hfss_bc.pdf

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14. 0 Release

Chapter1.4:BoundaryConditionsIntroductiontoANSYSHFSSIntroductiontoANSYSHFSS

Chapter4:Introduction

14. 0 Release

IntroductiontoANSYSHFSS

Chapter4:BoundaryConditions


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ANSYS, Inc. Proprietary

2009 ANSYS, Inc. All rights reserved.February 23, 2009

Inventory #002593

2011 ANSYS, Inc.4-2 Release14.0February 1, 2012

Boundary Conditions This chapter describes the basics for applying boundary conditions. Boundary conditions enable you to control the

characteristics of planes, faces, or interfaces between objects. Boundary conditions are important to understand and

are fundamental to the solution of Maxwells equations.

Why are they Important? The wave equation that is solved by HFSS is derived from the differential form of Maxwells Equations. For these

expressions to be valid, it is assumed that the field vectors are single-valued, bounded, and have a continuous

distribution along with their derivatives. Along boundaries or sources, the fields are discontinuous and the derivatives

have no meaning. Therefore boundary conditions define the field behavior across discontinuous boundaries.

As a user of HFSS you should be aware of the field assumptions made by boundary conditions. Since boundary

conditions force a field behavior we want to be aware of the assumptions so we can determine if they are appropriate

for the simulation. Improper use of boundary conditions may lead to inconsistent results. When used properly, boundary conditions can be successfully utilized to reduce the model complexity. In fact, HFSS

automatically uses boundary conditions to reduce the complexity of the model. When a 3D object is assigned a

conducting material, HFSS assigns a finite boundary condition to the conductor instead of solving for the fields inside of

the metal.

The model complexity usually is directly tied to the solution time and computer resources so it is a competitive

advantage to utilize them whenever possible.

Boundary Conditions

0

B

D

t

DJH

tBE


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Why are they critical?

Any current injected into a system must return to the source

DC

Chooses path of least resistance

AC

Chooses path of least inductance

A signal propagates between the signal trace and its reference plane

Reference plane is just as important as signal trace!

Why do I care?

Many real designs have nonideal return paths

These effects are only captured by full-wave simulators

Failure to maintain the correct return path will

Limit correlation to measurements

Mask or create design problems

Port and Boundary setup is the most common source of error in model setup

Excitations & Boundary Conditions


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2011 ANSYS, Inc.4-4 Release14.0February 1, 2012Port1

Port2 Port3

No DCReturn Path

No DC Return Path


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2011 ANSYS, Inc.4-5 Release14.0February 1, 2012ort1

Port2 Port3

DCReturn Path

DC Return Path


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DC RFReturn Path

S21

S11

S31

AC and DC Return Path


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Common Boundary Conditions There are three types of boundary conditions. The first two are largely the users responsibility to define them or ensure

that they are defined correctly. The material boundary conditions are transparent to the user.

Excitations

Wave Ports (External)

Lumped Ports (Internal)

Surface Approximations

Symmetry Planes

Perfect Electric or Magnetic Surfaces

Radiation (absorbing) boundary surface

Perfectly matched layer (PML)

Strictly not boundary condition, but effectively behaves like one

Finite Element-Boundary Integral (FEBI)

Background or Outer Surface

Finite conductivity surface

Impedance surface

Layered impedance

Lumped RLC boundary

Master/slave (linked or periodic) boundaries

Screening impedance

Material Properties

Boundary between two dielectrics

Finite Conductivity of a conductor

Common Boundary Conditions


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How the Background Affects a Structure The background is a hidden region that is automatically defined by HFSS. The background surrounds the geometric

model and fills any space that is not occupied by an object. Any object surface that touches the background is

automatically defined to be a Perfect E boundary and given the boundary name outer. You can think of your structure

as being encased with a thin, perfect conductor.

If it is necessary, you can change a surface that is exposed to the background to have properties that are different from

outer: To model losses in a surface, you can redefine the surface to be either a Finite Conductivity or Impedance

boundary.

To model a surface to allow waves to radiate infinitely far into space, redefine the surface to be radiation

boundary.

The background can affect how you make material assignments. For example, if you are modeling a simple air-filled

rectangular waveguide, you can create a single object in the shape of the waveguide and define it to have the

characteristics of air. The surface of the waveguide is automatically assumed to be a perfect conductor and given theboundary condition outer, or you can change it to a lossy conductor.

Background or Outer Surface


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Boundary Condition Precedence The order in which boundaries are assigned is important in HFSS. Latter assigned boundaries take precedence over

former assigned boundaries.

For example, if one face of an object is assigned to a Perfect E boundary, and a hole which lies in the same plane as

this surface is assigned a Prefect H boundary, then the Perfect H will override the Perfect E in the area of the hole, and

the E field will pass through the hole. If this operation were performed in the reverse order, then the Perfect E boundary

would cover the Perfect H boundary, and no field would penetrate. Once boundaries have been assigned, they can be re-prioritized by selecting HFSS > Boundaries > Re-prioritize.

The order of the boundaries can be changed by clicking on a boundary and dragging it further up or down in the list.

NOTE: Ports will always take the highest precedence in an area of overlap

Boundary Condition Precedence


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Technical Definition of Boundary Conditions Perfect E Perfect E is a perfect electrical conductor, also referred to as a perfect conductor. This type of boundary

forces the electric field (E-Field) perpendicular to the surface. There are also two automatic Perfect E assignments:

Any object surface that touches the background is automatically defined to be a Perfect E boundary and given the

boundary condition name outer.

Any object that is assigned the material pec (Perfect Electric Conductor) is automatically assigned the boundary

condition Perfect E to its surface and given the boundary condition name smetal.

Perfect H Perfect H is a perfect magnetic conductor. Forces E-Field tangential to the surface.

Natural for a Perfect H boundary that overlaps with a perfect E boundary, this reverts the selected area to its

original material, erasing the Perfect E boundary condition. It does not affect any material assignments. It can be

used, for example, to model a cut-out in a ground plane for a coax feed.

Technical Definitions


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Perfect E is perfect electrical conductor (PEC) Forces E-field perpendicular to surface

Represents metal surfaces, ground planes, ideal cavity walls, etc.

Infinite ground plane option simulates effects of infinite ground plane in post-processing radiated fields

Perfect H is perfect magnetic conductor (PMC) Forces H-field perpendicular to surface and E-field tangential

Does not exist in real world

Useful boundary constraint for electromagnetic models

Represents openings in metal surfaces, etc.

Perfect E and Perfect H Boundaries

Perfect E Boundary Perfect H Boundary

When you define a solid object as a

perf_conductor, a Perfect E boundary

condition is applied to its exterior surfaces.

E-field Parallel to surface

E-field Perpendicular to surface


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Surface Loss Modeling Finite ConductivityA Finite Conductivity boundary enables you to define the surface of an object as a lossy

(imperfect) conductor. HFSS applies this boundary for lossy metal materials. To model a lossy surface, you provide

loss in Siemens/meter and permeability parameters. Loss is calculated as a function of frequency. It is only valid for

good conductors. Forces the tangential E-Field equal to Zs(n x Htan). The surface impedance (Zs) is equal to,

(1+j)/(), where:

is the skin depth, (2/())0.5 of the conductor being modeled, is the frequency of the excitation wave, is theconductivity of the conductor, is the permeability of the conductor

Impedance a resistive surface that calculates the field behavior and losses using analytical formulas. Forces the

tangential E-Field equal to Zs(n x Htan). The surface impedance is equal to Rs + jXs, where:

Rs is the resistance in ohms/square, Xs is the reactance in ohms/square

Layered Impedance Multiple thin layers in a structure can be modeled as an impedance surface. See the Online

Help for additional information on how to use the Layered Impedance boundary.

Lumped RLC a parallel combination of lumped resistor, inductor, and/or capacitor surface. The simulation is similar

to the Impedance boundary, but the software calculate the ohms/square using the user supplied R, L, C values.

Infinite Ground Plane Generally, the ground plane is treated as an infinite, Perfect E, Finite Conductivity, orImpedance boundary condition. If radiation boundaries are used in a structure, the ground plane acts as a shield for far-

field energy, preventing waves from propagating past the ground plane. To simulate the effect of an infinite ground

plane, check the Infinite ground plane box when defining a Perfect E, Finite Conductivity, or Impedance boundary

condition.

NOTE: Enabling the Infinite Ground Plane approximation ONLY affects post-processed far-field radiation patterns.

It will not change the current flowing on the ground plane.



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Surface Loss Modeling All methods utilize an equivalent surface impedance applied to the field as it travels across the surface

Model / Mesh Simplification

jZs

1

2

Zs specified as /sq

0.7mil Copper

500 in N ickel 500 in Gold

0.7mil Copper

500 in N ickel 500 in Gold

Finite Conductivity Impedance

Layered Impedance Lumped RLC

Parallel RLC Circuit

t >>

Zs,input

Zs,AuLAu

Zs,NiLNi

Zs,CuLCu

Zs,input

Zs,AuLAu

Zs,NiLNi

Zs,CuLCu

*Lossassumes

current

penetrates

1skin

depth

*Usedimplicitlyforgoodconductingobjectsthat

arenotsolvedinside

*ModelsastackupofthinmaterialsasTL

*Workswithmetalsanddielectrics

)( tantan

HnZE s

t


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Conducting traces often modeled as 2D objects for electromagnetic simulations More computationally efficient since fewer meshing surfaces required

Good approximation for many structures operating in skin depth regime

Trace Thickness Effects on Planar Antenna

Patch antenna modeled with

2D sheet

Patch antenna modeled with

3D object

Frequency response of both models


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Planar filters which use edge coupling to operate require 3D objects (finite thickness)

for modeling conducting traces Applications whose performance depends upon closely-coupled traces

End-coupled, parallel-coupled, hairpin filters, etc.

Trace Thickness Effects on Planar Filter

Edge-coupled filter modeled with 2D sheets

Edge-coupled filter modeled with 3D objects

Frequency response of both models


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Radiating Boundary Conditions Radiation boundaries, also referred to as absorbing boundaries, enable you to model a surface as electrically open:

waves can then radiate out of the structure and toward the radiation boundary. The system absorbs the wave at the

radiation boundary, essentially ballooning the boundary infinitely far away from the structure and into space. Radiation

boundaries may also be placed relatively close to a structure and can be arbitrarily shaped. This condition eliminates

the need for a spherical boundary. For structures that include radiation boundaries, calculated S-parameters include

the effects of radiation loss. When a radiation boundary is included in a structure, far-field calculations are performedas part of the simulation.

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 equallyin 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.

Finite Element Boundary Integral (FEBI) is an alternative to Radiation and PML boundaries for radiating designs.

The FEBI boundary is a hybrid FEM (Volume) and IE solver (Radiating Surface). FEBI is a reflection-less boundary

that can be applied to arbitrarily shaped volumes. Requires an HFSS-IE license.



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Mimics continued propagation beyond boundary plane Absorption achieved via 2nd order radiation boundary

Absorbs best when incident energy flow is normal to surface

Distance from radiating structure

Place at least /4 from strongly radiating structure

Place at least /10 from weakly radiating structure

Must be concave to all incident fields from within modeled space

Radiation Boundary

Boundary is /4 away from

horn aperture in all directions


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Radiation Boundary:

Incidence Angle Dependency

Radiation boundary functions

well for incident angles less

than 25-30

RadiationBoundary

RadiationBoundary

Poorabsorptionofradiationboundaryaffectsradiationpattern


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Example probe-fed circular patch Varied distance between absorbing boundary condition (ABC) and antenna

/20, /10, /8, /4, /2, 3 /4,

Examined impact on return loss and gain

Impact of Distance to ABC

/4 and caseswithin 13 MHz of

each other

(0.1%)

0.2 dB

variation


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Perfectly Matched Layer (PML) Fictitious lossy anisotropic material which fully absorbs electromagnetic fields

Two types of PML applications

PML objects accept free radiation if PML terminates free space

PML objects continue guided waves if PML terminates transmission line

Guidelines for assigning PML boundaries

Use PML setup wizard for most cases

Manually create a PML when base object is curved or inhomogeneous

Perfectly Matched Layer (PML)

PMLSetup

Wizard

ParameterstoDefine:

1. DefinePMLThickness

2. SpecifyMinimumFrequency(FromSweep)

3. SpecifyMinimumDistancefromAntennato

PML(MeasureinModel)

1.

2.

3.


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PML Incidence Angle Dependency

PML functions well forincident angles less than

65-70 Betterabsorptionleadstobetterconsistencyinthepatterns

PML

PML


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Example probe-fed circular patch Varied distance between PML and antenna

/20, /10, /8, /4, /2, 3/4

Examined impact on return loss and gain

Impact on Distance to PML

/8 and 3/4 cases

within 28 MHz of

each other (0.3%)


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FEBI Mesh truncation of infinite free space into a finite computational domain

Alternative to Radiation or PML

Hybrid solution of FEM and IE

IE solution on outer faces

FEM solution inside of volume

FE-BI Advantages

Arbitrary shaped boundary

Conformal and discontinuous to minimize solution volume

Reflection-less boundary condition

High accuracy for radiating and scattering problems

No theoretical minimum distance from radiator

Reduce simulation volume and simplify problem setup

Finite Element Boundary Integral

FEM Solution

in Volume

IE Solution

on Outer Surface

Fields at outer surface

Iterate

Free space

(No Solution Volume)

FE-BI

Arbitrary shape


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IE-Regions An extension of FEBI that supports a true mixed domain FEM and IE solution

Metal objects can be solved directly with an IE solution applied to surface

Removes need for air box to surround metal objects

Dielectric regions can be replaced with an IE Region on the boundary of uniform dielectric material

Solution inside of dielectric is solved using IE

IE-Regions

FE-BI

IE Region

Surface current on

metal block

FEMOnlySolution HybridFEMIESolution


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Peak gain vs. Airbox sizing ABC needs at least /4 spacing from antenna element to yield accurate far field results

PML and FE-BI accurately predicts gain, even as close as /30

Distance from Radiator

Distance From

Antenna

/30 /2


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Technical Definition of Boundary Conditions (Continued) Symmetry - represent perfect E or perfect H planes of symmetry. Symmetry boundaries enable you to model only part

of a structure, which reduces the size or complexity of your design, thereby shortening the solution time. Symmetry

boundaries, as opposed to a simple Perfect E or H plane, should be used when the plane cuts across a port. In this

instance, the port has a different amount of power, voltage, and current associated with it, and thus a different

impedance. To make a port with a symmetry plane look like a full-sized port, you must use the Impedance Multiplier in

the boundary wizard. For a single Symmetry H boundary, the Impedance Multiplier is 0.5.

For a single Symmetry E boundary, the Impedance Multiplier is 2.

Other considerations for a Symmetry boundary condition:

A plane of symmetry must be exposed to the background.

A plane of symmetry must not cut through an object drawn in the 3D Modeler window.

A plane of symmetry must be defined on a planar surface.

Only three orthogonal symmetry planes can be defined in a problem



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Symmetry Planes Allows for modeling portion of entire structure

For Driven Modal solutions

Two symmetry options are available

Use perfect E when electric field is perpendicular to symmetry plane

Use perfect H when electric field is tangential to symmetry plane

Involve further implications to boundary manager and fields post-processing

May need to specify impedance multiplier

Existence of symmetry boundary allows for near- and far-field calculation of entire structure

User Parameters

Type

Impedance multiplier

Symmetry Plane

Conductive edges on

all four sides

Waveguide contains symmetric propagating

mode which could be modeled using half thevolume vertically or horizontally.

Perfect E Symmetry

(bottom)

Perfect H Symmetry

(left side)


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Impedance Multiplier When symmetry is used, Zpi and impedance line-dependent Zpv and Zvi calculations will be incorrect since entire port

aperture is not represented

Impedance is halved for model with Perfect E symmetry plane

Impedance is doubled for model with Perfect H symmetry plane

Port impedance multiplier is renormalizing factor used to obtain correct impedance

Value applied to all ports

Global parameter set during assignment of any port

Symmetry Plane Impedance Multiplier

Rectangular WG

(No Symmetry)

Half Rectangular WG

(Perfect E Symmetry)

Impedance Multiplier = 2

Half Rectangular WG

(Perfect H Symmetry)

Impedance Multiplier = 0.5


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Perfect E Symmetry (top)

Perfect H Symmetry

(right side)

TE20 mode in full modelProperly represented

with Perfect E symmetry

Mode cannot occur

with Perfect H

symmetry

Field Symmetry Geometric symmetry does not necessarily imply field symmetry for higher-order modes

Symmetry boundaries can act as mode filters

Next higher propagating waveguide mode is not symmetric about vertical center plane of waveguide

Therefore one symmetry case is valid while the other is not

Use caution when using symmetry planes to assure that real behavior is not filtered out by boundary conditions

Symmetry Plane Mode Implications


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Technical Definition of Boundary Conditions (Continued) Master / Slave - 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. Some considerations for Master/Slave boundaries:

They can only be assigned to planar surfaces.

The geometry of the surface on one boundary must match the geometry on the surface of the other boundary.

Screening Impedance - Used to efficiently represent periodic screens or grids with impedance boundary condition



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Master/Slave Boundaries Used to model unit cell of periodic structure

Also referred to as linked or periodic boundaries

Master and slave boundaries are always paired

Fields on master surface are mapped to slave surface with a phase shift

Phase shift specified either as absolute phase value or using scan angle

Constraints

Master and slave surfaces must be identical in shape and size

Coordinate systems must be created to identify point-to-point

correspondence

Parameters

Master/slave pairing

UV coordinate systems Phase shift method

Master/Slave Boundaries

Unit Cell Model of Waveguide Array

WG Port

(bottom)

Ground Plane

Master

BoundarySlave

Boundary

V-

axis

U-axis


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Screening Impedance Used to efficiently represent periodic screens or grids with impedance boundary condition

Can be anisotropic (different values in x and y directions)

Can be frequency-dependent

Periodic grid characterized by unit cell

Dynamic link support to import impedance values from unit cell

Includes effects of polarization

Parameters

Resistance and reactance (/square)

Coordinate system if anisotropic

HFSS design for dynamic link

Screening Impedance Boundary

PerfectE

PerfectE

PerfectH

PerfectH

WavePort

WavePort

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