The 3D Berenger's and uniaxial perfectly matched layers used for the truncation of the FDTD computations are theoretically investigated respectively in the discrete space, including numerical dispersion and impedance characteristics. Numerical dispersion for both PMLs is different from that of the FDTD equations in the normal medium due to the introduction of loss. The impedance in 3D homogeneous Berenger's PML medium is the same as that in the truncated normal medium even in the discrete space, however, the impedance in 3D homogenous UPML medium is different, but the discrepancy smoothly changes as the loss in the UPML medium slowly change. Those insights acquired can help to understand why both 3D PMLs can absorb the outgoing wave with arbitrary incidence, polarization, and frequency, but with different efficiency.
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