Techniques
Electronic Military & Defense Annual Resource, 6th Edition
9
the FEM is strong when a full scattering matrix is required
for a situation with more than a dozen antennas, while
the FDTD method can perform wide frequency sweeps
efficiently. We will continue to focus on the MoM, though,
because of its ability to handle rotor-blade modulation.
Rotor-Blade Modulation
The problem depicted in Figure 2 becomes worse when
trying to account for the movement of the rotor blades.
The antenna pattern shown, with its many maxima and
minima, is valid for only one snapshot of the rotor position.
Rotor movement will make the pattern undulate. Analysis
of the effect of rotor-blade position on the installed antenna
performance involves a nonchanging rotorcraft body (static
part) and a rotating set of rotor blades (dynamic part).
One can reduce the computational time significantly if the
solution of the static part can be reused when analyzing a
new rotor blade position. The Numerical Green's Function
(NGF) utility
5
in the MoM can serve this purpose. Although
the solution (the electrical currents and fields) of the static
part changes all the time when the blades rotate, the main
part of the "number crunching" is done only once and then
is reused. Thus, new full-model solutions for every blade
position are generated at a low cost (Figure 3). Note that the
movement of the blades changes the pattern by several dB.
In some directions, the difference easily exceeds 5 dB.
Figure 2: Turnstile antenna placed on a rotorcraft and the corresponding
radiation pattern
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