A bow tie antenna is a common type of broadband antenna that is similar to a 2-dimensional conical dipole. Its lower frequency is a function of length and flare angle, $\alpha$, and is usually a little lower than a thin wire dipole of the same length. The input impedance is a function of frequency and $\alpha$, and the real part is typically between 70 and 500 ohms.
In this tutorial, the bow tie antenna geometry is constructed using two triangular pieces of metal, where $\alpha$ is approximately 70 degrees and the length is 40 mm. This generates an average input impedance near 300 ohms with a lower frequency of 2 GHz.
This tutorial utilizes the following skills:
- Creating bow tie antenna geometry by using the polygon tool to define the sheet body.
- Assigning a PEC material to the bow tie in order to to define the antenna's electromagnetic properties.
- Adding a voltage source with 300 ohm resistor to represent the antenna's connection to a generator.
- Verifying the grid and enabling XACT mesh.
- Using a far zone sensor to save the 3-D radiation pattern during simulation.
- Running the simulation with the specified frequencies of interest at 6.7 and 20 GHz.
- Viewing the simulation's S-parameter, far zone gain, and impedance results.
Step 1: Set the Project Properties
Setting the frequency range of interest is the first step in the workflow because XFdtd uses this information when constructing the grid, creating a waveform, and setting the bounds of output plots. This simulation focuses on the antenna's performance from 2 GHz to 25 GHz.
- Open the editor window by selecting Edit ❯ Project Properties.
- In the editor's Frequency Range of Interest tab, set the Minimum to 2 GHz.
- Set the Maximum to 25 GHz.
- Under the Display Units tab, change the Time setting by selecting nanoseconds (ns) from the drop-down menu.
- Click Done to close the editor.
Step 2: Create Antenna Geometry
The size of the antenna can be determined using basic geometric formulas and the values given in the overview for $\alpha$ and length—approximately 70 degrees and 40 mm, respectively. Users should note that the exact size of the bow tie is not critical, so the vertices are rounded to intergers for simplicity.
- Right-click on the Parts branch of the Project Tree, and choose Create New ❯ Sheet Body from the drop-down menu.
- Under the Edit Profile tab, enter the part's name by typing Bow Tie into the Name box.
- Select the Polygon tool from the drop-down menu in the Shapes toolbar.
- Click on (−1 mm, 0 mm) to select the left-hand triangle's first vertex.
- Use the mouse wheel to zoom as needed.
- Click on (−20 mm, 14 mm) as the triangle's second vertex.
- Click on (−20 mm, −14 mm) as the triangle's third vertex.
- Press Enter on the keyboard to close the triangle.
- Repeat for the right-hand triangle or press Tab to enter the vertices as (1 mm, 0 mm), (20 mm, 14 mm), and (20 mm, −14 mm).
- Click Done in the upper-right corner to finish the Bow Tie geometry.
Step 3: Define and Assign a PEC Material
A material needs to be associated with the geometry in order to define the electromagnetic properties of the antenna material, so a material first needs to be defined and then assigned to the bow tie. Assuming the bow tie is made of a perfect electric conductor (PEC) material is a good approximation in this case because the impedance and radiation patterns would be similar to those of a very good conductor, such as copper. XF's definitions node allows users to define a set of electromagnetic properties once and use it in multiple places, although this tutorial is basic and requires just one.
First, define the antenna material.
- In the Definitions branch of the Project Tree, right-click on Materials and select New Material Definition from the drop-down menu.
- Enter the material's name by typing PEC into the highlighted field.
- Press Enter to add the new material to the project.
- Double-click the new material in the Project Tree to open the Material Editor.
- Under the Electric tab, select Perfect Conductor from the Type drop-down menu.
- If desired, navigate to the Appearance tab to change the PEC material's display color.
- Click Done to finish the PEC material.
Then, assign the material to the Bow Tie.
- Click and drag the PEC material from the Definitions branch of the Project Tree and drop it on top of the Bow Tie in the Parts branch of the Project Tree.
Verify that the PEC material has been assigned to the Bow Tie by expanding the Parts branch of the Project Tree to see that the PEC shortcut appears underneath the Bow Tie.
Step 4: Add a Circuit Component Excitation
The feed circuitry serves as the point where a feed line would connect to the antenna. For this example, it consists of a voltage source in series with a source resistor that needs to be added between the two halves of the bow tie. This design represents a generator that connects directly to the antenna, and is similar to a generator attached to a perfectly matched feed line that connects to the antenna. Unless interaction with the feed line itself is relevant to the problem, this scenario is an efficient and common way to simulate a bow tie antenna.
First, set the location of the feed.
- Right-click on the Circuit Components branch of the Project Tree and select New Circuit Component with ❯ New Feed Definition.
- Click on the middle picker tool found next to the Y coordinate of Endpoint 1 to specify each point using the mouse.
- Click on (-1, 0, 0) to select your first endpoint.
- Click on (1, 0, 0) to select your second endpoint.
- Click Done to apply the changes.
Then, define the component's properties.
- In the Definitions branch of the Project Tree, double-click on 50 ohm Voltage Source located underneath the Circuit Component Definitions node.
- In the editor window that opens, change the Resistance to 300 ohm.
- Name the component by typing 300 ohm Voltage Source into the Name box.
- Note that the Waveform is defined as Automatic.
- Click Done to close the editor.
Observe that XF generated an Automatic waveform located in the Waveforms branch of the Project Tree. Users can double-click to open the Waveform editor and see that the graph matches the project's Frequency Range of Interest.
Step 5: Check the Mesh
XF's gridding and meshing controls are used to discretize the CAD geometry into cell edges. This discretized representation is passed to the calculation engine, so it is important to double-check it before creating a simulation. Using the mesh controls provides a view of the geometry edges as they coincide with, but do not follow, the grid lines. This creates a jagged, staircase-like edge that can be smoothed out by enabling XACT mesh for increased simulation accuracy.
- Double-click on Mesh in the Project Tree to open the mesh controls.
- Click XY Plane in the upper-left corner of the mesh controls to view the mesh along the XY plane.
- Drag the Z Slice sliding control to the right until the mesh coincides with the Bow Tie at slice 13.
- Use the sidebar button to select View from +Z (top).
- Compare the grid to the Bow Tie geometry.
- Use the sidebar button to toggle the part's visibility.
- Right-click on Bow Tie in the Project Tree, then Gridding/Meshing ❯ Enable XACT Mesh.
Verify that XACT removed the staircasing effect, causing the mesh to follow the Bow Tie in the Geometry window.
- Double-click on Mesh in the Project Tree to close the mesh controls.
Step 6: Request Results
The last step before running a simulation is requesting results, which is done through sensors. This project utilizes two sensors: port and far zone. The port sensor was created by XF as the circuit component was defined, and it allows users to view S-parameters and impedance. The far zone sensor saves the antenna's far-zone radiation pattern at certain frequencies once the simulation is performed. The desired frequencies will be input when the simulation is launched.
- Right-click on Far Zone Sensors in the Sensors branch of the Project Tree, then select New Far Zone Sensor to open the editor across the top of the Geometry window.
- Under the Geometry tab, leave the default start and stop settings for bothTheta (i.e., 0 degrees and 180 degrees, respectively) and Phi (i.e., 0 degrees and 360 degrees, respectively).
- Change the Theta and Phi increments to 2 deg.
- Under the Properties tab, name the sensor by typing 3D Far Zone into the Name box.
- Be sure that the Collect Steady State Data option is checked.
- Click Done to save the changes and complete the creation of the project.
Step 7: Create and Run a Simulation
Once the project is complete, it must be saved in order to create a new simulation and write output files. When creating a simulation, XF provides options for collecting S-parameters and allows users to specify the frequencies at which the far zone pattern will be saved.
First, save the project.
- Save the project by selecting File ❯ Save Project As.
- Name the project by typing Bow Tie Antenna into the space provided.
- Click Save.
Then, create and run a simulation.
- Click the Simulations button in the upper-right corner of XF to open the Simulations window.
- Select FDTD from the Create Simulation drop-down menu or click FDTD if it has been automatically selected.
- In the Create FDTD Simulation window, enter the simulation name by typing Broadband Bow Tie Simulation into the Name box.
- Under the Setup S-Parameters tab, be sure the Compute S-Parameters option is checked and the port is selected.
- Under the Frequencies of Interest tab, check the Collect Steady-State Data box.
- Under Frequencies, click the button on the right side of the window to add the frequencies at which the far zone pattern will be saved.
- Enter 6.7 GHz as the first frequency.
- Click the button and enter 20 GHz as the second frequency.
- Click Create and Queue Simulation.
Watch the simulation's progress by selecting it in the Simulations window and clicking the Output tab in the bottom portion of the window.
Step 8: View the Far Zone Results
Once the simulation is complete, the results may be viewed by using the results browser. The far zone sensor defined earlier requested data over the full range of theta and phi angles, which XF considers to be a 3-D pattern and will plotted it in the geometry window by default.
Load a 3-D far zone sensor.
- Click on the Results button in the upper-right corner of XF to open the Results browser.
- Filter the results by selecting Broadband Bow Tie Simulation as the Simulation Name, and Gain as the Result Type.
- Double-click on the result line in the lower portion of the Results browser to plot the 3-D gain pattern in the Geometry window.
- Select Theta from the Viewing drop-down menu.
- Select desired frequency from the Frequency drop-down menu.
- Click Apply to display the Viewing selection in the Geometry window.
- Click Unload to close the result in the Geometry window.
View a 2-D slice.
- In the lower portion of the Results browser, right-click on the result line and select Create Line Graph from the drop-down menu.
- Select Polar as the Graph Type, Line as the Plot Type, and Total as the Component.
- View a 360 degree slice around the XY plane by selecting Phi as the Independent Axis and entering 45 (i.e., 90 degrees) as the Theta value.
- Click View.
Step 9: View the Component's Results
Broadband and discrete frequency S-parameter results are available for the voltage source. The broadband results will cover the project's frequency range of interest from 2 GHz to 25 GHz, and the discrete frequency results are available for the two frequencies that were defined when creating the simulation.
Plot broadband S-parameter results.
- In the Results browser, select the Simulation Name (e.g., Broadband Bow Tie Simulation), the Sensor (e.g., Component), the Domain (e.g., Frequency), and the Result Type (e.g., S-Parameters).
- Double-click on the result line in the lower portion of the browser to plot S11 v. Frequency.
View a table of steady-state output.
- In the Results browser, change the Domain to Discrete Frequencies.
- Double-click on the result line to view the table of steady-state output for the port.
Plot broadband impedance results.
- In the Results browser, change the Domain to Frequency and the Result Type to Impedance.
- Double-click on the result line to plot the real and imaginary Impedance v. Frequency.