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Anechoic Chamber Absorbing Electromagnetic Waves
Introduction
An anechoic chamber is a measurement facility for antenna characterization, electromagnetic interference (EMI), and electromagnetic compatibility (EMC) tests. By absorbing electromagnetic waves inside the chamber and blocking the incoming signals from the outside, it creates a virtually infinite space that has almost zero internal reflection and does not suffer from unwanted external RF noises, so the device-under-test in the chamber can be accurately measured without any interference. This model simulates a biconical antenna, popularly used in EMI and EMC tests, which is located at the center of a small anechoic chamber. The computed far-field radiation pattern and S-parameter (S11) demonstrate that the microwave absorbers reduce reflection from the walls significantly without distorting antenna performance.
Figure 1: A state-of-the-art anechoic chamber built in a small room (3.9m ? 3.9m ? 3.3m). It consists of microwave absorbers on thin conductive walls. Two side walls are not included in this figure.
Model Definition
The shape of the absorbers is configured with the array of pyramidal objects to steer the propagation direction of the incident field on the absorbers reflected back not to the radiation source, but toward the surface of the adjacent absorbers. The radiation-absorbent material (RAM), conductive carbon-loaded foam in the pyramidal shaped absorber, is modeled using a low conductive material (σ = 0.5 S/m). So the electromagnetic waves illuminated on the absorber has the process of partial reflection and partial transmission with subsequent attenuation that is repeated until the wave reaches the base of the pyramid. The amplitude of the field at the base of the pyramid is drastically reduced. Thus, the reflection from the absorbers at this point is marginal.
The exterior of the chamber is finished with a perfect electric conductor (PEC) to model metallic surfaces that insulate the chamber from the outside RF noises.
The imported biconical antenna geometry is identical to the one used in another application library example, Modeling a Biconical Antenna for EMI/EMC Testing (Ref. 1). This reference model is simulating the same antenna geometry but the antenna is enclosed by a numerical version of an anechoic chamber that is a perfectly matched layer (PML).
The metallic surfaces of the antenna are also configured by PEC. A lumped port with a 50 Ω reference impedance is assigned to the gap located at the center of the two structures composed of hexagonal frames. All domains except for the absorbers is filled with the air.
The simulation frequency is set to 240 MHz.
Results and Discussion
The far-field polar plot as a function of azimuth angle is visualized in Figure 2. The plotting plane is perpendicular to the dominant polarization of the antenna so it is the H-plane radiation pattern. Just like the radiation pattern of the biconical antenna surrounded by the PML in Ref. 1, it is isotropic since the reflection from the chamber walls ,that are made of the lossy conductive pyramidal form array, is negligible. The computed S-parameter (S11) is around -10 dB that is very close to the value evaluated at 240 MHz in Ref. 1, which also indicates that the reflection from the chamber walls is marginal.
Figure 3 shows one way to enhance the quality of the results postprocessing by utilizing solution set selections and uniform custom colors. The contour of the norm of electric fields in a dB-scale is plotted in a realistic view of an anechoic chamber. The exterior metallic walls are visualized with the norm of electric fields using the GrayScale color table.
Figure 2: The far-field radiation pattern on the H-plane of the biconical antenna at 240 MHz. It is isotropic as expected.
Figure 3: The contour plot of the norm of the electric field (dB-scaled). The strength of the field is gradually decaying inside absorbers.
References
1. Modeling a Biconical Antenna for EMI/EMC Testing, COMSOL Application Libraries.
Notes About the COMSOL Implementation
The example model is memory intensive and may require more than 20 GB RAM. The goal of this model is not to simulate an antenna but to design a state-of-the-art anechoic chamber and validate it based on the performance of the antenna. It is recommended to use a PML instead of absorber models to simulate antennas efficiently. The same biconical antenna with the PML (Ref. 1) may need less than 3 GB memory. Note that the anechoic model is not designed in full compliance with well-known standards such as CISPR and ANSI.
Application Library path: RF_Module/EMI_EMC_Applications/anechoic_chamber
Model Instructions
From the File menu, choose New.
New
In the New window, click  Model Wizard.
Model Wizard
1
In the Model Wizard window, click  3D.
2
In the Select Physics tree, select Radio Frequency > Electromagnetic Waves, Frequency Domain (emw).
3
Click Add.
4
Click  Study.
5
In the Select Study tree, select General Studies > Frequency Domain.
6
Click  Done.
Study 1
Step 1: Frequency Domain
Define the study frequency ahead of performing any frequency-dependent operation such as building mesh. The physics-controlled mesh uses the specified frequency value.
1
In the Model Builder window, under Study 1 click Step 1: Frequency Domain.
2
In the Settings window for Frequency Domain, locate the Study Settings section.
3
In the Frequencies text field, type 240[MHz].
Geometry 1
Block 1 (blk1)
1
In the Geometry toolbar, click  Block.
2
In the Settings window for Block, locate the Size and Shape section.
3
In the Width text field, type 3.9.
4
In the Depth text field, type 3.9.
5
In the Height text field, type 3.3.
6
Locate the Position section. From the Base list, choose Center.
7
Click  Build Selected.
8
Click the  Wireframe Rendering button in the Graphics toolbar.
Pyramid 1 (pyr1)
1
In the Geometry toolbar, click  More Primitives and choose Pyramid.
2
In the Settings window for Pyramid, locate the Size and Shape section.
3
In the Base length 1 text field, type 0.15.
4
In the Base length 2 text field, type 0.15.
5
In the Height text field, type 0.4.
6
In the Ratio text field, type 0.
7
Locate the Position section. In the x text field, type -1.425.
8
In the y text field, type -1.425.
9
In the z text field, type -1.6.
Block 2 (blk2)
1
In the Geometry toolbar, click  Block.
2
In the Settings window for Block, locate the Size and Shape section.
3
In the Width text field, type 0.15.
4
In the Depth text field, type 0.15.
5
In the Height text field, type 0.05.
6
Locate the Position section. In the x text field, type -1.5.
7
In the y text field, type -1.5.
8
In the z text field, type -1.65.
Union 1 (uni1)
1
In the Geometry toolbar, click  Booleans and Partitions and choose Union.
2
Select the objects blk2 and pyr1 only.
3
In the Settings window for Union, locate the Union section.
4
Clear the Keep interior boundaries checkbox.
Array 1 (arr1)
1
In the Geometry toolbar, click  Transforms and choose Array.
2
Select the object uni1 only.
3
In the Settings window for Array, locate the Size section.
4
In the x size text field, type 20.
5
In the y size text field, type 20.
6
Locate the Displacement section. In the x text field, type 0.15.
7
In the y text field, type 0.15.
Union 2 (uni2)
1
In the Geometry toolbar, click  Booleans and Partitions and choose Union.
Select all objects in the array as shown in the figure below.
2
In the Settings window for Union, locate the Union section.
3
Clear the Keep interior boundaries checkbox.
Mirror 1 (mir1)
1
In the Geometry toolbar, click  Transforms and choose Mirror.
2
Select the object uni2 only.
3
In the Settings window for Mirror, locate the Input section.
4
Select the Keep input objects checkbox.
Rotate 1 (rot1)
1
In the Geometry toolbar, click  Transforms and choose Rotate.
2
Select the object uni2 only.
3
In the Settings window for Rotate, locate the Input section.
4
Select the Keep input objects checkbox.
5
Locate the Rotation section. In the Angle text field, type 90.
6
Locate the Point on Axis of Rotation section. In the x text field, type -0.15.
7
In the z text field, type 0.15.
8
Locate the Rotation section. From the Axis type list, choose y-axis.
Rotate 2 (rot2)
1
In the Geometry toolbar, click  Transforms and choose Rotate.
2
Select the object rot1 only.
3
In the Settings window for Rotate, locate the Rotation section.
4
In the Angle text field, type 0 90 180 270.
Union 3 (uni3)
1
In the Geometry toolbar, click  Booleans and Partitions and choose Union.
2
Select the objects mir1, rot2(1), rot2(2), rot2(3), rot2(4), and uni2 only.
3
In the Settings window for Union, locate the Selections of Resulting Entities section.
4
Find the Cumulative selection subsection. Click New.
5
In the New Cumulative Selection dialog, Create a set of absorber selections that will make easier to set up the physics and material.
6
type Absorbers in the Name text field.
7
Click OK.
Sphere 1 (sph1)
1
In the Geometry toolbar, click  Sphere.
2
In the Settings window for Sphere, locate the Selections of Resulting Entities section.
3
Find the Cumulative selection subsection. Click New.
4
In the New Cumulative Selection dialog, Create a set of far-field selections.
5
type Far-field in the Name text field.
6
Click OK.
Import 1 (imp1)
1
In the Geometry toolbar, click  Import.
2
In the Settings window for Import, locate the Source section.
3
In the Filename text field, type anechoic_chamber_antenna.mphbin.
4
Locate the Selections of Resulting Entities section. Find the Cumulative selection subsection. Click New.
5
In the New Cumulative Selection dialog, Create a set of antenna geometry selections.
6
type Antenna in the Name text field.
7
Click OK.
8
In the Geometry toolbar, click  Build All.
9
Click the  Wireframe Rendering button in the Graphics toolbar.
10
Click the  Transparency button in the Graphics toolbar.
11
In the Model Builder window, click Geometry 1.
Adjust the graphics window settings as you prefer for the remaining modeling steps.
12
Click the  Transparency button in the Graphics toolbar.
13
Click the  Wireframe Rendering button in the Graphics toolbar.
14
Click the  Zoom In button in the Graphics toolbar.
Add Material
1
In the Materials toolbar, click  Add Material to open the Add Material window.
2
Go to the Add Material window.
3
In the tree, select Built-in > Air.
4
Click the Add to Component button in the window toolbar.
5
In the Materials toolbar, click  Add Material to close the Add Material window.
Materials
Absorbers
1
In the Model Builder window, under Component 1 (comp1) right-click Materials and choose Blank Material.
2
In the Settings window for Material, type Absorbers in the Label text field.
3
Locate the Geometric Entity Selection section. From the Selection list, choose Absorbers.
4
Locate the Material Contents section. In the table, enter the following settings:
Property
Variable
Value
Unit
Property group
Relative permittivity
epsilonr_iso ; epsilonrii = epsilonr_iso, epsilonrij = 0
1
1
Basic
Relative permeability
mur_iso ; murii = mur_iso, murij = 0
1
1
Basic
Electric conductivity
sigma_iso ; sigmaii = sigma_iso, sigmaij = 0
0.5
S/m
Basic
Electromagnetic Waves, Frequency Domain (emw)
Perfect Electric Conductor 2
1
In the Physics toolbar, click  Boundaries and choose Perfect Electric Conductor.
2
In the Settings window for Perfect Electric Conductor, locate the Boundary Selection section.
3
From the Selection list, choose Antenna.
Lumped Port 1
1
In the Physics toolbar, click  Boundaries and choose Lumped Port.
Add a lumped port at the center of the antenna. Zoom in a few of times to get a clear view.
2
In the Settings window for Lumped Port, locate the Boundary Selection section.
3
Click  Paste Selection.
4
In the Paste Selection dialog, type 6310 in the Selection text field.
5
Click OK.
For the first port, wave excitation is on by default.
Far-Field Domain 1
1
In the Physics toolbar, click  Domains and choose Far-Field Domain.
2
In the Settings window for Far-Field Domain, locate the Domain Selection section.
3
From the Selection list, choose Far-field.
Far-Field Calculation 1
1
In the Model Builder window, expand the Far-Field Domain 1 node, then click Far-Field Calculation 1.
2
In the Settings window for Far-Field Calculation, locate the Boundary Selection section.
3
Click  Clear Selection.
4
From the Selection list, choose Far-field.
Study 1
Step 1: Frequency Domain
In the Study toolbar, click  Compute.
Results
Electric Field (emw)
1
In the Settings window for 3D Plot Group, locate the Plot Settings section.
2
Clear the Plot dataset edges checkbox.
Multislice 1
1
In the Model Builder window, expand the Electric Field (emw) node, then click Multislice 1.
2
In the Settings window for Multislice, locate the Expression section.
3
In the Expression text field, type 20*log10(emw.normE).
4
Locate the Multiplane Data section. Find the Y-planes subsection. In the Planes text field, type 0.
5
Find the Z-planes subsection. In the Planes text field, type 0.
6
Click to expand the Range section. Select the Manual color range checkbox.
7
In the Minimum text field, type -40.
8
In the Maximum text field, type 20.
9
In the Electric Field (emw) toolbar, click  Plot.
Electric Field, Logarithmic (emw)
In the Model Builder window, under Results click Electric Field, Logarithmic (emw).
2D Far Field (emw)
1
In the Model Builder window, click 2D Far Field (emw).
2
In the Settings window for Polar Plot Group, locate the Axis section.
3
Select the Manual axis limits checkbox.
4
In the r minimum text field, type 0.
5
In the r maximum text field, type 1.
6
In the 2D Far Field (emw) toolbar, click  Plot.
Compare the reproduced plot to Figure 2.
3D Far Field, Gain (emw)
In the Model Builder window, click 3D Far Field, Gain (emw).
3D Plot Group 6
1
In the Results toolbar, click  3D Plot Group.
2
In the Settings window for 3D Plot Group, locate the Color Legend section.
3
Clear the Show legends checkbox.
4
Locate the Plot Settings section. Clear the Plot dataset edges checkbox.
Volume 1
1
Right-click 3D Plot Group 6 and choose Volume.
2
In the Settings window for Volume, locate the Expression section.
3
In the Expression text field, type 1.
4
Locate the Coloring and Style section. From the Coloring list, choose Uniform.
5
From the Color list, choose Custom.
6
On Windows, click the colored bar underneath, or — if you are running the cross-platform desktop — the Color button.
7
Click Define custom colors.
8
Set the RGB values to 0, 64, and 128, respectively.
9
Click Add to custom colors.
10
Click Show color palette only or OK on the cross-platform desktop.
Selection 1
1
Right-click Volume 1 and choose Selection.
2
In the Settings window for Selection, locate the Selection section.
3
Click  Paste Selection.
4
In the Paste Selection dialog, type 4, 6, 10 in the Selection text field.
5
Click OK.
Surface 1
1
In the Model Builder window, right-click 3D Plot Group 6 and choose Surface.
2
In the Settings window for Surface, locate the Expression section.
3
In the Expression text field, type 1.
4
Locate the Coloring and Style section. From the Coloring list, choose Uniform.
5
From the Color list, choose White.
Selection 1
1
Right-click Surface 1 and choose Selection.
2
In the Settings window for Selection, locate the Selection section.
3
From the Selection list, choose Antenna.
Surface 2
1
In the Model Builder window, right-click 3D Plot Group 6 and choose Surface.
2
In the Settings window for Surface, locate the Coloring and Style section.
3
From the Color table list, choose GrayScale.
Selection 1
1
Right-click Surface 2 and choose Selection.
2
In the Settings window for Selection, locate the Selection section.
3
Click  Paste Selection.
4
In the Paste Selection dialog, type 3, 485, 12215 in the Selection text field.
5
Click OK.
Cut Plane 1
1
In the Results toolbar, click  Cut Plane.
2
In the Settings window for Cut Plane, locate the Plane Data section.
3
From the Plane list, choose ZX-planes.
Contour 1
1
In the Model Builder window, right-click 3D Plot Group 6 and choose Contour.
2
In the Settings window for Contour, locate the Data section.
3
From the Dataset list, choose Cut Plane 1.
4
Locate the Expression section. In the Expression text field, type 20*log10(emw.normE+1e-5).
Adding 1e-5 to the log expression improves the color variation in the contour plot.
5
Locate the Levels section. In the Total levels text field, type 100.
6
In the 3D Plot Group 6 toolbar, click  Plot.
See Figure 3 to compare the reproduced plot.
Port Probe
The S11 value in the table should be around -10 dB.