EM.Tempo Tutorial Lesson 1: Analyzing A Center-Fed Resonant Dipole Antenna

 Tutorial Project: Analyzing A Center-Fed Resonant Dipole Antenna Objective: In this project, you will construct a center‐fed resonant dipole antenna, analyze it and visualize its near and far field characteristics. Concepts/Features: PEC Objects Lumped Source Port Definition Mesh Density S/Z/Y Parameters Radiation Pattern Field Sensor Observable Cartesian and Polar Graphs Minimum Version Required: All versions ' Download Link: EMTempo_Lesson1

What You Will Learn

This tutorial will guide you through all the necessary steps to set up and perform a basic FDTD simulation and visualize and graph the simulation results. Specifically, you will use the wire dipole wizard to create the geometry of a resonant half-wave dipole antenna in the project workspace. You will examine the port definition observable for computing S/Z/Y parameters. You will also define far-field radiation pattern and near-field sensor observables.

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

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 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:

 Name EMTempo_Lesson1 Millimeters GHz 1GHz 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.Tempo, 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.

Creating the Dipole Antenna Geometry

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

 EM.Tempo's wizard toolbar.
 The dipole antenna geometry in the project workspace and the highlighted additions to the navigation tree.

The geometry of a dipole antenna appears at the center of the project workspace. A close look at the physical structure reveals that it consists of two thin vertical cylinders connected through a short vertical line at the center. In the navigation tree on the left, you see three objects listed under a perfect electric conductor (PEC) material group called "DIPOLE":

1. ANCHOR (Feed Line)
2. Dipole_Arm_1
3. Dipole_Arm_2

Also note that a blue domain box is placed around your dipole antenna structure. By default, the domain box is placed at an offset of 0.25λ0 away from the largest bounding box of your physical structure. By default, a convolutional perfectly matched layer (PML) boundary condition is assumed at all the six faces of the domain box. This is required for modeling an open-boundary problem. For this project, you will accept the default computational domain and the default PML boundary conditions.

Examining the Length of the Dipole

The geometry of the dipole antenna created by the wizard 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:

$\lambda_0 = \frac{c}{f} = \frac{3\times 10^8}{1 \times 10^9} = 0.3m = 300mm$

A half-wave dipole, therefore, should have a length of 150mm. Now open the Variable Dialog by clicking the 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 0.47*λ0. This is the total length of the dipole antenna and is slightly less than half free-space wavelength as is typical in the design of dipole antenna of finite radius. The radius of the wire, which is indeed the radius of the two cylindrical arms, is set to 0.002*λ0. These parameters can easily be changed to any arbitrary value, expression, or other definitions. The height of the two cylindrical arms is defined by a variable called "arm_length", which takes into account portion of the joining vertical line at the center.

 The variables dialog.

Select and highlight the item "Dipole_Arm_1" in the navigation tree, right-click on it and select Properties... from the contextual menu. The property dialog of the bottom cylinder opens up on the lower right corner of the screen. Note that the height of the cylinder is set equal to the variable "arm_len" and its radius is set equal to the variable "wire_rad".

 The property dialog of one of the cylinder objects.

Examining the Source & Observables

EM.Cube wizards take care of most of the things you need to build a complete project including the excitation source and simulation variables. You cannot run a simulation if your physical structure doesn't have an excitation source. Also, if you don't define an observable for your project, there will be no output data at the end of a simulation. The wire dipole wizard automatically inserts a Lumped Source, a Far-Field Radiation Pattern observable and a Port Definition observable in your project. You are set to go!

Before running an FDTD simulation of your dipole antenna, let's examine the properties of the lumped source. Select the item "LS_1" under Lumped Sources in the Sources section of the navigation tree. Right-click on it and select Properties... from the contextual menu. In the lumped source dialog, you will see the name "Feed_Line_ANCHOR" in the drop-down list labeled Line Object. Also, note that the value of the Offset parameter is set equal to "0.5*feed_len", which places the source at the center of the middle vertical line. The internal impedance of the voltage source is set equal to 75Ω.

 A lumped source must always be associated with a PEC or thin wire line object that is parallel to one of the principal axes.

The far-field observable called "FF_1" is defined to compute the far-field radiation pattern of the dipole antenna. You can open the property dialog of the far-field observable in the same way. Note that the spherical theta and phi angle increments are both set to 5°. The port definition observable called "PD_1" is initiated to calculate the port characteristics of the antenna, i.e., its S, Z and Y parameters. According to the property dialog of the port definition observable, the reference port impedance is 75Ω and the S/Z/Y parameters will be computed over the frequency range [fc - bw/2, fc + bw/2], where fc is the center frequency and bw is the bandwidth of the project.

 EM.Tempo's lumped source dialog. EM.Tempo's radiation pattern dialog. EM.Tempo's port definition dialog.

Examining the FDTD Mesh

EM.Tempo generates a “staircase” Yee mesh of your physical structure. To be able to capture the details of your dipole's very fine cylindrical arms, the mesh should have adequate resolution. EM.Tempo’s default “Adaptive” FDTD mesh generator exactly does that. To view the mesh, click the Show/Generate Mesh button of the Simulate Toolbar or alternatively use the keyboard shortcut Ctrl+M. The resolution of the mesh is determined by its Mesh Density expressed in effective wavelength. EM.Tempo's default mesh density of 20 cells/λeff. However, the wizard automatically set the mesh density to 50 cells/λeff for this particular structure. Open the FDTD Mesh Settings dialog by clicking the Mesh Settings button of Simulate Toolbar or using the keyboard shortcut Ctrl+G. The FDTD Mesh Settings dialog offers a large number of parameters that can be used to fine-tune the Yee mesh to better approximate and represent your physical structure.

 EM.Tempo's mesh settings dialog.

It is often very informative to examine the FDTD mesh grid. Enable the XY mesh grid plane by right clicking the XY Grid Plane item in the “Discretization” section of the navigation tree and selecting Show from the contextual menu. You can see that the grid lines are more densely placed around the location of the cylindrical arms. Next, enable the YZ mesh grid plane by right clicking the YZ Grid Plane item in the “Discretization” section of the navigation tree and selecting Show from the contextual menu.

 The XY mesh grid plane. The YZ mesh grid plane.

Running the FDTD Simulation

At this time, your project is ready for FDTD simulation. To run a quick simulation of your physical structure, click the Run Button of the Simulate Toolbar to open up the Simulation Run Dialog. Or alternatively, use the keyboard shortcut Ctrl+R, or select the menu item Simulate → Run… The simplest simulation mode in EM.Tempo is Wideband Analysis, which is its default Simulation Mode. In this mode, your physical structure is taken “As Is” and its mesh is passed to the FDTD simulation engine along with the necessary information regarding the sources and observables.

 EM.Tempo's Simulation Run dialog.

Before you run your first FDTD simulation in EM.Tempo, let’s take a closer look at the FDTD simulation engine’s settings. Click the Settings button next to the “Select Engine” drop-down list to bring up the FDTD Engine Settings dialog. The convergence section ("Termination Criterion") of this dialog offers three criteria for terminating the FDTD time marching scheme. The first one is a Power Threshold, which normally has a default value of -30dB. The second one is a Maximum Number of Time Steps, which normally has a default value of 10,000. The third option is labeled “Both”, which means that both of the above termination criteria will be considered until one of them is met. For this particular structure, the wizard set the power threshold equal to -50dB and the maximum number of time steps to 20,000.

 For highly resonant structures, it is recommended that you establish a much more demanding convergence criterion with a lower power threshold value and a larger number of time steps.
 EM.Tempo's Simulation Engine Settings dialog.

To run the simulation, click the Run button of the Simulation Run Dialog. A separate window pops up displaying messages from the simulation engine. In four separate fields, the engine reports the current time step, elapsed time, performance in MCells/second, and convergence status. Once the simulation is completed, you can close the message window and return to the project workspace. The navigation tree and data manager are now populated with simulation results.

 EM.Tempo's simulation output window.

Plotting Scattering and Impedance Parameters

A list of all the 2D output data files generated at the end of a simulation can be viewed in EM.Tempo’s Data Manager. To open this dialog, click the Data Manager button of Simulate Toolbar, or use the keyboard shortcut Ctrl+D, or select the menu item Simulate → Data Manager. Select the file "DP_S11.CPX" from the list by clicking on its name and highlighting its row in the table. Click the Plot button of the dialog. A PyPlot graph window pops up that shows the magnitude and phase of the S11 parameter. If you move the mouse around the graph, you can read the values of the graph on the Status Bar of the graph window. The figure below shows that the dip of the plot is at 977MHz with a value of -33.4dB

 The data manager dialog.
 Plot of the return loss (S11 parameter).

Next, select the file "DP_Z11.CPX" from the list and plot it. This time the graph shows the real and imaginary parts of the Z11 parameter. It can be seen that at 977MHz, the input resistance of the dipole is 73.75Ω.

 Plot of the input impedance (Z11 parameter).