposure for the general public (averaged over 30 min and the whole body) for this frequency is 48.37 V/m.

Based on the collected EMF measurement results, several frequency ranges were selected for further analysis. The bands allocated for broadcasting services were excluded: the range of 87.5–108 MHz (radio broadcasting) and the frequency of 184.5 MHz (DVB-T), as well as the frequency of 225 kHz (AM radio). During the analysis of the selected data, no signal was identified for which the electric field intensity would decrease as the distance from the wind turbine rotor increased. In conclusion, the values obtained by the authors of this paper were several orders of magnitude lower than the limits legally allowed in Poland. Furthermore, we did not identify any significant radio signals that could have been generated by the wind farm or its equipment (such as the transformer station, etc.). Taking into account the above, and the fact that for the MF band the mechanism of radio waves reflection from wind turbines is negligible or does not degrade the operation of systems, it should be stated that the impact of interference, both primary—generated EMF (where the wind farm is the direct source of the signal) and secondary—due to reflection (where the wind farm is the source of the reflected signal), introduced by the wind farms on the bands in question will also be negligible. This applies to all types of farms: those operating on land as well as at sea (OWFs).

For the purposes of analyzing the possibility of radio shadowing being caused by the wind farms’ presence on the propagation route of radio signals of MF systems, measurements were carried out for both ends of the band: the lower part of the band (DGPS—301 kHz) as well as its upper range—2715 kHz. In both cases, analyses were made for two scenarios: (a) the wind farm obscuring the direction of arrival of the MF signal and (b) the wind farm not obscuring the MF signal propagation path. The analyses were also supported by simulation data obtained in the GRWAVE tool, which were used to model the propagation attenuation. The results collected during measurements near the Jasna, Wrzeście, and Wojciechowo wind farms clearly showed that the wind farm does not significantly affect systems operating in the lower and upper MF bands. Similar conclusions and observations were made in earlier measurement campaigns carried out by the authors in 2013 and 2021. Therefore, it can be concluded that wind farms will not cause radio shadowing for radio systems operating in the entire MF band, most likely including wireless stations utilizing the higher LF as well as lower HF range of frequencies. It should be emphasized that maritime conditions may somehow affect MF radiowave propagation, but our analyses indicate that the presence of wind turbines on the propagation path has very little effect on signal attenuation in that band. Consequently, we conclude there is no need to develop a novel, mathematical model of MF signal reflection from the turbines, as this effect is either negligible or even non-existent.

It should be emphasized that the authors are also researching the impact of offshore wind farms (those deployed at sea) on future radio systems, which is important, because in the next few years such structures will be built in the waters of the Baltic Sea (in Poland’s EEZ). The next stage of the research will be measurement verification at sea, which will allow for the refinement of mathematical models enabling the determination of the actual impact of OWFs on radio communication systems. The same transmitters (i.e., Rozewie and Ustka stations) will be utilized for the planned research, while the receiving station will be installed on a vessel that will move around the wind farm at a speed sufficient to collect at least 50 measurements over distance of 40λ [22] for each measurement location (especially in front of and behind the farm). We plan to exploit two different antenna heights during the measurements, reflecting large and small vessels. In addition, the National Institute of Telecommunications is working on the development and standardization of VDES [25] (VHF Data Exchange System) and R-Mode [17,18] (Ranging-Mode) systems, including the aspects of MF band propagation. The authors’ intention is, therefore, to conduct thorough studies on the impact of OWFs on these systems, in particular the delays they introduce in the context of ranging and locating ships at sea. That specific issue is addressed in another project the NIT is currently involved in, which is the Ormobass project (Operational R-Mode Baltic Sea System for Robust Maritime Traffic and New Maritime Applications), co-financed under the EU Interreg Baltic Sea Region Programme [26].