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EDA http://www.edajx.com

Trai ning Workbook Sl ot Antenna Page 1 of 23

Ansoft HFSS Version 8 / 8.5

Training Workbook

Slot Antenna

EDA

http://www.edajx.com
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Conventions:

Capitalized text refers to menu items (i.e. choose MODEL/MEASURE is an instructionto click on the Measure command under the Model menu)

Bold Faced text is the text that is to be entered from the keyboard.

Italicized text will signify the names of buttons that you will click on
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Introduction:

We will create a model of a stripline fed annular-ring slot antenna. The objective

will be to duplicate the results presented in the paper by C. Chen, et al, Stripline-fed

arbitrarily shaped printed-aperture antennas,IEEE Trans. On Ant. And Propagat., Vol. 45,

No. 7, July 1997, pp. 1186-1198, specifically the antenna described in Figure 11 of the paper.

The stripline is spaced midway between the two ground layers and has a width of 2.6 mm.

The ground layers are spaced 3.14 mm apart and the dielectric has an r=2.2. The annular-

ring slot on the upper ground plane has an inner radius of 9.2 mm and an outer radius of

13.2 mm. The stripline ends at the point directly below the center of the ring. In the paper

data for this structure that was computed using the BEM (assuming infinite substrate and

ground layers) is presented. Measured data is also presented, but details on the size of the

actual ground/substrate layers of the experimental model were not given. The measured

data had a resonance at 5.64 GHz. for this antenna. The |S11| was -25dB at that point.

The computed data presented had a resonance at 5.59 GHz with |S11| -31.5dB.

We will attempt to model this antenna using a finite sized substrate/ground layers

and an ABC. For this case lets use a substrate/ground that is square with dimension of 60

mm. The adapt frequency will be 5.3 GHz., which has a free space wavelength of

approximately 57 mm.

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Sequence of Steps:

1. From the Maxwell Control Panel open the Project Manager. Create a new HFSS projecttitled Slot_ant. You should see the HFSS Executive Commands panel. Click on

Draw to enter the 3D modeler.

The window in the upper left is the XY 2D drawing window. The lower left and lower

right windows are two the YZ and XZ drawing windows, while the upper right window

display a 3D-drawing window. All objects are drawn in the drawing windows. 2D and 3D

objects can then be extruded, rotated, etc. to create more complex shapes. These shapes

can then be unioned, subtracted, etc. to create the required model.

2. First lets set the defaults. Go to OPTIONS/UNITS. Under the resulting menu you willbe able to change the drawing units, the default is mm, which is what we will use here.

3. We will let the aperture plane lie in the XY plane. Note that the axes shown in theupper left window are the X and Y-axes. Notice that after the mouse is clicked in the

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XY plane, the Z coordinate is unchecked and its value is frozen. This will keep all

objects drawn in the XY plane at this elevation in Z. For this example, leave Z

unchecked and its value set to 0.

4. We will now draw the two circles that make up the annular ring centering the ringabout the origin. Select LINES/CIRCLE. The modeler is now waiting for you to input

the center of the circle. Click your mouse at the origin. The XYZ coordinates column

will show zero value. Make sure the two entries are 0 and click on Enter. Since the

inner radius is 9.2-mm, we will draw this circle with a radius of 9.2-mm. Type a value

of 9.2 in the Radius Box. Type a value of 24 in the Num segments Box. This tells the

modeler how many segments to use in drawing the circle (i.e. an angle of 30 gives 12

segments, an angle of 10 gives 36,). In this case 24 segments will produce a circle

segment every 15 degrees. Type the name inner_cir in the Name Box. You can change

the color of this object by clicking on the color button on the dialog box. Click on Enter

to create the closed and covered 2D-circle object.

Pick a color from this palette and click on the Execute.

5. Repeat the same procedure to draw the outer radius of the annular-ring. The circleshould be centered at the origin and have a radius of 13.2 mm. Call this object opening.

6. Next we will draw the 2D object that will become the top ground layer. Draw thisobject in the XY plane. Since the grounds and stripline are very thin compared to a

wavelength we will make them 2D objects and can define them to be PEC (Perfect

Electric Conductor) in the boundary setup. Select LINES/RECTANGLE. You will now

have to enter the two corners of the rectangle. We can input the exact dimensions for

the starting point of the rectangle directly in the X and Y boxes in the left-hand column.

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Set the values: X = -30 and Y = -30. Click Enterto set the first point. In the X and Y size

boxes type the values: X = 60 and Y = 60. Call this object upper_gnd. Change colors if

you wish. Click Enterto create the 60-mm x 60-mm rectangle.

7. We will now copy upper_gnd and Sweep this 2D object to create the 3D dielectric layer.Select EDIT/SELECT upper_gnd. Click OK. Select EDIT/COPY. Select EDIT/PASTE.

This creates a new object called upper_gnd1. Select EDIT/SELECT upper_gnd1. Select

EDIT/ATTRIBUTES/BY CLICKING Upper_gnd1. Click OK. Type the new name

dielect. Change the color then click OK and Cancel. Now select

SOLIDS/SWEEP/ALONG VECTOR dielect and click OK. Enter a Z value of 3.14 (the

minus sign since we wish to sweep in the z direction). Click on Enter. Click the

mouse on empty space on the left column. Type the F key. This will bring all the

windows to full screen. The 3D view of the object should be something like:

dielect

Inner_cir

opening

Upper_gnd

8. We will also need a ground plane on the lower surface of the dielectric. We will nowcopy upper_gnd and create the object lower_gnd. Select EDIT/SELECT upper_gnd.

Click OK. Select EDIT/COPY and then EDIT/PASTE. This creates a new object called

upper_gnd2 Next we will change the object attributes by selecting

EDIT/ATTRIBUTES/BY CLICKING Upper_gnd2. Click OK. Type the new name

lower_gnd. Change the colorthen click OKand Cancel.

9. Select EDIT/SELECT lower_gnd and click OK. Next select ARRANGE/MOVE. In thebox next to Z in the left-hand column enter 3.14 (the distance and direction we wish to

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move this new object). Click on Enter. This will move the object lower_gnd to the

bottom of the dielectric.

10.Next we will subtract the opening from the upper_gnd to eliminate object overlap.Select SOLIDS/SUBTRACT. Highlight upper_gnd and click OK. Next highlight

opening and click OK. The resulting object will be the upper_gnd with a hole in the

center. Note that the second object is eliminated in this step to reduce the number of

unneeded model objects.

11.At this point it would be a good idea to explore some of the features of the modeler notalways used in creating a model. Select EDIT/VISIBILITY/BY ITEM. In this window

you can turn the display of the various objects in the model on or off. Turn on and off

different objects and use the Show and Hide button to see the results. Make sure before

you click on Ok button that you turn the display of all the objects back on. Next click

the right mouse button on the 3D view. Select Rotate. Move the mouse while pressing

the left mouse button. The object will rotate in 3D. Next double click the right mouse

button in the upper right corner of the 3D window. This adjusts the 3D view back to

the original default viewpoint.

12.Now would be a good point to draw in the stripline. Again it is midway between thetwo ground layers so we need to change our 2D drawing window to the z = -1.57mm

plane. Click the mouse in the XY plane and check Z by clicking on it. Enter 1.57 here

and click the check mark to make it disappear. This will freeze the XY plane to a Z

elevation of 1.57mm. Use the LINES/RECTANGLE menu to draw in the rectangle

that will be the stripline. For the first corner use X = -30; Y = -1.3. For size values use X

= 30; Y = 2.6. Call this new object strip_L and give it an appropriate color. Select from

the menu FILE/SAVE. It is a good idea to periodically perform this step in the event of

unexpected computer power loss. During the save, a box may pop up asking to

expand the background. Go ahead and expand the background if this occurs because

we want to eliminate any overlapping objects.

13.We next need to define the surrounding box that we will assign the abc to in a laterstep. Click the mouse in the XY plane. Enter 16.5 here and freeze the XY plane to a Z

elevation of 16.5mm. Use the LINES/RECTANGLE menu to draw in the rectangle

that will be the stripline. For the first corner use X = -30; Y = -45. For size values use X

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= 75; Y = 90. Call this new object abc and give it an appropriate color. Next we will

sweep this to create the rectangular box for the radiation boundary. Sweep this up

33mm and call it abc. It is spaced 15 mm from the other 3 sides of the device and is 16.5

mm above and 13 mm below. The reason for the smaller spacing below is we know

in advance that the radiation is predominantly above the device, so the abc spacing

below is not as critical as above. In fact we could have probably used /10 (6 mm)

there and gotten satisfactory results. The model should now look like this:

14.The last critical features to be added to the model are the 2D object that we will defineto be the port. We do not want to use the entire back face of the model as the port.

That would take the two ground layers entirely inside the cross-section and the 2D

solver would treat them as signal lines. We need the edges of the port to touch the two

grounds. We will thus have to draw a 2D rectangle on the back face that will be our

port. Set and freeze the X value in the YZ plane to X = -30. Draw a rectangle in the YZ

view with the first corner at the values: Y= -15; Z= -3.14.Set the size of the rectangle to

Y= 30; Z= 3.14. Call this object port.

15.Before we leave the modeler it would be a good idea to add a virtual object. There area large number of node points in the annular ring section. The corners of the abc lie on

the order of 30 mm from these nodes and the mesher maker will be drawing many

very narrow tetrahedras from the ring node points to the abc corners. These will not be

well-formed tets and will slow the solution convergence. To eliminate many of these

we can place an air box directly above the annular section called a virtual object. Most

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of the tets from the annular ring should terminate on this object. Set the XY plane to an

elevation of Z = 5. Draw a rectangle on this plane that has its first corner at: X = Y = -15;

and size of X = Y = 30. Call this object virtl. Sweep this object up a distance of Z = 5.

16.That completes the model definition. Select FILE/EXIT and click Yes. Click theVisibility button that appears below the 3D view of the model. This will list all the

objects of this model:

abc

dielect

inner_cir

lower_gnd

port

strip_L

upper_gnd

virt1

background {visible is set to no for this object}

and then Click Done to close the visibility box.

17.Once complete you are returned to the Executive Commands window where a checkmark now appears next to the Draw button and the Setup Material button is now

on (i.e. the button is not greyed out any more). Click on this button and the material

definition window appears.

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Trai ning Workbook Sl ot Antenna Page 10 of

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18.Here we will need to assign the material properties to the various objects contained inthe model. To define the materials click on the object name in the Objects sub-window.

Choose the proper material in the Material Definition subwindow and click on Assign.

A check mark should appear next to the object name signifying that it has been defined.

For a dielectric, highlight dielect and select MATERIAL/ADD. Type in the name

local22 in the Material Attributes box and 2.2 in the Rel. Permittivity box. Then click

EnterandAssign. Assign vacuum to the objects abc and virtl.

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19.In alphabetical order the objects and their definitions are:

Object Material definition

abc dielectric with r=1.0, tan=0, r=1.0, tan=0(=vacuum, the default)

dielect dielectric with r=2.2, tan=0, r=1.0, tan=0

virtl vacuum

The boundaries upper_gnd, lower_gnd, inner_cir, port, and strip_L are all 2D objects that

we will assign boundary conditions to in the next step. All these objects will be assigned

as PEC (Perfect Electric Conductor) except for the port that will be vacuum.

20.Again once all objects have been defined click on Exit and Yes to return to theExecutive Commands window where the Setup Materials button now has a check

mark next to it and the Setup Boundaries/Sources button is now on.

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21.Once the materials have been properly defined the last major task is to define the portsand boundaries of the model. Click on the Setup Boundaries/Sources button. The

following screen should now appear:

22.Let us define the port first. The name port1 should appear in the Name box the firsttime the boundary manager is opened. None of the buttons in the Boundary/Source

column should be on, as we have not selected anything to define this boundary yet.

Select the radio button next to Object in the Graphical Pick box. Select from the menu

EDIT/SELECT/BY NAME. Highlight the port name in the list and click Done. Note

that the lower middle box contains Number of Modes and has the default value of 1

entered in the data box. For this problem we are only interested in the dominant mode

on the stripline so 1 mode is sufficient. In cases where more than one mode propagates

or where higher order modes may be important to the given port solution this is where

you chose to add more modes to the resulting 2D port computation. Click Assign to

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create port1. The port is now completely defined so it will now appear in the Name

Assigned box.

23.We can next define the abc. Click on Boundary Radio Button, which is directly belowthe 3D view of the model. Click Perfect E and drag the mouse down then click Radiation.

This will set the boundary type to a radiation boundary. Type abc in the Name dialog

box. Move the mouse onto the 3D view and the mouse arrow is displayed as an arrow

with a small cube. Click the left mouse button on the abc object in the 3D view. Click

Assign to create the abc boundary. A warning message appears showing an

overlapping boundary. Click OKto continue. Port boundaries always take precedence

over other boundaries. Again once all is complete the new name will appear in the

Name Assigned box.

24.Next we need to define the PEC objects. The upper_gnd with the annular ring cut intoit, inner_cir, and lower_gnd can be assigned together. Click Radiation and drag the

mouse down then click Perfect E. Name this boundary metal. Select multiple objects

inner_cir, upper_gnd, and lower_gnd while holding down the Control key from the

menu pick EDIT/SELECT/BY NAME. Click Assign. Repeat these steps to define PEC

on strip_L and name it strip.

25.At this point we are done. Click on the MODEL/BOUNDARY DISPLAYto reviewthe boundary definition and see if it was done correctly. Note the solver will actually

create the initial 3D mesh here. Select each of the boundary definitions and click on

Toggle Display to view the boundary. You can change the colors by using the color

button shown.

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26.Check all of the boundaries and make sure the annular ring is properly defined. Clickon

Close. You can at this stage define an Impedance and Calibration line. To do so

highlight port1 in the Name Assigned column. Then you would click the mouse on the

white square next to Use Impedance Line. At this point you can define a line on the

port that tells the impedance calculator where to integrate to get the voltage

(Impedance Line). The calibration line orients the fields in each port to be in the same

direct avoiding possible 180 deg. phase errors in the resulting [S] values Calibration

lines are usually copied from the Impedance. For this project, Impedance Lines and

Calibration Lines are not important and so this step can be skipped. Click on

FILE/EXIT. Note: the polarization line is only for square and circular waveguides,

trying to use it for other port cross-sections will yield erroneous results.

27.At this point you will be back at the Executive Commands window again. At this stagewe need to define the solution parameters and the model is ready for simulation. Click

on Setup Solution.

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28.Radiation boundary seeding is automatically taken care of in HFSS version 6 so therewill be no need to use Mesh Options to manually set one in this case. The current mesh

should be around 550 Tetrahedra. In the Solution Setup window you can now decide

how many modes to include in the solution (we are only using 1 mode so this

subwindow can be ignored). You can also select how accurate you want the 2D port

simulation to be by setting the Ports Fields Accuracy percent. The default is 2% and for

this problem that should be sufficient.

29.Under the top subwindow in HFSS Solution Setup we will setup the stopping criteria.Set Allowable Delta S to 0.02, Adapt Frequency to 5.3 and Number of Adaptive passes

to 7. We also want to do an ALPS (Adaptive Lanczos Pade Sweep) or Fast Sweep

about the Adapt Frequency, so click on the check button next to Sweep and Fast. For a

fast sweep you will need to input the start and end frequencies, plus the number of

steps between these two points. For this problem set these values to 5, 6 and 100

number of steps. Note that the number of divisions only affects the number of [S]

points computed. You can always post process on any frequency in the solution range

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once the simulation is complete. Click on Ok and you should be back at the Executive

Commands window. At this point you can click on Solveand the FEM Full-Wave

simulation will start.

30.Since we are doing a training problem we will not simulate the model at this time. Asolved copy of this project has been supplied and we can start our Post Processing on

that.

31. Once the solved project has been copied to your directory open it and you should seethe main window. You will notice that all the Post Processing buttons are now on.

Lets first look at the convergence data. The menu above the 3D view has 4 main

buttons. The Model is self-explanatory. Click on Profile. You will see the solution

statistics for this simulation including total time at the bottom. Use right slide bar to

scan up and down this data. Notice that the last pass was Pass 5. The simulation

reached the 0.02 delta S criteria at that point and went directly in to the sweep

calculation. More detailed information about convergence is found when you click

Convergence. Return to Profile and slide down to the adaptive pass 5 information.

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32.The memory value listed under emsolve is RAM used (90 MB here). The disk usage isthe amount of hard disk space used (26 MB here). We can see from this information

that this required approx. 90MB of RAM, 26MB of disk space and took roughly 30 min

for this simulation.

33.Now click onMatrix to view the S parameter data. For the sweep S_1 you can use theSlide Bar on the right to select any frequency in the sweep to view the [S] data. Click

on the S_1 data. This will show the results for the frequency sweep.

34.The value of S11 (= 0.08 mag 112 deg) is shown. The Gamma is the propagationconstant of the mode computed in the 2D solver. The Port Z0 (Zpi) is the computed

impedance for that mode. Note it is approx. 50 . These values can be shown by

selecting the Radio Buttons on the right.

35.Once you have finished here click on Post. Under this button there are 3 choices:Fields{visualization of fields and antenna patterns},

Matrix Data{manipulation of S-Parameter Data},

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Matrix Plot{plotting of S-Parameters, Impedance, etc}.

36.SelectMatrix Data. Select COMPUTE and you will see RENORMALIZE, De-EMBED,Y MATRIX, and Z MATRIX. For this demonstration we will not use all of these. For

now lets compute the input impedance of the antenna. We need to first de-embed out

the added feed line. We will de-embed to the center of the ring. The length of the

stripline is 30 mm. Click on S_1 to select the S-Parameter data. Next click on

COMPUTE/De-EMBED. Enter a value of 30 for the de-embedding distance and click

on Set distance. Next set De-embedding Direction into Object. To verify the correct

distance, 3D picture of the model in the right hand screen. An arrow should appear on

the model showing the distance from the port to the end of the line. Click on Ok. A

button D_1 should now appear in the matrix list. Click on this button since we want to

compute the impedance of the de-embedded response. Now select COMPUTE/Z

MATRIX. You will then have the choice of which port impedance to use. Since we did

not set an Impedance line, the default of Zpi is correct here. Click on the Z matrix radio

button in the View box. You will see the de-embedded input impedance, which 45.8

Ohm 5.7 deg at 5.64GHz. Click on the FILE/EXITand save changes Yes. You are now

back to the Executive Commands window.

37.The last menu pick in Post Process, Matrix Plot, is the one feature here you will likelyuse the most. Go to PLOT/NEW PLOT. You will see the various sweeps listed in the

left most column. The next column lists the various data types you can plot. The

default should be S_1. This will plot the S11 data from this sweep. The 1_1 in the

arguments for this term signifies port 1 mode 1. It is the coupling from port1 mode 1 to

port 1 mode 1 or S11. Select the dB option and click Plot. The plot should look like:

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38.Notice the resonance here is at approx. 5.64 GHz that agrees well with the computedand measured resonant frequency from the paper. This is very good agreement

considering we are simulating a small finite sized antenna and comparing the results to

a more idealized model. The measured data from the paper had S11 at approx. 24.5 dB

at the resonance point, which also agrees well with this plot. To check on the accuracy

of the Fast Sweep data the solver includes here a term called the Error Function. Click

on PLOT/NEW PLOTand the error function will be displayed. The sweep data isaccurate for those values of error function smaller than 15 dB to 20 dB. We can see

that that means this response is across the entire 5 to 6 GHZ range. Again return to the

Executive Commands HFSS window.

39.Now click on POST PROCESS/FIELD. This will take a short amount of time as HFSSis loading the data into memory. At this point you can change the frequency under

consideration by going to DATA/EDIT SOURCES. Click on the Specify Frequency

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button and you can input the frequency you want. This is the window where you can

set the input power (in magnitude and phase) for each port in the simulation. Since we

will want the results at the frequency of 5.64 GHz, set this value and click Ok.

40.To change the view of the model you can move the mouse over the 3D view and selectRotate by clicking the left mouse button. Move the mouse to the upper left corner of

the 3D view and double click the left mouse button. This puts the view into the normal

orthogonal mode. Move the mouse arrow directly over the Z-axes and double click the

mouse again. This should produce a view that looks down on the XY plane. You

should see a top view of the antenna. We want to view the |E| on the aperture

plane to see if the field mode agrees with that of Figure 12 of the paper. We can view

the fields as shade or arrow plots on any cutplane in the solution space. To do that we

may define a cutplane. HFSS version 6 already predefines the common ones. We will

not need to create a cut plane here; but if one is needed in the future, use

GEOMETRY/CREATE/CUTPLANE. The aperture plane is the XY plane, and it

already exists, so select PLOT/FIELD. We can now plot any of several field choices in

several different forms on this plane. Select Mag E in the Plot Quantity column (this

will plot the magnitude of the electric field in phasor form). From the On Geometry

Column select Surface xy and click Ok. The Scalar Surface Plot box appears.

Experiment with these settings to create contour as well as shaded plots. Plot limits

and colors can also be adjusted from here. Select the Filled radio button. Next select

Ok to view the filled (shaded) plot. After some processing you will see the plot of the

field being drawn on the screen. The resulting plot should look like the following

picture. For this report, the slot antenna geometry was added onto the MagE Field

pattern plot.

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41.This agrees well with the mode pattern figure from the paper.42.In this window we can also plot |E| in a time animated form. If you have time try that

next by selecting Phase Animation Check Mark under Plot Type and Mag E under

Phase Value. The rest is the same.

43.Since this is an antenna, we will go next to RADIATION/COMPUTE/FAR FIELD. Thewindow that comes up is the Computer Far Field window. Lets plot the two principal

planes so set Phi to start at 0 and stop at 90 for 1 step. Next set Theta to go from 0 to 360

in 90 steps. Click on Ok. After the computation is complete a Plot Far Field box

appears. Selecting PLOT/FAR FIELD can also open this box. Click the dB check box

and click Ok. A plot should appear that is similar to the one below:

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44.An interesting option is to click on RADIATION/DISPLAY DATA/FAR FIELD. Thiswill display a sub window with the key antenna far field parameters including

directivity (note it is not in dB) and radiated power. The solver computes the latter, by

integrating the Poynting vector over the abc and keeping the real part of the result.

The Panel should look like the following:

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45.By selecting RADIATION/COMPUTE/NEAR FIELD you can plot the field at specifieddistances from the antenna, not necessarily in the far zone. The solver will integrate

the complete Greens function over the abc to accurately compute the near zone fields.

Spend some time exploring this and the Far Field windows.

46. That completes this tutorial.

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