Study Of Key Techniques Of TDPO And A Hybrid TDPO/FDTD Approach

The Time-Domain Physical-Optical (TDPO) approximation is developed for the analysis of transient scattering by electrically large PEC objects. The development is based on the inverse Fourier transformation of frequency domain surface-current density and physical optical expressions. The estimation of requested computer memory, sampling criterion of time discretization and the hidden surface removal algorithm of incident wave are discussed. The transient scattering and RCS in a wide band of typical target are computed. Numerical results show that TDPO method requires relatively small amounts of computer memory. It is an efficient algorithm for analyzing the wide band characteristic of electrically large objects.A parallel TDPO algorithm combined with MPI platform is presented. It is applied to overcome a drawback that too much time and prohibitive computation resources are needed for the analysis of electromagnetic characteristic of extra electrically large objects. The speedup factor and the efficiency for parallel TDPO are measured on a distributed computer cluster. Numerical results show that the speed up ratio is approximately equal to N, where N is the number of processors, illustrating the high efficiency and good performance of the parallel TDPO.The Time-Domain iterative Physical-Optical (TD-IPO) approach is presented for the analysis of transient scattering problems with coupling effects between the parts of electrically large PEC scatterer. In this approach, the TDPO approximation is used to obtain the original surface electric current on the scatterer. The surface electric current is also called the zeroth order TD-IPO surface current. The coupling effects are taken into account by iteratively computation and a modified TD-IPO surface current is obtained. A new surface current that extraordinary approaches the real current on the scatterer surface is obtained by adding the modified current to the original surface current. In this way, we can obtain more accurate results than that obtained by TDPO approximation.A time-domain hybrid approach that combines the finite-difference time-domain (FDTD) method with time domain Physical Optics (TDPO) is presented as well. The approach can be applied to the analysis of the backscattering problem of combinative objects including a Large-Size structure (LS) and a Small-Size structure (SS) with respect to the wavelength of interest. The TDPO and FDTD are taken to treat the scattering by LS and SS, separately. When dealing with the coupling of SS to LS, the near-to-near field extrapolation technique based on Kirchhoff's surface integral representation is used and a sequential transfer method is developed. According to the time domain calculation sequence in FDTD, the contribution of SS to LS is transferred directly to far zone observation point. The sequential transfer method has some advantages in high efficiency and small amounts of computer memory. For far zone back scattering, the influence of LS onto SS can be obtained by using the reciprocity theorem.The coupling and penetration of electromagnetic pulse with lapped joints at junction of conducting screens are simulated by the finite difference time domain (FDTD) method. For obtaining the far zone transmission field easily, the radiation of equivalent electromagnetic current sheets in a quarter-space is replaced by the radiation of equivalent electromagnetic current sheets and their images in free space based on the equivalence principle. This equivalent scheme avoids the difficulty of computing the Green's function with the boundaries of lapped joints. During extrapolating, sub-area extrapolation is applied which saves computing time. Finally, by analyzing the variation of coupling field, the coupling resonant characteristic of lapped joints is obtained.The near-to-near/far field transformation based on Kirchhoff's surface integral representation in FDTD simulations is presented. The calculation of any of field components depends only on the value of the same field component over the FDTD output boundary. The radiation field outside the shielding enclosure from square, rectangle aperture, multi aperture and the rule of EM wave with different polarized direction are described. The simulation results lead to some useful conclusions. The energy radiated from a square aperture is less than the one from other shape's aperture. The energy radiation will attain the maximum while the polarization of incident wave is parallel to the short side of a rectangular aperture. The energy radiated from the multi an aperture is less than that from a single aperture provided their apertures area being the same.