Training · Radiation An ‘absorbing boundary condition,’ used at the periphery of a project in ... Perfect E Symmetry (top) Perfect H Symmetry (right side) TE20 Mode in WR90 Properly

smart software for high-frequency design5-1

Ansoft HFSS Version 7Training

Section 5: Boundary Module

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SynopsisGeneral Overview

Boundary Types, Definitions, and Parameters Source Types, Definitions, and Parameters Interface Layout

Assigning Boundaries Face Selection Precedence Assumptions (the ‘outer’ Boundary)

Boundary Setup Exercise Part 1: Define Boundaries inExample Model

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


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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-providedresistance and reactance in =Ω/

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 astructure 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 replicatinggeometry (e.g. an antenna array)


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HFSS Boundary Descriptions: Perfect E andPerfect H/Natural

Parameters: None Perfect E is a perfect electrical conductor*

Forces E-field perpendicular to the surface Represent metal surfaces, ground planes,

ideal cavity walls, etc. Perfect H is a perfect magnetic conductor

Forces H-field perpendicular to surface, E-field tangential

Does not exist in the real world, butrepresents useful boundary constraint formodeling

Natural denotes effect of Perfect H appliedon top of some other (e.g. Perfect E)boundary

‘Deletes’ the Perfect E condition,permitting but not requiring tangentialelectrical fields.

Opens a ‘hole’ in the Perfect E plane

Perfect E Boundary*

Perfect H Boundary

‘Natural’ Boundary

larperpendicuE

continuousE

parallelE

*NOTE: When you define a solid object as a‘perf_conductor’ in the Material Setup, aPerfect E boundary condition is applied to itsexterior surfaces!!


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HFSS Boundary Descriptions: FiniteConductivity

Parameters: Conductivity andPermeability

Finite Conductivity is a lossyelectrical conductor

E-field forced perpendicular, as withPerfect E

However, surface impedance takesinto account resistive and reactivesurface losses

User inputs conductivity (insiemens/meter) and relativepermeability (unitless)

Used for non-ideal conductoranalysis*

Finite Conductivity Boundary

gattenuatinlarperpendicuE ,

*NOTE: When you define a solid objectas a non-ideal metal (e.g. copper,aluminum) in the Material Setup module,and it is set to ‘Solve Surface’, a FiniteConductivity boundary is automaticallyapplied to its exterior faces!!


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HFSS Boundary Descriptions: Impedance

Parameters: Resistance andReactance, ohms/square (=Ω/χ)

Impedance boundary is a direct, user-defined surface impedance

Use to represent thin film resistors Use to represent reactive loads

Reactance will NOT vary withfrequency, so does not representa lumped ‘capacitor’ or ‘inductor’over a frequency band.

Calculate required impedance fromdesired lumped value, width, and length

Length (in direction of current flow) ÷Width = number of ‘squares’

Impedance per square = DesiredLumped Impedance ÷ number ofsquares

EXAMPLE: Resistor in Wilkenson Power Divider

Resistor is 3.5 mils long (in direction of flow) and4 mils wide. Desired lumped value is 35 ohms.

squareN

RR

N

lumpedsheet /40

875.35

875.045.3

Ω===

==


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HFSS Boundary Descriptions: RadiationParameters: None

A Radiation boundary is an absorbingboundary condition, used to mimiccontinued propagation beyond theboundary plane

Absorption is achieved via a second-order impedance calculation

Boundary should be constructed correctlyfor proper absorption

Distance: For strong radiators (e.g.antennas) no closer than λ/4 to anystructure. For weak radiators (e.g. abent circuit trace) no closer than λ/10to any structure

Orientation: The radiation boundaryabsorbs best when incident energyflow is normal to its surface

Shape: The boundary must beconcave to all incident fields fromwithin the modeled space

Note boundary does notfollow ‘break’ at tail endof horn. Doing sowould result in a convexsurface to interiorradiation.

Boundary is λ/4 away fromhorn aperture in all directions.


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HFSS Boundary Descriptions: Radiation,cont.

Radiation boundary absorption profilevs. incidence angle is shown at left

Note that absorption falls offsignificantly as incidence exceeds 40degrees from normal

Any incident energy not absorbed isreflected back into the model,altering the resulting field solution!

Implication: For steered-beam arrays,the standard radiation boundary maybe insufficient for proper analysis.

Solution: Use a Perfectly MatchedLayer (PML) construction instead.

Incorporation of PMLs is covered inthe Advanced HFSS training course.Details available upon request.

-100

-80

-60

-40

-20

0

20

Ref

lect

ion

Coe

ffic

ient

(dB

)

0 10 20 30 40 50 60

theta (deg)

Reflection Coefficient (dB)

70 80 90

Reflection of Radiation Boundary in dB, vs.Angle of Incidence relative to boundarynormal (i.e. for normal incidence, θ = 0)


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HFSS Boundary Descriptions: Symmetry

Parameters: Type (Perfect E or Perfect H) Symmetry boundaries permit modeling of

only a fraction of the entire structure underanalysis

Two Symmetry Options: Perfect E : E-fields are perpendicular to the

symmetry surface Perfect H : E-fields are tangential to the

symmetry surface Symmetry boundaries also have further

implications to the Boundary Manager andFields Post Processing

Existence of a Symmetry Boundary willprompt ‘Port Impedance Multiplier’ verification

Existence of a symmetry boundary allows fornear- and far-field calculation of the ‘entire’structure

Conductive edges, 4 sides

This rectangular waveguide contains asymmetric propagating mode, which couldbe modeled using half the volumevertically....

Perfect E Symmetry (top)

...or horizontally.

Perfect H Symmetry(left side)


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HFSS Boundary Descriptions: Symmetry,cont.

Geometric symmetry does notnecessarily imply field symmetryfor higher-order modes

Symmetry boundaries can act asmode filters

As shown at left, the next higherpropagating waveguide mode isnot symmetric about the verticalcenter plane of the waveguide

Therefore one symmetry case isvalid, while the other is not!

Implication: Use caution whenusing symmetry to assure that realbehavior in the device is not filteredout by your boundary conditions!!

Perfect E Symmetry (top)

Perfect H Symmetry(right side)

TE20 Mode in WR90

Properly represented withPerfect E Symmetry

Mode can not occur properlywith Perfect H Symmetry


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HFSS Boundary Descriptions: Master/SlaveBoundaries

Parameters: Coordinate system,master/slave pairing, and phasing

Master and Slave boundaries are usedto model a unit cell of a repeatingstructure

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 onthe master surface, with a phase shift.

Constraints: The master and slave surfaces must be

of identical shapes and sizes A coordinate system must be identified

on the master and slave boundary toidentify point-to-point correspondence

Unit Cell Model of End-Fire Waveguide Array

WG Port(bottom) Ground Plane

Perfectly Matched Layer(top)

Slave BoundaryMaster Boundary

Origin

V-axis

U-axis


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HFSS Source List Port

Most Commonly Used Source. Its use results in S-parameter outputfrom HFSS.

Two Subcategories: ‘Standard’ Ports and ‘Gap Source’ Ports Apply to Surface(s) of solids or to sheet objects

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 ‘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


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HFSS Source Descriptions: Port

Parameters: Mode Count, Calibration,Impedance, Polarization, Imp. Multiplier

A port is an aperture through whichguided electromagnetic field energy isinjected into a 3D HFSS model. Thereare two types:

Standard Ports: The aperture is solvedusing a 2D eigensolution which locatesall requested propagating modes

Characteristic impedance iscalculated from the 2D solution

Impedance and Calibration Linesprovide further control

Gap Source Ports: Approximated fieldexcitation is placed on the gap sourceport surface

Characteristic impedance isprovided by the user during setup

EXAMPLE STANDARD PORTS

EXAMPLE GAP-SOURCE PORTS


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HFSS Source Descriptions: Incident WaveParameters: Poynting Vector, E-field Magnitude and Vector

Used for radar cross section (RCS)scattering problems.

Defined by Poynting Vector(direction of propagation) and E-field magnitude and orientation

Poynting and E-field vectors mustbe orthogonal.

Multiple plane waves can becreated for the same project.

If no ‘ports’ are present in themodel, S-parameter output is notprovided

Analysis data obtained by post-processing on the Fields using theField Calculator, or by generatingRCS Patterns

In the above example, a plane incident wave isdirected at a solid made from dielectrics, to viewthe resultant scattering fields.


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HFSS Source Descriptions: Voltage Drop andCurrent Source

Example VoltageDrop (between

trace and ground)

Example CurrentSource (along trace

or across gap)Parameters: Direction and Magnitude

A voltage drop would be used toexcite a voltage between two metalstructures (e.g. a trace and a ground)

A current source would be used toexcite a current along a trace, oracross a gap (e.g. across a slotantenna)

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 thecurrent flow is also required


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HFSS Source Descriptions: Magnetic BiasParameters: Magnitude andDirection or Externally Provided

The magnetic bias source is usedonly to provide internal biasing H-field values for models containingnonreciprocal (ferrite) materials.

Bias may be uniform field (enterparameters directly in HFSS)...

Parameters are direction andmagnitude of the field

...or bias may be non-uniform(imported from externalMagnetostatic solution package)

Ansoft’s 3D EM FieldSimulator provides thisanalysis and output

Apply source to selected 3D solidobject (e.g. ferrite puck)


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Sources/Boundaries and EigenmodeSolutions

An Eigenmode solution is a direct solution of the resonantmodes of a closed structureAs a result, some of the sources and boundaries discussed sofar 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 areplacement


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The HFSS Source/Boundary Setup Interface

Side Window Coordinate Fields and Snap Options

Source/Boundary List Shows all sources andboundaries currentlyassigned to the projectand their status; allowsselection for viewing,editing, and deletion

Source/Boundary Control Allows Naming, contains executioncontrols (Assign, Clear, Units...)

Boundary Attributes Field Region Layout changes to provideentry fields for selected source or boundarycharacteristics and options.

Source/Boundary Drop-Down Lists all source or boundary types,based on radio button selected

Graphical View Window Shows geometry, permitspoint-and-click selection, vector definition, andassignment.

Source/Boundary Selection Buttons

Menu and Toolbar

Pick Options Controls selection optionsin graphical window


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Boundary Manager: Object/Face Selection The Graphical Pick options (1)

control the result of clicking in thegraphical view window.

Object: mouse-click selectsexterior of entire object

Face: mouse-click selectsclosest face of object

Boundary: mouse-click selectsclosest existing boundarycondition (if any)

To shift your focus to an object orface deeper into the model, use theright mouse menu (2) choice NextBehind, or the hotkey “N”

Selected faces will highlight in a gridpattern; selected objects will havetheir wireframe highlighted

Multiple faces may be selectedsimultaneously; a second clickdeselects already-selected faces

1.

2.

NOTE: The same graphical view manipulationshortcuts for rotation, panning, and zooming foundin the Draw module also work here; the visibilityicon also assists object/face selection by ‘hiding’exterior objects.


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Boundary Manager: Object/Face Selection,cont.

The Edit menu (3) provides furtherSelect options, including FacesIntersection

Faces intersection opens a listbox containing all objects inthe model

Selecting two touching objectsfrom the list will prompt theinterface to automatically findall intersecting faces

Note: only exterior faces inintersection are selected, notfaces of one object which areinside the volume of the other

The Edit menu Select option ByName (4) provides a list of all facesin the model, numbered and sortedby object, for selection.

4.

3.


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Boundary Assignment: General Procedure

Select Source orBoundary radio button,and desired type fromthe drop-down listing

Select the face or faceson which you wish toapply thesource/boundarycondition

(Above 2 stepsinterchangeable)

Fill in any necessaryparameters for thesource/boundary

Name thesource/boundary, andpress the Assign button

1. Select source or boundary and type

2. Select face(s)

3. Fill in Parameters as necessary4. Name and Assign

5. New Boundary willappear in list


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Boundary Assignment: Precedence Boundary assignments are

order dependent: Boundaries assigned later

supercede thoseassigned earlier over anyshared surfaces

Ports are the exception;they always supercedeany earlier or laterassignments

Ports will sort to thebottom of theboundary list toreflect this fact

Boundaries can bere-prioritized usingthe Model menu

In the pictured example, the ‘radiation’boundary overlays the orange rectangle(on the back face) which was earlierassigned as the port. Ports, however,always take precedence, and show at thebottom of the boundary listing.


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Boundary Assignment: Default Boundary Any exterior face of the

modeled geometry notgiven a user-definedboundary condition isassumed to be a Perfect E

Default boundary calledouter

Imagine entire modelburied in solid metalunless you instructotherwise

To view boundaries andsee if you missed anassignment, use theBoundary Display pick fromthe Model menu

Graphical window showsboth user and auto-assigned boundaries


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Boundary Setup Exercise Part 1 We will practice by

assigning boundaries to aCoax to Microstriptransformer model

This exercise is only Part 1of the entire operation;excitation assignment willbe covered after a detaileddescription of HFSSsources and portassignment

In the Maxwell ProjectManager, find the projectentitled “bnd_exer” andOpen it

Once open, proceed toSetup Boundaries/Sources

NOTE: The model for this exercise is nearlyidentical to that used in the Material Setupexercise, but has been split in half along the axisof the microstrip and coax feed to demonstratesymmetry boundary application as well.


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Boundary Setup Exercise: Trace MetalizationNOTE: Since solid Material parameters are alreadyapplied, there is already a boundary on the exterior of themetal objects “pin”, “pin1”, and “pin2”. We only need toapply the surface metalization for the actual microstriptrace line, and define outer radiation, ground plane, andsymmetry boundaries.

1. Select the Boundary radio Button.

2. From the list of available boundaries, select Perfect E.

3. Set the Graphical Pick option to Face.

4. Click in the graphical window as if you are touching thetrace. The nearest face of the air box will highlight, since itis between your view and the trace.

5. Right-click to bring up the pop-up menu and select NextBehind, or use the “N” key on the keyboard to shift focusdeeper. Continue this operation until the trace is selected.

NOTE: If you appear to have selected the bottom-mostface of the model, you have gone too far. Use the right-click menu to pick Deselect All and start over.

6. In the Name field, type in “trace_metal”, and click theAssign button.

7. The boundary should appear in the boundary list at left.

1. 2.

3.

4.

5.

6.

7.


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Boundary Setup Exercise: Radiation1. The Boundary radio button should remain selected.

2. From the list of available boundaries, select Radiation.

3. Leave the Graphical Pick option set to Face.

4. Click in the graphical window to touch the air volumesurrounding the structure on the three faces indicated. Youmay wish to rotate to facilitate your selection.

NOTE: Had this model been constructed with the air solidsitting on top of the substrate solid, instead of containingthe substrate solid, we would have to pick specific faces onthree sides of the substrate object as well.

5. In the Name field, type in “absorbing”, and click theAssign button.

6. The boundary should appear in the boundary list at left.

NOTE: We have assigned a Radiation boundary overwhere the microstrip port will need to be! This will besuperceded in a step in part 2 of this exercise, following theSource discussion.

1. 2.

3.

4. (Back, right side, top)

5.

6.


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Boundary Setup Exercise: Ground Plane1. The Boundary radio button should remainselected.

2. From the list of available boundaries, selectPerfect E.


4. Either rotate the model view to bring the lowerface to the front, and click on it, or click as thoughtouching the lower face of the air volume and usethe “N” key to shift focus deeper to the lowersurface of the air volume and substrate.

5. In the Name field, type in “ground_plane”, andclick the Assign button.

6. The boundary should appear in the boundary listat left.

NOTE: Since this is being assigned a Perfect Eboundary, we could have allowed the automatic“outer” boundary to take care of this face if wewished.

1. 2.

3.

4.

5.

6.


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Boundary Setup Exercise: Symmetry Plane1. The Boundary radio button should remain selected.

2. From the list of available boundaries, select Symmetry.


4. Click on the face of the model which bisects themicrostrip trace and coax. Once a face is selected, theoptions for the Symmetry boundary appear below thegraphical view. Click again in the model to select the cutfaces of the ‘thru_hole_in_wall’ and “coax_outer” cylindersas well. (You may wish to zoom in to assure you have thecorrect faces selected.)

NOTE: Again, if we had defined our air volume to sit atoprather than to contain the substrate, we would need toselect the substrate face too.

5. In the parameter space for the boundary, click theradio button for Perfect H type symmetry (E-fieldstangential to surface).

6. In the Name field, type in “mag_symmetry”, and clickthe Assign button.

7. The boundary should appear in the list at left.

THIS CONCLUDES PART 1 OF THE BOUNDARYSETUP EXERCISE. DO NOT EXIT THE BOUNDARY/SOURCE MANAGER.

1. 2.

3.

4.

5.

6.

4.

4.

7.


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HFSS Ports: A Detailed LookThe Port Solution provides the excitation for the 3D FEMAnalysis. Therefore, knowing how to properly define andcreate 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 requestedSince Port Assignment is so important, the following slides willgo into further detail regarding their creation.


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HFSS Ports: Setup InterfaceName Field Ports are always named“portN”. Box also includesAssign, Clear, and Optionsbuttons.

Mode Entry Field Set port mode solution requirements.Set polarization. Shows impedanceand calibration definitions applied, ifany.

Lumped Gap Source Port Option Activating enables Port Impedance entry fields.

Impedance and Calibration Line Fields ‘Edit Line’ dropdown allows setting,clearing, and relating Imped. and Calib.lines.

Impedance Multiplier Field Use if symmetry planes intersect ports.


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HFSS Port Selection: Standard or GapSource?

When would you choose touse a Gap Source Port over aStandard Port?

When the model has tightly-spaced lines

When ‘backing’ the portwould be too disruptive ofinternal fields

When a port referencelocation is difficult todetermine using a Standardport

When you’d like to use avoltage gap, but want S-parameter output

Gap Source Ports (blue)


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HFSS Ports: SizingA port is an aperture through which aguided-wave mode of some kindpropagates

For transmission line structures entirelyenclosed in metal, port size is merely thewaveguide interior carrying the guidedfields

Rectangular, Circular, Elliptical, Ridged,Double-Ridged Waveguide

Coaxial cable, coaxial waveguide, square-ax, Enclosed microstrip or suspendedstripline

For unbalanced or non-enclosed lines,however, field propagation in the airaround the structure must also be included

Parallel Wires or Strips Stripline, Microstrip, Suspended Stripline Slotline, Coplanar Waveguide, etc.

A Coaxial Port Assignment

A Microstrip Port Assignment(includes air above substrate)


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HFSS Ports: Sizing, cont.The port solver only understandsconductive boundaries on its borders

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 portedge termination!!

Result: Moving the port edges too closeto the circuitry for open waveguidestructures (microstrip, stripline, CPW,etc.) will allow coupling from the tracecircuitry to the port walls!

This causes an incorrect modal solution,which will suffer an immediatediscontinuity as the energy is injected pastthe port into the model volume

Port too narrow (fields coupleto side walls)

Port too Short(fields couple to top wall)


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HFSS Ports: Sizing Handbook IMicrostrip Port Sizing Guidelines

Assume width of microstrip trace is w Assume height of substrate dielectric

is hPort Height Guidelines

Between 6h and 10h Tend towards upper limit as dielectric

constant drops and more fields existin air rather than substrate

Bottom edge of port coplanar with theupper face of ground plane

(If real structure is enclosed lowerthan this guideline, model the realstructure!)

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

w

h

6h to10h

10w, w ≥ hor

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

Note: Port sizing guidelines are notinviolable rules true in all cases. Forexample, if meeting the height andwidth requirements outlined result in arectangular aperture bigger than λ/2on one dimension, the substrate andtrace may be ignored in favor of awaveguide mode. When in doubt,build a simple ports-only model andtest.


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HFSS Ports: Sizing Handbook II

Stripline Port Sizing Guidelines Assume width of stripline trace is w Assume height of substrate dielectric is h

Port Height Guidelines Extend from upper to lower groundplane,

hPort Width Guidelines

8w, for microstrip profiles with w ≥ h 5w, or on the order of 3h to 4h, for

microstrip profiles with w < hBoundary Note: Can also make sidewalls of port Perfect H boundaries

wh

8w, w ≥ hor

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


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HFSS Ports: Sizing Handbook III

Slotline Port Guidelines Assume slot width is g Assume dielectric height is h

Port Height: 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 at least 3g to either side

of slot, or 7g total minimum Port boundary must intersect both side

ground planes, or they will ‘float’ andbecome signal conductors relative tooutline ‘ground’

g

Approx 7g minimum

h

Larger of 4h or 4g


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HFSS Ports: Sizing Handbook IVCPW Port Guidelines

Assume slot width is g Assume dielectric height is h Assume center strip width is s

Port Height: 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, orthey will ‘float’ and become additional signalconductors along with the center strip.

Larger of approx. 10g or 10s

s

h

Larger of 4h or 4g

g


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HFSS Ports: Sizing Handbook V; Gap SourcePorts

Gap Source ports behave differently fromStandard Ports

Any port edge not in contact with metal structureor another port assumed to be a Perfect Hconductor

Gap Source Port Sizing (microstrip example): “Strip-like”: [RECOMMENDED] No larger than

necessary to connect the trace width to theground

“Wave-like”: No larger than 4 times the stripwidth and 3 times the substrate height

The Perfect H walls allow size to be smaller thana standard port would be

However, in most cases the strip-like applicationshould be as or more accurate

Further details regarding Gap Source Portsizing available as a separate presentation

Perfect H

Perfect H

Perfect E

Perfect E

Perfect H

Perfect H

Perfect E

Perfect H


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HFSS Ports: Spacing from Discontinuities Structure interior to the modeled volume may

create and reflect non-propagating modes These modes attenuate rapidly as they travel

along the transmission line If the port is spaced too close to a discontinuity

causing this effect, the improper solution will beobtained

A port is a ‘matched load’ as seen from themodel, but only for the modes it has beendesigned to handle

Therefore, unsolved modes incident upon it arereflected back into the model, altering the fieldsolution

Remedy: Space your port far enough fromdiscontinuities to prevent non-propagating modeincidence

Spacing should be on order of port size, notwavelength dependent

PortExtension


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HFSS Ports: Single-Direction Propagation Standard ports must be

defined so that only oneface can radiate energy intothe model

Gap Source Ports have nosuch restriction

Position Standard Ports onthe exterior of the geometry(one face on background) orprovide a port cap.

Cap should be the samedimensions as the portaperture, be a 3D solidobject, and be defined asa perfect conductor in theMaterial Setup module

Port on Exterior Face of Model

Port Inside Modeled Air Volume;Back side covered with Solid Cap


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HFSS Ports: Mode Count Ports should solve for all propagating modes

Ignoring a mode which does propagate will resultin incorrect S-parameters, by neglecting mode-to-mode conversion which could occur atdiscontinuities

However, requesting too many modes in the fullsolution also negatively impacts analysis

Modes in cutoff are more difficult to calculate; S-parameters for interactions between propagatingand non-propagating modes may not convergewell

What if I don’t know how many modes exist? Build a simple model of a transmission line only,

or run your model in “Ports Only” mode, andcheck!

You can alter the mode count before running thefull solution.

Degenerate mode ordering is controlled withcalibration lines (see next slide)

Circular waveguide, showing twoorthogonal TE11 modes and TM01mode (radial with Z-component).Neglecting the TM01 mode fromyour solution would cause incorrectresults.


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HFSS Ports: Degenerate Modes 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)’ Thus, mode-to-mode S-parameters may be

referenced incorrectly To enforce numbering, use a calibration line

and polarize the first mode to the line OR, introduce a dielectric change to slightly

perturb the mode solution and separate thedegenerate modes

Example: A dielectric bar only slightly higher inpermittivity than the surrounding medium willconcentrate the E-fields between parallelwires, forcing the differential mode to bedominant

If dielectric change is very small (approx. 0.001or less), impedance impact of perturbation isnegligible

For parallel lines, a virtual objectbetween them aids mode ordering.Note virtual object need not extendentire length of line to help at port.

In circular or square waveguide, use thecalibration line to force (polarize) the modenumbering of the two degenerate TE11modes. This is also useful because withouta polarization orientation, the two modesmay be rotated to an arbitrary angle insidecircular WG.


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HFSS Ports: Phase Calibration A second purpose of the calibration line is to

control the port phase references The 2D port eigensolver finds propagating

modes on each port independently The zero degree phase reference is chosen at

a point of maximum E-field intensity on the portface.

This occurs twice, with 180 degreesseparation, for each 360 degree cycle

Therefore the possibility exists for the softwareto select inconsistent phase references fromport to port, resulting in S-parameter errors

All port-to-port S-parameter phases, e.g.S21, will be off by 180 degrees

Solution: The calibration line defines thepreferred direction for the zero degreereference on each port.

Which of the above fieldorientations is the zero

degree phase reference?Calibration Line defines...


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HFSS Ports: Impedance Definitions HFSS provides port characteristic

impedances calculated using the power-current definition (Zpi)

Incident power is known excitation quantity Port solver integrates H-field around port

boundary to calculate current flow For many transmission line types, the power-

voltage or voltage-current definition ispreferred

Slot line, CPW: Zpv preferred TEM lines: Zvi preferred

HFSS can provide these characteristicimpedance values, as long as an impedanceline is identified

The impedance line defines the line alongwhich the E-field is integrated to obtain avoltage

Often it can be identical to the calibration line

For a Coax, the impedance line extendsradially from the center to outer conductor (orvice versa). Integrating the E-field along theradius of the coaxial dielectric provides thevoltage difference. In many instances, the impedance andcalibration lines are the same!


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HFSS Ports: Impedance Multiplier When symmetry is used in a model, the

automatic Zpi and impedance line-dependant Zpv and Zvi calculations willbe incorrect, since the entire portaperture is not represented.

Split the model with a Perfect Esymmetry case, and the impedance ishalved.

Split the model with a Perfect Hsymmetry case, and the impedance isdoubled.

The port impedance multiplier is just arenormalizing factor, used to obtain thecorrect impedance results regardless ofthe symmetry case used.

The impedance multiplier is applied toall ports, and is set during theassignment of any port in the model.

Whole Rectangular WG(No Symmetry)

Impedance Mult = 1.0

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


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Source Setup Exercise: Part 2 We will now complete the

Setup Boundaries/Sources exercise alreadybegun, by adding the twonecessary Ports to theproblem

Ports will use bothcalibration andimpedance lines, but willrequire only one mode oneach terminal


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Source Setup Exercise: Coaxial Port1. Select the Source radio button.

2. The source list should set by default to Port.

3. Zoom in on your model, or otherwise orientit so you have clear visual access to theextended face of the coaxial line. Click on theface to select it. The port parameter entryfields will now appear.

4. Leave the port name as “port1”, and thenumber of modes as “1”.

5. Check the box for Use Impedance Line.This enables the Edit Line dropdown menubeside it. In the Edit Line dropdown, pick Set...

6. The side window will prompt you to SetImpedance Start. Define a starting point for theimpedance line by clicking in the graphicalwindow to snap to the vertex on the innerconductor, at the topmost point where itintersects the symmetry plane. (A first clickmay be necessary to activate the windowbefore selecting vertices.)

7. Click the Enter button in the side window toconfirm your point selection. The window nowshifts to request the vector information.(proceed to next page)

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Source Setup Exercise: Coaxial Port, cont.The interface will now show a vector from thestarting point you defined to the origin. (This ismerely a default ‘guess’ at the intendedendpoint.) The side window shows vector entryfields.

8. In the graphical window, snap to the pointradial from the starting point (on the outerconductor radius, at the topmost intersectionwith the symmetry plane). The vector fieldsshould update to reflect a Z-directed vector.

9. Press the Enter button on the side windowto confirm the vector end point. The sidewindow interface closes, and the completedimpedance line is displayed as a red vectorwith the letter “I”.

10. Check the box for Use Calibration Line. Inthe enabled Edit Line dropdown to the right,pick Copy Impedance. The vector will nowupdate to include a “C” indication.

11. Press the Assign button to complete theport creation. The boundary list will nowupdate to show “port1”

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Source Setup Exercise: Microstrip PortRotate and resize the graphical window so thatyou have visual access to the microstriptermination end of the model.

1. Source radio button and Port type arealready selected.

2. Click on the 2D rectangle provided for themicrostrip port face. If the entire face of the airvolume is selected, use the Next Behind menupick or “N” hotkey to shift the selection.

3. Leave the port name as “port2”, and thenumber of modes as “1”.

4. Use Impedance Line should remain checkedfrom the prior port assignment. In the Edit Linedropdown, pick Set...

5. The side window will prompt you to SetImpedance Start. Define a starting point for theimpedance line by clicking in the graphicalwindow to snap to the vertex on the trace, atthe point where it intersects the symmetryplane.

6. Click the Enter button in the side window toconfirm your point selection. The window nowshifts to request the vector information.(proceed to next page)

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Source Setup Exercise: Microstrip Port, cont.The interface will now show a vector from thestarting point you defined to the prior port’sending point. The side window shows vectorentry fields.

7. In the graphical window, snap to the pointwhere the ground plane intersects the symmetryface.

8. Press the Enter button on the side window toconfirm the end point. The side window interfacecloses, and the completed line is displayed.

9. Use Calibration Line should already bechecked. In the enabled Edit Line dropdown tothe right, pick Copy Impedance.

10. Before assigning the port, we need to set theImpedance Multiplier for the model. Enter avalue of 0.5.

11. Press the Assign button to complete the portassignment. You will receive an overlapwarning, because the port overlays the earlier“radiation” boundary. After the overlap warningmessage is dismissed “port2” will show in theboundary list.

12. We are now done with boundaryassignment. To verify our assignment, pickBoundary Display from the Model menu.

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Source Setup Exercise: Verifying BoundariesHFSS will now perform only the initial meshingnecessary to subdivide the problem intotetrahedra, so that actual boundary application tothe finite element mesh can be viewed.

13. The list of assigned boundaries is on the left.Note that it contains both boundaries we created,plus the boundaries “i_pinn” and “outer”. The“i_pinn” boundaries were assigned as a result ofassigning a finite conductivity metal -- copper --to the pin objects. The “outer” boundary isapplied to any surface of the model we did nototherwise define. Highlight “outer” in theboundary listing.

14. Press the Toggle Display button. The meshon the selected boundary is displayed, indicatingthe surfaces on which this boundary is applied.Note that it provides the Perfect E definition onthe outer conductor of the coax, on the outerconductor of the thru hole, and on the front faceof the model which represents the metal modulewall.

15. If you wish you may continue displayingadditional boundaries. When you are through,press the Close button to return to the SetupBoundaries window. There, pick Exit from theFile menu and save when prompted. (Theoverlap warning will repeat on exit.)

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