The absorbing boundaries should act such that the incident fields and waves propagate through them without any back reflection.
In an open boundary problem like an antenna, some kind of absorbing boundary conditions such as a perfectly matched layer (PML) must be used to emulate the free space. In a shielded structure, all objects are enclosed within a perfect electric (or magnetic) conductor box. At the boundaries of the computational domain, proper boundary conditions must be enforced.
Since FDTD is a finite domain numerical technique, the computational domain of the problem must be truncated. 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. 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. In the Finite Difference Time Domain (FDTD) method, a discretized form of Maxwell’s equations is solved numerically and simultaneously in both the 3D space and time. 10 Time Domain Simulation of Periodic Structures.9 Normalization of Frequency-Domain Field Data.8 The Relationship Between Excitation Waveform and Frequency-Domain Characteristics.3 Why Does FDTD Need Domain Termination?.2 Differential Form of Maxwell's Equations & the Yee Cell.