© 3M 2007. All Rights Reserved.

Modeling Techniques for Evaluation of

RFID Tag Antennas In Situ

Riki Banerjee

3M Company

Outline

� RFID operation

� RFID tag design

� Evaluation techniques In Situ

� Conclusions

What is Radio Frequency Identification (RFID)?

Typical RFID system components

Tag

Antenna

Silicon

Chip

Packaging and

Construction

ReaderAntenna(s)

Reader

Hardware

Software

Different Types of RFID Tags

Various Readers

Handhelds Portals Reader Pads

Standard readers can be configured for the application.

Common Frequencies of RFID

0.12 m2.45 GHz

0.33 m915 MHz

22.1 m13.56 MHz

2400 m125 kHz

Wavelength (λ)Frequency

Antennas less than λ/10 predominately only have

stationary fields.

Antennas greater than λ/10 create stationary and significant radiated

fields.

Radiated fields create longer read range.

Passive RFID

� Advantages

� No battery to replace

� Small Form Factor

� Low Cost

� Disadvantages

� Less range

� Chip Turn-on power limits the range

2 Chip turns on.

3 Chip listens for reader commands.1 Reader sends signal.

Tag backscatters information.45 Reader receives tag information.

Passive RFID Summary

What affects performance?

� Providing enough signal to turn on tag chip� Reader power and antenna size

� Reader-to-tag distance

� Tag antenna size and chip turn-on voltage

� Orientation of reader and tag antennas

� Presence of objects in close proximity to tag

� Speed of movement of tag in and out of reader fields

� Propagation medium between reader and tag

Advantages

� No battery to

replace

� Small Form Factor

� Low Cost

Disadvantages

� Less range

915 MHz and 2.45 GHz Passive RFID

� Longer read ranges (1m to 9 m) compared to LF/HF tags

� Propagating waves are more strictly limited by regulatory agencies

� Read performance affected by signal reflections off and blockage by objects along reader-to-tag propagation path (multipath)

� Frequency bands shared by other active services

� Strongly effected by presence of nearby non-metallic objects

� Tags available to work on metal are usually larger and thicker.

Metal mount tag

Supply chain tags

The important parameters in RFID Antenna Design

Gain: Radiation and Directionality of Power

Alien Squiggle RFID Tag and 3D

simulation of power

Smith Chart of Alien Squiggle

Impedance

Impedance: Must be matched to IC chip

915 MHz RFID Tag Design

� Chips are inexpensive (no on-chip matching), therefore very

capacitive.

� Antenna must be designed to provide the conjugate match.

� Traditional 50 ohm antenna designs are not used for RFID.

� Design is done using CST Microwave Studio – Transient Solver

Open (add space)

boundary conditions.

50 ohm discrete S-parameter port

915 MHz RFID Tag Parameters

Gain of the Tag

Impedance of the tag is inductive.

Assessing Tag Bandwidth

Can be done using CST Design Studio.

1

cap pFcap pF

1

Chip impedance is inserted at the port.

Sources of Changes for In Situ in Performance

Reader-to-tag link for RFID IC power-up

[ ][ ]

Γ−Γ−

= rrtttr GpPG

r

FP

πλ

π 411

4

222

2

2

where:

Gt, Gr = reader and tag antenna gains

F = Eactual/Efree space

Γt, Γr = reader and tag antenna reflection coefficients

p = polarization mismatch loss

Pt = transmit power

Pr = received power at tag

Changes in impedance match, gain, and field will cause changes in the read range of the tag.

Impedance/Gain Change: Tag within in an item

Impedance/Gain Change – Tag in use cases

� In use, we were seeing more loss than the impedance and gain would predict.

Evaluating Problems due to Absorption

� Load tag with chip impedance

� Hit structure with a plane wave.

� Evaluate voltage seen at the load.

The hand alone caused 2 dB of loss.

Evaluating Problems due to Absorption

� Load tag with chip impedance

� Hit structure with a plane wave.

� Evaluate voltage seen at the load.

The addition of other body parts caused 9.7 dB of loss.

A 10 dB decrease in field will result in a 70% decrease in read range.

Conclusions

� Modeling can be used to parse out the detailed affects

of changes in performance.

� RFID tags are expected to perform in complex

environments under complex conditions.

� Modeling can demonstrate feasibility and assist with

improvements in the design.