边界与端口设置
电子科技大学
贾宝富
2-1
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 /Square
Layered Impedance
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
Lumped RLC
Master and Slave
 ‘Linked’ boundary conditions for unit-cell studies of infinitely replicating
geometry (e.g. a slow wave circuit & an antenna array)
PML (Perfect Match Layer)
2-2
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
2-3
Perfect H for 2D Aperture (I)

Monopole Over a Ground
plane
Perfect H
Perfect H Surface Interior to the
Problem Space Behaves Like an
Infinitely Thin 2D Aperture
2-4
Perfect H for 2D Aperture (II)

Small Hole Can be “Cut” in infinitely Thin Septum
Between the Upper and Lower Guide Using a Perfect
H Surface at the Hole
Perfect H
2-5
HFSS Boundary Descriptions: Finite
Conductivity

E perpendicu lar , attenuating

Parameters: Conductivity and
Permeability

Finite Conductivity is a lossy
electrical conductor

Finite Conductivity Boundary



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*
2-6
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
2-7
HFSS Boundary Descriptions: Layered Impedance


Parameters: Surface Roughness; Layer; Thickness/Type;
Materials
用于定义多层均匀材料组成的边界。如在某种涂敷吸波材料散
射特性的计算中，可以使用这种边界。
2-8
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
2-9
HFSS Boundary Descriptions: Radiation,
cont.

Reflection of Radiation Boundary in dB, vs.
Angle of Incidence relative to boundary normal
(i.e. for normal incidence,  = 0)
Radiation boundary absorption profile
vs. incidence angle is shown at left

20
Reflection Coefficient (dB)
Refle cti on Coe fficien t (dB)
0

-20
-40

-60
-80
-100
0
10
20
30
40
50
60
70
80
theta (deg)
ETM
θ
90

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.
2-10
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
2-11
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)
2-12
HFSS Boundary Descriptions: Lumped RLC

Parameters: Resistance; Inductance; Capacitance
2-13
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
2-14
HFSS Boundary Descriptions: PML
由物体表面创建
PML层
2-15
HFSS Boundary Descriptions: PML
2-16
HFSS Boundary Descriptions: PML
由三维物体创建
PML层
2-17
HFSS Boundary Descriptions: PML
2-18
HFSS Source List

Wave Port and Lumped Port
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
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.
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
2-19
HFSS Source Descriptions: Wave Port
2-20
HFSS Source Descriptions: Wave Port
EXAMPLE WAVE PORTS

Parameters: Mode Count,
Calibration, Impedance,
Polarization


EXAMPLE LUMPED PORTS
A port is an aperture through which
guided electromagnetic field
energy is injected into a 3D HFSS
model.
Wave 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
2-21
Impedance and Polarization Lines


Impedance line and polarization line are optional in port setup.
They are located in the port and have a starting point and an end point.
I and/or P Line
Port = cross section
of waveguide
2-22
Impedance Line




Without impedance line, HFSS computes port impedance from power
and current: Zpi
With impedance line, a voltage can be defined:  Edl .
Two more port impedances result: Zpv and Zvi .
These are not the same for non-TEM transmission lines.
2-23
Polarization Line

Imposes polarization in case of ambiguity,
e.g. in square or circular guides with degenerate modes.
Port = cross section
of square waveguide
2-24
HFSS Source Descriptions: Lumped Port

Parameters: Mode Count,
Calibration, Impedance,
Polarization


A port is an aperture
through which guided
electromagnetic field
energy is injected into a
3D HFSS model.
Lumped Ports:
Approximated field
excitation is placed on the
gap source port surface

Characteristic
impedance is
provided by the
user during setup
2-25
HFSS Source Descriptions: Incident Wave
2-26
HFSS Source Descriptions: Incident Wave

In the above example, a plane incident wave is
directed at a solid made from dielectrics, to view
the resultant scattering fields.
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



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
2-27
HFSS Source Descriptions: Voltage Drop and Current
Source
Voltage Drop
Current Drop
2-28
HFSS Source Descriptions: Voltage Drop and
Current Source

Parameters: Direction and Magnitude

Example Current
Source (along trace
or across gap)


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
2-29
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:





Wave Ports and Lumped 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
2-30
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)
2-31
HFSS Ports: A Detailed Look


The Port Solution provides the excitation for the 3D FEM
Analysis. Therefore, knowing how to properly define and
create a port is paramount to obtaining an accurate analysis.
Incorrect Port Assignments can cause errors due to...









...Excitation of the wrong mode structure
...Bisection by conductive boundary
...Unconsidered additional propagating modes
...Improper Port Impedance
...Improper Propagation Constants
...Differing phase references at multiple ports
...Insufficient spacing for attenuation of modes in cutoff
...Inability to converge scattering behavior because too many
modes are requested
Since Port Assignment is so important, the following slides will
go into further detail regarding their creation.
2-32
HFSS Port Selection: Wave Port or Lumped Port?

什么时候你选择 Lumped
Port 而不是 Wave Port呢?

当模型中导线之间的间
隙太小时；

当使用Wave port很难确
定一个端口的参考定位
时；

当你希望使用电压降，
而不是S参数作为输出
时。
Lumped Ports (blue)
2-33
HFSS Ports: Sizing

A port is an aperture through which a
guided-wave mode of some kind
propagates

A Coaxial Port Assignment
For transmission line structures entirely
enclosed in metal, port size is merely the
waveguide interior carrying the guided
fields



A Microstrip Port Assignment
(includes air above substrate)
Rectangular, Circular, Elliptical, Ridged,
Double-Ridged Waveguide
Coaxial cable, coaxial waveguide,
squareax, Enclosed microstrip or
suspended stripline
For unbalanced or non-enclosed lines,
however, field propagation in the air
around the structure must also be included



Parallel Wires or Strips
Stripline, Microstrip, Suspended Stripline
Slotline, Coplanar Waveguide, etc.
2-34
HFSS Ports: Sizing, cont.

Port too narrow (fields couple
to side walls)
The port solver only understands
conductive boundaries on its borders




Result: Moving the port edges too close
to the circuitry for open waveguide
structures (microstrip, stripline, CPW,
etc.) will allow coupling from the trace
circuitry to the port walls!

Port too Short
(fields couple to top wall)
Electric conductors may be finite or perfect
(including Perfect E symmetry)
Perfect H symmetry also understood
Radiation boundaries around the
periphery of the port do not alter the port
edge termination!!
This causes an incorrect modal solution,
which will suffer an immediate
discontinuity as the energy is injected past
the port into the model volume
2-35
HFSS Ports: Sizing Handbook I
10w, w  h
or
5w (3h to 4h), w < h

Microstrip Port Sizing Guidelines



6h to
10h
Assume width of microstrip trace is w
Assume height of substrate dielectric
is h
Port Height Guidelines

Between 6h and 10h

w
h
Note: Port sizing guidelines are not
inviolable rules true in all cases. For
example, if meeting the height and
width requirements outlined result in a
rectangular aperture bigger than /2
on one dimension, the substrate and
trace may be ignored in favor of a
waveguide mode. When in doubt,
build a simple ports-only model and
test.



Tend towards upper limit as dielectric
constant drops and more fields exist
in air rather than substrate
Bottom edge of port coplanar with the
upper face of ground plane
(If real structure is enclosed lower
than this guideline, model the real
structure!)
Port Width Guidelines


10w, for microstrip profiles with w  h
5w, or on the order of 3h to 4h, for
microstrip profiles with w < h
2-36
HFSS Ports: Sizing Handbook II

Stripline Port Sizing Guidelines

8w, w  h
or
5w (3h to 4h), w < h


Port Height Guidelines

w
h

Extend from upper to lower groundplane,
h
Port Width Guidelines



Assume width of stripline trace is w
Assume height of substrate dielectric is h
8w, for microstrip profiles with w  h
5w, or on the order of 3h to 4h, for
microstrip profiles with w < h
Boundary Note: Can also make side
walls of port Perfect H boundaries
2-37
HFSS Ports: Sizing Handbook III

Slotline Port Guidelines



Port Height:

Approx 7g minimum

Larger of 4h or 4g
Should be at least 4h, or 4g (larger)
Remember to include air below the
substrate as well as above!

g
h
Assume slot width is g
Assume dielectric height is h

If ground plane is present, port should
terminate at ground plane
Port Width:


Should contain at least 3g to either side
of slot, or 7g total minimum
Port boundary must intersect both side
ground planes, or they will ‘float’ and
become signal conductors relative to
outline ‘ground’
2-38
HFSS Ports: Sizing Handbook IV

CPW Port Guidelines



Larger of approx. 10g or 10s

s
h
Port Height:

Larger of 4h or 4g
Assume slot width is g
Assume dielectric height is h
Assume center strip width is s

g
Should be at least 4h, or 4g (larger)
Remember to include air below the substrate
as well as above!


If ground plane is present, port should
terminate at ground plane
Port Width:

Should contain 3-5g or 3-5s of the side
grounds, whichever is larger


Total about 10g or 10s
Port outline must intersect side grounds, or
they will ‘float’ and become additional signal
conductors along with the center strip.
2-39
CPW Wave Ports: Starting Recommendations
Wave Port Size
The standard recommendation for most CPW wave ports is a rectangular aperture
Port width should be no less than 3 x the overall CPW width, or 3 x (2g + w)
Port height should be no less than 4 x the dielectric height, or 4 h
Wave Port Location
The wave port should be centered horizontally on the CPW trace
If the port is on GCPW, the port bottom edge should lie on the substrate bottom ground plane
If the port is on ungrounded CPW, the port height should be roughly centered on the CPW metal layer
Wave Port Restrictions
As with all wave ports, there must be only one surface normal exposed to the field volume
Port should be on exterior model face, or capped by a perfect conductor block if internal
The wave port outline must contact the side grounds (all CPWs) and bottom ground (GCPW)
The wave port size should not exceed lambda/2 in any dimension, to avoid permitting a rectangular waveguide
modal excitation
3 (2g + w)
3 (2g + w)
4h minimum
4h minimum
w
w
h
g
h
g
Ungrounded CPW
Grounded CPW
(Port height centered on trace)
(Port height begins at lower ground)
2-40
HFSS Ports: Sizing Handbook V; Lumped Ports
Perfect E

Lumped ports behave differently from Wave
Ports

Perfect H
Perfect H

Perfect E
Lumped Port Sizing (microstrip example):

Perfect H

Perfect H
Any port edge not in contact with metal structure
or another port assumed to be a Perfect H
conductor
“Strip-like”: [RECOMMENDED] No larger than
necessary to connect the trace width to the
ground
“Wave-like”: No larger than 4 times the strip
width and 3 times the substrate height

Perfect H

Perfect E

The Perfect H walls allow size to be smaller than
a standard port would be
However, in most cases the strip-like application
should be as or more accurate
Further details regarding Lumped Port sizing
available as a separate presentation
2-41
HFSS Port Selection Example: Parallel
Traces
Spaced by 8 or more times Trace Width
Inputs sufficiently isolated that no coupling behavior should occur
Sufficient room for Wave port apertures around each trace
Use Wave Ports as shown
Spaced by 4 – 8 times Trace Width
Inputs still fairly isolated, little to no coupling behavior should
occur
Insufficient room for Wave port apertures around each trace
without clipping fringing fields
Use Lumped Ports as shown
Spaced by less than 4 times Trace Width
Traces close enough to exhibit coupling
Even and Odd modes possible; N modes total for N
conductors and one ground reference [odd mode shown at
right]
Lumped Ports from trace to ground neglect coupling behavior
and are no longer appropriate
Use multi-mode Wave Port
Terminal line assignments can permit extraction of Sparameters referenced to each ‘trace’
2-42
HFSS Ports: Spacing from Discontinuities

Structure interior to the modeled volume may
create and reflect non-propagating modes

Port
Extension

If the port is spaced too close to a discontinuity
causing this effect, the improper solution will be
obtained



These modes attenuate rapidly as they travel
along the transmission line
A port is a ‘matched load’ as seen from the
model, but only for the modes it has been
designed to handle
Therefore, unsolved modes incident upon it are
reflected back into the model, altering the field
solution
Remedy: Space your port far enough from
discontinuities to prevent non-propagating mode
incidence

Spacing should be on order of port size, not
wavelength dependent
2-43
HFSS Ports: Single-Direction Propagation
Port on Exterior Face of Model

Wave ports must be defined
so that only one face can
radiate energy into the
model


Position Wave Ports on the
exterior of the geometry
(one face on background) or
provide a port cap.

Port Inside Modeled Air Volume;
Back side covered with Solid Cap
Lumped Ports have no
such restriction
Cap should be the same
dimensions as the port
aperture, be a 3D solid
object, and be defined as
a perfect conductor in the
Material Setup module
2-44
HFSS Ports: Mode Count

Ports should solve for all propagating modes


However, requesting too many modes in the full
solution also negatively impacts analysis


Circular waveguide, showing two
orthogonal TE11 modes and TM01
mode (radial with Z-component).
Neglecting the TM01 mode from
your solution would cause incorrect
results.
Modes in cutoff are more difficult to calculate; Sparameters for interactions between propagating
and non-propagating modes may not converge
well
What if I don’t know how many modes exist?



Ignoring a mode which does propagate will result
in incorrect S-parameters, by neglecting modeto-mode conversion which could occur at
discontinuities
Build a simple model of a transmission line only,
or run your model in “Ports Only” mode, and
check!
You can alter the mode count before running the
full solution.
Degenerate mode ordering is controlled with
calibration lines (see next slide)
2-45
HFSS Ports: Degenerate Modes


In circular or square waveguide, use the
calibration line to force (polarize) the mode
numbering of the two degenerate TE11
modes. This is also useful because without
a polarization orientation, the two modes
may be rotated to an arbitrary angle inside
circular WG.
Degenerate modes have identical impedance,
propagation constants
Port solver will arbitrarily pick one of them to
be ‘mode(n)’ and the other to be ‘mode(n+1)’



To enforce numbering, use a polarize the first
mode to the line
OR, introduce a dielectric change to slightly
perturb the mode solution and separate the
degenerate modes

For parallel lines, a virtual object
between them aids mode ordering.
Note virtual object need not extend
entire length of line to help at port.
Thus, mode-to-mode S-parameters may be
referenced incorrectly

Example: A dielectric bar only slightly higher in
permittivity than the surrounding medium will
concentrate the E-fields between parallel
wires, forcing the differential mode to be
dominant
If dielectric change is very small (approx. 0.001
or less), impedance impact of perturbation is
negligible
2-46
HFSS Ports: Impedance Definitions

HFSS provides port characteristic
impedances calculated using the powercurrent definition (Zpi)



For many transmission line types, the powervoltage or voltage-current definition is
preferred

For a Coax, the impedance line extends
radially from the center to outer conductor (or
vice versa). Integrating the E-field along the
radius of the coaxial dielectric provides the
voltage difference.
In many instances, the impedance and
calibration lines are the same!


Incident power is known excitation quantity
Port solver integrates H-field around port
boundary to calculate current flow
Slot line, CPW: Zpv preferred
TEM lines: Zvi preferred
HFSS can provide these characteristic
impedance values, as long as an impedance
line is identified

The impedance line defines the line along
which the E-field is integrated to obtain a
voltage
2-47
HFSS Ports: Impedance Multiplier

Whole Rectangular WG
(No Symmetry)
Impedance Mult = 1.0
When symmetry is used in a model, the
automatic Zpi and impedance linedependant Zpv and Zvi calculations will
be incorrect, since the entire port
aperture is not represented.

Half Rectangular WG
(Perfect E Symmetry)
Impedance Mult = 2.0


Half Rectangular WG
(Perfect H Symmetry)
Impedance Mult = 0.5

...and for Quarter Rectangular WG?
(Both Perfect E and H Symmetry)
Impedance Mult. = 1.0
Split the model with a Perfect E
symmetry case, and the impedance is
halved.
Split the model with a Perfect H
symmetry case, and the impedance is
doubled.
The port impedance multiplier is just a
renormalizing factor, used to obtain the
correct impedance results regardless of
the symmetry case used.
The impedance multiplier is applied to
all ports, and is set during the
assignment of any port in the model.
2-48