Lecture 11 HFSS Boundary Conditions

Customer Training Material

Chapter 3.1 Boundary Conditions Primer

Introduction to HFSS

ANSYS, Inc. Proprietary © 2011 ANSYS, Inc. All rights reserved.

Release 13.0 January 2011

L3.1-1


Excitations 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

L3.1-2



User-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 – – – – – – – – –

∂B ∂t ∂D ∇×H = J + ∂t ∇⋅D = ρ ∇⋅B =0

∇×E =−

• Excitations – Wave ports (external) – Lumped ports (internal)



L3.1-3


Perfect 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

E-field Perpendicular to surface E-field Parallel to surface 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.


L3.1-4



Excitations


• 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



L3.1-5


Driven 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.)


L3.1-6



Excitations


• Example Solution Types:

Mode 1 (Even Mode)

Integration Line

Mode 2 (Odd Mode)

Integration Line Port1

Modal

2 Modes

Port2 2 Modes

Modes to Nodes Transformation

T1

T2 T1

T1 Port1

SPICE Differential Pairs

Terminal

T2


Port2 T2 Release 13.0 January 2011

L3.1-7


Ports


• 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


L3.1-8



Wave 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 1

Port 4

Port 3 Port 2



L3.1-9


Port 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  1  ∇ ×  ∇ × E (x , y ) − k02ε r E ( x, y ) = 0  µr 

• 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


L3.1-10



Wave 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 ANSYS, Inc. Proprietary © 2011 ANSYS, Inc. All rights reserved.


L3.1-11


Wave 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

8w, w ≥ h or 5w (3h to 4h), w < h

10w, w ≥ h or 5w (3h to 4h), w < h

w h 6h to 10h 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. L3.1-12



Wave Port Sizing Guidelines


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

• 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

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

– Port boundary must intersect both side ground planes or they will ‘float’ and become signal conductors Approx 7g minimum

– Total width ~10g or ~10s – Port outline must intersect both side grounds or they will ‘float’ and become signal conductors Larger of approx. 10g or 10s

Larger of 4h or 4g Larger of 4h or 4g

g

s h h

g

For Driven Modal solutions, use Zpv for impedance calculation



L3.1-13


Lumped 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

Dipole element with lumped port

Zo

Uniform electric field User-defined Zo


L3.1-14



Lumped vs Wave Ports for Planar Filters

• Wave ports can be used to feed printed transmission lines

• Lumped ports can be used to feed printed transmission lines

– S-parameters normalized to computed characteristic impedance – Multiple propagating modes possible – De-embedding available as postprocessing operation – Must touch background object (or be backed by conducting object)

– S-parameters normalized to userspecified characteristic impedance – Single mode propagation – No de-embedding operations available – Must be located inside model



L3.1-15



Lumped vs Wave Ports for Planar Filters


• Same results obtained from both port types

Lumped Ports

Wave Ports


L3.1-16



Wave Ports vs Lumped Ports


Wave port

Lumped port

External Faces

Internal to Model

Higher order modes

Yes

No

De-embedding

Yes

No

Re-normalization

Yes

Yes

Setup complexity

Moderate

Low

Accessibility


L3.1-17


Lecture 11 HFSS Boundary Conditions

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