Electronic Military & Defense Annual Resource

6th Edition

Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.

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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 10 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

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