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The theoretical modeling of FSS presented in this thesis was done by High Frequency Structure Simulator (HFSS).This software is the standard tool for 3D-full wave electromagnetic simulations . It integrates simulations, modeling, visualization and automation to learn a problem quickly because of its user friendly interface . This chapter describes a general overview of Ansoft HFSS and a step-wise procedure is described to model and simulate an FSS. There are many optional panels in the Ansoft HFSS window as shown in Figure 1.Following are the non-compulsory panels to interact with HFSS .
a. 3D Modeler Window tree.
b. Progress Window.
c. Property Window.
d. Message Manger.
e. Project Manger.
3D Modeler user interface: The design of the model can be created in this area which can be further divided into following three parts.
3D Modeler Design Tree: This is the very important part of 3D Modeler user interface where one can access the structural elements and also the parameters of the elements can be controlled from this part .
Context menus: This is the easy way to access those commands which may be frequently used for the active project by clicking right on the graphical interface.
Figure 1 HFSS user interface.
Graphical Interface: In this area user may interact with the structural elements and the unit cell of the project can be designed in this graphical interface .
Project Manger with Project tree
The complete details of the active project can be observed in the project manager window which is shown in Figure 2. The geometric model, material assignments, boundary conditions and other important information for each project can be monitored in project manger tree.
The properties window is shown in Figure 3 consists of two tabs which are called the command tab and attribute tab. Actions selected in the history tree are displayed in the command tab while the information about the material and its properties are displayed in attribute tab.
Figure 2 Project manager window.
Figure 3 Properties window.
Progress window is shown in above Figure 4, which displays the progress of the solution when a simulation of a particular project is in process.
The development of a project can be observed in the message manager window such as errors in the design which help a user to redesign the project and remove the specific errors to simulate the design successfully. Window of message manger is shown in Figure 5.
Figure 4 Progress window.
Figure 5 Message manager window.
How to Simulate a Periodic Structure/FSS
Example: 900MHz Bandstop FSS
To understand the functionality of this simulation tool, we will create and simulate a simple dipole 900MHz band stop FSS. As discussed earlier the distance between the two elements will be .
Unit cell of 900 MHz Bandstop FSS.
To design and simulate an infinite array of periodic element is impossible in HFSS so a unit cell for FSS may be designed with periodic boundary conditions.Following are the basic steps to design and simulate a 900MHz Band stop FSS.
a. Open the HFSS
b. Some basic commands of HFSS
c. Create the FR4 substrate
d. Create the dipole element
e. Create the vacuum or waveguide to solve for S-Parameters.
f. Allocate the Master and slave boundaries
g. Allocate the perfect E boundary
h. Assign the Floquet port
i. Setup the Analysis
j. Setup the sweep
k. Viewing the result.
Comprehensive details of the above listed steps is given below to understand the design and simulation of FSS
How to Open the HFSS
a. To open the HFSS click on start>all programs>ansoft>HFSS as shown in Figure 6.
b. For new project and to save it, click on file>new >save as and write the name of the project (900 MHz FSS).
c. We may use the file>save command to save the active program or Ctrl+S command can be used to save the active programme in a particular location in your system.
Figure 6.How to open the HFSS
Some Basic commands of HFSS
Before the designing of FSS, one should be aware about some basic commands of HFSS which are very useful for a new user. We can also get the particulars for every command the Help menu of HFSS.
Help. We may get the help from the help menu by doing the following or directly open the online help menu by pressing F1 key.
1. Click on the Help bar menu>search.
2. Click on the Help bar menu>contents.
3. Click on the Help bar menu>index.
Draw. To draw an object on 3D modeller window , click on draw and pick one object from the menu> rectangle, line, square, ellipse, circle, cone, sphere etc depends upon the design of FSS and click on 3D modeller window press the left mouse and drag towards right and then towards up. In 3D window modeller tree double click on create box; a properties window will be appeared where we can put the parameter of the design according to the required design.
Copy. From this command, we can copy an image or object from the 3D modeler window. Click on object or image and then click right>edit>copy or we may use directly Ctrl+C to copy an object or image from the graphical window.
Paste. To paste the object or image on the desired location click right at the desired location>edit>paste or click on the desired location and press Ctrl+V.
Select face, object or edge. We can select a face, object or edge of any object by clicking right in 3D modeller window and by selecting select face, select object or select edge etc. or by using simple commands F,O and E respectively.
Rotate Model Centre, Screen Centre and Current Axis. For rotation of object, screen and axis click on view menu>rotate>rotate model center or rotate screen center or rotate current axis by clicking on the object or screen.
Zoom in, Zoom Out. To magnify or shrink the contents, in the view window click zoom in and zoom out and click on the object which is required for zoom in or out. For zoom out we may also use ¿½+¿½ , ¿½=¿½ or Ctrl+E sign to magnify the object and to shrink the object press ¿½¿½¿½ or Ctrl-E, on each press 5 % of object will be magnified or contracted depends upon the executed command.
Zoom. On the view menu, click zoom (Ctrl+Shift+Drag), to zoom in; press and hold the left mouse and drag it on the object towards the view window (upward) and for zoom out do the same but drag the mouse towards the bottom (downward) of the view menu.
Subtract. We can subtract one object from another by selecting both the objects and right click on 3D modeller window>edit>subtract. A window will be appeared and object in the top register will be subtracted from the object in the second register but both of these objects or quantities should be of same kind (scalar or vector).
Unite. If one object is placed on another and we want to make them one object we can do this by selecting the objects to be united and right click >bolean>unite.
Export 3D Moel Files; We may export HFSS 3D models file; right click on 3D modeller grahphical interface>export data.
Validate: The validation check on project is so important ;it is essential to run this command before the execution of analysis.By the exaction of this command HFSS confirms that all the required steps were taken and all the parameters are ok. To execute this command we may click validation check from the HFSS menu.
Analyse All. To run the designed model after the validation check,we may click on analyse all in the HFSS menu.
Create the FR4 Substrate
Open a new project and name it 900MHz Bandstop FSS.
a. Pick a box from draw menu, click on an arbitrary point in the graphical interface window to set the starting point and after that make a box as described earlier to create FR4 substrate.
b. In 3D modeler design tree right click on vacuum>properties>select definition>enter the name of FR4_epoxy and click on OK.
c. Click on create box and set the parameters of FR substrate in properties window as shown in Figure 7.
Create the Dipole Element
a. Click on draw>box, and after that click on an arbitrary point in the graphical interface window to set the starting point and after that make a box to create the dipole element for 900 MHz FSS.
b. In 3D modeler design tree right click on vacuum>properties>select definition>enter the name copper and click on OK.
c. Expand solids tree in 3D window modeler and click on copper>create box> and put the parameters of the copper element as shown in Figure 8 and put the cursor in the graphical interface window click right and after that edit>duplicate>around axis.
d. To unite both the elements in the 3D graphical window select both the elements and after that click on modeler>boolean >unite.
Create the Vacuum or Waveguide
Pick a box from the draw menu, click on an arbitrary point in the graphical interface window to set the starting point and after that make a box as described earlier.
a. Expends solid in 3D modeler design window and click on vacuum> create box and set the parameters of vacuum as shown in Figure 9.
Figure 7 FR4 parameters
Figure 8 Di-pole element parameters.
b. To fit the object in the view click on view menu>fit all>all views.
c. To see the 3D view of the object click on View>rotate >rotate model center; screen center or current axis and
d. To select object, FR4, dipole or vacuum we may press the key O.
Figure 9 Vacuum parameters.
e. To Select the face of vaccum, we may press F and to select the edge of the particular object press E.
Allocate the Master and Slave boundaries
Master and slave boundaries to the vacuum object can be assigned as follows.
a. pick the face of unit cell in the graphical interface by pressing the key F and on the face of the object and click HFSS>boundaries>assign>master or right click on the selected face of the object and accept assign>master.The master boundary dialog will be displayed after that accepts the default name as master 1.
b. From the drop down menu select U vector as exposed in the Figure 10, click on new vector from the menu of vector A, the measure dialog plus create line dialog will be displayed, click on the lower left corner and drag cursor to the right and after that click, a vector U will be displayed and check the reverse direction box against V vector and click OK.
c. Now select the opposite face of that particular object from the graphical interface window allocate the slave by choosing HFSS>boundaries>assign slave.
d. In the general tab a slave dialog will be appeared with the name slave1. After selecting the master1 as the master boundary, U Vector will be sketched. Other settings will remain same and the dialog will be closed after clicking on OK button as shown in Figure 11.
e. The same procedure will be repeated after selecting the other two faces for Master 2 and Slave 2. But do not mark the reverse direction dialog for the V vector for Master 2 boundary, but Slave 2 boundary will be checked.
f. Now we can see in project manger window by expanding the boundaries, master 1, master 2, slave 1 and slave 2 boundaries respectively.
g. By clicking on any one from boundaries menu(master 1,slave 1) the object will be looked like as revealed in Figure 12 (a) and Figure 12 (b).
Figure 10 Master 1 boundary of 900 MHz unit cell.
Figure 11 Slave 1 boundary of 900 MHz unit cell.
Figure 12 (a) Master 1 and slave 1
Figure 12 (b) Master 2 and slave 2 for 900MHz Bandstop FSS
Assign the Perfect E Boundary
In this section, a perfect E boundary will be applied to the dipole.
a. Press ¿½O¿½ and select the dipole from the 3D Modeler graphical interface to assign the perfect E boundary
b. From the context menu assign the perfect E boundary to dipole, as show in Figure 13 and allow the default name, click the OK button to lock the dialog. The perfect E1 boundary will be appeared in project manger window under the boundary icon.
Allocate the Floquet Ports
Floquet ports to the upper and lower faces of the unit cell can be assigned as follows.
a. Press key F and choose the face of the unit cell and pick the floquet command from the context menu or HFSS menu.
b. A window with ¿½Floquet Port¿½ name will be appeared; select the default name floquet port 1, from the lattice coordinate system select new vector from the drop down menu of A as shown in Figure 14.After the selection of new vector two small windows, create Line and measure data will be displayed as shown in Figure 15.
c. Click the cursor on the very left corner of the face, drag it towards right till the right corner of the face and press the left mouse, black icon is displayed as shown in Figure 15 which is the right corner of the model.
d. Select the new vector from the drop down menu of vector B as shown in Figure 16, again create line and measure data windows will be appeared. Click the cursor again on the very left corner of the face, drag it towards the very second corner of the model and press the left mouse, click the next button.
Figure 13 Perfect E boundaries
Figure 14 Lattice coordinate system for vector A
Figure 15 Create line and measure data windows
Figure 16 Lattice coordinate system for vector B
Figure 17.Phase delays
Figure 18.Floquet Port 1, Polarization states TE and TM modes.
e. Modes Setup will be appeared as shown in the above Figure 18 with TE and TM Polarization states. Click next till Post Processing window and click finish.
f. Rotate the model and select the opposite face of the Floquet Port 1.
g. Select next and accept all the default values till the Post Processing window and click finish.
h. It is important to note that one mode should participate in 3D refinement otherwise the design will be unable to simulate.
i. After assigning the floquet port 1 and floquet port 2 expend the excitation tree in the project manger window,floquet port 1 and floquet port 2 will be appeared.
j. From the Project Manger window expand the excitation menu and click on Floquet Port 1 and Floquet Port 2; model will be displayed in 3D graphical interface as shown in Figure 19.
Figure 19 (a) Floquet Port
Figure 19 (b) Floquet Port (b).
Setup the Analysis
To setup the analysis
a. Expand the analysis tree in the project manger window, select add solution setup by clicking right on the analysis .
b. This opens the solution setup dialog as shown in Figure 20.On the General tab, set the Solution frequency to 1 GHz.
c. Put the maximum number of passes to 10, and the Maximum Delta S as 0.02.
d. On the Options tab, check the do lamda refinement box,
e. Put the value of the lambda target as 0.2 and 30 % maximum refinement per pass, with 3 minimum number and 2 minimum converged passes.
f. Click OK after the selection of first order basis function,
Setup the Sweep
To setup the Sweep:
a. Right click on setup1 in the project tree, and select add frequency sweep. The Edit Sweep dialog will be appeared as shown in Figure 21 and accept the default Name ¿½Sweep 1¿½.From the discrete sweep drop down menu; pick Interpolating sweep.
b. For Frequency setup pick the linear setup write the starting Frequency 400 MHz, Stop Frequency 1500 MHz and step size 10 MHz. Click the display button to view all the frequencies.
Figure 20 Solution setup of 900MHz bandstop FSS
Figure 21 Frequency setup of 900 MHz bandstop FSS.
Viewing the Results
To see the rectangular plot of 900 MHz Bandstop FSS.
a. . From the Project Manager Tree click right on Results icon and click Create Modal Solution Data Report >Rectangular Plot. Select S parameter from category list and dB from function add trace and close the window.
b. To validate the design click on HFSS>Validation. If we have done any mistake during the design, it will be shown in the window. If the design is OK no error will be appeared as shown in the Figure 22, afterward click HFSS> analyze all, processing on the designed FSS will be started.
c. When the processing will be finished click on Results>XY Plot>dB and a rectangular plot will be appear as shown in the above Figure 23
Figure 22 Validation checks for 900 MHz Bandstop FSS.
Figure 23 Rectangular plots of 900 MHz Bandstop FSS (Reflection and Transmission result).