Ground effects for VHF/HF antennas on helicopter airframes

Abstract

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 helicopter in the presence of a lossy ground. The equivalent sources of the radiation system are obtained by solving an FEM problem in conjunction with an absorbing 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 converted into a set of properly combined Hertzian dipoles referred to as the Sommerfeld problem. Instead of evaluating the Sommerfeld integral rigorously, Norton's approximation is used to improve the overall computational efficiency. The validation of this method is accomplished in two steps: first, the FEM is compared with the finite-difference time-domain method (FDTD) in the 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 analytic solutions. Finally, this technique is extended to analyze an antenna on a helicopter above a lossy ground.Index Terms-Ground effects, HF/VHF antennas, helicopter airframe.

Cômputo do campo elétrico irradiado por um canal de descarga atmosférica com ênfase no efeito do solo

“…Este fato é de fundamental importância, uma vez que à medida que a resistividade aumenta o solo distancia da aproximação de condutor elétrico perfeito. Tendo como objetivo a validação destes resultados buscou-se na literatura modelos condizentes com o sistema físico apresentado neste trabalho [9,13]. Dessa forma gerou-se a Figura 8 para comparação e uma maior análise de sensibilidade.…”