EM.Picasso Tutorial Lesson 8: Analyzing A CPW-Fed Folded Dipole Slot Antenna
What You Will Learn
In this tutorial you will learn how to construct and simulate slot structures. You will define coupled ports to model coplanar waveguide (CPW) structures.
Creating the Base One-Port Coplanar Waveguide Line
Open the variables dialog and change the definition of the following variables:
|Variable Name||Original Definition||New Definition|
The wizard creates the geometry of a one-port coplanar waveguide in the project workspace. The default coplanar waveguide structure is made up of four rectangle strip objects grouped under a slot trace called "CPW":
The objects wizards create are not only highly parameterized, but they are also usually linked to one another. This means that you can move them or rotate them together without affecting their parameterization. But they are rules to follow. The object named "ANCHOR" is the one which you should translate or rotate. Most other objects initially created by a wizard are linked to the anchor and follow its translation or rotation.
As you saw in the previous tutorial lesson, slot structures in EM.Picasso are modeled as magnetic currents on an infinite ground plane. The slot trace "CPW" is sandwiched between the finite dielectric layer and the top half-space. Open EM.Picasso's Stackup Settings dialog to see the substrate layer hierarchy.
Keep in mind that the default setting of the bottom half-space is PEC when you create a new project. When working with slot structures, make sure that the bottom half-space material is set to vacuum to represent the free space.
|Slot traces are assumed to lie on an infinite, horizontal, PEC ground plane. The finite objects belonging to a slot trace group represent cut‐out parts off this ground plane. Metal and slot trace groups cannot be collocated on the same Z‐plane.|
Examining the Coupled Sources & Port Definition
The wizard created two scattering wave port sources called "WP_1" and "WP2". Open the property dialog of both sources and inspect their properties. You will notice that the two sources are out of phase. They have been set to have a phase difference of 180° to excite coplanar waveguide's dominant odd mode.
The wizard also initiated a port definition for the two wave port sources. If you open the port definition dialog, you will notice that there is only one port in the list rather than two. The only define port called "Port_1" has been associated with both sources "WP_1" and "WP_2". In other words, the two sources have been coupled to each other, creating a single port. Select and highlight "Port_1" in the table and click the Edit button of the dialog. This opens the "Edit Port" dialog from which you can modify the source associations. Close the dialogs and return to the project workspace.
Drawing the Additional Slot Segments
Open the variables dialog again and change the definition of variable "center_len" as shown below. This will turn the objects "ANCHOR" and "Slot_2" into small joint squares.
Next, draw the following five rectangle strip objects in the project workspace:
|Label||Object Type||LCS Origin||Length||Width|
|Rect1||Rectangle Strip||(0, -20mm, 1.2mm)||2mm||34mm|
|Rect2||Rectangle Strip||(0, 20mm, 1.2mm)||2mm||34mm|
|Rect3||Rectangle Strip||(-6mm, 0, 1.2mm)||2mm||74mm|
|Rect4||Rectangle Strip||(-3mm, -38mm, 1.2mm)||8mm||2mm|
|Rect5||Rectangle Strip||(-3mm, 38mm, 1.2mm)||8mm||2mm|
After making all the changes and adding all the new slot segments, your physical structure should look like this:
Running a PMOM Analysis of the Slot Antenna
Before running the simulation, let’s take a look at the planar mesh of your slot antenna. The wizard set Mesh Density for this structure to 40 cells/λeff. It is recommended that you use a higher mesh density for slot traces (PMC) compared to PEC traces. As you would expect, EM.Picasso extends both feed lines with scattering wave ports on them to 2λg in the mesh view. For a coplanar waveguide transmission line, λeff = λ0/√εeff, where εeff ≈ (εr + 1)/2 when the medium above the slot is vacuum and the one beneath it is a dielectric of permittivity εr.
Define a far-field radiation pattern observable and set both the theta and phi angle increments to 1° in the radiation pattern dialog. Run a quick single-frequency PMOM analysis of your folded slot antenna. The port characteristics are reported as:
S11: -0.019578 -0.075183j
Z11: 47.549339 -7.193173j
Y11: 0.020560 +0.003110j
Keep in mind that since EM.Picasso models slot traces as perfect magnetic conductors (PMC), the electric surface current distribution is zero everywhere. Therefore, under the current distribution node "CD_1" in teh navigation tree, you should look at the magnetic current distribution plots instead. Note that the magnetic current density has units of V/m, which is the same as that of electric field.
Visualize the 3D radiation pattern of the slot antenna. The radiation pattern is typical of a dipole antenna as you would expect. The discontinuity at θ = 90°, is due to the presence of the infinite dielectric substrate layer.
Examining the Resonant Behavior of the Slot Antenna
Next, you will run a frequency sweep of your folded dipole slot antenna to examine its frequency response and resonant behavior. Run an adaptive frequency sweep of your physical structure with the following parameters:
|Min. Number of Frequency Samples||5|
|Max. Number of Frequency Samples||15|
At the end of the sweep simulation, graph three data files: “S11_RationalFit.CPX”, “Z11_RationalFit.CPX” and “VSWR_RationalFit.DAT”. You will see that around 1.65GHz, the magnitude of S11 (return loss) dips into a deep minimum representing a very good impedance match. Also note that the slot antenna features a 10-dB return loss bandwidth of more than 330MHz.
The real and imaginary parts of the input impedance are initially plotted on two separate graphs. You can merge them into a single graph by selecting the menu item Edit → Combine Plots from the "Edit Menu" of EM.Grid. You can track the data points on EM.Grid plots. In the impedance graph, click on the "Tracker" tab of the left side bar. As you move the mouse on the graph, you can read the value of the horizontal axis (frequency) and the corresponding value of the real and imaginary parts of Z11-parameter in the settings panel. You can see that around 1.65GHz, the imaginary part of Z11 (i.e. input reactance) vanishes and the antenna resonates.
In the voltage standing wave ratio (VSWR) graph, click on the "Edit Plots" tab of the left side bar and check the box labeled Show Minima. A small tooltip appears on the graph showing the minimum VSWR to be 1.068. Next, click on the "Edit Graph" tab of the left side panel. You can modify many parameters related to the appearance of the graph from this panel. You can change the margins on the four sides of the graph box and also change the format of the axis labels.