In the Finite Difference Time Domain (FDTD) method, a discretized form of Maxwellâs equations is solved numerically and simultaneously in both the 3D space and time. During this process, the electric and magnetic fields are computed everywhere in the computational domain and as a function of time starting at t = 0. From knowledge of the primary fields in space and time, one can compute other secondary quantities including frequency domain characteristics like scattering [[parameters]], input impedance, far field radiation patterns, radar cross section, etc.
A time domain simulation like FDTD offers several advantages over a frequency domain simulation. In certain applications, you may seek the time domain signature or behavior of a system. For example, the transient response of a circuit or an antenna might be of primary interest. In other applications, you may need to determine in the wideband frequency response of a system. In such cases, using a frequency domain technique, you have to run the simulation engine many times to adequately sample the specified frequency range. By contrast, using the FDTD method requires a single-run simulation. The temporal field data are transformed into the Fourier domain to obtain the wideband frequency response of the simulated system. Among other advantages of the FDTD method is its versatility in handling complex geometries and inhomogeneous material compositions as well as its superb numerical stability. It is worth noting that unlike frequency domain methods like the finite element method (FEM) or method of moments (MoM), the FDTD technique does not involve numerical solution of large ill-conditioned matrix equations that are often very sensitive to the mesh quality.
Like every numerical technique, the FDTD method has disadvantages, too. Adding the fourth dimension, time, to the computations increases the size of the numerical problem significantly. Unfortunately, this translates to both larger memory capacity requirements and longer computation times. Note that the field data are generated in both the 3D space and time. [[EM.Cube]]'s [[FDTD Module]] uses a staircase "Yee" mesh to discretize the physical structure. This works perfectly well for rectangular objects that are oriented along the three principal axes. Difficulties start to appear for highly curved structures or slanted surfaces and lines. As a result, the quantization effect might compromise the geometrical fidelity of your structure. [[EM.Cube]] provides a default adaptive FDTD mesher that can capture the fine details of geometric contours, slanted thin layers, surfaces, etc. to arbitrary precision. However, due to the stability criterion, smaller mesh cells lead to smaller time steps, hence longer computation times. Another disadvantage of the FDTD technique compared to naturally open-boundary methods like MoM is its finite-extent computational domain. This means that to model open boundary problems like radiation or scattering, absorbing boundary conditions are needed to dissipate the incident waves at the walls of the computational domain and prevent them from reflecting back into the domain. The accuracy of the FDTD simulation results depends on the quality of these absorbers and their distance from the actual physical structure. [[EM.Cube]]'s [[FDTD Module]] provides high quality perfectly match layer (PML) terminations at the boundaries, which can be placed only a quarter wavelength or less from your physical structure.