EM.Picasso Tutorial Lesson 2: Designing A Patch Antenna With A Recessed Feed

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Tutorial Project: Designing A Patch Antenna With A Recessed Feed
Picasso L2 Fig title.png

Objective: In this project, you will build a patch antenna with a recessed microstrip feed line and will vary the feed design parameters to achieve a good impedance match.

Concepts/Features:

  • Wizard
  • Scattering Wave Port
  • Scattering Parameters
  • Variable
  • Uniform Frequency Sweep
  • Adaptive Frequency Sweep
  • Parametric Sweep

Minimum Version Required: All versions

'Download2x.png Download Link: EMPicasso_Lesson2


What You Will Learn

In this tutorial you will create the parameterized geometry of a microstrip-fed patch antenna with a recessed feed using a wizard. You will learn how to perform frequency sweeps as well as parametric sweeps of various design variables.

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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_Lesson2
Length Units Millimeters
Frequency Units GHz
Center Frequency 2.4GHz
Bandwidth 0.5GHz

In the previous tutorial lesson, you saw that a patch antenna with an edge-connected microstrip feed line doesn't provide a satisfactory impedance match. In this lesson, you will use a recessed feed design to improve the impedance matching.

Creating the Patch Geometry with a Recessed Feed

Click on the Microstrip-Fed Patch Wizard button of the Wizard Toolbar or select the menu item Tools → Antenna Wizards → Microstrip-Fed Patch Antenna. A dialog pops up asking you if you would like a patch design with a recessed feed. This time, answer "Yes" and let the wizard draw the geometry of the patch antenna with the recessed feed.

The patch antenna geometry with the recessed feed in the project workspace.

The wizard automatically created a PEC trace called "PATCH_PEC" in the navigation tree containing four rectangle strip objects:

  1. "ANCHOR": representing the square patch antenna
  2. "Feed": representing the microstrip feed line
  3. "Flap_1": representing the upper flap on the feed edge
  4. "Flap_2": representing the lower flap on the feed edge

Also, note that wizard placed the scattering wave port not that the junction between "ANCHOR" and "Feed", but an offset equal to the depth of the recessed feed. This established the phase plane for the computation of the S-parameters. If you open the property dialog of the source "WP_1", you will find that the value of its "Offset" parameter has been set equal to the variable "recess_dep".

The scattering wave port/source dialog.

Open the variables dialog and make the following changes:

Variable Name Original Definition New Definition
patch_len floor(0.5*lambda0_unit*100/sqrt(er))/100 42
recess_dep 0.015*to_meters 15
recess_wid 0.005*to_meters 5

The length of the original patch antenna created by the wizard was frequency-dependent as the definition of its variable "patch_len" involved the variable "lambda0_unit", which itself depended on the project center frequency "fc". By defining numeric values for the three variables "patch_len", "recess_dep" and "recess_wid", you have now turned them into independent variables.

Attention icon.png Only independent variables can be designated as sweep variables for performing parametric sweeps.
The variables dialog showing the new definitions of some variables.

The figure below shows the planar mesh of your antenna structure with a mesh density of 30 Cells/λeff:

The planar mesh of the patch antenna geometry with the recessed feed.

Running a Frequency Sweep of the Resonant Patch Antenna

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.Picasso'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 2.15GHz and 2.65GHz for the start and stop frequencies, respectively, and keep the number of frequency samples at the default value of 11.

EM.Picasso'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". Note that you can make multiple file selections for plotting using the Ctrl or Shift keys.

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

With EM.Picasso, 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". From the figures below you can clearly see the resonance of the antenna at 2.4GHz. However, the input impedance of the patch antenna at 2.4GHz is about (66 + j99)Ω, which is far from an acceptable impedance match.

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.

Running a Parametric Sweep of the Feed Recess Depth

At this time, you are going to vary the depth of the feed recess to see if it improves the return loss (|S11|). Open the simulation run dialog and select Parametric Sweep as the simulation mode. You will notice a red box next to the drop-down list. This means that you are not ready to run a simulation because some parameters haven't been set yet.

Setting parametric sweep as the simulation mode in EM.Picasso's run dialog.

Click the Settings button next to the Simulation Mode drop-down list to open the Parametric Sweep Settings dialog.

EM.Picasso's parametric sweep settings dialog.

On the left side of the dialog you will see a list of all the available independent variables already defined in your project. Select and highlight "recess_dep" and click the right arrow --> button in the middle of the dialog to move the selected variable to the Sweep Variables list on the right. A new dialog titled "Edit Sweep Variable" opens up. Accept the Uniform variable type and enter 2, 16, 2, for the start, stop and step values of the sweep variable, respectively. This will create a list of sweep variable values: {2, 4, 6, ..., 14, 16}.

Setting the bounds and number of samples of the sweep variable.

Close this dialog and return to the parametric sweep settings dialog. You will see the specifications of the sweep variable in the table on the right.

The parametric sweep settings dialog showing "recess_dep" as the active sweep variable.

Close the parametric sweep settings dialog and return to the run dialog. Click the Run key to start the parametric sweep. After the completion of the sweep simulation, open the data manager and plot the data file "S11_Sweep.CPX". Note that the return loss is minimized for the value recess_dep = 14mm.

Plots of the magnitude and phase of the S11 parameter of the patch antenna as a function of the feed recess depth at f = 2.4GHz.

Running a Parametric Sweep of the Feed Recess Width

Open the variable dialog and change the value of the variable "recess_dep" to 14mm.

Adjusting to the optimal value of the feed recess depth.

Next, you are going to vary the width of the feed recess to see how far you can improve the return loss (|S11|). Open the simulation run dialog and then the parametric sweep settings dialog. Select and highlight "recess_dep" in the sweep variables table on the right side of the dialog. Click the left arrow <-- button in the middle of the dialog to move the selected sweep variable back to the Independent Variables list on the left. Now, select "recess_wid" from the left table and move it to the right table to make it the active sweep variable. In the "Edit Sweep Variable" dialog, accept the Uniform variable type and enter 1, 8, 1, for the start, stop and step values of the sweep variable, respectively. This will create a list of sweep variable values. Return to the parametric sweep settings dialog as shown in the figure below.

The parametric sweep settings dialog showing "recess_wid" as the active sweep variable.

Run the new parametric sweep and then plot the data file "S11_Sweep.CPX". Note that the return loss is minimized for a value of recess_dep between 2mm and 3mm.

Plots of the magnitude and phase of the S11 parameter of the patch antenna as a function of the feed recess width (recess_dep = 14mm) at f = 2.4GHz.

Analyzing the Patch Antenna with the Optimal Recessed Feed

You already set the value of the variable "recess_dep" to 14mm in the previous part. Now open the variables dialog again and change the value of the variable "recess_wid" to 2.5mm.

The variables dialog showing the optimal values of the design variables "recess_dep" and "recess_wid".

After the above changes, your patch antenna structure should look like this:

The geometry of the patch antenna structure with the optimal recessed feed.

Run a quick single-frequency PMOM analysis of the optimal structure. At the end of the simulation, the port characteristics are reported as follows:

S11: 0.011933 +0.142894j

S11(dB): -16.869538

Z11: 49.134305 +14.336804j

Y11: 0.018756 -0.005473j

The return loss has dramatically improved down to -16.87dB. Visualize the current distribution and 3D radiation pattern of the antenna as shown in the figures below. Notice how the standing wave pattern has diminished on the microstrip feed line due to the improved impedance match.

The plot of total electric current (JTOT) distribution of the patch antenna.
The 3D radiation pattern plot of the patch antenna.

Finally, run an adaptive frequency sweep of your patch antenna with the optimal recessed feed over the frequency range [2GHz, 3GHz]. After the completion of the sweep simulation, open the data manager and plot the data files "S11_RationalFit.CPX" and "Z11_RationalFit.CPX". The graphs of the S and Z parameters are shown in the figures below. A very good resonance and impedance match is accomplished at 2.4GHz.

Plots of the magnitude and phase of the S11 parameter of the patch antenna with the optimal recessed feed.
Plots of the real and imaginary parts of the Z11 parameter of the patch antenna with the optimal recessed feed.

 

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