An investigation of near fields for shipboard antennas using the numerical electromagnetics code (NEC)

The numerical electromagnetics code (NEC) was used to evaluate the admittance, the average power gain, and the electric near and far fields of a monopole antenna ( lambda /5) mounted on a cubical box ( lambda /3 per side) over a perfectly conducting ground plane. Two models of the box, using surface patches and wire grids, were evaluated. The monopole was positioned at the center, the edge, and at a corner of the box top surface. NEC results were examined and compared with experimental data and with results from ´PATCH´ another independent electromagnetic modeling code. Contour and three-dimensional amplitude and phase plots of the near electric fields are presented. Radiation patterns were calculated to relate far-field and near-field behavior of the antenna. It is found that wire grid modeling gave essentially identical results to those using surface patches, attesting to the equivalence of the two numerical techniques in NEC (less than 2 dB differences).<>