EM.Picasso Tutorial Lesson 4: Designing A Circularly Polarized Probe-Fed Patch Antenna

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Tutorial Project: Designing A Circularly Polarized Probe-Fed Patch Antenna
Picasso L4 Fig title.png

Objective: In this project, you will build a patch antenna with a probe feed and will vary the location of the feed to get good circular polarization.

Concepts/Features:

  • CubeCAD
  • PEC Via Object
  • Probe Source
  • Circular Polarization
  • Current Distribution
  • Radiation Pattern
  • Axial Ratio
  • Variable
  • Parametric Sweep

Minimum Version Required: All versions

'Download2x.png Download Link: EMPicasso_Lesson4


What You Will Learn

In this tutorial you will analyze a probe-fed patch antenna and will become familiar with embedded via sets and probe sources. You will also define new variables and learn about radiation characteristics such as axial ratio. You will run a parametric sweep of the probe feed location and examine the circular polarization of your patch antenna.

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Getting Started

Open the EM.Cube application and switch to EM.Picasso. Start a new project with the following parameters:

Starting Parameters
Name EMPicasso_Lesson4
Length Units Millimeters
Frequency Units GHz
Center Frequency 1.575GHz
Bandwidth 0.2GHz
Substrate Configuration
Number of Finite Layers 1
Top Half-Space Vacuum
Middle Layer εr = 3.38, σ = σm = 0, thickness = 1.524mm
Bottom Half-Space PEC

Creating the Probe-Fed Patch Geometry

Click on the Probe-Fed Patch Wizard ProbeFedPatchWizardIcon.png button of the Wizard Toolbar or select the menu item Tools → Antenna Wizards → Probe-Fed Patch Antenna.

EM.Picasso's wizard toolbar.

The wizard creates the geometry of the patch antenna in the project workspace.

The geometry of the probe-fed patch antenna in the project workspace.

Open the variables dialog and change the definition of the following variables:

Variable Name Original Definition New Definition
er 2.2 3.38
h 0.0015*to_meters 1.524
feed_rad 0.0025*to_meters 1

The variables list now looks like this:

The variables dialog showing the new values of some variables.

A Note on Via Objects

The wizard created two trace groups on the navigation tree:

  1. A PEC trace group called "PEC_1" containing a rectangle strip object called "ANCHOR"
  2. A PEC via (embedded object) set group called "VIA_FEED" containing a circle strip object called "Probe_feed"

In the last three tutorial lessons, you worked with PEC trace groups. In this tutorial lesson, you have a new type of objects called "PEC Via Objects", which are used to model "embedded objects" inside the substrate layers. An embedded object sets is defined inside a finite substrate layer. All the objects belonging to this set extend vertically across the whole host layer. Therefore, they are three-dimensional prismatic objects from a geometric point of view. Right-click on the "VIA_FEED" item in the navigation tree and select Properties... from the contextual menu to open the PEC via set dialog. You can see that the via set group has been associated with the "Substrate" layer, which is part of the project's background structure.

The PEC via set dialog.

Open EM.Picasso's stackup manager. This dialog has two tabs: Layer Hierarchy and Embedded Sets. In the layer hierarchy tab, in the middle row representing the finite layer called "Substrate", you will see the names of all the embedded object sets inside that layer in a column labeled “Vias”. In this case, there is only one via set called "VIA_FEED". In addition, you will see the word “(EMB)” next to the name of the host layer, indicating that it contains embedded objects. The embedded sets tab, on the other hand, shows a list of all the defined embedded sets, their material composition, their host layer and their height, which is always equal to the thickness of the host layer. Note that each background layer can host several different embedded sets.

The "Layer Hierarchy" tab of the stackup manager.
The "Embedded Sets" tab of the stackup manager.
Attention icon.png In EM.Picasso, vias and embedded object sets extend vertically all the way across the height of their host layer. Under an embedded set group, you simply draw a surface object representing the base or cross section of a via or an embedded object. The base is then extruded automatically by EM.Picasso to create a 3D prismatic object that extends across the host layer.
The extended prismatic via object called "Probe_feed" under the rectangular patch object called "ANCHOR" in its freeze state.

The wizard also defined a probe source called "PS_1" to excite your patch antenna. A probe source always needs to be associated with a PEC via set. It creates a gap discontinuity across the via object, where a voltage source is connected. Right-click on the item "PS_1" under Probe Gap Circuits in the Sources section of the navigation tree and select Properties... from the contextual menu to open the probe source dialog. Examine the various parameters of probe gap circuit, which can serve either as a source or as a passive RLC load.

The probe source dialog.

Running a PMOM Analysis of the Probe-Fed Patch Antenna

The wizard set the mesh density equal to 30 cells/λeff. View the mesh of your probe-fed patch antenna. Note how the rectangular mesh of the patch has been perturbed around the location of the via using smaller triangular cells to provide a transition to the probe.

The planar mesh of the probe-fed patch antenna.

Run a quick single-frequency PMOM analysis of your probe-fed patch antenna. The port characteristics of the structure are reported at the end of the PMOM analysis:

S11: 0.351007 +0.406269j

S11(dB): -5.402150

Z11: 60.703043 +69.300071j

Y11: 0.007152 -0.008165j

Visualize the current distribution and 3D radiation pattern of your antenna. You will see a typical sinusoidal current distribution along the X-axis and an edge-singular behavior along the top and bottom edges of the patch. which are very “hot”. The radiation pattern is that of a typical linearly polarized antenna.

The total electric surface current distribution (JTOT) of the probe-fed patch antenna.
The 3D radiation pattern of the probe-fed patch antenna.

Modifying the Patch Geometry for Circular Polarization

In order to design a circularly polarized (CP) patch antenna, you are going to make some changes to the geometry of your patch structure and its feed location. First, your square patch will become rectangular with unequal X and Y dimensions. Second, you will move the probe feed inward and off the X-axis. Open the variables dialog and change the definition of the following variables:

Variable Name Original Definition New Definition
len 0.48*lambda0_unit/sqrt(er) 52
feed_x feed_ratio*len 15

Next, define two new variables called "feed_y" and "wid" according to the table below. You can do this using the Add button of the variable dialog.

Variable Name Definition
feed_y 0
wid 51
Defining a new variable "wid" from the ground up.

The variables list now looks like this:

The variables dialog showing the modified and newly added variables.

Keep in mind that variables by themselves don't do anything unless they are attached to your objects' properties. Open the property dialog of the patch object "ANCHOR" and via object "Probe_feed" and make the changes shown in the figures below:

Changing the properties of the patch object to reflect the new variable "wid".
Changing the properties of the via object to reflect the new variable "feed_y".
Attention icon.png After you define a new variable, make sure to attach is to the properties of one or more objects in your project.

A measure of the circular polarization of an antenna is its axial ratio. An axial ratio of 1 represents a perfect circular polarization. Open property dialog of the radiation pattern observable "FF_1" the wizard created for your project. In the section titled "Additional Radiation Characteristics", check the box labeled Axial Ratio (AR).

Defining Axial Ratio as one of the radiation characteristics to be computed in the radiation pattern dialog.

Now run a parametric sweep of the variable "feed_y" with the following parameters:

Sweep Variable Name feed_y
Sweep Variable Type Uniform
Start Value -10
Stop Value 0
Step Value 1
The parametric sweep settings dialog shown "feed_y" as the sweep variable.

At the end of the parametric sweep, open the data manager and plot the data files "S11_Sweep.CPX" and "FF_1_ARU_Sweep.DAT". The figures below show that the axial ratio is minimized for a value of the sweep variable "feed_y" between -4mm and -3mm, where the return loss also reaches a minimum.

Graph of S11 of the patch antenna as a function of the variable feed_y.
Graph of the axial ratio of the patch antenna as a function of the variable feed_y.

Analyzing the CP Patch Antenna

In the last part of this tutorial lesson, change the location of the probe feed by setting feed_y = -3.5mm. Then, run a quick single-frequency analysis of your circularly polarized patch antenna to examine it radiation characteristics in more detail. The port characteristics of the antenna are reported at the end of simulation as:

S11: 0.341778 -0.249927j

S11(dB): -7.464786

Z11: 82.781025 -50.417060j

Y11: 0.008812 +0.005367j

The current distribution plot shows all four edges of the patch excited and “hot”, which is an indicator of a circularly polarized patch.

The Jx component of the surface current distribution on the CP patch antenna.
The Jy component of the surface current distribution on the CP patch antenna.
The total surface current distribution on the CP patch antenna.

The 3D radiation pattern now looks circularly symmetric.

The 3D radiation pattern of the CP patch antenna.

Plot the 2D polar radiation pattern graphs. Open the Data Manager and select and plot the data files “FF1_Pattern_Polar_YZ.ANG”, “FF1_Pattern_Polar_ZX.ANG” and “FF1_Pattern_Polar_Custom.ANG”. The last data file corresponds to the user-defined "Custom" Phi-plane, which is φ = 45° by default. You can see from the figures below that the Eθ and Eφ components of the far fields stay very close to each other over a wide theta range on both sides of the zenith. This is another good indicator of circular polarization.

The 2D polar graph of the YZ-plane radiation pattern of the CP patch antenna.
The 2D polar graph of the ZX-plane radiation pattern of the CP patch antenna.
The 2D polar graph of the custom plane (φ = 45°) radiation pattern of the CP patch antenna.

Finally, plot the axial ratio graphs as a function of elevation angle in the YZ, ZX and custom φ = 45° planes. To do so, select the data files “Axial_Ratio_YZ.DAT”, “Axial_Ratio_ZX.DAT” and “Axial_Ratio_Custom.DAT” in the data manager. You may have to change the default axes settings to get more informative graphs.

The Cartesian graph of the YZ-plane axial ratio of the CP patch antenna.

The PyPlot window has a number of controls that let you change the settings of your graph using your mouse. For example, using the Pan/Zoom button Py zoom icon.png, you can pan the graph with the left mouse button and zoom it in or out with the right mouse button. A combination of the two operations usually gives you an ideal scaling of your graph. As you move the mouse on the graph, you can read the value of the horizontal axis (X) and the corresponding value of the total electric field distribution along the X direction. You can also customize the graphs and change the scale of the graph axes. Click the Graph Settings button and uncheck the Auto box. Enter values -4 and 4 for Min and Max, respectively, for X-Axis and set the No. Major Intervals to 8 in "Axis Settings" panel. Similarly, for Y-Axis enter values 0, 5, and 5 for Min and Max, and No. Major Intervals respectively.

Changing X- and Y-axis scale in the Graph Settings dialog.

Note that AR < 1.1 at the boresight and stays close to 1 over a wide angular range (field of view) on both sides of θ = 0.

The Cartesian graph of the YZ-plane axial ratio of the CP patch antenna.
The Cartesian graph of the ZX-plane axial ratio of the CP patch antenna.
The Cartesian graph of the custom plane (φ = 45°) axial ratio of the CP patch antenna.

 

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