NEC, NECGS, and MININEC numerical models of LF top-hat monopole antennas

Abstract

The US Navy needs to increase the power handling capability of its current VLF and LF top-hat monopole antennas. This is most efficiently studied via numerical analysis of candidate antennas. Previous computer models based on the Numerical Electromagnetics Code (NEC-3) were shown to be unacceptable. The reasons for the inaccuracies were discovered to be NEC's inability to accurately model the effects of step changes of radius on adjacent portions of the structure. This thesis investigates two additional numerical models. The first uses the MININEC SYSTEM which has been shown to be accurate for stepped-radius antennas, and the second approximates a top-hat monopole antennas by a wire-cage structure. The latter employs NECGS, a special version of NEC-3 which efficiently uses radial symmetry. The performance parameters of interest which were calculated are static capacitance, resonant frequency, effective height, and radiation resistance. The results of these new models are compared to scale- model measurements and to the numerical results of the earlier NEC-3 study. The MININEC SYSTEM product acceptable values but is limited in the number of unknowns used to describe the model. NECGS results indicate that an acceptable wire-cage equivalency to a top-hat monopole may not exist