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. Furthermore,
if the observation points are confined only in the upper hemisphere,
then the asymptotic solution for the radiated field due to the source
can be obtained by using the Reflection Coefficient Method (RCM)
or Norton's approximation.
In this study, 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. To illustrate the capability of the method,
the radiated fields due to the VHF/HF antenna on a helicopter model
are predicted. As for the VHF antenna, a 12.75-cm monopole mounted
on the NASA scaled helicopter model is investigated when its height
from different grounds is varied. In addition, 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 sea water
or a stratified ground.
Figure 1: Apache full-scale helicopter model with a 14 ft towel-bar antenna on the left hand side and a 24-ft inverted-L antenna on the right hand side both of which are used at the HF band.
Figure 2: Influence of soil type ground on the pitch-plane radiation pattern when a 12.75-cm VHF monopole is mounted on the NASA scale model.
Figure 3: The space and the surface wave
arising from a 14-ft towel-bar antenna on the Apache full-scale
model when the helicopter is flying over sea.
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