Methods of suppressing sky-wave radiation from vertical antennas over imperfect ground

Ground wave radar (GWR) employs a ground wave mode in the detection of ships and icebergs. Reflected radar signals from the ionosphere can interfere with the principal ground wave mode. If the field strength of the ground wave is less than that of the ionospheric reflected wave, the range of the radar may be limited to the height of the ionospheric layer. This reduces the radar´s maximum detection range. GWR antennas must be able to minimize both transmission and/or reception of signal levels in the vertical and near vertical directions. Radiation patterns of vertically polarized antennas create a null in the vertical direction which suits the applications of GWR. It is well known that increases in maximum gain and reduction in half power beam width in the radiation pattern of a test vertical whip monopole are possible by modifying the geometry. The present work compares radiation characteristics of a number of geometrical modifications to the test whip monopole. These modifications include variation of the monopole length and its height over ground as well as the size of the ground wire grid. The data are calculated using the numerical electromagnetics code (NEC-2) which is based on the moment method. The feasibility of achieving a custom design antenna for GWR applications is illustrated.