North Hoyle Windfarm Report
3 QinetiQ VHF communications 3.1 Overview The use of VHF communications within the maritime community is wide spread. It is used for both ship-to-shore and ship-ship communication. It is essential that such communications are free from interference induced by intermediary structures since they are used in emergencies. The theoretical results have shown that the shadow at VHF frequencies behind a wind turbine tower is relatively shallow and should not adversely affect the normal operations of any VHF communication system. The VHF trial was designed to assess the depth of shadow behind wind turbine and compare the trial results with those expected theoretically. The trial consisted of traversing a course that passed within 5m behind turbine 26 . A continuous communication to the receiver set up on the shore at Prestatyn was used. The track data along the course was recorded to provide an indication of when the vessel was in the turbine shadows, thus affecting the signal. The antenna and receiver set up at Prestatyn is shown in Figure 3-1. Link margins of 2dB, 3dB, 4dB and 5dB were employed to estimate the depth of shadow experienced. The link margin is the strength of the signal received above the noise level. In free space at a fixed range the link margin was found to be 17dB (i.e. the signal is 50 times stronger than the noise level). We added an attenuation of 16dB to reduce the link margin to 1dB above the noise level and this was used as the baseline for all the VHF tests. Figure 3-1: The VHF antenna and receiver set up at Prestatyn In free space, away from the wind turbines, to get the link margin of 1dB required an attenuation of 16dB to be added in series with the receiver antenna. The results from all the different link margins are plotted together in Figure 3-2 and in Figure 3-3. The first of these figures shows the courses taken by the vessel when a 2dB and 3dB link margin was being used. In each case the uncertainty in our measurement is 1dB. On the graphs, the loss of signal is represented by the sudden drop in northing on the track. This "drop" shows the point at which the VHF signal was lost. The projection of the turbine shadows are shown as thick black lines. In Figure 3-2 it can be seen that the shadows from turbines 26 and 21 have contributed to a loss in the VHF signal. It can also be noted in the figure that with a 2dB link margin there is a loss in the signal that occurs between the easting values of 301913m and 301942m. Similarly another loss, not attributable to any turbines exists around the easting value of 302075m. These are the result of interference from other sources, such as another broadcasts on the same VHF channel. Turbine 21 is approximately 500m from the path of the launch. At this distance behind a wind turbine the shadow predicted is approximately 2dB (at 150MHz). Considering that the uncertainty in the link margins is of the order of 1dB, our experimental results are in very good agreement with the predictive work undertaken previously [1]. Figure 3-2: Position of VHF signal loss relative to turbine positions with a 2dB and 3dB link margin Figure 3-3 shows the position at which a signal loss was observed when the link margins were 4dB and 5dB. Here the signal loss only occurs in the shadow of turbine 26. This is expected since the 2dB and 3dB link margin results (Figure 3-2) showed the shadow of turbine 21 at 500m to be only 2dB to 3dB. A further experiment to find the depth of shadow immediately behind a wind turbine was undertaken. This test involved adjusting the link margin when immediately behind a turbine in the shadow until the signal was regained. We found that the depth of shadow at this position behind a turbine was around 10dB. Figure 3-3: Position of VHF signal loss relative to turbine positions with a 4dB and 5dB link margin The shadows found experimentally agree with the theoretical results outlined in the original study [1]. The affects are small and will not effect the VHF systems used in the wind farm unless the link margin between the transmitter and receiver is very low. This will only occur at long range and other effects caused by other users on the VHF channel are likely to present a greater problem.
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