5-1 Section 6: Boundary Module Getting Started: Ansoft HFSS 8.0.

5-1

Section 6: Boundary Module

Getting Started: Ansoft HFSS 8.0

5-2

Synopsis

General 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 in Example 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-provided resistance and reactance in /Square

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 Master and Slave

‘Linked’ boundary conditions for unit-cell studies of infinitely replicating geometry (e.g. a slow wave circuit & an antenna array)

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HFSS Boundary Descriptions: Perfect E and Perfect 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, 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

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, a Perfect E boundary condition is applied to its exterior surfaces!!

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Perfect Hfor 2D Aperture (I)

Monopole Over a Ground plane

Perfect H Surface Interior to the Problem Space Behaves Like an Infinitely Thin 2D Aperture

Perfect H

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Perfect Hfor 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

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HFSS Boundary Descriptions: Finite Conductivity

Parameters: Conductivity and Permeability

Finite Conductivity is a lossy electrical conductor

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*

Finite Conductivity Boundary

gattenuatinlarperpendicuE ,

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

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HFSS Boundary Descriptions: Impedance

Parameters: Resistance and Reactance, ohms/square (/)

Impedance boundary is a direct, user-defined surface impedance

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

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

Impedance per square = Desired Lumped Impedance number of squares

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

875.04

5.3

5-9

HFSS Boundary Descriptions: Radiation

Parameters: None A Radiation boundary is an absorbing

boundary condition, used to mimic continued propagation beyond the boundary plane

Absorption is achieved via a second-order 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

Note boundary does not follow ‘break’ at tail end of horn. Doing so would result in a convex surface to interior radiation.

Boundary is /4 away from horn aperture in all directions.

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HFSS Boundary Descriptions: Radiation, cont.

Radiation boundary absorption profile vs. incidence angle is shown at left

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.

-100

-80

-60

-40

-20

0

20

Refl

ect

ion

Co

effi

cien

t (d

B)

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 boundary normal (i.e. for normal incidence, = 0)

ETM

θ

5-11

HFSS Boundary Descriptions: Symmetry

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

only a fraction of the entire structure under analysis

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 and Fields Post Processing

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

Conductive edges, 4 sides

This rectangular waveguide contains a symmetric propagating mode, which could be modeled using half the volume vertically....

Perfect E Symmetry (top)

...or horizontally.

Perfect H Symmetry(left side)

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HFSS Boundary Descriptions: Symmetry, cont.

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

Symmetry boundaries can act as mode filters

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 E Symmetry (top)

Perfect H Symmetry(right side)

TE20 Mode in WR90

Properly represented with Perfect E Symmetry

Mode can not occur properly with Perfect H Symmetry

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HFSS Boundary Descriptions: Master/Slave Boundaries

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

Master and Slave boundaries are used to model a unit cell of a repeating structure

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.

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 to identify 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 output

from 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 (I)

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HFSS Source Descriptions: Port (II)

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

A port is an aperture through which guided electromagnetic field energy is injected into a 3D HFSS model. There are two types:

Standard 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

Gap Source Ports: Approximated field excitation is placed on the gap source port surface

Characteristic impedance is provided by the user during setup

EXAMPLE STANDARD PORTS

EXAMPLE GAP-SOURCE PORTS

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HFSS Source Descriptions: Incident Wave (I)

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HFSS Source Descriptions: Incident Wave (II)

Parameters: 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 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 post-processing on the Fields using the Field Calculator, or by generating RCS Patterns

In the above example, a plane incident wave is directed at a solid made from dielectrics, to view the resultant scattering fields.

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HFSS Source Descriptions: Voltage Drop and Current Source (I)

Voltage Drop

Current Drop

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HFSS Source Descriptions: Voltage Drop and Current Source (II)

Example Voltage Drop (between

trace and ground)

Example Current Source (along trace

or across gap)

Parameters: Direction and Magnitude 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

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HFSS Source Descriptions: Magnetic Bias

Parameters: Magnitude and Direction or Externally Provided

The magnetic bias source is used only to provide internal biasing H-field 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)

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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: 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

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The HFSS Source/Boundary Setup Interface

Side Window Coordinate Fields and Snap Options

Source/Boundary List Shows all sources and boundaries currently assigned to the project and their status; allows selection 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 the graphical view window.

Object: mouse-click selects exterior of entire object

Face: mouse-click selects closest face of object

Boundary: mouse-click selects closest existing boundary condition (if any)

To shift your focus to an object or face deeper into the model, use the right mouse menu (2) choice Next Behind, or the hotkey “N”

Selected faces will highlight in a grid pattern; selected objects will have their wireframe highlighted

Multiple faces may be selected simultaneously; a second click deselects already-selected faces

1.

2.

NOTE: The same graphical view manipulation shortcuts for rotation, panning, and zooming found in the Draw module also work here; the visibility icon also assists object/face selection by ‘hiding’ exterior objects.

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Boundary Manager: Object/Face Selection, cont.

The Edit menu (3) provides further Select options, including Faces Intersection

Faces intersection opens a list box containing all objects in the model

Selecting two touching objects from the list will prompt the interface to automatically find all intersecting faces

Note: only exterior faces in intersection are selected, not faces of one object which are inside the volume of the other

The Edit menu Select option By Name (4) provides a list of all faces in the model, numbered and sorted by object, for selection.

4.

3.

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Boundary Assignment: General Procedure

Select Source or Boundary radio button, and desired type from the drop-down listing

Select the face or faces on which you wish to apply the source/boundary condition

(Above 2 steps interchangeable)

Fill in any necessary parameters for the source/boundary

Name the source/boundary, and press 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 will appear in list

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Boundary Assignment: Precedence

Boundary assignments are order dependent:

Boundaries assigned later supercede those assigned earlier over any shared surfaces

Ports are the exception; they always supercede any earlier or later assignments

Ports will sort to the bottom of the boundary list to reflect this fact

Boundaries can be re-prioritized using the Model menu

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

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Boundary Assignment: Default Boundary

Any exterior face of the modeled geometry not given a user-defined boundary condition is assumed to be a Perfect E

Default boundary called outer

Imagine entire model buried in solid metal unless you instruct otherwise

To view boundaries and see if you missed an assignment, use the Boundary Display pick from the Model menu

Graphical window shows both user and auto-assigned boundaries

5-29

Boundary Setup Exercise Part 1

We will practice by assigning boundaries to a Coax to Microstrip transformer model

This exercise is only Part 1 of the entire operation; excitation assignment will be covered after a detailed description of HFSS sources and port assignment

In the Maxwell Project Manager, find the project entitled “bnd_exer” and Open it

Once open, proceed to Setup Boundaries/Sources

NOTE: The model for this exercise is nearly identical to that used in the Material Setup exercise, but has been split in half along the axis of the microstrip and coax feed to demonstrate symmetry boundary application as well.

5-30

Boundary Setup Exercise: Trace Metalization

NOTE: Since solid Material parameters are already applied, there is already a boundary on the exterior of the metal objects “pin”, “pin1”, and “pin2”. We only need to apply the surface metalization for the actual microstrip trace line, and define outer radiation, ground plane, and symmetry 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 the trace. The nearest face of the air box will highlight, since it is between your view and the trace.

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

NOTE: If you appear to have selected the bottom-most face 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 the Assign button.

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

1. 2.

3.

4.

5.

6.

7.

5-31

Boundary Setup Exercise: Radiation

1. 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 volume surrounding the structure on the three faces indicated. You may wish to rotate to facilitate your selection.

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

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


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

1. 2.

3.

4. (Back, right side, top)

5.

6.

5-32

Boundary Setup Exercise: Ground Plane


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


4. Either rotate the model view to bring the lower face to the front, and click on it, or click as though touching the lower face of the air volume and use the “N” key to shift focus deeper to the lower surface of the air volume and substrate.

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


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

1. 2.

3.

4.

5.

6.

5-33

Boundary Setup Exercise: Symmetry Plane


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


4. Click on the face of the model which bisects the microstrip trace and coax. Once a face is selected, the options for the Symmetry boundary appear below the graphical view. Click again in the model to select the cut faces of the ‘thru_hole_in_wall’ and “coax_outer” cylinders as well. (You may wish to zoom in to assure you have the correct faces selected.)

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

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

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

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

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

1. 2.

3.

4.

5.

6.

4.

4.

7.

5-34

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.

5-35

HFSS Ports: Setup Interface

Name Field Ports are always named “portN”. Box also includes Assign, Clear, and Options buttons.

Mode Entry Field Set port mode solution requirements. Set polarization. Shows impedance and calibration definitions applied, if any.

Type OptionWave port & Lumped Gap Source.

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.

5-36

HFSS Port Selection: Standard or Gap Source?

When would you choose to use a Gap Source Port over a Standard Port?

When the model has tightly-spaced lines

When ‘backing’ the port would be too disruptive of internal fields

When a port reference location is difficult to determine using a Standard port

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

Gap Source Ports (blue)

5-37

HFSS Ports: Sizing

A port is an aperture through which a guided-wave mode of some kind propagates

For transmission line structures entirely enclosed in metal, port size is merely the waveguide interior carrying the guided fields

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

Coaxial cable, coaxial waveguide, square-ax, 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.

A Coaxial Port Assignment

A Microstrip Port Assignment (includes air above substrate)

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HFSS Ports: Sizing, cont.

The port solver only understands conductive 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 port edge termination!!

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!

This causes an incorrect modal solution, which will suffer an immediate discontinuity as the energy is injected past the port into the model volume

Port too narrow (fields couple to side walls)

Port too Short(fields couple to top wall)

5-39

HFSS Ports: Sizing Handbook I

Microstrip Port Sizing Guidelines Assume width of microstrip trace is w Assume height of substrate dielectric

is h Port Height Guidelines

Between 6h and 10h 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

w

h

6h to 10h

10w, w hor

5w (3h to 4h), 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.

5-40

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,

h Port Width Guidelines

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

w

h

8w, w hor

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

5-41

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’ and become signal conductors relative to outline ‘ground’

g

Approx 7g minimum

h

Larger of 4h or 4g

5-42

HFSS Ports: Sizing Handbook IV

CPW 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, or they will ‘float’ and become additional signal conductors along with the center strip.

Larger of approx. 10g or 10s

s

h

Larger of 4h or 4g

g

5-43

HFSS Ports: Sizing Handbook V; Gap Source Ports

Gap Source ports behave differently from Standard Ports

Any port edge not in contact with metal structure or another port assumed to be a Perfect H conductor

Gap Source Port Sizing (microstrip example): “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

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 Gap Source Port sizing available as a separate presentation

Perfect H

Perfect H

Perfect E

Perfect E

Perfect H

Perfect H

Perfect E

Perfect H

5-44

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 be obtained

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

PortExtension

5-45

HFSS Ports: Single-Direction Propagation

Standard ports must be defined so that only one face can radiate energy into the model

Gap Source Ports have no such restriction

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

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

Port on Exterior Face of Model

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

5-46

HFSS Ports: Mode Count

Ports should solve for all propagating modes Ignoring a mode which does propagate will result

in incorrect S-parameters, by neglecting mode-to-mode conversion which could occur at discontinuities

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

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

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, and check!

You can alter the mode count before running the full solution.

Degenerate mode ordering is controlled with calibration lines (see next slide)

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.

5-47

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 the degenerate modes

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

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.

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.

5-48

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 port face.

This occurs twice, with 180 degrees separation, for each 360 degree cycle

Therefore the possibility exists for the software to select inconsistent phase references from port 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 the preferred direction for the zero degree reference on each port.

Which of the above field orientations is the zero

degree phase reference? Calibration Line defines...

5-49

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 is preferred

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

Often it can be identical to the calibration line

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!

5-50

HFSS Ports: Impedance Multiplier

When symmetry is used in a model, the automatic Zpi and impedance line-dependant Zpv and Zvi calculations will be incorrect, since the entire port aperture is not represented.

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.

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

5-51

Source Setup Exercise: Part 2

We will now complete the Setup Boundaries/ Sources exercise already begun, by adding the two necessary Ports to the problem

Ports will use both calibration and impedance lines, but will require only one mode on each terminal

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Source Setup Exercise: Coaxial Port

1. Select the Source radio button.

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

3. Zoom in on your model, or otherwise orient it so you have clear visual access to the extended face of the coaxial line. Click on the face to select it. The port parameter entry fields will now appear.

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

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

6. The side window will prompt you to Set Impedance Start. Define a starting point for the impedance line by clicking in the graphical window to snap to the vertex on the inner conductor, at the topmost point where it intersects the symmetry plane. (A first click may be necessary to activate the window before selecting vertices.)

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

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5-53

Source Setup Exercise: Coaxial Port, cont.

The interface will now show a vector from the starting point you defined to the origin. (This is merely a default ‘guess’ at the intended endpoint.) The side window shows vector entry fields.

8. In the graphical window, snap to the point radial from the starting point (on the outer conductor radius, at the topmost intersection with the symmetry plane). The vector fields should update to reflect a Z-directed vector.

9. Press the Enter button on the side window to confirm the vector end point. The side window interface closes, and the completed impedance line is displayed as a red vector with the letter “I”.

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

11. Press the Assign button to complete the port creation. The boundary list will now update to show “port1”

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5-54

Source Setup Exercise: Microstrip Port

Rotate and resize the graphical window so that you have visual access to the microstrip termination end of the model.

1. Source radio button and Port type are already selected.

2. Click on the 2D rectangle provided for the microstrip port face. If the entire face of the air volume is selected, use the Next Behind menu pick or “N” hotkey to shift the selection.

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

4. Use Impedance Line should remain checked from the prior port assignment. In the Edit Line dropdown, pick Set...

5. The side window will prompt you to Set Impedance Start. Define a starting point for the impedance line by clicking in the graphical window to snap to the vertex on the trace, at the point where it intersects the symmetry plane.

6. Click the Enter button in the side window to confirm your point selection. The window now shifts 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 the starting point you defined to the prior port’s ending point. The side window shows vector entry fields.

7. In the graphical window, snap to the point where the ground plane intersects the symmetry face.

8. Press the Enter button on the side window to confirm the end point. The side window interface closes, and the completed line is displayed.

9. Use Calibration Line should already be checked. In the enabled Edit Line dropdown to the right, pick Copy Impedance.

10. Before assigning the port, we need to set the Impedance Multiplier for the model. Enter a value of 0.5.

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

12. We are now done with boundary assignment. To verify our assignment, pick Boundary Display from the Model menu.

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Source Setup Exercise: Verifying Boundaries

HFSS will now perform only the initial meshing necessary to subdivide the problem into tetrahedra, so that actual boundary application to the 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 of assigning a finite conductivity metal -- copper -- to the pin objects. The “outer” boundary is applied to any surface of the model we did not otherwise define. Highlight “outer” in the boundary listing.

14. Press the Toggle Display button. The mesh on the selected boundary is displayed, indicating the surfaces on which this boundary is applied. Note that it provides the Perfect E definition on the outer conductor of the coax, on the outer conductor of the thru hole, and on the front face of the model which represents the metal module wall.

15. If you wish you may continue displaying additional boundaries. When you are through, press the Close button to return to the Setup Boundaries window. There, pick Exit from the File menu and save when prompted. (The overlap warning will repeat on exit.)

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5-1 Section 6: Boundary Module Getting Started: Ansoft HFSS 8.0.

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Transcript of 5-1 Section 6: Boundary Module Getting Started: Ansoft HFSS 8.0.