Simulating Unit Cell Problems PML and Master Slave Boundary Conditions Array Functionality in HFSS Ansoft High Frequency Structure Simulator v10: Advanced Notes
Simulating Unit Cell Problems Simulating Unit Cell Problems Many problems are comprised of multiple repetitions of a particular structure, e.g. an antenna array, frequency selective surface etc. These problems can be simulated in HFSS as a single unit cell with linked (Master-Slave) boundary conditions, and then post processed using the built in array functionality. This section looks at the functions in HFSS which enable this including: Two different types of advanced boundary conditions available in HFSS: Perfectly Matched Layers (PMLs) Periodic boundary conditions Master and Slave boundaries. Array post processing functionality in HFSS. Ansoft High Frequency Structure Simulator v10: Advanced Notes 2
Perfectly Matched Layer (PML) What are Perfectly Matched Layers? Perfectly Matched Layers (PMLs) are fictitious materials that fully absorb the electromagnetic fields acting upon them. There are two types of PML applications: free space termination and reflectionfree termination of guided waves. In free space termination, all PML objects must be included in a surface that radiates into free space equally in every direction. PMLs can be superior to radiation boundaries in this case because PMLs enable radiation surfaces to be located closer to radiating objects, reducing the problem domain. Any homogenous isotropic material, including lossy materials such as ocean water, can surround the model. In reflection-free termination of guided waves, the structure continues uniformly to infinity. The termination surface of the structure radiates in the direction in which the wave is guided. Reflection-free PMLs are superior to free space or radiation boundary terminations in this kind of application. Reflection-free PMLs are also superior for simulating phased array antennas because the antenna radiates in a certain direction. Ansoft High Frequency Structure Simulator v10: Advanced Notes 3
Perfectly Matched Layer (PML) Implementation in HFSS HFSS uses an adaptive PML: In classic implementation, one needs several layers, one over the other, to achieve desired attenuation. With adaptive PML, one layer is enough. Adaptive meshing takes care of the rest. Advantages: Easier implementation. More robust. Smaller mesh. HFSS contains a PML setup wizard for: Perfectly Matched Layer object creation. Material creation and assignment. PML boundaries can also be set-up manually. HFSS automatically identifies PML objects by a naming convention: Any object with a name beginning with the letters PML is identified as a PML and is subject to: special adaptive meshing. incident-wave treatment. user-defined radiation surfaces during post processing. Ansoft High Frequency Structure Simulator v10: Advanced Notes 4
Perfectly Matched Layer (PML) Radiation boundary versus PML Radiation Boundary Condition Sensitive to the incident angle, less accurate for non-normal incidence. Fully automatic. Easy to use. Radiation boundaries need to be placed around λ/4 away from radiating objects. Perfectly Matched Layer (PML) Accurate, boundary has zero reflection. A fictitious biaxial anisotropic material. Reasonably automatic to create using PML setup wizard. More accurate for calculating radiation parameters. PMLs can be brought much closer to radiating objects (as close as λ/10), resulting in a smaller problem space and smaller mesh. Ansoft High Frequency Structure Simulator v10: Advanced Notes 5
Perfectly Matched Layer (PML) Automatic PML creation Three basic steps: 1. Create device objects. 2. Select surfaces for PML objects to be created on. (note you may want to create a face list at this point for post processing later on. This can be done using 3D Modeler > List > Create > Face List.) Here radiation is allowed through three faces. Two other faces will later be assigned symmetry boundaries. 3. Launch PML setup wizard: Ansoft High Frequency Structure Simulator v10: Advanced Notes 6
Perfectly Matched Layer (PML) Automatic PML creation, continued. PML setup wizard has a two step process, firstly creating the PML cover objects: Specify layer thicknesses normally set this to be λ/10 of the lowest frequency to be solved for Ansoft High Frequency Structure Simulator v10: Advanced Notes 7
Perfectly Matched Layer (PML) Automatic PML creation, continued. PML cover is added to the box on all radiation surfaces: Ansoft High Frequency Structure Simulator v10: Advanced Notes 8
Perfectly Matched Layer (PML) Automatic PML creation, continued. Secondly defining the PML material properties: Note for this problem the fields radiate equally in free space in all directions hence PML Objects Accept Free Radiation is selected. Set minimum frequency to be solved in problem Minimum radiating distance is the minimum distance between the boundary and any radiating object. Ansoft High Frequency Structure Simulator v10: Advanced Notes 9
Perfectly Matched Layer (PML) The PML setup wizard: Automatically creates PML materials. Automatically calculates PML material matrices. PML material properties are automatically assigned to the cover objects using default names. Ansoft High Frequency Structure Simulator v10: Advanced Notes 10
Perfectly Matched Layer (PML) Open-ended waveguide results Magnitude of S11 of an open-ended waveguide. Note that the boundary PML is closer to aperture than the radiation boundary and requires fewer tetrahedra for better accuracy. PML: d/λ=0.15, 1782 tetrahedra ABC: d/λ=0.32, 6736 tetrahedra Mesh of an open-ended waveguide The non-uniform PML mesh is evident. Ansoft High Frequency Structure Simulator v10: Advanced Notes 11
Perfectly Matched Layer (PML) Creating PMLs manually The PML setup wizard can only create rectangular PML objects. If another shape of PML object is required (e.g. to terminate a circular waveguide) then PMLs must be created manually: Draw the PML object at the radiation surface, and then select it. Give the object a name with the prefix PML. Object names that start with PML are necessary for HFSS to recognize them as PMLs. Ansoft High Frequency Structure Simulator v10: Advanced Notes 12
Perfectly Matched Layer (PML) Creating PMLs manually, cont. Launch the PML setup wizard. Select use selected object as PML cover. Choose the corresponding base object. Enter the thickness of the PML layer object. Select the orientation of the PML object in terms of the direction of outward propagation in this case radiation would be in the y-direction. Ansoft High Frequency Structure Simulator v10: Advanced Notes 13
Perfectly Matched Layer (PML) Creating PMLs manually, cont As this is a waveguide termination, PML Objects Continue Guided Wave option is selected. The propagation constant at the minimum frequency must then be entered. The minimum radiating distance is specified as before. Once completed the summary box will appear as for the automatic PML creation (see page 10). Ansoft High Frequency Structure Simulator v10: Advanced Notes 14
Perfectly Matched Layer (PML) Post processing Far Field data with PMLs To insert a far field setup where a PML has been used, you need to first create a face list of the radiating surfaces (note sometimes it is easier to create this when you first select the faces for creating the PML objects): Ansoft High Frequency Structure Simulator v10: Advanced Notes 15
Perfectly Matched Layer (PML) Post processing Far Field data with PMLs This list appears in the lists section of the model tree. When you create a far field radiation setup, under the Radiation Surface tab select Use Custom Radiation Surface and select this face list from the drop down menu. Ansoft High Frequency Structure Simulator v10: Advanced Notes 16
Master and Slave Boundary Conditions Master and Slave Boundaries Master and slave boundaries enable you to model planes of periodicity where the E-field on one surface matches the E-field on another to within a phase difference. They force the E-field at each point on the slave boundary match the E-field to within a phase difference at each corresponding point on the master boundary. They are useful for simulating devices such as infinite arrays. Unlike symmetry boundaries, E does not have to be tangential or normal to these boundaries. The only condition is that the fields on the two boundaries must have the same magnitude and direction (or the same magnitude and opposite directions). When creating matching boundaries, keep the following points in mind: Master and slave boundaries can only be assigned to planar surfaces. These may be the faces of 2D or 3D objects. The geometry of the surface on one boundary must match the geometry on the surface of the other boundary. For example, if the master is a rectangular surface, the slave must be a rectangular surface of the same size. If the mesh on the master boundary does not match the mesh on the slave boundary exactly, the solution will fail. Normally HFSS automatically forces the mesh to match on each boundary; however, in some cases, the mesh cannot be forced to match. To prevent the solution from failing, create a virtual object on the slave boundary that exactly matches any extra object on the master boundary, or create a virtual object on the master boundary that exactly matches any extra object on the slave boundary. Ansoft High Frequency Structure Simulator v10: Advanced Notes 17
Master and Slave Boundary Conditions Master and Slave Boundaries To make a surface a master or slave boundary, you must specify a coordinate system that defines the plane on which the selected surface exists. When HFSS attempts to match the two boundaries, the two coordinate systems must also match each other. If they do not, HFSS will transpose the slave boundary to match the master boundary. When doing this, the surface to which the slave boundary is assigned is also transposed. If, after doing this, the two surfaces do not occupy the same position relative to their combined defined coordinate system, an error message appears. For example, consider the following figure: To match the coordinate system of the master boundary, the coordinate system on the slave boundary must rotate 90 degrees counterclockwise; however, when this is done, you get the following: The two surfaces do not correspond and thus the mesh will not match, causing an error message. The angle between the axes defined by the u point and v point must be identical for the master and slave boundary. Ansoft High Frequency Structure Simulator v10: Advanced Notes 18
Master and Slave Boundary Conditions Assigning Master boundary Ensure plane is set to that of desired face Select face for master boundary and launch master boundary assignment Under Coordinate system select new vector to assign U Ansoft High Frequency Structure Simulator v10: Advanced Notes 19
Master and Slave Boundary Conditions Assigning Master boundary Note reverse direction changes orientation of V with respect to U Ansoft High Frequency Structure Simulator v10: Advanced Notes 20
Master and Slave Boundary Conditions Assigning Slave boundary Ensure plane is set to that of desired face Select face for slave boundary and launch slave boundary assignment Select Master boundary slave is associated with Ansoft High Frequency Structure Simulator v10: Advanced Notes 21
Master and Slave Boundary Conditions Assigning Slave boundary Define directions for U and V on slave boundary remember these must match the Master boundary assignment Ansoft High Frequency Structure Simulator v10: Advanced Notes 22
Master and Slave Boundary Conditions Assigning Slave boundary You have the option to relate the slave boundary s E-fields to the master boundary s E-fields in one of the following ways: Select Scan Angles, and then enter the φ scan angle in the Phi box and the θ scan angle in the Theta box. The phase delay is calculated from the scan angles; however, if you know the phase delay, you may enter it directly in the Phase Difference box below. Select Field Radiation, and then enter the phase difference, or phase delay, between the boundaries E-fields in the Phase Difference box. Ansoft High Frequency Structure Simulator v10: Advanced Notes 23
Array Functionality in HFSS Simulating arrays in HFSS HFSS enables you to compute antenna array radiation patterns and antenna parameters for designs that have analyzed a single array element. You can define array geometry and excitation. HFSS models the array radiation pattern by applying an "array factor" to the single element s pattern. Two array geometry types are supported: The "regular uniform array" geometry defines a finite 2D array of uniformly spaced, equal-amplitude elements. The regular array type may be scanned to a user-specified direction. Scan direction can be specified in terms of spherical coordinate angles in the radiation coordinate system. The regular array geometry type also allows scan specification in terms of differential phase shifts between elements. The "custom array" geometry allows for greater flexibility. It defines an arbitrary array of identical elements distributed in 3D space with individual user-specified complex weights. The basic procedure for using the array functionality in HFSS is: 1. Set up a HFSS model of a single unit cell. 2. Apply Master-Slave boundary pairs to the sides of the unit cell for which you want the array to extend. 3. Run the HFSS simulation: This simulates an infinite array comprised of identical elements, which extends to infinity on the sides where the Master-Slave boundaries are placed. All elements in the array are assumed to be fed in an identical manner to the single unit cell. 4. Post process for a finite array using HFSS > Radiation > Antenna Array Setup Ansoft High Frequency Structure Simulator v10: Advanced Notes 24
Array Functionality in HFSS Notes on using the array functionality in HFSS When using antenna array functionality Master-Slave boundaries are always necessary. Using Master-Slave boundaries simulates the effect of an element in an infinite array, and coupling effects of the element in this infinite array are taken account of. These coupling effects are simulated as if all elements in the array were excited with the same magnitude and phase, as applied to the unit cell. Plotting the far field of an infinite array simulation will give you the far field of the single element, as if it was in the presence of the whole array and all elements were excited in the same manner. When inserting any array function within HFSS, the coupling effects modelled within the array setup are based on those obtained for the infinite array. Because of this the custom array function is only really suitable for a general taper in magnitude and phase across the array. HFSS does not really generate an active match for each element but assumes the same match for each based upon the infinite array coupling. So anything other than gradual tapers in magnitude and phase will cause errors. The antenna array factor in HFSS neglects the edge effects in the array. Thus for an element to really be considered part of the infinite array as simulated, they should be at lease two elements in from the edges. When using an array setup, the far field will be that of the whole array. If two pairs of Master-Slave boundaries are used either side of an element, diagonal coupling effects will be taken account of. Better radiation results can be obtained if both the radiation surfaces and the master slave surfaces are setup to be part of the custom radiation surface used by the far field setup. This is particularly the case for small arrays with only a few infinite elements. When using the scan angle capability in HFSS always make sure that the scan angles in the Master-Slave setup are the same as those in the array setup. It may make sense to use a variable to achieve this. For array cases where the above assumptions are not appropriate always simulate the complete array in HFSS. Ansoft High Frequency Structure Simulator v10: Advanced Notes 25
Array Functionality in HFSS Setting up a regular array Select the menu item HFSS > Radiation > Antenna Array Setup When the Antenna Array Setup dialogue appears select the radial button next to Regular Array Setup. A new tab appears at the top labelled Regular Array select this tab. Ansoft High Frequency Structure Simulator v10: Advanced Notes 26
Array Functionality in HFSS Setting up a regular array, cont. Enter the array parameters: First Cell Position: this is the xyz coordinates of the first unit cell which you will reference the others to (xyz coordinates of the centre of your unit cell project). Directions: Use the U and V vectors to specify which directions (x, y, and/or z) the array extends in. Note these are different to the U and V vectors used for the Master-slave boundary setup. Distance Between Cells: Specify the desired distance between elements in the U and V directions. Number of Cells: Specify the desired number of cells in the U and V directions. Scan Definition: This can be specified two ways: Use Scan Angles: Specify the theta and phi scan angles. Use Differential Phase Shift: Specify a phase shift between adjacent element in U and V directions. This will then be applied uniformly across the array. Click OK. Ansoft High Frequency Structure Simulator v10: Advanced Notes 27
Array Functionality in HFSS Setting up a custom array Select the menu item HFSS > Radiation > Antenna Array Setup When the Antenna Array Setup dialogue appears select the radial button next to Custom Array Setup. A new tab appears at the top labelled Custom Array select this tab. Ansoft High Frequency Structure Simulator v10: Advanced Notes 28
Array Functionality in HFSS Setting up a custom array, cont. Click Import Definition Link to the text file which describes the array definition. The text file takes the format: N x_1 y_1 z_1 A_1 P_1 x_2 y_2 z_2 A_2 P_2... x_n y_n z_n A_N P_N where: x_1 is the x-coordinate position of the first element, which will take the model drawing unit. y_1 is the y-coordinate position of the first element, which will take the model drawing unit. z_1 is the y-coordinate position of the first element, which will take the model drawing unit. A_1 is the amplitude weight of the first element. P_1 is the phase weight for the first element. Example: a square 3 x 3 custom array, with uniformly weighted elements, and separated by 0.6729 user units: 9 0.0 0.0 0.0 1.0 0.0 0.6729 0.0 0.0 1.0 0.0 1.3458 0.0 0.0 1.0 0.0 0.0 0.6729 0.0 1.0 0.0 0.6729 0.6729 0.0 1.0 0.0 1.3458 0.6729 0.0 1.0 0.0 0.0 1.3458 0.0 1.0 0.0 0.6729 1.3458 0.0 1.0 0.0 1.3458 1.3458 0.0 1.0 0.0 Ansoft High Frequency Structure Simulator v10: Advanced Notes 29
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