EM.Libera Tutorial Lesson 1: Analyzing A Center-Fed Wire Dipole Antenna

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Tutorial Project: Analyzing A Center-Fed Wire Dipole Antenna
Libera L1 Fig title.png

Objective: In this project, a wire dipole is modeled and analyzed.

Concepts/Features:

  • Wizard
  • Line Object
  • Wire Dipole
  • Wire MoM Analysis
  • Gap Source
  • S-Parameters
  • Z-Parameters
  • Current Distribution
  • Radiation Pattern

Minimum Version Required: All versions

'Download2x.png Download Link: EMLibera_Lesson1

What You Will Learn

This tutorial will guide you through all necessary steps required to set up and perform a basic Wire Method of Moments (WMOM) simulation and visualize and graph the simulation results. In particular, you will model the radiation characteristics of a wire dipole antenna.

Attention icon.png We strongly recommend that you read through the first few tutorials and study them carefully before setting up your own projects.

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Getting Started

Open the EM.Cube application by double-clicking on its icon on your desktop. By default, EM.Cube opens a blank project with the name “UntitledProj0” in its CubeCAD Module. You can start drawing objects and build up your physical structure right away. Or you can initiate a new project by selecting the New Fdtd newb.png button of the System Toolbar or using the keyboard shortcut Ctrl+N. This opens up the New Project Dialog, where you can enter a title for your new project and set its path on your hard drive. From the same dialog, you can also set the project’s length units, frequency units, center frequency and bandwidth.

The project workspace.
The New Project dialog.

For this tutorial lesson, set the following parameters in the new project dialog:

Starting Parameters
Name EMLibera_Lesson1
Length Units Millimeters
Frequency Units GHz
Center Frequency 1GHz
Bandwidth 1GHz

Click the Create button of the dialog to accept the settings. A new project folder with your given name is immediately created at your specified path.

To navigate to EM.Libera, simply select its icon from the Module Toolbar on the left side of the screen. Selecting the module icon changes the contents of the navigation tree to reflect the types of objects supported by the current module.

Constructing the Wire Dipole Antenna Geometry

Click on the Wire Dipole Wizard WireDipoleIconx.png button of the Wizard Toolbar or select the menu item Tools → Antenna Wizards → Wire Dipole Antenna.

EM.Libera's wizard toolbar.

The geometry of a wire dipole antenna appears at the center of the project workspace. When you drew a large object, it may extend beyond the current view's screen. You can zoom to fit your structure into the screen using the keyboard shortcut Ctrl+E or by clicking the Zoom Extents Fdtd zoomextents.png button of View Toolbar.

The dipole antenna geometry in the project workspace and the highlighted additions to the navigation tree.

In EM.Libera, the geometric objects in the project workspace are grouped together based on their common material or surface properties. You can see that the wizard created a "Thin Wire" group called "WIRE_DIPOLE" under Thin Wires in the Physical Structure section of the navigation tree. The name of the group is displayed in bold letter, meaning that it is the currently active group in the navigation tree. All the new object you draw will belong to the active material group. To see the properties of the group "WIRE_DIPOLE", select its name in the navigation tree, right-click and select Properties... from the contextual menu. This opens up he Thin Wire dialog. From the property dialog, you can see that all the objects belonging to this group share the same red color and the same wire radius equal to "wire_rad".

The thin wire dialog.

Examining the Length of the Dipole

The structure the wizard created for you is fully parameterized. The objective of this tutorial lesson is to teach the basics of running a simulation rather than parameterizing a geometrical construction. Therefore, we will not get into the details of defining variables at this point. Note that at the center frequency fc = 1GHz, the operating wavelength is:

[math] \lambda_0 = \frac{c}{f} \approx \frac{3\times 10^8}{1 \times 10^9} = 0.3m = 300mm [/math]

A half-wave dipole, therefore, should have a length of 150mm. Now open the Variable Dialog by clicking the Variable icon tn.png button on the Simulate Toolbar or selecting the menu item Simulate → Variables.... You will see a variable called "dipole_len", whose value has been set to 0.5*λ0. The radius of the wire is set to 0.002*λ0. These parameters can easily be changed to any arbitrary value, expression, or other definitions.

The variables dialog.

Select and highlight the item "dipole" in the navigation tree, right-click on it and select Properties... from the contextual menu. The property dialog of the line object opens up at the lower right corner of the screen. Note that the length of the lineis set equal to the variable "dipole_len".

The property dialog of the line object.

Examining the Gap Source & Simulation Observables

Your dipole antenna must be excited using some type of source. The wizard define a gap source for your project and placed it at the center of the dipole. A wire gap source must always be associated with an existing line object in the project workspace. It creates an infinitesimal gap on the line and connects a voltage source across the gap. Right-click on the item "LC_1" under Wire Gap Circuits in the “Sources” section of the navigation tree, and select Properties… from the contextual menu. The Wire Gap Circuit dialog opens up, where you can see the gap source has been associated with the line object called "dipole".

The location of the source on its host line object can be controlled via the Offset parameter of the dialog. Note that the wizard sets the value of the offset parameter equal to "0.5*dipole_len". This causes the gap source to always stay at the center of the dipole.

The wire gap source's property dialog.

EM.Libera by itself doesn't generate any output data at the end of a simulation. You need to define simulation observables for your project. The wizard already defined a default current distribution observable. Right-click on the item "CD_1" under Current Distributions in the "Observables" section of the navigation tree and select Properties... from the contextual menu to open the property dialog of this observable.

The current distribution dialog.

The wizard also initiated a far-field radiation pattern observable. Right-click on the item "FF_1" under Far-Field Radiation Patterns in the navigation tree and select Properties... from the contextual menu. The Radiation Pattern dialog opens up. The values of the "Angle Increment" parameter for both Theta and Phi have been set equal to 1°.

The far-field radiation pattern dialog.

Ports are defined to calculate the scattering, impedance and admittance parameters of a one-port or multiport structure. Your dipole antenna is a one-port structure. The wizard already initiated a port definition for your dipole structure. Right-click on the item "PD_1" under Port Definitions in the navigation tree and select Properties... from the contextual menu. In the Port Definition dialog, you will see that the gap source LC_1 has been associated with Port 1 and its port reference impedance has been set to 75Ω.

The port definition dialog.

Examining the Mesh of the Wire Structure

The quality of the simulation results in a method of moments (MoM) simulation greatly depends on the quality and resolution of its mesh. The mesh of a line object consists of a number of linear cells along its length. The resolution of EM.Libera's mesh is controlled by a parameter called Mesh Density, which has a default value of 10 Cells/λeff. For resonant structures, a higher mesh density is recommended. A low mesh density may fail to provide an adequate level of numerical accuracy. Extremely high mesh densities, on the other hand, may lead to numerical instability.

The mesh properties can be accessed by clicking the Mesh Settings Fdtd meshsettings.png button of the Simulate Toolbar or using the keyboard shortcut Ctrl+G or via the menu item Simulate → Discretization → Mesh Settings. Since a wire dipole antenna is a resonant structure, the wizard automatically set the value of mesh density to 30 Cells/λeff.

EM.Libera's mesh settings dialog.

To view the mesh, click the Show/Generate Mesh Fdtd meshshow.png button of the Simulate Toolbar or alternatively use the keyboard shortcut Ctrl+M. To exit the mesh view mode and return to the normal view mode, use the keyboard’s Esc key or click the Show/Generate Mesh Fdtd meshshow.png button of the Simulate Toolbar one more time.

A Note on EM.Libera's Simulation Engines

EM.Libera features two simulation engines: one based on Wire Method of Moments (WMOM) and the other based on Surface Method of Moments (SMOM). These two engines are totally independent of each other and never work together. The choice of simulation engine is made by EM.Libera, and the user cannot control it. SMOM is EM.Libera's default solver and can handle metallic surfaces and solids and dielectric objects. If your physical structure contains at least one curve geometric object, then WMOM is set as the simulation engine. In that case, all the surface and solid geometric objects are discretized as metallic wireframe structures. Since your dipole structure in this project is made of a line object, EM.Libera will use the WMOM solver to simulate your antenna.

Running a Single-Frequency Simulation of Your Dipole Antenna

At this time, your project is ready for simulation. Click the Run Fdtd runb.png Button of the Simulate Toolbar or use the keyboard shortcut Ctrl+R or select the menu item Simulate → Run... to open up the Simulation Run Dialog. You will see that the "Simulation Engine Type" is automatically set to "Wire MoM Solver".

EM.Libera's simulation run dialog.

The simplest simulation mode in EM.Libera is “Single-Frequency Analysis”. In this mode, your physical structure is taken “As Is” and its mesh at the center frequency of the project is passed to the simulation engine along with the necessary information regarding the sources and observables. Start the simulation by clicking the Run button of the dialog. Another window opens up which reports the progress of the simulation. Once the WMOM simulation is complete, the output window will report the calculated S, Z and Y parameters:

S11: 0.099202 +0.248841j

S11(dB): -11.441001

Y11: 0.009744 -0.005224j

Z11: 79.712742 +42.738593j

Visualizing the Simulation Results

Once the simulation is completed, the navigation tree is populated with simulation results under the current distributions and radiation patterns nodes. EM.Cube’s computational modules usually generate two types of data: 2D and 3D. Examples of 2D data are Cartesian and polar radiation patterns. 2D data are graphed in EM.Grid. Examples of 3D data are near-field and current distributions and 3D radiation patterns. 3D data are visualized in EM.Cube’s project workspace, and the plots are usually overlaid on the physical structure.

First, you will visualize the current distribution on the dipole antenna. The "Current Distribution" section of the navigation tree plots both electric currents (J) and magnetic currents (M). Each current distribution observable contains a list of twelve amplitude and phase plots for all the six current components: Jx, Jy, Jz, and Mx, My, Mz. There are also two additional plots for the magnitude of total electric current and total magnetic current. In the case of your project, since you have only a metal wire, M = 0. From the figure below, you can see the familiar sinusoidal current distribution on the wire with its peak at the center and zero current at the two ends.

The total current distribution on the wire dipole.

Next, visualize the 3D radiation pattern of the dipole antenna. You will see the familiar donut shape with a directivity of 1.64.

The total 3D radiation pattern of the half-wave dipole antenna.

A list of all the 2D output data files generated at the end of a simulation can be viewed in EM.Picasso’s Data Manager. To open this dialog, click the Data Manager Fdtd datamanagerb.png button of Simulate Toolbar, or use the keyboard shortcut Ctrl+D, or access the menu item Simulate → Data Manager, or right-click on the Data Manager item in the “Observables” section of the navigation tree and select Open Data Manager… The S/Z/Y parameters are written into complex data files with a “.CPX” file extension. These ASCII data files are called “S11.CPX”, “Z11.CPX” and “Y11.CPX”, respectively. You can view the contents of the these files using the View button of data manager.

EM.Libera's Data Manager dialog.

Besides the 3D visualization of the radiation patterns, you can plot 2D graphs of the patterns at certain plane cuts. The 2D radiation patterns can be plotted as both Cartesian and polar graphs. In the data manager dialog, spot Cartesian pattern data files with a “.DAT” file extension as well as the polar (angular) data files with a “.ANG” file extension. The figure below shows the Cartesian and polar radiation pattern plots in the YZ plane cut. These are the data files called "FF_1_PATTERN_Cart_YZ.DAT" and "FF_1_PATTERN_Polar_YZ.ANG", respectively. To plot them, select their name or row in the Data Manager's list and highlight them and then click the Plot button. Note that you can make multiple file selections for plotting using the Ctrl or Shift keys. Besides the three principal XY, YZ and ZX plane cuts, there are also data files for one additional user defined Phi-plane cut, which by default is calculated at φ = 45°.

The 2D Cartesian graph of the YZ-plane radiation pattern.
The 2D polar graph of the YZ-plane radiation pattern.

Running a Frequency Sweep of the Dipole Antenna

In this part of the tutorial lesson, you are going to run a frequency sweep of the dipole antenna.

Attention icon.png Many of EM.Cube's wizards create parameterized geometries whose dimensions are functions of the project's operating frequency or wavelength. Before running a frequency sweep of your physical structure, you have to review such frequency-dependent variables and possibly replace their definitions with numeric values.

In this project, the variables "dipole_len" and "wire_rad" are frequency-dependent as their definitions involve the independent variable "lambda0_unit", which itself depends on the project center frequency "fc". Open the variables dialog and make the following changes:

Variable Name Original Definition New Definition
dipole_len lambda0_unit*0.5 150
wire_rad lambda0_unit*0.002 0.6

To change the definition of a variable, select and highlight its name in the variables list and click the Edit button of the dialog to open the "Edit Variable" dialog. In this dialog, replace the definition of the selected variable with a numeric value or any other expression.

Changing the definition of variable "dipole_len".
The variables dialog showing the new definitions of some variables.

Now open the simulation run dialog and select Frequency Sweep from the drop-down list labeled Simulation Mode.

Setting frequency sweep as the simulation mode in EM.Libera's run dialog.

Click the Settings button next to this drop-down list to open the Frequency Sweep Settings dialog. The default frequency sweep type is Uniform, which you keep intact. Enter 0.5GHz and 1.5GHz for the start and stop frequencies, respectively, and keep the number of frequency samples at the default value of 11.

EM.Libera's frequency sweep settings dialog.

Close this dialog to return to the run dialog and click the Run key to start the frequency sweep. After the completion of the sweep simulation, open the data manager and plot the data files "S11_Sweep.CPX" and "Z11_Sweep_CPX" in EM.Grid.

Plots of the magnitude and phase of the S11 parameter of the dipole antenna as a function of frequency.
Plots of the real and imaginary parts of the Z11 parameter of the dipole antenna as a function of frequency.

With EM.Libera, you can also perform an adaptive frequency sweep of your physical structure. This sweep starts with a few frequency samples in the beginning and then inserts more frequency samples in between and uses a rational function interpolation to achieve a smooth frequency response. Open the frequency sweep settings dialog again and this time choose the radio button Adaptive for the sweep type. Accept the default values of the adaptive sweep parameters and run and new adaptive frequency sweep simulation of your planar structure. The program may give a warning that during the adaptive sweep, current distribution and far-field radiation pattern data will not be produced. Ignore the warning and continue. Also, after a number of sweep iterations, the program may pop up a message saying the convergence criterion hasn't been met and will ask you whether to continue the sweep process. In that case, reply "No" and stop the sweep. Too many adaptive sweep iterations may sometime lead to spurious spikes in the frequency response.

Setting the sweep type to "Adaptive" in the frequency sweep settings dialog.

At the end of the sweep simulation, open the data manager and plot the data files "S11_RationalFit.CPX" and "Z11_RationalFit.CPX" in EM.Grid. From the figures below you can see that the dipole antenna's return loss |S11| reaches a minimum of -31.3dB at 957MHz. Also, the imaginary part of the input impedance of the dipole antenna Im(Z11) vanishes at about 955MHz, where its real part (input resistance) is about 71Ω at this frequency. In other words, the dipole antenna resonates at about 955MHz.

Plots of the magnitude and phase of the S11 parameter at the end of an adaptive frequency sweep.
Plots of the real and imaginary parts of the Z11 parameter at the end of an adaptive frequency sweep.

 

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