problem of exitation and boundaries

pml boundaries

When I use a plane wave in free space to couple the energe into my model, how long should I choose the distance between the plane wave and the suface of my model? should be a quarter of wavelength or more? My model is EBG, the boundaries of the outspace of ebg should be set as master、slave or only radiation if I use HFSS? Should the face which is set as plane wave exitation be assigned as ”radiation" boundary? what is the different about "radiation" and "PML"?
Thanks for your time.

Added after 17 minutes:

when I use CST to simulate EBG structure, I use "unit cell" as the x,y directions' boundary, “open” as z direction's boundary. The exitation is from z direction, is set up at port "Zmax". Frequency domain solver is used to simulate. Method is “General purpose", Frequency range is from 300-450. Adapt.Freq. is 450. Excitation mode is TE00. broadband freq. sweep. But I found the results are different at high freq section when I change the distance between the exitation and pbg structure. why?

site:www.edaboard.com pml radiation boundary

As far as I know, for antenna pattern simulation, radiation boundary and PML boundary are interchangeable.

PML stands for Perfectly Matched Layer. We can grasp its mechanism by analogy with microwave absorbers.

Actually there's a substantial difference between radiation boundaries and PML. The numerical problem of meshing requires a finite volume to be used for Finite Element method. PML acts as an absorber, in order to avoid reflections at the domain edges. The reflections, in such a case, are due to discontinuities between the discretized domain/volume containing the structure and the non-discretized domain/volume. Using PML, this numerical reflections are minimized. Radiation is a boundary, used to indicate that wave can propagate through those directions towards infinity (free space). Here's a definition I found on a presentation, for HFSS:

Radiation Boundary vs. PML
-Recommended spacing for radiation boundary is
lambda0/4 while for PML, it is lambda0/6.
-In general, PML boundaries can be placed closer to the
structure
-Depending on the angle of incidence, some reflections
will exist for radiation boundaries
-PML boundaries have zero reflections
-Radiation boundaries is a boundary condition
--Definition is the 2nd order boundary condition that
approximates free-space
-PML is part of the solution space
-Definition is a set of “fictitious” biaxial anisotropic
material

An example of use of PML is at www.emtalk.com/tut_3.htm.

For characterizing an EBG unit cell, have a look at the attached file. It may be helpful. Another solution, with respect to the one you find in this file, is to enforce an incident plane wave and use E-H boundaries and a radiation boundary in place of the waveport (in HFSSv11 is possible to choose a radiation boundary with incident fields). Another way for characterizing a unit cell is to consider periodic boundaries (master-slave) and enforce the incident plane wave by correctly setting a floquet's port.

Hope this helps.

I.