Circularly Polarized Antenna for GPS Applications

One way to generate circular polarization from a microstrip patch antenna is to truncate the patch radiator. This example is tuned around the GPS frequency range. The axial ratios are calculated to show the degree of circular polarization.

Figure 1: A truncated microstrip patch antenna fed by a probe generates circular polarization along the main radiation direction. Perfectly matched layers enclosing the model domain are not shown in this figure.

Model Definition

A circularly polarized microstrip patch antenna design begins by adding a square metallic patch on top of a 60 mil substrate with a ground plane. The patch size is approximately estimated by a half wavelength inside the substrate;

where c0 is the speed of light, f0 is frequency, and εr is the relative permittivity of a substrate. This estimated value is only an initial guess number and the size needs to be tuned precisely for the intended frequency.

The basic square or rectangular patch radiator generates a linear polarization. By truncating two diagonally paired corners of the patch, the antenna can produce a circular polarization; electric fields with a fairly equal magnitude and ~90 degree phase difference between two orthogonal components; x- and y-axis field components.

A rigid coaxial cable filled with Teflon (εr = 2.1) is added on the bottom of the substrate and the outer conductor of the coaxial cable is connected to the ground plane. The inner conductor pin of the cable is extended through the dielectric part of the substrate and shorted to the patch on the top surface. All metal parts including the patch, ground plane, inner and outer conductors of the coaxial cable are modeled as perfect electric conductors.

A coaxial lumped port is used to excite the antenna. It is known that the input impedance on the edge center of a patch is very high while the input impedance around the center of the patch is quite low. The port location is optimized between these two points to get the best matching to the reference impedance 50 Ω with a coaxial probe feed.

The antenna is modeled in a spherical air domain. The air domain is truncated with Perfectly Matched Layers (PMLs) which absorb all outgoing radiation.

All domains except the PMLs are meshed by a tetrahedral mesh with maximum element size of five elements per wavelength so that the wave is well-resolved. The parts in the coaxial cable are meshed more finely to provide good resolution of the curved surfaces. The PMLs are swept with a total of five elements along the absorbing direction.

Results and Discussion

The default plot is modified to show the electric fields only on xy-plane (Figure 2). In general, a linearly polarized patch antenna provides strong fields on two parallel radiating edges. However, the truncated patch antenna shows the radiating fields confined at each corner of the patch. The antenna performs almost equally at every azimuthal angle in terms of the field intensity magnitude.

The axial ratio, which is a measure of the circularity of the polarization, is plotted in Figure 3. In the positive z direction, the evaluated value is less than 3 dB.

The 3D far-field radiation pattern is shown in Figure 4. Because the size of the ground plane is bigger than that of the radiating patch, it blocks the backward radiation and make the pattern directive in the positive z direction.

Figure 2: All corners are excited almost equally. This condition is necessary for the antenna to create a circular polarization.

Figure 3: The minimum axial ratio is observed at the antenna boresight (the positive z direction).

Figure 4: The 3D far-field pattern is directed to the positive z direction due to the ground plane.

RF_Module/Antennas/circularly_polarized_antenna

Modeling Instructions

From the menu, choose .

New

In the window, click

Model Wizard

In the window, click

In the tree, select .

Click .

Click

In the tree, select .

Click

Study 1

Step 1: Frequency Domain

In the window, under click .

In the window for , locate the section.

In the text field, type 1.57542[GHz].

Geometry 1

In the window, under click .

In the window for , locate the section.

From the list, choose .

Global Definitions

Parameters 1

In the window, under click .

In the window for , locate the section.

In the table, enter the following settings:


Name	Expression	Value	Description
thickness	60[mil]	0.001524 m	Substrate thickness
patch_w	50.28[mm]	0.05028 m	Patch width
probe_l	10[mm]	0.01 m	Probe location
ch_d	3.5[mm]	0.0035 m	Chamfer distance

Here, mil refers to the unit milliinch.

Geometry 1

Sphere 1 (sph1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 100[mm].

Click to expand the section. In the table, enter the following settings:


Layer name	Thickness (mm)
Layer 1	20[mm]

Click

Click the

button in the toolbar.

Click the

button in the toolbar.

Work Plane 1 (wp1)

In the toolbar, click

In the window for , click

Work Plane 1 (wp1)>Plane Geometry

In the window, click .

Work Plane 1 (wp1)>Square 1 (sq1)

In the toolbar, click

In the window for , locate the section.

In the text field, type patch_w.

Locate the section. From the list, choose .

Work Plane 1 (wp1)>Square 2 (sq2)

In the toolbar, click

In the window for , locate the section.

In the text field, type 100[mm].

Locate the section. From the list, choose .

Work Plane 1 (wp1)>Chamfer 1 (cha1)

In the toolbar, click

On the object , select Points 1 and 3 only.

It might be easier to select the correct points by using the window. To open this window, in the toolbar click and choose . (If you are running the cross-platform desktop, you find in the main menu.)

In the window for , locate the section.

In the text field, type ch_d.

Extrude 1 (ext1)

In the window, right-click and choose .

In the window for , locate the section.

In the table, enter the following settings:


Distances (mm)
thickness

Cylinder 1 (cyl1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 0.5.

In the text field, type thickness+2.

Locate the section. In the text field, type -probe_l.

In the text field, type -2.

Cylinder 2 (cyl2)

In the toolbar, click

In the window for , locate the section.

In the text field, type 2.05.

In the text field, type 2.

Locate the section. In the text field, type -probe_l.

In the text field, type -2.

Click

The finished geometry should look like this.

Definitions

Perfectly Matched Layer 1 (pml1)

In the toolbar, click

Select Domains 1–4 and 11–14 only.

In the window for , locate the section.

From the list, choose .

Electromagnetic Waves, Frequency Domain (emw)

Perfect Electric Conductor 2

In the window, under right-click and choose .

In the window for , locate the section.

Click

In the dialog box, Select all metal parts.

type 15, 20-21, 24-25, 28-32, 34, 42-47 in the text field.

Click .

Lumped Port 1

In the toolbar, click

and choose .

Select Boundary 26 only.

In the window for , locate the section.

From the list, choose .

For the first port, wave excitation is by default.

Far-Field Domain 1

In the toolbar, click

and choose .

Add Material

In the toolbar, click

to open the window.

Go to the window.

In the tree, select .

Click in the window toolbar.

In the toolbar, click

to close the window.

Materials

Material 2 (mat2)

In the window, under right-click and choose .

Select Domains 6 and 7 only.

In the window for , locate the section.

In the table, enter the following settings:


Property	Variable	Value	Unit	Property group
Relative permittivity	epsilonr_iso ; epsilonrii = epsilonr_iso, epsilonrij = 0	3.38	1	Basic
Relative permeability	mur_iso ; murii = mur_iso, murij = 0	1	1	Basic
Electrical conductivity	sigma_iso ; sigmaii = sigma_iso, sigmaij = 0	0	S/m	Basic

Material 3 (mat3)

Right-click and choose .

Select Domain 8 only.

In the window for , locate the section.

In the table, enter the following settings:


Property	Variable	Value	Unit	Property group
Relative permittivity	epsilonr_iso ; epsilonrii = epsilonr_iso, epsilonrij = 0	2.1	1	Basic
Relative permeability	mur_iso ; murii = mur_iso, murij = 0	1	1	Basic
Electrical conductivity	sigma_iso ; sigmaii = sigma_iso, sigmaij = 0	0	S/m	Basic