Lecture 11 HFSS Boundary Conditions

L3.1-1ANSYS, Inc. Proprietary

© 2011 ANSYS, Inc. All rights reserved.Release 13.0

January 2011

Introduction to HFSS

Customer Training Material

Chapter 3.1

Boundary Conditions Primer




January 2011

Customer Training MaterialExcitations and Boundary Conditions

• Excitations and Boundary Conditions– Majority of HFSS errors are related to improper usage of excitations and boundary conditions

– Boundary conditions are important because they significantly impact electromagnetic solution

• They determine model scope

– To truncate infinite space to finite volume, HFSS applies PEC boundary to surface surrounding geometric

model

• They can reduce model complexity

– Boundary conditions can be used to reduce solution time and computing resource demands

TE10 Cavity

Resonator Pyramidal

Horn

Antenna




January 2011

Customer Training MaterialUser-Defined Boundary Conditions

• Surface approximations– Perfect E surface

– Perfect H surface

– Finite conductivity surface

– Impedance surface

– Layered impedance

– Lumped RLC boundary

– Symmetry planes

– Radiation (absorbing) boundary surface

– Perfectly matched layer (PML)

• Strictly not boundary condition, but effectively behaves like one

– Master/slave (linked or periodic) boundaries

– Screening impedance

• Excitations– Wave ports (external)

– Lumped ports (internal)

0=⋅∇

=⋅∇

∂

∂+=×∇

∂

∂−=×∇

B

D

t

DJH

t

BE

ρ




January 2011

Customer Training MaterialPerfect E and Perfect H Boundaries

• 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.

• Parameters– None

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




January 2011

Customer Training MaterialExcitations

• Provide means for energy to enter and exit model

• Types of excitations– Ports

• Wave ports

• Lumped ports

• Floquet ports

– Voltage sources

– Current sources

– Magnetic biases

– Incident waves

• Plane waves

• Hertzian dipole

• Cylindrical wave

• Gaussian beam

• Linear antenna wave

• Far-field wave

• Near-field wave

• Only ports provide S-parameters– This presentation will focus on this type of excitation




January 2011

Customer Training MaterialDriven Modal vs Driven Terminal Solutions

• Driven modal– S-matrix solution expressed in terms of incident and reflected powers of waveguide modes

– Always used by wave solver

– Integration lines set phase between ports and modal voltage integration path (Zpv and Zvi)

– Use for modal-based S-parameters of passive, high-frequency structures such as microstrips, waveguides, and

transmission lines

• Driven terminal– S-matrix solution expressed in terms of linear combination of nodal voltages and currents for wave port

– Equivalent “modes-to-nodes” transformation performed from modal solution

– Use for terminal-based S-parameters of multi-conductor transmission line ports (with several quasi-TEM modes, etc.)




January 2011

Customer Training MaterialExcitations

• Example Solution Types:

T1T1T1T1 T2T2T2T2

Integration LineIntegration LineIntegration LineIntegration Line

Mode 1Mode 1Mode 1Mode 1

(Even Mode)(Even Mode)(Even Mode)(Even Mode)

Integration LineIntegration LineIntegration LineIntegration Line

Mode 2Mode 2Mode 2Mode 2

(Odd Mode)(Odd Mode)(Odd Mode)(Odd Mode)

Modes to NodesModes to NodesModes to NodesModes to Nodes

TransformationTransformationTransformationTransformation

SPICESPICESPICESPICE

Differential PairsDifferential PairsDifferential PairsDifferential Pairs

ModalModalModalModalPort1Port1Port1Port1 Port2Port2Port2Port2

TerminalTerminalTerminalTerminalPort1Port1Port1Port1 Port2Port2Port2Port2

T1T1T1T1

T2T2T2T2

T1T1T1T1

T2T2T2T2

2 Modes2 Modes2 Modes2 Modes 2 Modes2 Modes2 Modes2 Modes




January 2011

Customer Training MaterialPorts

• Ports are unique type of boundary condition– Allow energy to flow into and out of structure

– Defined on 2D planar surface

– 2D field patterns serve as boundary conditions for full 3D problem

• Incorrect port setup will produce incorrect results– If port fields are incorrect, then solution will be incorrect

– Assumed boundary condition on port edges should always be considered

Initial Mesh

Seeding and Lambda Refinement (Single Frequency)

Port Solution(Adaptive)

Full Volumetric

Solution




January 2011

Customer Training MaterialWave Ports

• External port type

• Arbitrary port solver calculates natural waveguide field patterns (modes)– Assumes semi-infinitely long waveguide with same cross-section and material properties as port surface

• Recommended only for surfaces exposed to background object

• Supports multiple modes, de-embedding, and re-normalization

• Computes generalized S-parameters– Frequency-dependent characteristic impedance

– Perfectly matched at every frequency

Port 1Port 1Port 1Port 1

Port 2Port 2Port 2Port 2Port 3Port 3Port 3Port 3

Port 4Port 4Port 4Port 4




January 2011

Customer Training MaterialPort Solver

• Wave port solver solves two-dimensional wave equation

• Field pattern of traveling wave inside waveguide can be determined by solving Maxwell’s equations

• Wave equation is derived directly from Maxwell’s equations

• where– E(x,y) is phasor representing oscillating electric field

– k0 is free space wave number

– µr is complex relative permeability

– εr is complex relative permittivity

• 2D solver obtains excitation field pattern in form of phasor solution E(x,y)– Phasor solutions are independent of z and time

– Only after being multiplied by e-γz do they become traveling waves

– Different excitation field pattern is computed for each frequency point of interest

( ) 0),(,1 2

0 =−

×∇×∇ yxEkyxE r

r

εµ




January 2011

Customer Training MaterialWave Port Boundary Conditions

• All outer edges are assigned Perfect E boundary by default – Port is defined within waveguide

– Simple setup for enclosed transmission lines (coax, waveguide, etc.)

– Challenging setup for unbalanced or non-enclosed lines (microstrip, CPW, slotline, etc.)

• Symmetry or impedance boundaries also recognized at port edges

• For port on same surface as radiation boundary, default interface is Perfect E boundary

– Can set option to use radiation boundary on port edges during port solution

• Creating port edges too close to current-carrying lines will allow coupling from trace to port walls

– Causes incorrect modal solution which will suffer immediate discontinuity as energy is injected past port into model

Port too narrow

(fields coupled to sidewalls)Correct port size




January 2011

Customer Training MaterialWave Port Sizing Guidelines

• Microstrip port height between 6h and 10h

– Tend towards upper limit as dielectric

constant drops and fringing fields increase

– Make bottom edge of port co-planar with

upper face of ground plane

• Microstrip port width

– 10w for w ≥ h

– 5w, or on order of 3h to 4h, for w < h

• Extend stripline port height from upper to lower groundplane (h)

• Stripline port width

– 8w for w ≥ h

– 5w, or on order of 3h to 4h, for w < h

• Can also make side walls of port Perfect H boundaries

w

h

6h to

10h

10w, w ≥≥≥≥ h

or

5w (3h to 4h), w < h

Port sizing guidelines are not inviolable rules. If meeting height and width requirements result in rectangular aperture larger than

λ/2 in one dimension, the substrate and trace may be ignored in

favor of a waveguide mode. When in doubt, run a ports-only solution to determine which modes are propagating.

w

h

8w, w ≥≥≥≥ h

or

5w (3h to 4h), w < h




January 2011

Customer Training MaterialWave Port Sizing Guidelines

• Slotline port height at least 4h or 4g (whichever is larger)

– Include air above and below substrate

– 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

• Coplanar waveguide port height at least 4h or 4g (whichever is larger)

– Include air above and below substrate

– If ground plane is present, port should

terminate at ground plane

• Port width should contain 3-5g or 3-5s of side grounds (whichever is larger)

– Total width ~10g or ~10s

– Port outline must intersect both side grounds

or they will ‘float’ and become signal

conductors

g

Approx 7g minimum

h

Larger of 4h or 4g

For Driven Modal solutions, use

Zpv for impedance calculation

Larger of approx. 10g or 10s

s

h

Larger of 4h or 4g

g




January 2011

Customer Training MaterialLumped Ports

• Recommended only for surfaces internal to model– Single TEM mode with no de-embedding

– Uniform electric field on port surface

– Normalized to constant user-defined Z0

• Lumped port boundary conditions– Perfect E or finite conductivity boundary for port edges which interface

with conductor or another port edge

– Perfect H for all remaining port edges

Uniform electric field

User-defined Zo

Zo

Dipole element

with lumped port




January 2011

Customer Training MaterialLumped vs Wave Ports for Planar Filters

• Lumped ports can be used to feed printed transmission lines

– S-parameters normalized to user-specified characteristic impedance

– Single mode propagation

– No de-embedding operations available

– Must be located inside model

• Wave ports can be used to feed printed transmission lines

– S-parameters normalized to computed characteristic impedance

– Multiple propagating modes possible

– De-embedding available as post-processing operation

– Must touch background object (or be backed by conducting object)




January 2011

Customer Training MaterialLumped vs Wave Ports for Planar Filters

• Same results obtained from both port types

Lumped Ports

Wave Ports




January 2011

Customer Training MaterialWave Ports vs Lumped Ports

Wave port Lumped port

Accessibility External Faces Internal to Model

Higher order modes Yes No

De-embedding Yes No

Re-normalization Yes Yes

Setup complexity Moderate Low

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Lecture 11 HFSS Boundary Conditions

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