A SMALL PASSIVE UHF RFID TAG FOR METALLIC ITEM IDENTIFICATION
Mun Leng Ng, Kin Seong Leong, and Peter H. Cole
Auto-ID Laboratory, Scho ol of Electrical and Electronic Engineering, The University of Adelaide
{mng, kleong, cole}@eleceng.adelaide.edu.au
ABSTRACT
Passive UHF Radio Frequency Identification (RFID)
tags are potentially low cost and offer good read range
for object identification in various types of applications.
However, the performance of conventional passive UHF
RFID tags will be greatly affected when the tags are
attached to metallic objects. In this paper, a small
passive UHF RFID tag consisting a loop antenna de-
signed specifically for attaching to metallic objects is
presented. Theoretical design steps for this tag design,
as well as significant simulation results are included.
The results for practical read range performance mea-
surement of this tag are also presented.
1. INTRODUCTION
Radio Frequency Identification (RFID) is a technology
used for object identification via radio waves. A basic
RFID system consist of tags, one or more readers, and
a network system for data handling [1]. The first idea
of RFID has been traced back to as early as the 1940’s
[2]. However, it is only recently that RFID is receiving
increasing attention, especially in terms of research and
development, due to the widening of the applications
of this technology in various areas. Examples of RFID
applications can be found in [2][3]. Today, some of the
common operating frequencies for RFID are 125 kHz
(LF), 13.56 MHz (HF) and 860 - 960 MHz (UHF).
Passive UHF RFID tags are able to provide good
read ranges for object identification compared to LF or
HF RFID tags, and they are also seen as potentially low
cost. However, conventional planar passive UHF RFID
tags will suffer a degradation in performance when at-
tached to metallic objects or structures. The reason
to this may be either due to the detuning of the reso-
nant frequency of the RFID tag antenna or insufficient
interrogation field from the RFID reader reaching and
powering-up the tags.
The presence of metallic objects or structures are
certainly unavoidable. Hence, for widespread RFID
This work was supported by Auto-ID Laboratory, Adelaide.
Tagantenna
Tag
chip
Width (W )rec
Length( )Lrec
Height
(H )rec
x
y
z
Figure 1: Structure of the RFID tag with a rectangular
loop antenna.
deployment, passive UHF RFID tags that are suitable
for tagging metallic objects are needed. In this pa-
per, we present a small passive UHF RFID tag with a
simple rectangular loop antenna design that is suitable
for attaching to metallic objects. Design methodology
for this tag design is also presented. The design has
been evaluated through simulations and simple empir-
ical measurements. Significant calculation, simulation
and performance measurement results are presented in
this paper.
2. DESIGN CONCEPT
When designing the RFID tag suitable for attaching
to metallic objects, the first point considered is the
behaviour of the interrogation fields (from the RFID
reader) near a metallic surface. According to the theory
of boundary conditions, for magnetic field, there are
only tangential components and no normal components
of this field to the metallic surface [4]. The magnetic
field (tangential component) will be doubled when it is
very near the metallic surface.
The RFID tag design presented here exploits the
fact above by having a loop antenna oriented such that
the plane of the loop is perpendicular to the plane of the
metallic surface where the RFID tag will be attached
on. With this orientation, the idea is to have the rich
concentration of magnetic fields near the metallic sur-
face to couple to the loop antenna of the RFID tag.
Many different types of loop antenna can be con-
sidered for the tag antenna. The circular loop antenna