Abstract
Passive UHF RFID tags suffer from performance degradation when placed near conductors and high dielectric substances. Microstrip RFID tags offer a potential solution to this metal-water problem associated with passive UHF RFID tags. However, their use is limited by narrow bandwidth, manufacturing complexity, because of, the need cross-layered construction and therefore cost. We present a new antenna and matching circuit design using balanced feed that eliminates any reference to ground, provides moderate gain and has broadband impedance matching for low cost metal-mountable RFID tags.
Introduction
Auto–ID (automatic identification) technology enables identification and tracking of assets
and goods. RFID (Radio frequency identification) has evolved in the recent years as a near
ideal implementation of Auto–ID. Most UHF (ultra high frequency) RFID tags are dipoles
or some variation of it and thus have characteristics similar to that of a dipole. Dipoles
suffer performance degradation when placed near conductors, e.g. metals and high dielectric
materials like water. The performance of current RFID tags is therefore limited near such
materials. We call this the ‘metal-water’ problem. Microstrip antennas offer a potential
solution to the metal-water problem. The traditional microstrip antenna design principle of a
single unbalanced feed requires cross–layered structures. The designs proposed so far based
on Planar inverted F (PIFA) [1, 2, 3] are constructed using shorting walls or vias and
therefore are of limited commercial viability due to manufacturing complexity and
associated costs. The research motivation for this thesis is to determine weather a microstrip
antenna that is completely planar can be designed.
This thesis describes the theory, implementation, and discusses the performance
characteristics of a planar microstrip antenna with balanced feed and shorting stub matching
network. Such a microstrip antenna can be described in its working using traditional odd
mode circuit analysis. The planar microstrip antenna is a rectangular patch antenna with two
microstrip transmission feed lines driven 180º out of phase. The antenna is designed on a
very low profile substrate, has high performance and is cheap to manufacture.
This thesis is organized into 6 chapters. Chapter 2 deals with background of RFID and
related work. It comprises of two major parts, the first part gives a brief introduction to RFID
systems; the history of RFID, its current implementation, and standards are presented. In the
second part, basics of microstrip antenna along with some common feed techniques and
broadband designs are studied. The design of microstrip RFID tags using PIFA, and its
limitations are discussed. In chapter 3 we give a detailed description of the theory and
working of planar microstrip antenna with balanced feed. The circuit model and odd mode
analysis of the antenna are presented. Since this is the first known attempt at construction of
a balanced feed microstrip antenna, the effects of various antenna and substrate parameters
are studied in detail; an initial set of design parameter values is chosen based on theses
studies. Experimental procedures to characterize the materials used in the construction of the
planar microstrip antenna are presented. The planar microstrip antenna is designed using
these materials with the initial set of parameter values. The design is then optimized to
increase performance and reduce form-factor.
Validation of the design by prototype fabrication and experimental measurement of
performance characteristics is done and the results are presented in chapter 4. We found that
the tag performance does not follow measured gain, and there are also significant differences
between measured and simulated antenna gain. Chapter 5 discusses scope of further work
along with some preliminary results for broadband planar microstrip antennas with balanced
feed. The conclusions from this work are presented in chapter 6.
Madhuri Bharadwaj Eunni
B.E., Electronics and Communication Engineering,
A.M.A College of Engineering, Kancheepuram –Madras University,
India, May 2004
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