Analysis of HF antennas on a helicopter in the presence of a stratified ground

Analysis of antennas on a helicopter airframe including the surrounding environment, such as the ground, will provide much more reliable radiation characteristics for communication link budget predictions. In order to account for all reflections and diffractions from the airframe, as well as those from a stratified ground, a full-wave analysis should be used. Since the exact Green´s function for a stratified ground is available, the radiation problem of a helicopter above the ground reduces to a problem of finding the total equivalent source in the presence of the ground. In this paper, HF antennas, such as a 14-ft towel-bar and a 24-ft inverted-L on the AH-64 (Apache) full-scale helicopter, are analyzed in the presence of a stratified ground. In particular, the ground influence on the radiation pattern is investigated using the finite element method (FEM) in conjunction with Norton´s approximation. Therefore, the radiated field contains not only the space wave but also the surface wave which is important for a short observation distance. The lossy ground is defined by its relative permittivity and electric conductivity.