Ground effects for VHF/HF antennas on helicopter airframes

In this paper, the finite element method (FEM) is used to predict the space and surface wave radiation patterns of VHF/HF antennas mounted on a helico pter in the presence of a lossy ground. The equivalent sources of the radia tion system are obtained by solving an FEM problem in conjunction with an a bsorbing. boundary condition (ABC) or an impedance boundary condition (IBC) , From the equivalent sources, the total radiated field is calculated using the equivalence principle and superposition; the original problem is conve rted into a set of properly combined Hertzian dipoles referred to as the So mmerfeld problem. instead of evaluating the Sommerfeld integral rigorously, Norton's approximation is used to improve the overall computational effici ency, The validation of this method is accomplished in two steps: first, th e FEM is compared with the finite-difference time-domain method (FDTD) in t he absence of a lossy ground; second, the Hertzian dipole problem is solved in the presence of a lossy ground and the results are compared with analyt ic solutions, Finally, this technique is extended to analyze an antenna on a helicopter above a lossy ground.