3.3.1   Perfectly Matched Layer (PML)

Perfectly Matched Layer (PML) absorbing boundary condition (ABC) allows for specifying a thickness of absorbing layer (in FDTD cells) and the conductivity profile. There is a choice from two available profiles: Parabolic and Exponential. Choosing one of them means that the conductivity of the absorbing layer varies as:

Parabolic:                  

Exponential:             

where A and B are set by the user, and x=(distance from PML edge)/(PML thickness).

Typically the PML thickness varies between 4 and 8 cells, but can be optionally changed if required.

In general the PML ABC is more accurate than the MUR with superabsorption ABC and its performance is less dependent on the angle of incidence. However, some general remarks regarding the practical use of PML ABC and pointing out its disadvantages should be made at this point:

·        The PML ABC penetrates towards the circuit (from the plane where it is formally defined into the meshed area), thus its thickness (determined in cells) must be taken into account when placing other elements i.e. NTF (Near-To-Far transformation surfaces), to avoid their overlapping and simulation inaccuracies.

·        The distance between the PML ABC (considering the above) and NTF surface must be at least 3 cells.

·        The user should also be aware that the PML ABC should be placed at sufficient distance from the radiating element so that the radiation fields dominate there. When the absorbing boundary is placed in the area of high density of the close (inductive) fields of the antenna, it can significantly change its radiation patterns. The minimum distance depends on application but typically half-wavelength is recommended.

·        The PML ABC can be used when transformation of near fields to far fields in homogeneous medium other than air is of interest (i.e. in biomedical applications). The software automatically adjusts PML parameters to the surrounding medium. 

·        The PML ABC is more time and memory consuming than MUR with superabsorption ABC.

·        The sides of PML ABC must coincide with the grid termination (extend over the entire side of the meshed area) or a metal wall. They cannot interact with other elements except for other PML walls. Otherwise the FDTD algorithm instability may occur. This practically limits the application of PML to the antenna problems (when it is used as an absorbing box surrounding the antenna) or for a reflectionless termination of a rectangular waveguide.

·        The PML ABC cannot be placed in a region of boundary-conforming cells of shapes other that rectangular.

The PML ABC is not enabled in QW-V2D package.