Transcript Boundary

Getting Started: Ansoft
HFSS 8.0
Section 6: Boundary Module
5-1
Synopsis

General Overview




Boundary Types, Definitions, and Parameters
Source Types, Definitions, and Parameters
Interface Layout
Assigning Boundaries




Boundary Setup Exercise Part 1: Define Boundaries in
Example Model


Face Selection
Precedence
Assumptions (the ‘outer’ Boundary)
Details of Port Definition and Creation
 Size and Position
 Mode Count
 Degenerate Modes
 Calibration, Impedance, and Polarization
 Gap Source Ports
Boundary Setup Exercise Part 2: Add ports to Example Model
5-2
HFSS Boundary List






Perfect E and Perfect H/Natural
 Ideal Electrically or Magnetically Conducting Boundaries
 ‘Natural’ denotes Perfect E ‘cancellation’ behavior
Finite Conductivity
 Lossy Electrically Conducting Boundary, with user-provided
conductivity and permeability
Impedance
 Used for simulating ‘thin film resistor’ materials, with user-provided
resistance and reactance in /
Radiation
 An ‘absorbing boundary condition,’ used at the periphery of a project in
which radiation is expected such as an antenna structure
Symmetry
 A boundary which enables modeling of only a sub-section of a
structure in which field symmetry behavior is assured.
 “Perfect E” and “Perfect H” subcategories
Master and Slave
 ‘Linked’ boundary conditions for unit-cell studies of infinitely replicating
geometry (e.g. an antenna array)
5-3
HFSS Boundary Descriptions: Perfect E and
Perfect H/Natural

 Parameters: None
E perpendicu lar

Perfect E is a perfect electrical conductor*


Perfect E Boundary*

E parallel

Perfect H is a perfect magnetic conductor

Perfect H Boundary

E continuous


‘Natural’ Boundary
*NOTE: When you define a solid object as a
‘perf_conductor’ in the Material Setup, a
Perfect E boundary condition is applied to its
exterior surfaces!!
Forces E-field perpendicular to the surface
Represent metal surfaces, ground planes,
ideal cavity walls, etc.
Forces H-field perpendicular to surface, Efield tangential
Does not exist in the real world, but
represents useful boundary constraint for
modeling
Natural denotes effect of Perfect H applied
on top of some other (e.g. Perfect E)
boundary


‘Deletes’ the Perfect E condition, permitting
but not requiring tangential electrical fields.
Opens a ‘hole’ in the Perfect E plane
5-4
HFSS Boundary Descriptions: Finite
Conductivity


E perpendicular , attenuating
Parameters: Conductivity and
Permeability

Finite Conductivity is a lossy
electrical conductor


Finite Conductivity Boundary


*NOTE: When you define a solid object
as a non-ideal metal (e.g. copper,
aluminum) in the Material Setup module,
and it is set to ‘Solve Surface’, a Finite
Conductivity boundary is automatically
applied to its exterior faces!!
E-field forced perpendicular, as with
Perfect E
However, surface impedance takes
into account resistive and reactive
surface losses
User inputs conductivity (in
siemens/meter) and relative
permeability (unitless)
Used for non-ideal conductor
analysis*
5-5
HFSS Boundary Descriptions: Impedance

Parameters: Resistance and
Reactance, ohms/square (/)

Impedance boundary is a direct, userdefined surface impedance


EXAMPLE: Resistor in Wilkenson Power Divider
Resistor is 3.5 mils long (in direction of flow) and
4 mils wide. Desired lumped value is 35 ohms.
3.5
 0.875
4
Rlumped
35
Rsheet 

 40  / square
N
.875

Use to represent thin film resistors
Use to represent reactive loads
 Reactance will NOT vary with
frequency, so does not represent
a lumped ‘capacitor’ or ‘inductor’
over a frequency band.
Calculate required impedance from
desired lumped value, width, and length

N

Length (in direction of current flow) 
Width = number of ‘squares’
Impedance per square = Desired
Lumped Impedance  number of
squares
5-6
HFSS Boundary Descriptions: Radiation

Parameters: None

Boundary is /4 away from
horn aperture in all directions.

Note boundary does not
follow ‘break’ at tail end
of horn. Doing so would
result in a convex
surface to interior
radiation.
A Radiation boundary is an absorbing
boundary condition, used to mimic
continued propagation beyond the
boundary plane
 Absorption is achieved via a secondorder impedance calculation
Boundary should be constructed correctly
for proper absorption
 Distance: For strong radiators (e.g.
antennas) no closer than /4 to any
structure. For weak radiators (e.g. a
bent circuit trace) no closer than /10
to any structure
 Orientation: The radiation boundary
absorbs best when incident energy
flow is normal to its surface
 Shape: The boundary must be
concave to all incident fields from
within the modeled space
5-7
HFSS Boundary Descriptions: Radiation,
cont.

Radiation boundary absorption profile
vs. incidence angle is shown at left

20
Reflection Coefficient (dB)
Refl ection Co effi cie nt (d B)
0
-20

-40
-60

-80
-100
0
10
20
30
40
50
60
70
80
90
theta (deg)

Reflection of Radiation Boundary in dB, vs.
Angle of Incidence relative to boundary
normal (i.e. for normal incidence,  = 0)
Note that absorption falls off
significantly as incidence exceeds 40
degrees from normal
Any incident energy not absorbed is
reflected back into the model,
altering the resulting field solution!
Implication: For steered-beam arrays,
the standard radiation boundary may
be insufficient for proper analysis.
Solution: Use a Perfectly Matched
Layer (PML) construction instead.

Incorporation of PMLs is covered in
the Advanced HFSS training course.
Details available upon request.
5-8
HFSS Boundary Descriptions: Symmetry
Conductive edges, 4 sides

Parameters: Type (Perfect E or Perfect H)


This rectangular waveguide contains a
symmetric propagating mode, which could
be modeled using half the volume
vertically....
Perfect E Symmetry (top)
...or horizontally.
Symmetry boundaries permit modeling of
only a fraction of the entire structure under
analysis
Two Symmetry Options:



Symmetry boundaries also have further
implications to the Boundary Manager and
Fields Post Processing


Perfect H Symmetry
(left side)
Perfect E : E-fields are perpendicular to the
symmetry surface
Perfect H : E-fields are tangential to the
symmetry surface
Existence of a Symmetry Boundary will
prompt ‘Port Impedance Multiplier’ verification
Existence of a symmetry boundary allows for
near- and far-field calculation of the ‘entire’
structure
5-9
HFSS Boundary Descriptions: Symmetry,
cont.

TE20 Mode in WR90

Geometric symmetry does not
necessarily imply field symmetry
for higher-order modes
Symmetry boundaries can act as
mode filters

Perfect E Symmetry (top)
Properly represented with
Perfect E Symmetry


Mode can not occur properly
with Perfect H Symmetry
As shown at left, the next higher
propagating waveguide mode is
not symmetric about the vertical
center plane of the waveguide
Therefore one symmetry case is
valid, while the other is not!
Implication: Use caution when
using symmetry to assure that real
behavior in the device is not filtered
out by your boundary conditions!!
Perfect H Symmetry
(right side)
5-10
HFSS Boundary Descriptions: Master/Slave
Boundaries
Perfectly Matched Layer
(top)

Parameters: Coordinate system,
master/slave pairing, and phasing

Master Boundary
Slave Boundary
Master and Slave boundaries are used
to model a unit cell of a repeating
structure


V-axis

Origin
WG Port
(bottom)

U-axis
Constraints:

Ground Plane
Unit Cell Model of End-Fire Waveguide Array
Also referred to as linked boundaries
Master and Slave boundaries are
always paired: one master to one slave
The fields on the slave surface are
constrained to be identical to those on
the master surface, with a phase shift.

The master and slave surfaces must be
of identical shapes and sizes
A coordinate system must be identified
on the master and slave boundary to
identify point-to-point correspondence
5-11
HFSS Source List

Port




Incident Wave




Used for RCS or Propagation Studies (e.g. Frequency-Selective
Surfaces)
Results must be post-processed in Fields Module; no S-parameters
can be provided
Applies to entire volume of modeled space
Voltage Drop or Current Source



Most Commonly Used Source. Its use results in S-parameter output
from HFSS.
Two Subcategories: ‘Standard’ Ports and ‘Gap Source’ Ports
Apply to Surface(s) of solids or to sheet objects
‘Ideal’ voltage or current excitations
Apply to Surface(s) of solids or to sheet objects
Magnetic Bias


Internal H Field Bias for nonreciprocal (ferrite) material problems
Applies to entire solid object representing ferrite material
5-12
HFSS Source Descriptions: Port
EXAMPLE STANDARD PORTS

Parameters: Mode Count, Calibration,
Impedance, Polarization, Imp. Multiplier

A port is an aperture through which
guided electromagnetic field energy is
injected into a 3D HFSS model. There
are two types:

EXAMPLE GAP-SOURCE PORTS

Standard Ports: The aperture is solved
using a 2D eigensolution which locates
all requested propagating modes
 Characteristic impedance is
calculated from the 2D solution
 Impedance and Calibration Lines
provide further control
Gap Source Ports: Approximated field
excitation is placed on the gap source
port surface
 Characteristic impedance is
provided by the user during setup
5-13
HFSS Source Descriptions: Incident Wave

Parameters: Poynting Vector, Efield Magnitude and Vector


Used for radar cross section (RCS)
scattering problems.
Defined by Poynting Vector
(direction of propagation) and Efield magnitude and orientation


In the above example, a plane incident wave is
directed at a solid made from dielectrics, to view
the resultant scattering fields.

Poynting and E-field vectors must
be orthogonal.
Multiple plane waves can be
created for the same project.
If no ‘ports’ are present in the
model, S-parameter output is not
provided

Analysis data obtained by postprocessing on the Fields using the
Field Calculator, or by generating
RCS Patterns
5-14
HFSS Source Descriptions: Voltage Drop and
Current Source
Example Current
Source (along trace
or across gap)

Parameters: Direction and Magnitude


Example Voltage
Drop (between
trace and ground)

A voltage drop would be used to
excite a voltage between two metal
structures (e.g. a trace and a ground)
A current source would be used to
excite a current along a trace, or
across a gap (e.g. across a slot
antenna)
Both are ‘ideal’ source excitations,
without impedance definitions


No S-Parameter Output
User applies condition to a 2D or 3D
object created in the geometry

Vector identifying the direction of the
voltage drop or the direction of the
current flow is also required
5-15
HFSS Source Descriptions: Magnetic Bias

Parameters: Magnitude and
Direction or Externally Provided

The magnetic bias source is used
only to provide internal biasing Hfield values for models containing
nonreciprocal (ferrite) materials.



Bias may be uniform field (enter
parameters directly in HFSS)...
 Parameters are direction and
magnitude of the field
...or bias may be non-uniform
(imported from external
Magnetostatic solution package)
 Ansoft’s 3D EM Field
Simulator provides this
analysis and output
Apply source to selected 3D solid
object (e.g. ferrite puck)
5-16
Sources/Boundaries and Eigenmode
Solutions


An Eigenmode solution is a direct solution of the resonant
modes of a closed structure
As a result, some of the sources and boundaries discussed so
far are not available for an Eigenmode project. These are:

All Excitation Sources:





Ports
Voltage Drop and Current Sources
Magnetic Bias
Incident Waves
The only unavailable boundary type is:

Radiation Boundary
 A Perfectly Matched Layer construction is possible as a
replacement
5-17
The HFSS Source/Boundary Setup Interface
Menu and Toolbar
Side Window
Coordinate Fields and
Snap Options
Pick Options
Controls selection options
in graphical window
Source/Boundary List
Shows all sources and
boundaries currently
assigned to the project
and their status; allows
selection for viewing,
editing, and deletion
Source/Boundary Control
Allows Naming, contains execution
controls (Assign, Clear, Units...)
Graphical View Window
Shows geometry, permits
point-and-click selection,
vector definition, and
assignment.
Source/Boundary Selection Buttons
Source/Boundary Drop-Down
Lists all source or boundary types,
based on radio button selected
Boundary Attributes Field
Region Layout changes to provide
entry fields for selected source or boundary
characteristics and options.
5-18
Boundary Manager: Object/Face Selection

2.

1.

NOTE: The same graphical view manipulation
shortcuts for rotation, panning, and zooming found
in the Draw module also work here; the visibility
icon also assists object/face selection by ‘hiding’
exterior objects.

The Graphical Pick options (1)
control the result of clicking in the
graphical view window.
 Object: mouse-click selects
exterior of entire object
 Face: mouse-click selects
closest face of object
 Boundary: mouse-click selects
closest existing boundary
condition (if any)
To shift your focus to an object or
face deeper into the model, use the
right mouse menu (2) choice Next
Behind, or the hotkey “N”
Selected faces will highlight in a grid
pattern; selected objects will have
their wireframe highlighted
Multiple faces may be selected
simultaneously; a second click
deselects already-selected faces
5-19
Boundary Manager: Object/Face Selection,
cont.

3.
4.

The Edit menu (3) provides further
Select options, including Faces
Intersection
 Faces intersection opens a list
box containing all objects in
the model
 Selecting two touching objects
from the list will prompt the
interface to automatically find
all intersecting faces
 Note: only exterior faces in
intersection are selected, not
faces of one object which are
inside the volume of the other
The Edit menu Select option By
Name (4) provides a list of all faces
in the model, numbered and sorted
by object, for selection.
5-20
Boundary Assignment: General Procedure


2. Select face(s)
5. New Boundary will
appear in list
1. Select source or boundary and type
4. Name and Assign
3. Fill in Parameters as necessary


Select Source or
Boundary radio button,
and desired type from
the drop-down listing
Select the face or faces
on which you wish to
apply the
source/boundary
condition
 (Above 2 steps
interchangeable)
Fill in any necessary
parameters for the
source/boundary
Name the
source/boundary, and
press the Assign button
5-21
Boundary Assignment: Precedence

Boundary assignments are
order dependent:


In the pictured example, the ‘radiation’
boundary overlays the orange rectangle
(on the back face) which was earlier
assigned as the port. Ports, however,
always take precedence, and show at the
bottom of the boundary listing.
Boundaries assigned later
supercede those
assigned earlier over any
shared surfaces
Ports are the exception;
they always supercede
any earlier or later
assignments
 Ports will sort to the
bottom of the
boundary list to
reflect this fact
 Boundaries can be
re-prioritized using
the Model menu
5-22
Boundary Assignment: Default Boundary

Any exterior face of the
modeled geometry not
given a user-defined
boundary condition is
assumed to be a Perfect E



Default boundary called
outer
Imagine entire model
buried in solid metal
unless you instruct
otherwise
To view boundaries and
see if you missed an
assignment, use the
Boundary Display pick from
the Model menu

Graphical window shows
both user and autoassigned boundaries
5-23
Boundary Setup Exercise Part 1



NOTE: The model for this exercise is nearly
identical to that used in the Material Setup
exercise, but has been split in half along the axis
of the microstrip and coax feed to demonstrate
symmetry boundary application as well.

We will practice by
assigning boundaries to a
Coax to Microstrip
transformer model
This exercise is only Part 1
of the entire operation;
excitation assignment will
be covered after a detailed
description of HFSS
sources and port
assignment
In the Maxwell Project
Manager, find the project
entitled “bnd_exer” and
Open it
Once open, proceed to
Setup Boundaries/Sources
5-24
Boundary Setup Exercise: Trace Metalization
NOTE: Since solid Material parameters are already
applied, there is already a boundary on the exterior of the
metal objects “pin”, “pin1”, and “pin2”. We only need to
apply the surface metalization for the actual microstrip
trace line, and define outer radiation, ground plane, and
symmetry boundaries.
5.
1. Select the Boundary radio Button.
2. From the list of available boundaries, select Perfect E.
3.
3. Set the Graphical Pick option to Face.
4. Click in the graphical window as if you are touching the
trace. The nearest face of the air box will highlight, since it
is between your view and the trace.
7.
4.
5. Right-click to bring up the pop-up menu and select Next
Behind, or use the “N” key on the keyboard to shift focus
deeper. Continue this operation until the trace is selected.
NOTE: If you appear to have selected the bottom-most
face of the model, you have gone too far. Use the rightclick menu to pick Deselect All and start over.
6.
1.
2.
6. In the Name field, type in “trace_metal”, and click the
Assign button.
7. The boundary should appear in the boundary list at left.
5-25