COMSOL 6.4 - Polarized Circular Ports

This example of a circular waveguide demonstrates how to excite and terminate ports with degenerate port modes. In particular it shows how to model and excite the TE11 mode of circular waveguides in 3D.

Model Definition

A straight piece of circular waveguide with perfect metallic walls is excited by a linearly polarized TE11 mode at one end and ideally terminated at the other end.

The TE11 mode of a circular waveguide is degenerate, meaning that there is an infinite number of possible variants of the TE11 mode that only differ in polarization. Any type of polarization (for example circular polarization) of the TE11 mode can be constructed by or decomposed into two linearly polarized modes. The direction of polarization of the first one can be chosen freely and the second one is obtained from the first one by a rotation of 90 degrees around the waveguide axis.

As a general structure may change the polarization of the incident field as it is transmitted or reflected, ideal termination means that any circular waveguide port that operates in the TE11 mode need to have two port features which are tuned to mutually orthogonal, linear polarizations of the TE11 mode respectively. The reference directions for the two port features are subject to a manual choice but must differ by a rotation of 90 degrees around the waveguide axis.

The Port subfeature, Circular Port Reference Axis is used to determine the reference direction for the polarization of each mode/port by means of selecting two vertices (points) on the port circumference. In this example, extra vertices that are equally distributed along the port edge are added to the geometry to allow for the definition of the desired reference directions.

With the stipulated excitation using the two mutually orthogonal TE11 ports as boundary conditions, the following equation is solved for the electric field vector E inside the waveguide:

Results and Discussion

The first TE11 mode of the inport is shown in Figure 1.

Figure 1: The first TE11 mode of the inport

Depending on the details of the mesh, which in turn may depend on the origin of the CAD geometry, a mode that is rotated 180 degrees may be found.

The first TE11 mode of the outport is shown in Figure 2.

Figure 2: The first TE11 mode of the outport.

Depending on the details of the mesh, which in turn may depend on the origin of the CAD geometry, a mode that is rotated 180 degrees may be found.

The transmission coefficients between the inport and outport modes are shown in Figure 3.

Figure 3: The transmission coefficients between inport modes and outport modes are plotted as a function of frequency. As the port modes are misaligned by 45 degrees the transmission coefficients approach the -3dB level.

RF_Module/Tutorials/polarized_circular_ports

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

Global Definitions

Add a parameter for the operating frequency.

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
frq	c_const/0.03[m]	9.9931E9 1/s	Operating frequency

Study 1

Step 1: Frequency Domain

Define the study frequency ahead of performing any frequency-dependent operation such as building mesh. The physics-controlled mesh uses the highest frequency value in the specified range.

In the window, under click .

In the window for , locate the section.

In the text field, type range(0.9*frq,(1.5*frq-(0.9*frq))/10,1.5*frq).

Geometry 1

The geometry is essentially a cylinder.

Cylinder 1 (cyl1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 0.01.

In the text field, type 0.1.

Click

Click the

button in the toolbar.

You need to add a reference direction for the port polarization. Add a couple of lines on the cylinder end to generate extra vertices. This is done in a work plane.

Work Plane 1 (wp1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 0.1.

Work Plane 1 (wp1) > Plane Geometry

In the window, click .

Work Plane 1 (wp1) > Line Segment 1 (ls1)

In the toolbar, click

and choose .

In the window for , locate the section.

From the list, choose .

Locate the section. From the list, choose .

Locate the section. In the text field, type 0.01.

Locate the section. In the text field, type -0.01.

Work Plane 1 (wp1) > Rotate 1 (rot1)

In the toolbar, click

and choose .

Select the object only.

In the window for , locate the section.

In the text field, type 45 135.

Click

Ignore Edges 1 (ige1)

In the window, right-click and choose > .

On the object , select Edges 7, 8, 13, and 14 only.

In the window for , locate the section.

Clear the checkbox.

In the toolbar, click

Materials

Next, add a material for the interior (air) of the waveguide.

Material 1 (mat1)

In the window, under right-click and choose .

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	1	1	Basic
Relative permeability	mur_iso ; murii = mur_iso, murij = 0	1	1	Basic
Electric conductivity	sigma_iso ; sigmaii = sigma_iso, sigmaij = 0	0	S/m	Basic

Electromagnetic Waves, Frequency Domain (emw)

Set up one inport and three outports.

Port 1

In the toolbar, click

and choose .

Select Boundary 3 only.

It might be easier to select the correct boundary 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.

From the list, choose .

Circular Port Reference Axis 1

In the toolbar, click