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 Advanced Computational Tools For Rotorcraft Electromagnetics Simulation can efficiently solve the environmental electromagnetic challenges presented by rotorcraft, including antenna coupling, interference, and radar visibility. By Martin Vogel and C. J. Reddy R otorcraft pose numerous challenges to design engineers, such as proper analysis of the antenna link budget, mitigation of antenna co-site interference, prevention of electromagnetic interference (EMI) with on-board electronic systems, and reduction of radar cross section (RCS). Although all aircraft pose such challenges, for rotorcraft, the rapid movement of the rotors compounds them. However, a wide range of advanced simulation tools can be applied to rotorcraft electromagnetics. These methods include full-wave solvers like Method of Moments (MoM), Finite Element Method (FEM), and Finite Difference Time Domain (FDTD). With MoM, a technique exists to perform the simulation for the main body of the helicopter once, and then to reuse the result for many positions of the blades and/ or locations of the antennas. Given that the computational cost of rigorous methods scales rapidly with the problem size, this technique is particularly valuable for rotorcraft. Other simulation methods to alleviate the problem of computational cost include the Multilevel Fast Multipole Method (MLFMM), Physical Optics (PO) hybridized with MoM/MLFMM, and Uniform Theory of Diffraction (UTD) hybridized with MoM and Ray-Launching Geometrical Optics (RL-GO). Through several examples, this paper addresses the suitability of these solvers using commercially available software, Altair's FEKO. 1,2 Antenna Placement Almost all antennas in real life operate in the presence of a surrounding structure — usually the platforms on which they are mounted or the devices in which they are embedded — which will influence significantly the radiation characteristics of antennas. Since it is expensive to measure the radiation characteristics of an antenna mounted on a rotorcraft, simulations can be used to model accurately the interaction of the antenna with the rotorcraft. While solvers like MoM, FEM, and FDTD can be used to design the antennas by themselves, solvers like MLFMM, PO, and UTD can be used for antenna placement analysis, depending on the size of the rotorcraft together with the antenna at the operating frequency. Figure 1 depicts an antenna one might use for communication. The geometry is not complicated, and the antenna pattern is almost a textbook example of a desired pattern, with a broad main lobe and a small back lobe. However, once this antenna has been placed on a helicopter (Figure 2), the pattern is almost unrecognizable. The entire structure of the helicopter interacts with the radiated fields and scatters them. 3,4 The design engineer will have to evaluate several candidate locations for the antenna and study the link budget to determine whether the system will have acceptable performance. MoM is well-suited for this kind of study because, in that method, the surrounding air doesn't add to the computational cost. But what about the other rigorous full-wave simulation methods, such as the FEM and the FDTD? Once a volume of surrounding air has been included in the model, these methods also will provide accurate results. They can be advantageous when the model has an excessive amount of complicated, nonmetallic geometrical detail. Like the MoM, Electronic Military & Defense Annual Resource, 6th Edition 8 Figure 1: Turnstile antenna and its free-space radiation pattern at 350 MHz