Techniques
Co-Site Interference
Rotorcraft typically have more than one antenna, and the
existence of multiple antennas, co-located on the same
platform, may lead to interference. Analysis to ensure that
the communication systems operate as intended, without
interfering with each other, is similar to the previously
described approach. The MoM is suited for this kind of analysis
because the surrounding air doesn't add to the computational
cost, and the Numerical Green's Function enables the design
engineer to evaluate many antenna positions and rotor blade
positions without much additional cost.
Figure 4 shows an example where two blade antennas
have been placed on the helicopter. Blade antennas — a
kind of fin that is easy to add to an existing air frame without
causing much additional aerodynamic drag — are popular
on aircraft. As depicted in Figure 4, the installed antenna
operates around 130 MHz. The coupling between the two
antennas is, at most, -34 dB. Whether this is acceptable
depends on the rest of the system (filters, amplifiers, mixers)
and the specifications. If not, an antenna may have to be
moved or different filters used. In any case, the simulation
tool has solved the hardest part of the problem.
Electromagnetic Interference
Electronic systems inside the rotorcraft, too, may suffer
interference due to Electromagnetic Environmental Effects
(E3), (e.g., lightning, high-intensity radiated fields, or the
field transmitted by the helicopter's own antenna). Typically,
interference is picked up by electric cables connected to the
system since it is those cables that, due to their length, have
the ability to act as receiving antennas. A simulation tool with
a fully featured cable-analysis utility can be used to calculate
interference into the cables in the 3D environment caused
by external radiation, as well as radiation from the cables'
carrying of their own signals. The most useful solvers will
incorporate a wide range of cable cross sections, including
single conductor, coaxial cables, ribbon cables, twisted pair,
Electronic Military & Defense Annual Resource, 6th Edition
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Figure 5: Signal levels picked up by the cables inside the rotorcraft due to
radiation from the antenna
Figure 4: Blade antennas on a rotorcraft and the coupling between
them at different frequencies
Figure 3: Effect of rotor blade rotation on the antenna's performance