Modeling of a UHF RFID Tag

UHF RFID tags are widely used for identifying and tracking animals. This example simulates a passive radio-frequency identification (RFID) tag for the UHF frequency range.

With respect to the chip transponder’s complex impedance, a reflection coefficient is computed. This is done using an approach that differs from the conventional scattering parameter analysis method by a real reference impedance value.

Figure 1: The RFID tag’s geometry consists of copper traces patterned on an FR4 board that is enclosed by a low dielectric PTFE case. The surrounding air domain and perfectly matched layers, which are required for the simulation, are not included in this figure.

Model Definition

In this example, the RFID tag’s operating frequency is 915 MHz. At this frequency, the metal part of the RFID tag can be modeled as a perfect electrical conductor (PEC), because while the copper traces patterned on the FR4 board are geometrically very thin, they are much thicker than the skin depth.

The entire circuit board is inserted inside a lossless PTFE casing. The tag is modeled in a spherical air domain, which is enclosed by perfectly matched layers (PML) that absorb all outgoing radiation from the tag.

A lumped port with a reference impedance of 50 Ω is used on the location of an RFID chip. This is done to excite the tag and evaluate the input impedance of the tag’s antenna part, which is modeled as a meander line. An additional copper strip is placed adjacent to the meander line to control the impedance.

The conventional S-parameter works well only with a real reference impedance. However, the RFID chip's impedance is complex and the calculated S-parameter is not physical when a complex port reference impedance is used.

In Ref. 1, the power wave reflection coefficient term is introduced. It is applicable for evaluating the matching properties of an RFID tag:

where Zl is the complex load impedance and Zref is the complex reference impedance.

Results and Discussion

Figure 2 shows the default E-field norm on the xy-plane. The field distribution plot indicates that the electric field is symmetrically confined along the meander line, as well as in the area between the meander line and impedance matching strip.

The far-field radiation pattern of the tag is shown in Figure 3. Noticeably, the tag's radiation pattern looks very similar to the radiation pattern of a half-wave dipole antenna.

The evaluated impedance of the tag is around 18 + j124 Ω and the power wave reflection coefficient, in dB, is below −15 dB.

Figure 2: The E-field norm plot shows where the field is strongly confined in the tag.

Figure 3: The far-field radiation pattern resembles that of a half-wave dipole antenna.

Reference

1. K. Kurokawa, “Power Waves and the Scattering Matrix,” IEEE Transactions on Microwave Theory and Techniques, Volume 13, 1965.

RF_Module/Antennas/uhf_rfid_tag

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 915[MHz].

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
Zc	15-j*125[ohm]	(15-125i) Ω	Chip impedance

Geometry 1

In the window, under click .

In the window for , locate the section.

From the list, choose .

Import 1 (imp1)

In the toolbar, click

In the window for , locate the section.

Click

Browse to the model’s Application Libraries folder and double-click the file uhf_rfid_tag.mphbin.

Click

Click the

button in the toolbar.

Add a sphere for the air domain surrounding the RFID tag and perfectly matched layers that will be configured later on.

Sphere 1 (sph1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 150.

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.

Definitions

Variables 1

In the window, under right-click and choose .

Define a variable for calculating the reflection coefficient between two complex impedances.

In the window for , locate the section.

In the table, enter the following settings:


Name	Expression	Unit	Description
Gamma	(emw.Zport_1-conj(Zc))/(emw.Zport_1+Zc)		Reflection coefficient for complex impedance matching

Perfectly Matched Layer 1 (pml1)

In the toolbar, click

Select Domains 1–4 and 9–12 only.

These are all of the outermost domains of the sphere.

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 the boundary condition .

Click the

button in the toolbar, a couple of times to get a clear view of the RFID tag.

Select Boundaries 25, 27, and 54 only.

Lumped Port 1

In the toolbar, click

and choose .

Select Boundary 35 only.

For the first port, wave excitation is by default.

Click the

button in the toolbar.

Click the

button in the toolbar.

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 tree, select .

Click in the window toolbar.

In the toolbar, click

to close the window.

Materials

FR4 (Circuit Board) (mat2)

Select Domain 7 only.

Material 3 (mat3)

In the window, right-click and choose .

Select Domain 6 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