6

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SETTING UP A HOBBIES SOLUTION AND RUNNING A SIMULATION

6.0 SUMMARY

Once the geometry modeling is completed, the user can define a Higher Order Basis Based Integral Equation Solver (HOBBIES) project, create a mesh, and run simulations. The HOBBIES solution settings include specifying the operation mode, units, frequency, and so on. The operation mode determines whether HOBBIES solves either the radiation from an antenna or the scattering from an object excited by a plane wave. The unit option will define the units of frequency, coordinate, radius, voltage, and field. It also provides functions to convert the unit or the value of a variable into different units.

Assigning domains is an essential step in a HOBBIES project. In this chapter, the user will learn how to define domains and how to assign a domain to wires and quadrilateral surfaces. For complicated models involving multilayer structures, the user can make use of Layers to ease the job.

HOBBIES is capable of modeling both concentrated and distributed loading. The concentrated loading can model lumped elements such as resistors, inductors, capacitors, or their combinations. The latter is useful for modeling loading over a wire or a quadrilateral surface to simulate a coated structure, for example. Both types of loading are explained in detail later.

Two additional settings that may be involved in a HOBBIES solution are symmetry and edging. Symmetry properties can be used to speed up the analysis only if both the geometry and the excitation are symmetrical or anti-symmetrical with respect to the same basic coordinate plane. Edging is a manipulation that enables automatic meshing along the edges when the edge effect is a critical parameter for an accurate analysis.

Output options need to be set to display the current distribution, near-field distribution, and far-field radiation patterns. Other options include choosing the accuracy of integrals used in a simulation, setting the order of the current expansion, configuring the type of matrix inversion, and choosing the number of decimal digits for the elements of the impedance matrix.

The last step before running the simulation is to mesh the model. After meshing, the well-defined project can then be run in either serial or parallel mode using an in-core or out-of-core solver.

6.1 OPERATION MODE

Electromagnetic structures can be excited by voltage generators at their ports (antennas, microwave circuits, and electromagnetic compatibility (EMC) facilities) or by plane waves. Structures excited by voltage generators are called antennas, while those excited by plane waves are termed “scatterers.” The user can choose either of the two operation modes for antennas and of the two operation modes for scatterers, as shown in Figure 6.1.

Figure 6.1. Operation modes.

Antenna Modes

• All generators mode: all ports are excited simultaneously.
• One generator at a time mode: one port is excited at a time, while all others are short-circuited.

Scatterer Modes

• Bistatic RCS mode: the structure is excited by a plane wave defined in the Waves list (see Section 6.6.2), while scattered fields are calculated along the directions defined using the HOBBIESOutput SettingsScattering menu (see Section 6.9.3).
• Monostatic RCS mode: the structure is excited by a plane wave defined in the Wave list. And incoming directions are defined using the HOBBIESOutput SettingsScattering menu. Backscattered fields are calculated along the same directions as the incoming waves.

6.2 UNITS

UNITS

The units can be defined according to the problem parameters as follows:

• Frequency: GHz, MHz, kHz
• Coordinate: mm, m, km, ft (feet), in (inches), mils
• Radius (of wire): (μm, mm, m, ft, in, mils
• Voltage: mV, V, kV
• Field: μV/m, mV/m, V/m

The Units window is shown in Figure 6.2 (a)—(e) for Frequency, Coordinate, Radius, Voltage, and Field, respectively. For each unit, two options can be applied:

• Convert numbers and units: this option changes the units in the project and converts the corresponding numbers according to the relations between the new and previous units.
• Convert units: this option only changes the units, while the corresponding numbers remain the same.

Figure 6.2. Define units: (a) frequency, (b) coordinate, (c) radius, (d) voltage, (e) field.

6.3 FREQUENCY RANGE

An analysis is performed at one or more uniformly distributed frequency points. The frequency range is defined by the Start frequency, Stop frequency, and Number of frequencies, as shown in Figure 6.3.

The numerical parameters of the analysis are determined with respect to the Stop frequency. For example, the mesh used in a simulation is determined by the Stop frequency. How to generate a mesh is discussed in details in Chapter 5.

If the Number of frequencies is set to be 1, the analysis will be performed only at the Start frequency. Otherwise, the analysis will be performed at the frequencies that are uniformly distributed between the start and stop frequencies.

Figure 6.3. Define the frequency range and the number of frequencies to be simulated.

6.4 DOMAINS

6.4.1 Using the Domain List Window

The Domains menu contains three options: Define, Assign, and View.

Define domains

To define domains, click the Define option and the Define Domains list appears, as shown in Figure 6.4. Each domain consists of all bodies made of the same material. By default, Domain 0 consists of all bodies made of perfect electric conductor (PEC), and Domain 1 is free space (perfect vacuum by default). Boundary surfaces of domains are defined by Wires and Surfaces.

Figure 6.4. Define Domains window.

In the Define Domains list, one row of the list, which contains the domain number and the electric parameters, defines one domain. The following electric properties of the Domains are specified in the Define Domains list:

• Complex relative permittivity: ε_c = ε_r + jε_i
• Complex relative permeability: μ_c = μ_r + j μ_i
• Conductivity: σ (in S/m)

In addition, the following can be done in the Define Domains list:

• : add a new domain and edit the electrical parameters.
• : delete an existing domain.
• : go to the first page.
• : go to the previous page.
• : go to the next page .
• : go to the last page.
• : find a domain according to the domain number.

Assign domains

To assign the domains to which wires belong or the domains of metallic and dielectric surfaces, the Assign Domains list, shown in Figure 6.5, needs to be used.

• Wires: Each wire should have a domain specification. A wire belongs to one domain only, and it is always immersed in that domain.

1. Click the icon in the Wires Domain column and a drop-down menu appears. Select the domain in which the wire is immersed. Remember to choose the icon in the row that corresponds to the Layer to which the wire belongs. See Layers in Appendix A. By default, there will be just Layer0 only.
2. Click the Assign option in the Wires Domain column, and select the wire in the HOBBIES screen.
3. Press the Esc key.

The default domain number of a wire is 1, which means that the wire is immersed in free space.

Figure 6.5. Assign Domains window.

By using the Assign Domains list, the user can define:

• Metallic surfaces: A basic metallic surface is always immersed in one domain only. It is represented by a single electric current sheet. Two domains are used to define a basic metallic surface. One is the domain in which the surface is immersed, and the other is Domain 0.

1. Click the icon in the Basic Metallic Domain column and a drop-down menu appears. Select the domain in which the surface is immersed. Remember to choose the icon in the row that corresponds to the Layer to which the metallic surface belongs.
2. Click the Assign option in the Basic Metallic Domain column, and select the surface in the HOBBIES screen.
3. Press the Esc key.

The default domain numbers of a basic metallic surface are 1 and 0, which means that the surface is immersed in free space.

• Composite metallic surfaces: A composite metallic surface is placed between two domains. It is represented by two electric current sheets, each immersed in one of the neighboring domains. The two neighboring domains are used to define a composite metallic surface.

1. Click the icons one by one in the Composite Metallic Domain column, and select the domains in the drop-down menus. Remember to choose the icons in the row that corresponds to the Layer to which the composite surface belongs.
2. Click the Assign option in the Composite Metallic Domain column, and select the surface in the HOBBIES screen.
3. Press the Esc key.

• Dielectric surfaces: A dielectric surface represents the boundary surface between two domains. It is represented by equivalent electric and magnetic current sheets. Two domains are used to define a dielectric surface.

1. Click the icons one by one in the Dielectric Domain column, and select the domains in the drop-down menus. Remember to choose the icons in the row that corresponds to the Layer to which the dielectric surface belongs.
2. Click the Assign option in the Dielectric Domain column, and select the surface in the HOBBIES screen.
3. Press the Esc key.

Figure 6.6. Check or uncheck layers.

In addition, the following can be done in the Assign Domains list:

• : select all layers.
• : unselect all layers.
• : go to the first page.
• : go to the previous page.
• : go to the next page.
• : go to the last page.

View domains

The user can view domains in different colors to verify domain assignments by using the View option, as shown in Figure 6.7.

Figure 6.7. View Domains window.

For complex structures including multiple types of dielectrics, the View option helps the user to check domains. An example including eight dielectric domains is depicted in Figure 6.8. Check or uncheck the box of one domain, and all the structures including wires and surfaces belonging to that domain are displayed or hidden. By using this feature, it is possible to see the domain properties of the model clearly.

Figure 6.8. Hide and show one domain.

The Color and Transparency of each domain can be set by clicking the corresponding options in that row of the list. An example including eight dielectric domains is depicted in Figure 6.9.

Figure 6.9. Example of color and transparency options.

In addition, the following can be done in the View Domains list:

• : select all domains.
• : unselect all domains.
• : go to the first page.
• : go to the previous page.
• : go to the next page.
• : go to the last page.
• : find a domain according to the domain number.

6.4.2 Editing Domains Manually

Define the Domains of Metallic and Dielectric Surfaces

As discussed above, three types of quadrilateral surfaces are considered:

1. A basic metallic surface is always immersed in one domain only. It is represented by a single electric current sheet.
2. A composite metallic surface is placed between two domains. It is represented by two electric current sheets, each immersed in one of the neighboring dielectric domains.
3. A dielectric surface represents the boundary surface between two dielectric domains. It is accompanied by equivalent electric and magnetic current sheets.

Domains for wires and quadrilateral surfaces can also be edited through the Wire list and Surface list window, which appears by going to the menu HOBBIESStructureWires/Surfaces. The domain(s) of each surface should be defined according to the rules summarized in Table 6.1.

Table 6.1. Definition of first and second domains for different types of surfaces

The following are some examples regarding how to set up the domains.

In the first example, the metallic surface is of a basic type and is immersed in vacuum (first domain). Therefore, for the metallic surface, enter the numbers 1 and 0 (in this order) into the 1st and 2nd domain edit fields. The Surface list window is shown in Figure 6.10.

In the second example, the metallic surface should be immersed in a dielectric body, which belongs to the second domain. Click the 1st domain edit field, type in number 2, and press the Enter key. Click the 2nd domain edit field, type in number 0, and press the Enter key. The Surface list window is shown in Figure 6.11.

In the third example, the metallic surface is of a composite type and is placed at the boundary surface between a dielectric and the vacuum (first and second domains). Therefore, for the metallic surface type numbers −2 and −1 into the 1st and 2nd domain edit fields. The Surface list should look like that in Figure 6.12.

Note that for a dielectric body, the surfaces that form the body need to be a closed contour. And the domain settings for the surface can either be (i, j) or (−i, −j) depending on whether it is a dielectric surface or a composite metallic surface.

Figure 6.10. Surface list. (The first surface is immersed in the first domain.)

Figure 6.11. Surface list. (The first surface is immersed in the second domain.)

Figure 6.12. Surface list. (The first surface is placed at the boundary surface between a dielectric and vacuum.)

Define the Domain to Which a Wire Belongs

Wires are defined in the same way as for a basic metallic surface. In addition, the domain of a wire is considered to be the domain in which it is immersed.

Each row of the Wire list defines one wire. The default Domain edit field displays number 1, which means that the wire is immersed in vacuum (first domain). Click the Domain edit field, type in number 2, and press the Enter key. Then the first wire is immersed in the second domain. The Wire list window is shown in Figure 6.13.

Figure 6.13. Wire list. (The first wire is immersed in the second domain.)

6.5 LOADINGS

By default, wires and metallic surfaces are considered to be composed of a perfect electric conductor. Real material properties of wires and metallic surfaces (e.g., finite conductivity) can be taken into account by adding distributed loadings over the wires and surfaces. These loadings are defined by the Distributed Loadings editor.

Lumped elements inserted at the junction of a wire with other wires or surfaces are modeled by concentrated loadings. These loadings are defined by the Concentrated Loadings editor.

6.5.1 Distributed Loadings

Consider a thin layer of thickness δ, made of a homogeneous medium of parameters ε_m, μ₀, and σ_m , surrounded by a nonconducting homogeneous medium of parameters ε and μ₀. Such a layer is modeled by two dielectric surfaces. If |σ_m + jωε_m| >>| jωε| and δ <<λ, the layer can be replaced by a PEC metallic surface with a distributed loading of surface impedance:

So, the layer can be modeled by a single metallic surface. In this way, the number of unknowns is reduced at least by a factor of four.

Surface impedances for a resistive layer ( ε_m = ε ) and dielectric layer (σ_m = 0) are given in Table 6.2.

Table 6.2. Surface impedance

Consider a thin rod of radius a, made of a homogeneous medium of parameters ε_m , μ₀ , and σ_m , surrounded by a nonconducting homogeneous medium of parameters ε and μ₀. Such a rod is modeled by dielectric surfaces. If |σ_m + jωε_m | >>| jωε|, then the rod can be replaced by a PEC metallic wire with distributed loading of per-unit-length impedance:

In this way, we model the rod by a single wire and the number of unknowns is reduced by a factor of about ten for this case. The surface impedance of a wire is related to the per-unit-length impedance Z′ through

In particular, if the rod is hollow, we use Eq. (6.1) for Z_s, and Z′ is calculated according to Eq. (6.3).

Surface impedances for a resistive layer of thickness δ (ε_m = ε) and a dielectric rod (σ_m = 0) are shown in Table 6.3. In general, Eq. (6.1) can be written in the following form:

We see that surface impedance represents the parallel combination of distributed resistive and capacitive loadings and, as such, it can be specified in the Distributed Loadings editor.

Table 6.3. Surface impedance for a rod

Losses due to the skin effect can be taken into account if the wires and surfaces are modeled with a surface resistance:

where f is the frequency of operation and σ is the conductivity of the metallic wires and surfaces.

The Distributed Loadings window contains three types of loadings: Skin, Srfc and RLC.

• Skin tab (Figure 6.14): Use this tab to specify frequency dependent skin effect losses with conductivity σ.

  

  Figure 6.14. Using Skin tab to specify skin effect losses.

• Srfc tab (Figure 6.15): Use this tab to specify surface impedance in the R_s + jX_s format, where R_s and X_s are constant in a selected frequency range.

  

  Figure 6.15. Using Srfc tab to specify surface impedance.

• RLC tab (Figure 6.16): Use this tab to specify distributed loadings with R, L, C elements in a parallel or series configuration. A zero value means that the element is nonexistent.

Figure 6.16. Using RLC tab to specify distributed loadings.

In each tab for Skin, Srfc, and RLC, the following can be done:

• : add a new distributed loading and edit the parameters in the list.
  1. Click , and then an information window appears, as shown in Figure 6.17.

     

     Figure 6.17. Information window for specifying distributed loadings.

  2. Click OK in the information window, and select wires and/or surfaces with the mouse.
  3. Press the Esc key and a new loading will be added in the list.
  4. Edit the parameters in the list.
• : delete an existing distributed loading.
• : go to the first page.
• : go to the previous page.
• : go to the next page.
• : go to the last page.
• : find a loading according to the loading number.

Examples for distributed loadings over wires and surfaces are given in Figure 6.18 and Figure 6.19, respectively.

Figure 6.18. Distributed loadings over wires with Skin, Srfc, and RLC distributed loadings for wire 1, wire 2, and wire 3, respectively.

Figure 6.19. Distributed loadings over surfaces with Skin, Srfc, and RLC from left to right.

6.5.2 Concentrated Loadings

A concentrated loading is an element inserted at the junction of a wire with other wires or surfaces. It is completely determined by the two nodes of the wire to which it is connected and by its impedance. It is placed at the first of the two specified nodes. It can model lumped elements such as resistors, inductors, capacitors, or their combinations. A concentrated loading represents a special case of a distributed loading, when the loading is distributed over a surface or length whose dimensions are much smaller than the wavelength.

There are two types of concentrated loadings: R + jX and RLC.

• R + jX tab (Figure 6.20): Use this tab to specify the impedance in R + jX format, with R and X that are constant in the selected frequency range.

  

  Figure 6.20. R + jX tab for concentrated loadings.

• RLC tab (Figure 6.21): Use this tab to specify impedance with R, L, and C elements in a parallel or series configuration.

  

  Figure 6.21. RLC tab for concentrated loadings.

In each tab for R + jX and RLC, the following can be done:

• : add a new concentrated loading and edit the parameters in the list.
  1. Click , and then an information window appears, as shown in Figure 6.22.

     

     Figure 6.22. Information window for specifying concentrated loadings.

  2. Click OK in the information window and select the wire with the mouse.
  3. Press the Esc key and a new loading will be added in the list.
  4. Edit the parameters in the list.
• : delete an existing concentrated loading.
• : go to the first page.
• : go to the previous page.
• : go to the next page.
• : go to the last page.
• : find a loading according to the loading number.
• : label the concentrated loadings. See also in the Toolbar.

Figure 6.23. Concentrated loading on wires: (a) R + j X loading, (b) RLC loading.

6.6 EXCITATION

An electromagnetic structure can be excited by:

• Generators: in this case the antenna operation mode needs to be predefined;
• Waves: in this case the scatterer operation mode needs to be predefined. (See Operation Mode in Section 6.1)

6.6.1 Generators

A Generator is always connected to one node of a wire, and its reference direction is oriented towards the other node of the wire (from 1st node to 2nd node as shown in Figure 6.24).

Figure 6.24. Generators list.

There are three types of generators (there is no type 1):

• Type 2: coaxial transverse electromagnetic (TEM) frill excitation at the junction of two wires, as shown in Figure 6.25.
• Type 3: coaxial TEM frill excitation at the junction of a wire and a surface, as shown in Figure 6.26.
• Type 4: voltage generator (point or delta generator) at the junction of two wires or at the junction of a wire and a surface, as shown in Figure 6.27.

Figure 6.25. A type 2 generator.

Figure 6.26. A type 3 generator.

Figure 6.27. Type 4 generator: (a) a generator at a wire-wire junction, (b) a generator at a wire-surface junction.

The generator is completely defined by:

• Generator type: in Type field in the Generator list.
• The node of a wire at which the generator is placed: written in 1st Nodes field.
• The node of the same wire indicating the reference direction: written in 2nd Nodes field.
• Real and imaginary parts of the complex voltage: in Real and Imag fields, respectively,
• Inner and outer radius of the coaxial TEM frill: in 1st and 2nd Radius field. In the case of a delta generator (Type 4), these fields should be zero.

In the Generator list, the following can be done:

• : add a new generator and edit the parameters.
  1. Click , and then an information window appears, as shown in Figure 6.28.
  2. Click OK in the information window, and select the wire with the mouse.
  3. Press the Esc key and a new generator will be added in the list.
  4. Edit the parameters in the list.

Figure 6.28. Information window for generators.

In addition, the following commands can be applied in the Generator List.

• : delete an existing generator.
• : go to the first page.
• : go to the previous page.
• : go to the next page.
• : go to the last page.
• : find a generator according to the generator number.
• : label generators. See also icon in the Toolbar.

By default, a new generator introduced in a model is a delta generator (Type 4). A coaxial cable excitation can also be used by calling HOBBIESExcitationCoaxial Cable.

6.6.2 Waves

A plane wave is defined in the Wave list window, as shown in Figure 6.29.

Figure 6.29. Wave list window.

The plane wave is completely defined by:

• Incoming direction or incident direction in the ϕ and θ fields under Direction. Note that the reference coordinate system is spherical.
• Real and imaginary part of the complex ϕ -component of the electric field intensity in the Real and Imag E_φ fields, respectively.
• Real and imaginary part of the complex θ -component of the electric field intensity in the Real and Imag E_θ fields, respectively.

HOBBIES uses two arrows to denote a plane wave in the screen, as shown in Figure 6.30. The green (horizontal) arrow shows the incident direction and the red (vertical) one shows the polarization direction of the plane wave.

Figure 6.30. Arrows to indicate the polarization of the wave and the incident direction.

Similar to the Generator list, the following can be done in the Waves list:

: add a new plane wave and edit the parameters. By default the incident direction of the plane wave is ϕ = 0° and θ = 0°, and the electric field intensity is E_φ = 1 + j0 and E_θ = 0 + j 0 V/m.

: delete an existing plane wave.

: go to the first page.

: go to the previous page.

: go to the next page.

: go to the last page.

: find a plane wave according to the plane wave number.

6.7 SYMMETRY

Symmetry properties can be used to speed up the analysis only if both the geometry and the excitation are symmetrical or anti-symmetrical with respect to the same basic coordinate plane (XY, XZ, and/or YZ-plane).

For example, if one symmetry plane is used, only half of the original structure should be defined. As a result, the number of unknowns used in the analysis is approximately halved compared with the analysis of the original structure. It means that the maximum electrical size of the problem that can be handled by HOBBIES is at least doubled.

Symmetries about three basic coordinate planes are set in the Symmetry window (Figure 6.31):

• Symmetry X (YZ plane): to define the type of symmetry for the yOz-plane,
• Symmetry Y (XZ plane): to define the type of symmetry for the xOz-plane,
• Symmetry Z (XY plane): to define the type of symmetry for the xOy-plane.

Figure 6.31. Symmetry window.

For selected planes, choose the type of symmetry in the drop-down menu:

• Symmetry: the structure and the excitation are both symmetrical with respect to a basic coordinate plane. The user only needs to define the structure and excitation above the symmetry plane.
• Anti-Symmetry: the structure is symmetrical, and the excitation is anti-symmetrical with respect to a basic coordinate plane. The user only needs to define the structure and excitation at one side of the anti-symmetry plane.
• PMC: the structure is defined above a perfect magnetic conductor (PMC) that coincides with a basic coordinate plane.
• PEC: the structure is defined above a perfect electric conductor (PEC) that coincides with a basic coordinate plane.

Choosing different types of symmetry may affect the simulation result depending on the excitation. For some cases, when the Symmetry (Anti-Symmetry) plane is replaced by PMC (PEC):

• The field above the plane is not changed, while the field below the plane becomes equal to zero.
• The gain is increased by 3 dB—doubled in unnamed units, and the RCS is increased by 6 dB—4 times in unnamed units.
• Impedance of the antenna driven by a generator placed in the plane is doubled (halved).
• Admittance of the antenna driven by a generator placed in the plane is halved (doubled).

For other cases, the results could be different. Related examples can be found in Section 8.4.

The Symmetry and PMC planes are colored in red, while the Anti-Symmetry and PEC planes are colored in blue, as shown in Figure 6.32.

Figure 6.32. Symmetry options: (a) Symmetry plane (light red), (b) Anti-Symmetry plane (light blue), (c) PMC plane (red), (d) PEC plane (blue).

6.8 EDGE

Edge is a manipulation that enables automatic meshing along the edges when the edge effect is a critical parameter for an accurate analysis.

The edge effect occurs along the free edges of a metal quadrilateral surface and along the edges that represent surface-to-dielectric surface junctions. Edge meshing is performed by introducing additional narrow metallic and/or dielectric strips along the critical edges. It is up to the user to make decisions as to which of the edges are critical. A good example is a microstrip structure where edge effects may play a dominant role in the analysis.

Click the HOBBIESEdge option and the Edge list appears, as shown in Figure 6.33. In the Edge list, the following can be done:

• : add a new edge and edit the parameters.
• : delete an existing edge.
• : go to the first page.
• : go to the previous page.
• : go to the next page.
• : go to the last page.
• : find an edge according to the edge number.

Figure 6.33. Edge list.

To add an edge manipulation, click the icon . The Edge list window is shown in Figure 6.34. It consists of two Maximum Edge Width edit fields and the Domains selection list. The surfaces that are meshed are those at the boundary of two domains.

Figure 6.34. Edge list of the Auto edge type.

The related parameters are:

• Max. Edge Width [m]: defines the maximum width of the edge of the strip in m (or other selected unit). If the defined edge width exceeds the maximum value allowed by the geometry of the selected surface, then the Max. Edge Width [%] criterion is used.
• Max. Edge Width [%]: defines the maximum width of the edge strip in percent of the maximum dimension of the surface for which edging is performed. Recommended value is from 10 to 25.
• Domains: specify the domains of the surfaces for which the edging is performed. If All is selected, then all the surfaces at the boundary of two domains are meshed.

Note that the edge meshing is performed automatically when the simulation is performed. The user can preview the mesh from edging in the post-processing window after starting Pre-HOBBIES, which will be described in Chapter 7.

Example 1

A microstrip patch antenna is shown in Figure 6.35. To simulate the structure accurately, it is necessary to use the edge property in the model.

Click the HOBBIESEdge menu to open the Edge list, and add an edge by clicking the Add icon in Figure 6.36. Enter the value of the Max. Edge Width [m] as 0.003 and that of the Max. Edge Width [%] as 20.0. In the Domains drop-down menu, there are three options for the model, which are All, −1 −2, and 1 2. Figure 6.37 shows the edges corresponding to the three Domains options.

Figure 6.35. Perspective view of a microstrip patch.

Figure 6.36. Edging of the microstrip patch antenna.

Figure 6.37. Edges of the microstrip patch: (a) All Domains in top view, (b) All Domains in perspective view for the corner of the patch, (c) Domains −1 and −2 in top view, (d) Domains 1 and 2 in top view.

Example 2

A narrow wall slotted array is illustrated in Figure 6.38. Figure 6.39 displays the edges.

Figure 6.38. A slotted array.

Figure 6.39. Edges for slots in a waveguide of the array.

6.9 OUTPUT SETTINGS

The user can calculate four types of output results: Y, Z and S-parameters are calculated in Antenna Modes by default, while all other output results have to be specified manually.

The other three types of output results are specified below:

• Current: to define the density of points over surfaces or wires where the current is to be calculated.
• Near-Field: to define the points in Cartesian coordinates, where the near-field is to be calculated.
• Far-Field: to define the directions in the spherical coordinates, along which the radiation or scattering is to be calculated.

6.9.1 Current

Select HOBBIESOutput SettingsCurrent to display the Current configuration window (Figure 6.40); the user can choose:

• Automatic Density of Points
  • Low: to choose a low density of points over wires/surfaces where the current is to be calculated.
  • Middle: to choose a moderate density of points over wires/surfaces where the current is to be calculated.
  • High: to choose a high density of points over wires/surfaces where the current is to be calculated.
• Manual Density of Points
  • Set Ncp and Ncs: to set the number of points along the p- and s-coordinates in the Set Ncp and Ncs input field (referring to Section 4.1.3 for Ncp and Ncs).
  • No points (Clear All): to set the number of points to be zero.

Figure 6.40. Current settings window.

The user can also change the density of the sampling points for each particular wire or surface.

• Type a number in the Ncs edit field in the Wire list to set the number of points at which the current is calculated on the selected wire, as shown in Figure 6.41.
• Type numbers in the Ncp and Ncs edit fields in the Surface list to set the number points at which the current is calculated on the p- and s-components of the selected surface, as shown in Figure 6.42.

Figure 6.41. Ncs for a wire.

Figure 6.42. Ncp and Ncs for a surface.

6.9.2 Near-Field

The near-field is calculated at one or more uniformly spaced points in the Cartesian coordinate system.

Select the HOBBIESOutput SettingsNear-Field and the Near-Field dialog window appears (Figure 6.43); the user specifies:

• Number of Coordinates
• Start Coordinates
• Stop Coordinates

along the x- , y-, and z-coordinates respectively. The Domains drop-down menu should be set to All.

Select the Show near-field range on the screen check box, and the points at which the near-field will be calculated are displayed on the screen, as shown in Figure 6.44.

Figure 6.43. Near-field settings window.

Figure 6.44. Near-field range on the screen.

6.9.3 Far-Field

The far-field is calculated at one or more uniformly spaced angular directions. The angular direction is specified by the spherical ϕ- and θ-coordinates.

In Antenna Modes, Select HOBBIESOutput SettingsRadiation and the Radiation pattern dialog window opens (Figure 6.45). The user specifies:

• Number of directions
• Start directions
• Stop directions

in the corresponding edit fields for both ϕ- and θ-coordinates. In Scatterer Modes, the Radiation option changes to the Scattering option, accordingly, and the Scattering pattern window is similar to the window in Figure 6.45.

Figure 6.45. Radiation parameter settings window.

Select the Show radiation pattern range on the screen check box, and the angular directions in which the far-field will be calculated are displayed on the screen, as shown in Figure 6.46.

Figure 6.46. Far-field range on the screen.

If the PreferencesGeneralPopup messages option is set to be “Beginner” (refer to Appendix A for details), there will be a warning message shown as in Figure 6.47. The user can choose “No” to revise the output setting or choose “Yes” to continue to simulate the model. If the user chooses to continue, in the post-process window, the user will not be able to view the three dimensional (3D) radiation pattern as it exceeds the limit and another pop-up message will appear as seen in Figure 6.48. Similarly, for output setting of current and near-field, the user also needs to be aware of this limitation.

Figure 6.47. The pop-up warning window in the pre-process window when the output setting exceeds the limit.

Figure 6.48. The pop-up warning window in the post-process window when the output setting exceeds the limit.

Note that, all the two-dimensional (2D) graphs (i.e., 2D radiation pattern, 2D scattering pattern, 2D near-field, network parameters, or 2D frequency cut figure) will not be affected. This limitation only affects the 3D plots of the fields.

For the professional version, there is no such limit. The user can contact [email protected] for more information on the professional version of HOBBIES.

6.10 OPTIONS

HOBBIES automatically chooses the default parameters for the analysis, usually giving accurate results. However, in some cases, the results may not be satisfactory to the user. To improve the accuracy, the user should change some of the parameters or modify the initial model. In some cases, the user can even improve the efficiency of the analysis without losing the accuracy and stability of the computed results.

The parameters that can be changed manually include:

• Integral accuracy: the accuracy of the integrals used in the analysis.
• Current expansion: the order of the polynomial approximation of currents.
• Matrix inversion: the type of matrix inversion.
• Precision: single or double.

All these are shown in Figure 6.49.

Figure 6.49. Options window.

6.10.1 Choosing the Accuracy of the Integrals Used in the Analysis

During the computations, evaluation of many of the integrals has to be performed, and the accuracy of the solution can be improved by increasing the order of the numerical integration used. The accuracy of the overall analysis can be improved, but the central processing unit (CPU) time needed for the analysis is increased. HOBBIES automatically determines the orders of the numerical integration required, so that satisfactory results are obtained with minimum CPU time.

To manually change the accuracy for the numerical integration manually, select the Integral accuracy drop-down list (Figure 6.50) and set Integral accuracy to:

• Normal: to apply the default value, automatically defined by HOBBIES
• Reduced: to reduce the accuracy of the integrals and the time for the analysis
• Enhanced 1–10: to increase the accuracy of the integrals and hence the analysis time

If the results obtained using two or more successive grades show significant agreement, the results are considered stable.

The purpose of this option is to check the stability and convergence of the results. Greater stability of the results usually requires greater accuracy of the results.

Figure 6.50. Options window (Integral accuracy).

6.10.2 Choosing the Order of the Current Approximation

The electrical size of the structure to be analyzed is determined with respect to the Stop frequency specified in the Frequency dialog box (Section 6.3). When the Number of frequencies specified in the Frequency dialog box is 1, the analysis is performed at the Start frequency. In that case, increasing the Stop frequency increases the order of the current expansion in a relatively proportional way, still analyzing the structure at the Start frequency.

To change the orders of the current expansion for all surfaces and wires simultaneously, select the Current expansion drop-down list (Figure 6.51) and set the current expansion to:

• Normal: to apply the default value automatically defined by HOBBIES
• Reduced: to reduce the order of the current approximation and the time for the analysis
• Enhanced 1–10: to increase the orders and hence the analysis time

If the results obtained using two or more successive grades show significant agreement, the results are considered stable.

From the Current expansion drop-down list, the user can set orders of these polynomial expansions between 1 and 9. Note that the Current expansion drop-down list works when the current expansion order in the Wire list window (Figure 6.52) and Surface list window (Figure 6.53) are set as 0, which is the default value. The user can also manually change the order of the current expansion for each particular wire or surface, and the value cannot be larger than 9.

• Type the order in the Nds edit field in the Wire list to set the current expansion for the selected wire, as shown in Figure 6.52.
• Type the orders in the Degrees Ndp and Nds edit fields in the Surface list to set the current expansion for the p- and s-components of the selected surface, as shown in Figure 6.53.

Figure 6.51. Options window (Current expansion).

Figure 6.52. Edit the current expansion order in the Wire list window.

Figure 6.53. Edit the current expansion order in the Surface list window.

6.10.3 Choosing the Type of Matrix Inversion

The analysis method converts the integral equations of Chapter 1 into a single matrix equation. In some cases, the efficiency of the analysis can be significantly improved by choosing the type of matrix inversion. Select the Matrix inversion drop-down list. The Matrix inversion drop-down list offers three options: Reduced, Normal, and Avoided, as shown in Figure 6.54.

By default, the Matrix inversion drop-down list is set to normal. In this case, the matrix equation is solved directly using the standard LU decomposition procedure.

If the drop-down list is set to reduced, the matrix is considered to be symmetrical, and a symmetrical version of the matrix solution routine is used, reducing the CPU time by half. The matrix is symmetrical in the case of all metallic structures, except those having wire-to-surface junctions and junctions of wires of unequal radius. The matrix cannot be symmetrical if a structure contains dielectric parts. The application of this option to asymmetrical matrices will lead to erroneous results. Note that this option is only valid for the serial in-core code.

It is often of interest to repeat the same analysis, changing only the specification of the output results. If the drop-down list is set to A voided, HOBBIES avoids the calculation of the matrix and matrix inversion and uses the current coefficients obtained from the previous run of the same example. This possibility is used if the example has been run at least once with the matrix inversion option defined as normal or reduced. After this run, current coefficients are stored in the file with the extension .dis. It is correct to use these coefficients if, and only if, the new output data are calculated without changing the structure and excitation. This option cannot be used for multiple frequencies and multiple excitation analysis in this version .

Figure 6.54. Options window (Matrix inversion).

6.10.4 Choosing the Number of Decimal Digits for Representing the Impedance Matrix

The Precision drop-down list offers two options: Single and Double, as shown in Figure 6.55. When Single is chosen, each element of the matrix is represented using seven decimal digits. If Double is chosen, the number of decimal digits for the elements of the impedance matrix is 15. This leads to higher accuracy in the numerical calculations and decreases the accumulation of numerical errors in the matrix solution. On the other hand, the use of memory is nearly doubled if Double is chosen, so the maximum number of unknowns that can be analyzed is reduced by a factor of in contrast to the one generated by choosing Single.

To ensure accuracy, the parallel kernel only accepts double precision, no matter which precision the user selects.

Figure 6.55. Options window (Precision).

6.11 RUNNING SIMULATIONS

The next step is to run the HOBBIES simulation after the solution settings (Chapter 6) and the meshing (Chapter 5) are completed. The solution settings, especially the Domains, Loadings and Edge options, should be set before the meshing is done to make sure that the corresponding properties are correctly assigned to the meshed elements. In the case that the element sizes are larger than two wavelengths in the medium surrounding the model, the elements are refined automatically by the HOBBIES solver, when a simulation is run.

This section describes the HOBBIES solvers, which mainly include four types:

• Serial In-Core
• Serial Out-of-Core
• Parallel In-Core
• Parallel Out-of-Core

The four solvers are based on the higher order method of moments (MoM). The differences between the MoM solvers are listed in Table 6.4. The serial solvers, in-core and out-of-core (OOC), can run only one HOBBIES process at one time on a single CPU core (currently, it is common for CPUs to have multiple cores). The parallel solvers, in-core and out-of-core, can run multiple HOBBIES processes on multiple cores or CPUs with shared memory or distributed memory computers. In-core solvers use only RAM to store data when simulating models, while the out-of-core solvers use random access memory (RAM) (in-core buffer) and the hard disk to store the data when simulating models. These will be described in detail in the following sections.

Note that when executing each solvers or the Pre-HOBBIES option for the first time, the firewall or the anti-virus software may pop up a warning message asking whether to block or unblock the process. Please make sure to choose “unblock”; otherwise the HOBBIES kernel will not be able to run. Or the user may choose to turn off the firewall to avoid this. And for executing parallel kernels of HOBBIES, the user's account needs to be at least “Power User.” For personal computers, it is recommended to have the administrator's permission for a seamless execution. For the home edition of a Microsoft Windows 7 system (Microsoft Corporation, Redmond, WA), even the administrator account will not have the full permission by default. So the user may need to set manually the permission in the HOBBIES installation folder so that there is adequate permissions to read and write to the folders.

Table 6.4. Comparison of four method of moments (MoM) solvers

6.11.1 Setup Run Environment

Menu: Run HOBBIESSetup Run Environment

Click the Run HOBBIESSetup Run Environment menu and a window appears, as shown in Figure 6.56. The window includes two tabs: Parallel Environment and Solver Options.

If the message passing interface (MPI) status is Unbooted {Figure 6.56 (a)}, the user should register the MPI for parallel solvers. Input a user account with administrative rights and the corresponding password. If the account does not have a password, the user has to create one. Click Boot MPI and the setup of the MPI is completed when the status changes to Booted {Figure 6.56 (b)}. If the user wants to reboot the MPI, click Reset MPI and register it again.

The user can increase or decrease the number of processes that run the local host. The number of processes should not exceed the number of cores on the host. It is recommended that the number of processes be equal to the number of all the cores the user's computer has. When the user uses the parallel solvers of HOBBIES, the simulations will execute using all cores. Click Accept and the setup of the parallel environment is completed.

Figure 6.56. Parallel Environment tab: (a) MPI is unbooted, (b) MPI is booted.

The Solver Options tab includes variables for out-of-core solvers, as shown in Figure 6.57. The Physical RAM per core (MB) is the available physical memory per core when the user's computer is idle. Note that the total RAM for all cores should not exceed the amount of available physical memory. The In-core buffer per core (MB) is the physical memory per core used as the in-core buffer for the out-of-core solvers of HOBBIES. After the user inputs the Physical RAM per core (MB), click Get default value and HOBBIES will calculate the In-core buffer per core (MB) automatically. Typically, the In-core buffer per core (MB) is 80% of the Physical RAM per core (MB). The Hard-disk space per core (GB) is the free space per core on the logical partition where HOBBIES projects are located. It is not recommended to put HOBBIES projects in the primary partition where the operating systems are located when using the out-of-core solvers. The default values are 800, 640, and 50 for the three options.

Figure 6.57. Solver Options tab for out-of-core solvers.

6.11.2 Pre-HOBBIES

The Pre-HOBBIES option is used to predict the number of unknowns of a HOBBIES project, and thus, it helps the user to estimate the RAM requirement and CPU time needed for the simulation.

The RAM requirement is estimated through the equation RAM [Byte] = 16N² for double precision, where N is the number of unknowns. If the RAM requirement exceeds the total amount of available physical memory of the computer, it is strongly recommended to use the out-of-core solvers. Furthermore, if the RAM requirement exceeds the total amount of available hard-disk space, it will not be possible to simulate the project on that computer. See RAM and hard disk information in Section 6.11.1.

Click the Run HOBBIESPre-HOBBIES menu to see the information window including the number of unknowns as shown in Figure 6.58.

Figure 6.58. Information window of Pre-HOBBIES.

6.11.3 Serial Solvers

Once a project has been setup properly, the user may run the serial solvers by selecting Run HOBBIESSerial In-Core or Serial Out-of-Core. Select the in-core or out-of-core solver according to the estimation of RAM requirement as described in Section 6.11.2.

Note: If the amount of available physical memory on the computer is not enough for the serial in-core solver, HOBBIES will cancel the simulation.

If the amount of available physical memory of the computer is enough for the serial in-core solver but the serial out-of-core solver is selected, HOBBIES will continue to perform the simulation without cancellation.

Select Run HOBBIESSerial In-Core or Serial Out-of-Core and the process window appears, as shown in Figure 6.59. The window includes the Project name, Start time, UID, and Priority of the simulation. To stop the simulation, click the Terminate button and start the simulation again by clicking the Start button. The Close button closes the process window but does not terminate the simulation. To view the simulation process, click Run HOBBIESProcess Window.

Figure 6.59. Process window.

The Output view button allows the user to check the detailed process of the simulation. Click the Output view button and an information window appears (Figure 6.60).

The window contains useful information such as number of elements, number of excitations, number of unknowns, radiation directions, simulation time, and so on. Click the Close button to close the window. To open the window again, click Run HOBBIESView Process Info. If a project contains any incorrect input parameters, the Process errors window appears, as shown in Figure 6.61. The user needs to check the input data to continue to run the simulation. Useful information can be viewed by clicking the View errors button in the Process errors window. For example, if the radius of wires is set to be 0, the error information is given in Figure 6.62.

6.11.4 Parallel Solvers

Running the parallel solvers is similar to running the serial solvers. To run the parallel solvers, select Run HOBBIESParallel In-Core or Parallel Out-of-Core. Select the in-core or out-of-core solver according to the estimation of RAM requirement as described in Section 6.11.2.

The parallel solvers are based on MPI and thus can run on both shared memory (e.g., a multiple-core CPU computer) and distributed memory (e.g., a high-performance computing cluster) systems.

The number of processes and the number of hosts for running the parallel solvers are set in the Setup Run Environment window as described in Section 6.11.1.

Caution: When the parallel out-of-core solver is used the number of unknowns in the project has to be greater than one.

Figure 6.60. Output view window.

Figure 6.61. Process errors window.

Figure 6.62. Process errors window after clicking the View errors button.

Example

If a shared memory computer has two separate quad-core CPUs, which means that the total number of cores is 8, the parallel solvers allow a parallel solution with up to 8 parallel processes to be launched on this computer.

Take a HPC cluster as another example; it has 64 computing nodes, each of which has two quad-core CPUs (8 cores), and then the total number of cores is 64 × 8 = 512. The parallel solvers now allow a parallel solution with up to 512 parallel processes to be launched on this cluster.

6.12 CONCLUSION

This chapter summarizes all the various configurations required for a typical HOBBIES project, including the description of some of the important settings such as the operation mode, frequency band, units, domains, excitations, output settings. Some of the optional settings such as loadings, edging for calculation are also illustrated. With these necessary configurations properly set, the user can run the simulation with the serial in-core, serial out-of-core, parallel in-core, or parallel out-of-core solver to solve the problem of interest.