Terrengets påvirkning av jordbølgen på HF : måleresultater og analyse
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The two categories of HF operations are sky waves and ground waves. The focus in this report has been on ground wave, which consists of the direct wave, reflected wave and surface wave. The International Telecommunication Union (ITU) recommended ground wave prediction model for HF frequencies is GRWAVE. This model predicts the electrical field strength and transmission loss over a curved, homogeneous and smooth earth, and is not suitable for predictions for paths whose electrical properties are inhomogeneous and paths with irregular terrain. While some propagation tools are developed to overcome the problem of inhomogeneous ground there are no commercial tools that consider the topology between the transmitter and receiver other than diffraction. The first phase of measurements included calibration of test equipment and performing noise measurements in different surroundings. We found that especially (fast) trains made quite a lot of noise. In the second phase we measured the field strength over paths where the ground consists of either clay or sand to see if the field strength varies according to the difference in conductivity between the two paths. The terrain was rather flat so that the terrain effects could be neglected, and we saw a difference in field strength between the two paths. In the third phase we have measured the field strength over paths that have different topology. It seems that the terrain near the transmitter and receiver influence the field strength most. A factor called “clearance angle” seems to be relevant here. In the terrain profile between the transmitter and receiver this is the greatest angle at the position of the transmitter/receiver between the horizontal plane and a terrain obstacle.