PML and Master Slave Boundary Conditions

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PML and Master Slave Boundary Conditions
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Advanced Boundary Conditions

• This section looks at two different types of
  advanced boundary conditions available in HFSS
• Perfectly Matched Layers (PMLs)
• Periodic boundary conditions Master and Slave
  boundaries.

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Perfectly Matched Layer (PML)

• What are Perfectly Matched Layers?
• Perfectly Matched Layers (PMLs) are fictitious
  materials that fully absorb the electromagnetic
  fields acting upon them.
• There are two types of PML applications free
  space termination and reflection-free
  termination of guided waves.
• In free space termination, all PML objects must
  be included in a surface that radiates into free
  space equally in every direction. PMLs can be
  superior to radiation boundaries in this case
  because PMLs enable radiation surfaces to be
  located closer to radiating objects, reducing the
  problem domain. Any homogenous isotropic
  material, including lossy materials such as ocean
  water, can surround the model.
• In reflection-free termination of guided waves,
  the structure continues uniformly to infinity.
  The termination surface of the structure radiates
  in the direction in which the wave is guided.
  Reflection-free PMLs are superior to free space
  or radiation boundary terminations in this kind
  of application. Reflection-free PMLs are also
  superior for simulating phased array antennas
  because the antenna radiates in a certain
  direction.

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Perfectly Matched Layer (PML)

• Implementation in HFSS
• HFSS uses an adaptive PML
• In classic implementation, one needs several
  layers, one over the other, to achieve desired
  attenuation.
• With adaptive PML, one layer is enough.
• Adaptive meshing takes care of the rest.
• Advantages
• Easier implementation.
• More robust.
• Smaller mesh.
• HFSS contains a PML setup wizard for
• Perfectly Matched Layer object creation.
• Material creation and assignment.
• PML boundaries can also be set-up manually.
• HFSS automatically identifies PML objects by a
  naming convention
• Any object with a name beginning with the letters
  PML is identified as a PML and is subject to
• special adaptive meshing.
• incident-wave treatment.
• user-defined radiation surfaces during post
  processing.

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Perfectly Matched Layer (PML)

• Radiation boundary versus PML
• Radiation Boundary Condition
• Sensitive to the incident angle, less accurate
  for non-normal incidence.
• Fully automatic.
• Easy to use.
• Radiation boundaries need to be placed around l/4
  away from radiating objects.
• Perfectly Matched Layer (PML)
• Accurate, boundary has zero reflection.
• A fictitious biaxial anisotropic material.
• Reasonably automatic to create using PML setup
  wizard.
• More accurate for calculating radiation
  parameters.
• PMLs can be brought much closer to radiating
  objects (as close as l/10), resulting in a
  smaller problem space and smaller mesh.

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Perfectly Matched Layer (PML)

• Automatic PML creation
• Three basic steps
• Create device objects.
• Select surfaces for PML objects to be created on.
  (note you may want to create a face list at this
  point for post processing later on. This can be
  done using 3D Modeler gt List gt Create gt Face
  List.)

Here radiation is allowed through three
faces.Two other faces will later be assigned
symmetry boundaries.
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Perfectly Matched Layer (PML)

• Automatic PML creation, continued.
• PML setup wizard has a two step process, firstly
  creating the PML cover objects

Specify layer thicknesses normally set this to
be ?/4 of the lowest frequency to be solved for
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Perfectly Matched Layer (PML)

• Automatic PML creation, continued.
• PML cover is added to the box on all radiation
  surfaces

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Perfectly Matched Layer (PML)

• Automatic PML creation, continued.
• Secondly defining the PML material properties
• Note for this problem the fields radiate equally
  in free space in all directions hence PML
  Objects Accept Free Radiation is selected.

Set minimum frequency to be solved in problem
Minimum radiating distance is the minimum
distance between the boundary and any radiating
object.
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Perfectly Matched Layer (PML)

• The PML setup wizard
• Automatically creates PML materials.
• Automatically calculates PML material matrices.
• PML material properties are automatically
  assigned to the cover objects using default
  names.

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Perfectly Matched Layer (PML)

• Open-ended waveguide results
• Magnitude of S11 of an open-ended waveguide.
• Note that the boundary PML is closer to aperture
  than the radiation boundary and requires fewer
  tetrahedra for better accuracy.

PML d/l0.15, 1782 tetrahedra ABC d/l0.32,
6736 tetrahedra

• Mesh of an open-ended waveguide
• The non-uniform PML mesh is evident.

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Perfectly Matched Layer (PML)

• Creating PMLs manually
• The PML setup wizard can only create rectangular
  PML objects. If another shape of PML object is
  required (e.g. to terminate a circular waveguide)
  then PMLs must be created manually

Draw the PML object at the radiation surface, and
then select it.
Give the object a name with the prefix PML.
Object names that start with PML are necessary
for HFSS to recognize them as PMLs.
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Perfectly Matched Layer (PML)

• Creating PMLs manually, cont.
• Launch the PML setup wizard.

Select use selected object as PML cover. Choose
the corresponding base object.
Enter the thickness of the PML layer
object. Select the orientation of the PML object
in terms of the direction of outward propagation
in this case radiation would be in the
y-direction.
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Perfectly Matched Layer (PML)

• Creating PMLs manually, cont
• As this is a waveguide termination, PML Objects
  Continue Guided Wave option is selected.
• The propagation constant at the minimum frequency
  must then be entered.
• The minimum radiating distance is specified as
  before.

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Perfectly Matched Layer (PML)

• Post processing Far Field data with PMLs
• To insert a far field setup where a PML has been
  used, you need to first create a face list of the
  radiating surfaces (note sometimes it is easier
  to create this when you first select the faces
  for creating the PML objects)

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Perfectly Matched Layer (PML)

• Post processing Far Field data with PMLs
• This list appears in the lists section of the
  model tree.
• When you create a far field radiation setup,
  under the Radiation Surface tab select Use
  Custom Radiation Surface and select this face
  list from the drop down menu.

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Master and Slave Boundary Conditions

• Master and Slave Boundaries
• Master and slave boundaries enable you to model
  planes of periodicity where the E-field on one
  surface matches the E-field on another to within
  a phase difference. They force the E-field at
  each point on the slave boundary match the
  E-field to within a phase difference at each
  corresponding point on the master boundary. They
  are useful for simulating devices such as
  infinite arrays.
• Unlike symmetry boundaries, E does not have to be
  tangential or normal to these boundaries. The
  only condition is that the fields on the two
  boundaries must have the same magnitude and
  direction (or the same magnitude and opposite
  directions).
• When creating matching boundaries, keep the
  following points in mind
• Master and slave boundaries can only be assigned
  to planar surfaces. These may be the faces of 2D
  or 3D objects.
• The geometry of the surface on one boundary must
  match the geometry on the surface of the other
  boundary. For example, if the master is a
  rectangular surface, the slave must be a
  rectangular surface of the same size.
• If the mesh on the master boundary does not match
  the mesh on the slave boundary exactly, the
  solution will fail. Normally HFSS automatically
  forces the mesh to match on each boundary
  however, in some cases, the mesh cannot be forced
  to match. To prevent the solution from failing,
  create a virtual object on the slave boundary
  that exactly matches any extra object on the
  master boundary, or create a virtual object on
  the master boundary that exactly matches any
  extra object on the slave boundary.

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Master and Slave Boundary Conditions

• Master and Slave Boundaries
• To make a surface a master or slave boundary, you
  must specify a coordinate system that defines the
  plane on which the selected surface exists. When
  HFSS attempts to match the two boundaries, the
  two coordinate systems must also match each
  other. If they do not, HFSS will transpose the
  slave boundary to match the master boundary. When
  doing this, the surface to which the slave
  boundary is assigned is also transposed. If,
  after doing this, the two surfaces do not occupy
  the same position relative to their combined
  defined coordinate system, an error message
  appears.
• For example, consider the following figure
• To match the coordinate system of the master
  boundary, the coordinate system on the slave
  boundary must rotate 90 degrees counterclockwise
  however, when this is done, you get the
  following
• The two surfaces do not correspond and thus the
  mesh will not match, causing an error message.
• The angle between the axes defined by the u point
  and v point must be identical for the master and
  slave boundary.

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Master and Slave Boundary Conditions

• Assigning Master boundary

Ensure plane is set to that of desired face
Select face for master boundary and launch master
boundary assignment
Under Coordinate system select new vector to
assign U
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Master and Slave Boundary Conditions

• Assigning Master boundary

Note reverse direction changes orientation of V
with respect to U
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Master and Slave Boundary Conditions

• Assigning Slave boundary

Ensure plane is set to that of desired face
Select face for slave boundary and launch slave
boundary assignment
Select Master boundary slave is associated with
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Master and Slave Boundary Conditions

• Assigning Slave boundary

Define directions for U and V on slave boundary
remember these must match the Master boundary
assignment
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Master and Slave Boundary Conditions

• Assigning Slave boundary
• You have the option to relate the slave
  boundarys E-fields to the master boundarys
  E-fields in one of the following ways
• Select Scan Angles, and then enter the f scan
  angle in the Phi box and the q scan angle in the
  Theta box. The phase delay is calculated from the
  scan angles however, if you know the phase
  delay, you may enter it directly in the Phase
  Difference box below.
• Select Field Radiation, and then enter the phase
  difference, or phase delay, between the
  boundaries E-fields in the Phase Difference box.