# Wire-Grid and Sparse MoM Antennas: Past Evolution, Present Implementation, and Future Possibilities
## Why MoM?
### MoM Hybridization  
Each method is applied to the problem region for which it is best suited. The appropriate boundary conditions are applied at the borders between these regions. The method of boundary integral equations is used in combination with MoM to determine, based on the analysis of frequency-dependent lossless and lossy propagation, the characteristics of open multiconductor transmission lines in multilayer media [102]. A complete procedure for optimizing microwave and millimeter-wave planar circuits using MoM is presented in [103]. In this procedure, a new concept of electromagnetic optimization using MoM was proposed. It is based on the study of the smallest independent units of information, the so-called elementary invariant operands. None of the numerical methods are suitable for all required electromagnetic modeling problems. For example, MoM program codes are practically unsuitable for describing inhomogeneous nonlinear dielectrics. Finite element codes cannot effectively model large scattering problems. Multipole and diffraction-based approaches are not suitable for small complex geometries or problems that require accurate determination of the surface current. Unfortunately, there are tasks where all these features must be taken into account, for example, when evaluating radiation from a printed circuit board, and therefore, the analysis cannot be performed by any of these methods. There are some hyper-methods that result from combining MoM with other methods. One of the solutions is to combine two or more methods in one program code. Moreover, one of these methods is often MoM. For example, a hybrid method, which combines two numerical methods: the finite-difference formulation of alternating-direction implicit (ADI) in the time domain (ADI-FDTD) and the method of moments in the time domain (MoMTD), was proposed to simulate a short-pulse ground-penetrating radar (GPR) [104]. Another hybrid method based on the equivalence principle is presented in [105]. Additionally, the work describes the problem of antennas located on the ground. This problem has been split into two related equivalent problems: one for the antenna geometry and one for the ground geometry. The fields in each region can be modeled using the most appropriate numerical methods. FDM in the time domain was used for modeling the surroundings because it is well suited for modeling fields in inhomogeneous media, while MoM was used for antennas because it is well suited for modeling complex antennas in free space. The same hybrid approach (the combination of MoM and FDM in the time domain) was also applied to estimate the specific absorption ratio (SAR) of a rat inside a reverberation chamber (RC) [106]. This hybrid approach was an alternative method used to solve the problem of poor FDM convergence when analyzing the RC. Initially, RC with a dipole or helical antenna was numerically designed to operate at a frequency of 2 GHz. The MoM/FDM method was then used to calculate the average whole-body SAR (WBA-SAR) of a small animal inside the chamber case. The results were compared with those obtained using the FDM method to verify their accuracy. The combined MoM/FDM approach was also used to analyze the effects of millimeter waves on a rabbit's eye, using a dielectric lens antenna as the source of electromagnetic emission [107]. Mod was used and validated by comparing the calculated and measured electric fields emitted by the dielectric lens antenna. The hybrid method was then used to determine the specific absorption coefficient of the impact of millimeter waves on the eye of a rabbit placed in the focus of the antenna. In 2007, a highly efficient computational method based on a hybrid formulation of the physical optics moment method (MM-PO) combined with impedance matrix interpolation and dynamic adaptive frequency discretization was presented for broadband analysis of antennas radiating in the presence of conducting objects [108].