COMSOL 6.4 - Antenna Decoupling Using an Electromagnetic Band Gap Metamaterial

Antenna Decoupling Using an Electromagnetic Band Gap Metamaterial

An electromagnetic band gap (EBG) structure can be used to increase the isolation between antennas that are close to each other. This decoupling effect is not only a function of frequency but also of polarization and coupling-plane configuration. In this example, an EBG structure is placed between two antennas. The solution shows a marked decrease in coupling between the two antennas.

Figure 1: Two 90° bent monopole antennas over a thick substrate separated by electromagnetic band gap structures.

Model Definition

An electromagnetic band gap structure can strongly inhibit the propagation of an electromagnetic wave. Although there are many different structures that can exhibit a band gap, such structures are almost always initially designed at the unit cell level. That is, a single element of the EBG structure is assumed to be patterned infinitely through space, and a separate analysis is performed to find the band gaps.

Once the EBG structure has been engineered to achieve the desired band gap, for an infinite structure, it can be used in a finite-sized space. However, the properties of the structure change slightly because the assumption of infinite periodicity used to compute the band gaps does not hold any longer. Therefore, it is necessary to compute the performance of an EBG structure in real space.

This example starts with an EBG structure composed of metallic mushrooms that has already been engineered to have a band gap centered at 1.85 GHz. One row of these mushroom structures are placed between two antennas, as shown in Figure 1. A typical use of such an EBG structure is to provide isolation between the elements of an array antenna. This example uses only two antenna elements to demonstrate the concept.

The antenna elements are metal strips, fed by a coaxial cable, sitting above a dielectric substrate on top of the ground plane. Although these are not typical antenna elements, they are used because they highlight the effectiveness of the isolation provided by the mushroom structures. In this analysis only one antenna element is excited, while the other acts as a receiver to determine the coupling.

The antenna array structure is modeled inside an air sphere truncated by a Perfectly Matched Layer, which allows the antenna to radiate freely in all directions. The antenna characteristics are not of primary interest here, however.

The model can be run both with and without the EBG mushroom structures, of which only the former case is considered here. The model is simulated over a range of frequencies around the band gap to observe the change in S21.

Results and Discussion

Figure 2 plots S21 for the case of the antenna array with and without the EBG structure, showing a marked improvement when adding the EBG. Because the model only uses a single row of five mushroom elements, the maximum isolation is not exactly at the frequency predicted by the band-gap analysis.

Figure 2: The frequency response of the coupling between two antennas with/without EBG structures shows the decoupling effect around 2.2 GHz.

Figure 3: Electric field with EBG (left) and without EBG structures (right).

Two coaxial lumped ports are applied and S21 is observed with/without the five EBG structures. As shown in Figure 2, a part of the frequency band has stronger coupling with the EBG structures and the decoupling bandwidth is not wide. The center frequency of the decoupling band is also a function of the configuration of the coupling plane of the two antennas, and adding the EBG structures does not always guarantee better isolation between antennas.

Reference

1. M.N.M. Tan and others, “Antenna array enhancement using mushroom-like electromagnetic band gap (EBG),” Proc. 4th European Conf. Antennas and Propagation (EuCAP 2010), pp. 1–5, 2010.

RF_Module/EMI_EMC_Applications/antenna_ebg

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

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(2.1[GHz],10[MHz],2.35[GHz]).

You can alternatively set the simulation frequency range by clicking the Range button next to the Frequencies text field.

Geometry 1

In the window, under click .

In the window for , locate the section.

From the list, choose .

Create a substrate.

Substrate

In the toolbar, click

In the window for , type Substrate in the text field.

Locate the section. In the text field, type 60.

In the text field, type 80.

In the text field, type 12.

Locate the section. In the text field, type -30.

In the text field, type -40.

Click

Click the

button in the toolbar.

Create a mushroom.

Work Plane 1 (wp1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 12.

Click

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

In the toolbar, click

In the window for , locate the section.

In the text field, type 8.

Locate the section. From the list, choose .

In the text field, type -24.

Click

Mushroom

In the window, under > click .

In the window for , type Mushroom in the text field.

Mushroom Stem

In the toolbar, click

In the window for , type Mushroom Stem in the text field.

Locate the section. In the text field, type 12.

Locate the section. In the text field, type -24.5.

In the text field, type -0.5.

Click

Create an array of mushrooms.

EBG Structure

In the toolbar, click

and choose .

In the window for , type EBG Structure in the text field.

Select the objects and only.

Locate the section. In the text field, type 5.

Locate the section. In the text field, type 12.

Click

Create a coax inner conductor.

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 14.

Locate the section. In the text field, type -10.

In the text field, type -20.

Click

Create a coax outer conductor.

Cylinder 2 (cyl2)

In the toolbar, click

In the window for , locate the section.

In the text field, type 2.35.

In the text field, type 12.

Locate the section. In the text field, type -10.

In the text field, type -20.

Click

Create an antenna radiator.

Block 3 (blk3)

In the toolbar, click

In the window for , locate the section.

In the text field, type 25.

In the text field, type 2.

Locate the section. In the text field, type -10.

In the text field, type -21.

In the text field, type 13.

Click

Create a pair of 90-degree bent monopole antennas.

Copy 1 (copy1)

In the toolbar, click

and choose .

Select the objects , , and only.

In the window for , locate the section.

In the text field, type 40.

Click

Create a sphere for PML layers and an air domain.

Sphere 1 (sph1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 100.

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


Layer name	Thickness (mm)
Layer 1	30

Click

Click the

button in the toolbar.

In the window, click .

Definitions

Internal PEC Boundaries

In the toolbar, click

In the window for , type Internal PEC Boundaries in the text field.

Locate the section. From the list, choose .

Select Boundaries 15, 18, 25, 31, 32, 35, 36, 54, 58, 70, 71, 75, 87, 88, 111, and 120 only.

Perfectly Matched Layer 1 (pml1)

In the toolbar, click

Select Domains 1–4 and 20–23 only.

In the window for , locate the section.

From the list, choose .

Hide some domains to get a better view of the interior parts.

Hide for Physics 1

In the window, right-click and choose .

Select Domains 2 and 5 only.

Electromagnetic Waves, Frequency Domain (emw)

Perfect Electric Conductor 2

In the toolbar, click

and choose .

In the window for , locate the section.

From the list, choose .

Perfect Electric Conductor 3

In the toolbar, click

and choose .

Select Domains 7, 8, 11–14, 16, 18, 19, 24, and 25 only.

Lumped Port 1

In the toolbar, click

and choose .

Click the

button in the toolbar, two or three times.

Select Boundary 33 only.

In the window, click .

In the window for , locate the section.

From the list, choose .

For the first port, wave excitation is by default.

Lumped Port 2

In the toolbar, click

and choose .

Select Boundary 37 only.

In the window for , locate the section.

From the list, choose .

Add Material

In the toolbar, click

to open the window.

Go to the window.

In the tree, select > .

Click the button in the window toolbar.

In the toolbar, click

to close the window.

Materials

Air (mat1)

Override this material for the substrate and coaxial cable domains.

Substrate

In the window, right-click and choose .

In the window for , type Substrate in the text field.

Select Domain 6 only.

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
Electric conductivity	sigma_iso ; sigmaii = sigma_iso, sigmaij = 0	0	S/m	Basic

PTFE

Right-click and choose .

In the window for , type PTFE in the text field.

Select Domains 9 and 10 only.

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
Electric conductivity	sigma_iso ; sigmaii = sigma_iso, sigmaij = 0	0	S/m	Basic

Mesh 1

In the window, under click .

In the window for , locate the section.

From the list, choose .

Click

Click the

button in the toolbar.

Click the

button in the toolbar.

Click the

button in the toolbar, to reset the visibility state of the hidden domains in preparation of the results processing.

Study 1

In the toolbar, click

Results

Electric Field (emw)

In the window for , locate the section.

From the list, choose .

Click to expand the section. From the list, choose .

Multislice 1

In the window, expand the node, then click .

In the window for , locate the section.

Find the subsection. In the text field, type 0.

Find the subsection. From the list, choose .

In the text field, type 12.5.

Locate the section. In the text field, type 20*log10(emw.normE).

Locate the section. Clear the checkbox.

In the toolbar, click

Click the

button in the toolbar.

Click the

button in the toolbar.

The following instructions reproduce the frequency-response plot shown in Figure 2:

Global 1

In the window, expand the > node, then click .

In the window for , click to expand the section.

From the list, choose .

In the table, enter the following settings:


Legends
S11
S21 with EBG

S-Parameter (emw)

In the window, click .

In the window for , click to expand the section.

From the list, choose .

In the toolbar, click

To re-solve the model without EBG mushroom structures, do as follows:

Under > select Mushroom, Mushroom Stem, and EBG Structure, choose Disable.

Right-click and choose .

The computation takes about 10 minutes. Here, the table with computed S21 parameter values is imported for convenience.

Table 2

In the toolbar, click

Probe Table 1

In the window, click .

In the window for , locate the section.

Click

Browse to the model’s Application Libraries folder and double-click the file antenna_ebg_without_S21_parameter.txt.

Table Graph 1

In the window, right-click and choose .

In the window for , click to expand the section.

Select the checkbox.

From the list, choose .

In the table, enter the following settings:


Legends
S21 without EBG

In the toolbar, click

Smith Plot (emw)

In the window, under click .

Electric Field, Logarithmic (emw)

In the window, click .

Analyze the same model with a much finer frequency resolution using based on asymptotic waveform evaluation (AWE). When a device presents a slowly varying frequency response, the AWE method provides a faster solution time when running the simulation on many frequency points. The following example with the Adaptive Frequency Sweep can be computed five times faster than regular Frequency Domain sweeps with a same finer frequency resolution.

Electromagnetic Waves, Frequency Domain (emw)

Lumped Port 1

In the window, under > click .

In the window for , locate the section.

Click

In the dialog, type Lumped port 1 in the text field.

Click .

Lumped Port 2

In the window, click .

In the window for , locate the section.

Click

In the dialog, type Lumped port 2 in the text field.

Click .

Add Study

In the toolbar, click

to open the window.

Go to the window.

Find the subsection. In the tree, select > .

Click the button in the window toolbar.

In the toolbar, click

to close the window.

Study 2

Step 1: Adaptive Frequency Sweep

In the window for , locate the section.

In the text field, type range(2.1[GHz],2[MHz],2.35[GHz]).

Use a five times finer frequency resolution.

From the list, choose .

In the table, enter the following settings:


Asymptotic waveform evaluation (AWE) expressions
abs(comp1.emw.S11)

A slowly varying scalar value curve works well for AWE expression. Use abs(comp1.emw.S11) for this model.

Because such a fine frequency step generates a memory-intensive solution, the model file size will increase tremendously when it is saved. When only the frequency response of port related variables are of interest, it is not necessary to store all of the field solutions. By selecting the checkbox in the section, we can control the part of the model on which the computed solution is saved. We only add the selection containing these boundaries where the port variables are calculated. The lumped port size is typically very small compared to the entire modeling domain, and the saved file size with the fine frequency step is more or less that of the regular discrete frequency sweep model when only the solutions on the port boundaries are stored.

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