Lab1 Microstrip Line FEM FDTD

Lab 1: Microstrip Line

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In this lab, you will build a simple microstrip line to quickly familiarize yourself with the

EMPro User Interface and how to setup FEM and FDTD simulations. If you are doing

only the FEM or FDTD class, just skip the sections you are not interested in.

1. Getting Started

Once EMPro opens, use File > Save Project As to immediately define a project name

Lab1_ustrip_WG.ep and a location for your projects. You may want to first create a

folder such as C:\home\EMPro_Projects as a unique place to store EMPro projects.

2. Set Project Preferences

a. Select Edit > Application Preferences to bring up the Application Preferences

editor. Take a look at what is there. The Undo history size is not limited by

default. It should typically not be required to set a limit as only actions, and not

model states, are stored in memory.

The Clear Results on Project Load can be used to empty the Results window of

data from the previously loaded project. If unchecked, loading new projects adds

the results to those from previously loaded projects.

The Transparency Algorithm can be quite useful when trying to visualize models

with overlapping 3D shapes.


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b. Click on the Interface tab.

c. Check that the option Edit Properties is selected.. This will automatically open

the properties editor when you define a new material.

d. Click on the Modeling tab.

e. Here you can set the default colors used to display faces, edges, and/or vertices for

parts or materials when they are first created. You can also set the color of the

components (like voltage sources).

f. Press OK to apply the changes and close the editor.


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3. Set Modeling Units

a. In the pull-down menus, select Edit > Project Properties to bring up the Project

Properties Editor.

b. Click on the Display Units tab.

c. Check that the default Length units are millimeters.

d. Press Done.

Simulation Method

At the bottom left of the EMPro window are two buttons used to set the type of

simulations that will be performed. A geometry can be simulated using either FDTD or

FEM or both methods. Clicking the buttons enables so-called “user-interface skins” – that

is, some features are enabled or disabled based on this setting. For example, if FEM is

selected, notice that “External Excitations” becomes inactive (greyed out) because FEM

does not support such sources.

In this lab, we will use FEM first using features unique to this

method, so enable only FEM for now.


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4. Draw the Substrate

a. The Geometry Tools button in the Geometry window should be active by default.

Select Create > Extrude. This will bring you to the Edit Cross Section tab.

b. Click on the Specify Orientation tab and verify that the origin is located at

(0, 0, 0). The default drawing plane is the XY-plane.

c. Click on the Edit Cross Section tab. Ensure that all

snapping tools are selected to facilitate the correct

placement of points.

d. Double-click on the Z-axis to switch to a flat

XY-plane view or select the Top (-Z) orientation

using the View Tools button.

e. Select the Construction Grid button.

f. Set the minor grid Line Spacing to 1

mm.

g. Set Mouse Spacing to increments of

0.1 mm.

h. Select OK.


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i. Choose the Rectangle tool from the

shape icons.

j. Draw a rectangle from (0,0) to (15,20) in the

sketching plane. You can click the Tab key to

enter coordinates (but the window must be

active, that is, you have to have clicked inside

the drawing plane before starting the draw

action).

k. Click the Select/Manipulate tool, then

move the pointer over the bottom left

vertex, and with a right-click choose Edit Position from the pop-up menu.

l. Validate that U=0 mm and V=0 mm. Press OK when done.

m. Use the Select/Manipulate tool to highlight

the upper right vertex, and with the right mouse

button select Edit Position from the pop-up

menu.

n. Check that the values are U=15 mm and

V=20 mm and click OK.

o. Click on the Extrude tab.

p. Enter the value “2 mm” as the Extrude Distance in the W‟-direction.

Rotate the view and observe the arrow that indicates along which axis the sweep

will be performed. Experiment by changing U‟ and V‟ to see the effect on the

extrude axis. When done reset back to extrude only along W‟ direction. You can

also graphically drag the extrude vector.

q. Type Substrate in the Name field.

r. Press Done.

s. Note the new object in the Parts branch of the

Project Tree. You can open the Substrate branch

and double-click the Extrude entry to modify the

part at any time in the future (even after the part

has been combined with other parts using

Boolean operations).


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5. Draw the Microstrip Line

a. Select Create > Sheet Body. (A sheet body is simply a 2 Dimensional shape.)

b. Click on the Specify Orientation tab. Select the TOP (-Z) orientation from

the View Tools. Note that the last view you selected is remembered and the icon

changes to indicate which

plane you last selected (e.g, no

need to select from the list, just

click the icon).

c. Edit the Z-value to be 2 mm.

(This will move the drawing

plane to the top of the

substrate).

d. Click on the Edit Profile tab.

e. Select the Rectangle tool. Click on the point U’=0

mm, V’=9 mm on the left edge of the substrate (see the

coordinate readout at bottom right of the window or use

the Tab key).

f. Click a second point at U’=15 mm, V’=11 mm on the right edge of the substrate.

g. Select the Select/Manipulate tool , then move the pointer over the bottom left

vertex, and with a right-click, select Edit Position from the pop up menu.

h. Correct the values to be (0, 9) if necessary.

i. Repeat the check on the top, right vertex.

j. Name the sheet body Line.

k. Click Done and then Save the project.


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Note: Since no materials have been assigned, both objects appear with the default colors.

If you want to see outlines of the objects (along with their bounding box dimensions), just

select them in the Project Tree or in the Geometry window.

6. Define Materials

The assignment of materials can be done either after all the objects have been defined or

as each object is completed. The second approach has the benefit that the objects can be

more easily distinguished from each other.

A large set of materials can easily be inserted from the supplied library, but here you will

create your own defintions. These can also be stored as library parts and copied into any

other project.

a. Right-click on the Definitions > Materials branch of the Project Tree and select

New Material Definition.


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b. Type Cu in the Name field.

c. Select Isotropic in the Electric

drop-down list.

d. Under the Electric tab, use Type

Nondispersive, Entry Method Normal,

and type in 5.8e7 S/m for Conductivity.

e. In the APPEARANCE tab, pick

yellow for the Face Color and a light

shade of yellow for the Specular

Color. The Specular Color

determines how “bright” the object

will appear when the light source

reflects directly to the viewer. If you

plan to view object edges and/or

vertices, it is useful to pick other

shades of yellow for those items as

well.

f. Click Done to add the new material to

the Project Tree.

g. Add another material and define an Isotropic, Nondispersive material called

Dielectric using the Loss Tangent Entry Method with epsr=9.9 and tanD=0.005 at

1 GHz.

h. In the Appearance tab, select shades of green for the Face Color and Specular

Color.


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7. Drag-and-Drop Material Definitions

a. Drag the Dielectric material icon on top of the

Substrate object in the Parts branch .

b. Drag the Cu material icon on top of the Line object

under the Parts branch.

c. Experiment with the settings in the Appearance tab

of the Cu and Dielectric material defintions. Turn

Face/Edge/Vertex display on/off, change colors,

Vertex Size (0.4 is a good value), and notice the effect of Specular Color on the

object brightness when you rotate the faces towards the light source.

8. Define the Outer Boundary Conditions

The outer boundary conditions on each of the six sides of the model space must be set to

indicate how the EM simulators will interpret the space beyond them.

a. Double-click on the Simulation Domain >

Boundary Conditions branch of the Project

Tree to open the Boundary Conditions Editor.

b. Set the Lower Z boundary to PEC. This sets

the bottom of the substrate as a perfect

conductor to form the ground plane.

c. Note the Absorption Type is greyed

out because these only apply to

FDTD whose UI skin you turned off

earlier.

d. Click Done.


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9. Set Bounding Box Dimensions

a. Double-click the FEM Padding branch.

b. The FEM Padding Editor opens up. Set the lower and

upper X- and Y-padding to 0 mm. The lower

Z-padding must also be 0 mm to make sure the

bottom PEC surface touches the bottom of the

substrate. Set the upper Z-padding to 10 mm. Since

the FEM absorbing boundary acts like a resisitve

surface if placed too close to a structure, we want to

move it some multiple of the substrate thickness and line width away.

c. The FEM bounding box indicators are 3 white line segments terminated in red

dots. You can control bounding box visibility with the corresponding icon on the

right toolbar.


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10. Define Waveguide Ports for FEM Simulation

a. Right-click on the Circuit Components/Ports

branch and choose New Waveguide Port.

b. Click the Pick tool and then hover the mouse

over the substrate face at X=0 mm and click

to define the port. A transparent surface

appears to indicate the port cross section that

will be used by the FEM port solver.

c. On the Properties tab, change the name to

Waveguide Port1 and select the

50 ohm Voltage Source for the

Waveguide Port Definition. This will

result in 50 Ohm S-Parameters

(renormalized from the generalized

S-Parameters using the Power/Current

characteristic impedance computed by the

port solver). If you were to choose the

1W Modal Power Feed instead, you would

get the generalized S-Parameters instead.


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d. Click the Impedance Lines tab. Define the line along which the voltage for the

Zpv and Zvi characteristic impedances will be computed (the electric field for

Mode 1 will be integrated along this line). Click the Endpoint 1 Pick tool and then

click on the midpoint of the bottom substrate edge.

e. Click the Endpoint 2 Pick tool and then click on the midpoint of the strip edge.

You could also type the coordinates directly.

f. Click OK.

g. Repeat the procedure for the 2nd

port, changing the name to Waveguide Port2.

You have to rotate the model to be able to select the opposite face.

h. Save the project.


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11. Run an FEM Simulation

a. Click on the Simulations tab on the right workspace menu.

Choose New FEM Simulation.

b. Add an adaptive frequency sweep from 1 GHz to 10 GHz with 20 Sample Points

Limit using the Add To List button.

c. Click on the Setup Mesh/Discretisation tab. Define a Delta error of 0.02,

3 minimum adaptive passes and 10 maximum adaptive passes.


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d. Under the Matrix Solver tab, select the Direct matrix solver.

e. Click Create & Queue Simulation. In the Output tab, you can see entries for the

adaptive pass number, simulation frequency, mesh count, CPU and elapsed times

for the mesher, matrix size in number of unknowns, memory consumption in GB,

CPU and elapsed time for the matrix solver, and the DeltaS values compared to the

desired Delta Error:

(0.010[->0.020]). If Delta Error is met, the adaptive sweep proceeds with the

previous to last mesh (9200 unknowns – may vary from release to release due to

algorithm changes).


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12. View Results

a. Click on the Results tab to open the corresponding window.

b. Experiment with the column selections available to help identify the data by

right-clicking any of the column titles. The top part of the window has four fixed

columns that can be used to filter the data making it easier to find the data of

interest. The bottom part shows the actual data content based on the filter

selections you make above. The column settings are saved across sessions, but can

be reset using the application preference Clear Saved Layout Now.


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c. Click on S-Parameters under the Result Type column. Then click the Domain

column (make sure it is turned on) to put the Frequency data (AFS sweep) before

the discrete frequency points.

d. Right-click on the entry for S21 and choose View (default).

e. Notice that an entry is created under the

Graphs branch of the Project Tree. Here

you can rename, delete and open the graph.

f. Save the project.


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13. Create Project Templates

Templates can be configured with models, materials and sources already assembled to

form the basis of new designs. Templates also contain the Construction Grid settings and

any custom units you may have defined.

a. Use the pull-down menu Edit > Application Preferences and under the General

tab, notice the location on your computer to which templates are saved.

b. Select File> Manage Project Templates.

c. Click Create New and enter a name for your template in the dialog box such as

Lab1_template and then click Create.

d. Close the dialog box.

e. Templates can be used with the command File > New Project From Template.


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14. Define Discrete Sources

a. Make a copy of the project using File > Save Project As and call it

Lab1_ustrip_Int.ep.

b. Under the Results window, click on the Project Name Lab1_ustrip_WG and then

click Unlist Project.

c. Notice the warning icons that appear next to the graph entries. Moving the mouse

over them, you can see that the plots are now invalid because the project whose

data was being plotted is no longer in memory. Delete those two graphs (select

both and right-click on top to see the menus).

d. Select the Waveguide Ports and delete

them both.

e. Right-click on Circuit Components/Ports, and select New Circuit Component

with > No Definition.


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f. No new feed definition is needed because when

you created the waveguide ports, two

definitions were automatically created under the

Circuit Component Definitions branch.

g. Select the Pick tool for the location of Endpoint 1. Position the mouse over the

bottom substrate edge at X=0 mm. Click on the midpoint of the edge.

h. Select the Pick tool for Endpoint 2 and select the midpoint of the top microstrip

edge. An arrow indicating the component location appears.

i. Check the coordinates and correct them manually if necessary.

j. Click on the Properties tab and name the new component Port1.

k. Select the 50 ohm Voltage Source for the Component Definition pull-down

menu.

l. Click Done.


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m. Repeat the procedure to define Port2 at X=15 mm.

n. Double-click the FEM Padding branch and edit the

X-padding to be 5 mm in both directions. The voltage

sources are not allowed to touch an outer boundary.

o. Save the new project.

15. Simulate using FEM

a. Click the Simulations tab and click on New FEM Simulation. All the data

should still be set from the previous simulation.

b. Click Create & Queue Simulation. Check the Output tab until the simulation

finishes.

16. Compare the Results

a. Click the Results tab.

b. Use the List Project button to

reload the simulation results for

Lab1_ustrip_WG.

c. Select the two S21 entries from the

two simulations (use filters and

sort as necessary and use the CTRL key to select distinct entries). Right-click on

one of them and choose View (default).


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d. The results are quite different at higher frequencies due to the parasitic effect of the

voltage sources. Repeat the previous step, but this time choose

Create Line Graph and then select Phase.

e. Click on the Graph Properties

icon. On the left, click the

Axes Properties tab. Edit the

Min/Max phases to go from

-180 to +180 degrees.


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17. Define the FDTD Grid

a. Turn on the FDTD user interface skin to enable

FDTD specific features.

b. Hover the mouse cursor over the warning message inside the Circuit Component

Definitions branch – the 1 W Modal Power Feed is not valid in FDTD. Delete it

by right-clicking on it.

c. Notice the bounding box indicator for the FDTD simulator

with blue dots. FDTD bounding box sizes are typically

defined in terms of number of cells, while the FEM

bounding box is usually a half wavelength for free space

applications.

d. Double-click on Boundary Conditions in the Project Tree.

Now the Absorption Type is editable. The default

absorption model is Perfectly Matched

Layers - PML. This tends to be more

accurate than Liao, but it places

additional grid cells around the model

space defined by the number of Layers.

The default of 7 layers gives very good

absorption, but if memory is a problem,

you can reduce this to 5 and often get

similarily good results or use the Liao

boundary conditions.

e. Double-click the FDTD Grid entry in the Project Tree.


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f. The Size tab should be active by default. The Specify Padding button should be

selected.

g. Define the Target Base Cell Sizes as 1 mm for the X-, Y- and Z-directions. Use

Merge Ratios of 0.2 (cells smaller than 0.2*Base Cell Size=0.2 mm will be

merged).

h. Define Free Space Padding as 5 in both X- and Y-directions (side walls will be

1 mm*5=5 mm away). Set the Lower Z padding to 0 and the Upper Z padding to

10 cells (top wall will be 1 mm*10=10 mm away).

i. Press Done to close the Edit Grid dialog.

j. Right-click on the Substrate object in the

Geometry window or on its name in the

Project Tree, and select FDTD Gridding

Properties.

k. Select Use Automatic Grid Regions and

change the Z-directed Target to 0.5 mm.

It is generally recommended that

substrates are meshed with at least 3 cells

along their height for better accuracy.

The Target base cell size is the maximum value

a cell in that direction can have. A cell may be smaller than this size.

The Merge field sets the range over which the base cell size may be varied to obtain cells.

In the above case the use of the selection „ratio‟ means that in generating the grid the size

will not go below 1/5th

(20%) of the base cell size. This can also be set to an absolute size

by unchecking the Ratio flag.

The Free Space Padding determines the number of base cells added around the geometry‟s

bounding box.


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18. Creating the FDTD Mesh

The mesh is created as soon as we activate the mesh view. The grid is a set of brick-

shaped elements that is superimposed on the geometry. The cell edges do not necessarily

terminate at object boundaries. The material properties of the geometric objects are

mapped to the grid edges to define the simulation mesh for the FDTD solver. This

mapping introduces one of the principal errors in the FDTD analysis if one does not set

grid definitions well.

Check the Feed Placement in the Mesh

a. Double-click on the FDTD Mesh branch in the Project Tree

or click the mesh toggle icon on the right.

b. Select the “Right (-X)” view from the View Tools icon.

c. Press the Component Visibility icon along the right of the

Geometry window to turn off the Port Components (also works

for waveguide surfaces). This will let you see where the feed is applied in the

simulation mesh. If you have a component selected in the Project Tree, then it will

be visible regardless. You need to select something else to make it invisible.

d. The Mesh Cutplanes button whould be selected. Click the YZ Plane at

X-slice = 20 in order to view the slice of the mesh where the circuit component is

located. (The color of the components can be changed in the Modeling tab of Edit

> Application Preferences.) The cell edges above and below the voltage feed have

been automatically converted to a PEC wire along the length of the feed to make

sure that the voltage is actually applied between the ground plane and the

microstrip. They show up in a color that is hard to differentiate from the green

substrate. If you change the color of the dielectric to red, you can see the wire cell

edges.)


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e. Rotate the view to the other side and check that the feed is applied there as well by

changing the X-slice to 5.

f. Select 3D Mesh on the bottom left of the Geometry window and then select

Faces With Edge and rotate the geometry. Notice that the yellow microstrip

disappears. That‟s because by default, the geometry of the objects is superimposed

on the mesh, so it may appear that all the materials have been correctly assigned to

the mesh, when in fact, they are not. In this case, the copper of the microstrip has

been overwritten by dielectric. The reason is that the mesh priority is determined

by the position in the Parts list or by a numeric value that is nominally the same for

all new objects.

g. Drag the Line object above the Substrate object in the

Project Tree under Parts. The effect on the mesh

should be immediately visible. If you notice “holes”

in the mesh under the microstrip, don‟t worry about

these. In FDTD, the faces have no meaning. Only the

edges of the cells are used in the simulation. To see

what FDTD actually sees in a

simulation, turn on All Edges

and then turn off component

visibility.

h. When finished, dismiss the mesh

controls by clicking on the mesh

icon or by double-clicking the

FDTD Mesh branch of the Project

Tree.

i. Save the project.


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19. Editing Feed Components and Waveforms

a. To view the feed properties, double-

click the 50 ohm Voltage Source

entry under the Circuit Component

Definitions branch of the Project

Tree. The 1 Volt excitation with a

50 Ohm source impedance is the

typical source used for FDTD

simulations.

b. Click Done.

a. Double-click on the Broadband

Pulse definition under the

Waveforms branch.

b. Select Excite up to a Maximum Frequency and set the maximum frequency of

interest to 10 GHz and the signal level at that frequency to be -10 dBa.

The S-parameters will only be meaningful up to about 10 GHz because the largest mesh

cell of 1 mm cannot be larger than about 1/10th

of a wavelength. In the substrate at

10 GHz (epsr=9.9), we have a wavelength of about 10 mm. By limiting energy in the

pulse waveform to a lower band, the simulation will be faster.


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20. Setting Up the FDTD Simulation

a. Open the Simulations window by clicking on the Simulations

tab along the right edge of the EMPro window.

b. Select the New FDTD Simulation button in the upper-left of the

window.

c. Click on the

Setup S-Parameters tab.

Enable S-Parameters is on by

default. Leave Port1 as the

only active port. This will

give us S11 and S21.

Enabling the other port would

require a 2nd

simulation.

d. Click on the Specify Termination Criteria tab. The default values are sufficient

for a large class of problems. The simulation will stop if the convergence of -

30 dB has been reached or after a maximum of 50,000 time steps has been

simulated.


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e. Select Create & Queue Simulation to run this simulation.

f. Click on the Output tab to see the simulation progress.

g. View the Summary tab. The maximum frequeny referred to here is based on

largest cell size in free space. It does not take into account dielectrics which can

reduce the maximum usable frequency substantially.

21. Viewing the Results

FDTD simulation results can be viewed dynamically even before a simulation completes.

S-Parameter graphs update automatically as the simulation progresses.

a. Click on the Results tab along the right edge of the EMPro

window.

NOTE: Perform the next two steps only if you skipped the FEM part of

the lab above.

b. Notice the 4 columns at the top. These can be customized and are used to filter the

data available for plotting. Right-click on the column headers and select different

filters to familiarize yourself with the choices.

c. The columns at the bottom are the actual data which you can sort, select and plot.

Here too, right-click on the column headers to see the choices.


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e. Make one of the filter columns Simulation Name and select FDTD.... Make

another filter column Result Type and choose S-Parameters.

f. Double-click on [S2,1] for the default

graph. (If you right-click instead, you

see other options.)

g. Within the plot window, select the

Graph Properties icon.

h. Notice the 3 tabs on the upper, left side

of the plot. Choose the 2nd

tab - Axes

Properties. Define the X-axis range

from 0 to 10 GHz and Y-axis range from

0 to -3 dB.


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i. Take a look at the marker tools and choose Point

Marker to read off some values on the trace.

Notice that if you click on a point, the marker

readout and a reference dot will be placed on the

graph.

j. Try some of the other markers as well.

22. Saving Parts in a Library

One useful feature of EMPro is the ability to easily generate libraries of any of the objects

created in a project.

a. From the right side of the Main window, select the Libraries

tab.

b. To create a new Library press the + button in the Libraries

window. A File Manger window will open allowing you to

create a suitable directory to store your Libraries in. Use

Browse Folders, and create a new one with the right mouse

button called Libraries in the EMPro_Projects folder. Name

the new library MaterialLibrary.


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c. Click on the Filter Materials on the right side.

d. From the Project Tree in the Main window, select both Cu and Dielectric and drag

them on top of the Materials branch at the bottom of the Libraries window.

e. Close the Libraries window.

The stored materials will be used in a later lab exercise.

As can be seen from the Libraries window, almost any item located in the Project Tree can

be saved to a Library. This means standard parts, such as PCB connectors, can be reused

in a consistent manner. But even Sensor definitions (Far Field sensors, etc.) can be stored

in a library saving time and eliminating mistakes during simulation setup.

End of Lab Exercises

Drag and Drop

the items to be

used in other

projects to the

library


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Documents

Lab1 Microstrip Line FEM FDTD