The art of UHF RFID antenna design:
impedance matching and
size-reduction techniques
scare
Published in IEEE Antennas and Propagation Magazine, Vo.50, N.1, Jan. 2008
Gaetano Marroccoactivate
Dipartimento di Informatica Sistemi e Produzione
University of Roma “Tor Vergata”
Via del Politecnico, 1, 00133, Roma
Tel:+39 06 72597418, Fax:+39 06 72597460
Email: marrocco@disp.uniroma2.it
____________
Abstract
Radio Frequency Identification technology bad on the reader/tag paradigm is quickly permeating veral aspects of the everyday life. The electromagnetic rearch mainly concerns the design of tag antennas having high efficiency and small size and suited to complex impedance matching to the embedded electronics. Starting from the available but fragmented open literature, this paper prents a homogeneous survey of relevant methodologies for the design of UHF passive tag antennas with particular care to illustrate, within a common framework, the basic concepts of the mostly ud design layouts. The design techniques are illustrated by means of many non-commercial examples.
Keywords: antenna design, RFID, tag, T-match, meander line antenna, impedance matching, miniaturized antenna, PIFA, IFA.
1. Introduction
The idea of Radio Frequency IDentification (RFID) of objects and remote control of devices was first
introduced in the late 1948 by H. Stockman [1]. After the big efforts given by the development of the
microelectronic technology in the 1970s [2], and the continuing evolutions of the last decade [3], [4], RFID is
now imposing as a pervasive technology [5],[6]in everyday life [7] and in more advanced applications
involving logistics, inventory management, aided systems for disabled people, homeland and personal
curity, distributed nsor networks [8] and mobile healthcare [9]. A basic RFID system compris a radio-
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scanner unit, called reader, and a t of remote transponders, denoted as tags, which include an antenna and a
microchip transmitter with internal read/write memory. In passive tags, the required energy to drive the
microchip comes from the interrogation system itlf and a backscattering modulation is achieved when the
microchip acts as a switch to match or mismatch its internal load to the antenna.
Several frequency bands have been standardized for this technology. Low frequency (LF, 125-134 kHz) and high frequency (HF, 13.56 MHz) systems are the most mature and worldwide diffud technology. They
are bad on quasi-static magnetic flux coupling among the reader’s and tag’s coils. Ultra-high frequency
(UHF, 860-860 MHz) and microwave (2.4 GHz and 5.8 GHz) systems involve instead electromagnetic
interaction among true antennas, permit longer communication links and they are the emerging technology.
Together with the microchip power nsitivity, the tag antenna plays a key role in the overall RFID system performances, such as the overall size, the reading range and the compatibility with the tagged objects. Most
of the antennas for UHF omnidirectional tags are commonly fabricated as modified printed dipoles. The
design goal is to achieve inductive input reactance required for the microchip conjugate impedance matching,
and to miniaturize the antenna shape. Several tricks are ud, and the resulting tags sometimes exhibit
charming and nearly artistic layouts.
Although many tag configurations can be retrieved in the scientific papers, or even in the catalogs of commercial products, there is a lack of systematization of the design methodology. A first tutorial paper is available in [10], where the concept of conjugate impedance matching to the microchip is reviewed, some performance parameters are introduced and fabrication and measurement procedures are described in some detail.汤唯韩国获奖视频
This paper provides a unitary and general survey of the most ud design procedures for miniaturized tag antennas with complex impedance matched to the microchip load. Attention is devoted to the rationale and to the main features of basic configurations, by who modification and combination a great variety of tag layouts can be easily obtained. For each design solution, the role of the main geometrical parameters over the complex impedance tuning it is here investigated by intr
oducing matching charts which are a uful tool to suite a same antenna configuration to different kinds of microchip.
The rest of the paper is organized into four main Sections. Section 2 introduces veral techniques to achieve complex impedance matching, such as the T-mach, the proximity-loop and nested-slot layouts. Then (Section 3), miniaturization and bandwidth issues are addresd with references to meandered and inverted-F solutions corroborated by many examples. Other miscellaneous design issues are addresd in Section 4 and, finally, some measurement and test procedures for UHF tag are prented in Section 5.
2. Methods for conjugate impedance matching
Having fixed the effective power (EIRP R ) transmitted by the reader and the nsibility (P chip ) of the tag’s transponder, e.g. the RF power required to the microchip electronics to turn on and to perform back-scattering modulation, the maximum activation distance of the tag along the (θ,φ ) direction [10], under the hypothesis of polarization matching between reader and tag antennas, is given by
),(4),(max φθτπφθtag chip R G P EIRP f c d =
(1)
where G tag (θ,φ) is the tag gain and the factor 1||42≤+=A chip A
chip Z Z R R τ (2)
is the power transmission coefficient which accounts for the impedance mismatch between antenna (Z A =R A +jZ A ) and microchip (Z chip =R chip +jZ chip ). The microchip impedance depends on the input power and, since the transponder includes an energy storage stage, its input reactance is strongly capacitive and most of the available RFID ASICS in the UHF band exhibit an input reactance roughly ranging from -100Ω to -400Ω
[11], [12], [13], while the real part is about an order of magnitude smaller or less. The antenna impedance should be inductive in order to achieve conjugate matching, and a large impedance pha angle, atan(Z A )>45°, needs to be obtained. Beyond d max the power collected by the tag decreas below the microchip nsibility and the tag becomes unreachable.
To obtain low-cost devices, it is not feasible to u external matching networks involving lumped components and therefore the matching mechanisms have to be embedded within the tag antenna layout.
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Several feeding strategies can be adopted for antenna tuning. The most ud are modified versions of the well-known T-match, the proximity coupling to a small loop, and the inclusion of shaped slots. Uful configurations should permit a nearly independent tuning of resistance and reactance when acting on the tag geometrical parameters.de
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Some matching techniques will be now reviewed and compared having fixed, for clarity, the antenna maximum size to half a wavelength without taking care to the overall size which could be eventually un-practicable for real applications. The miniaturization issues are deferred to the next Section.
The matching capability of the considered feeding schemes will be summarized through an impedance charts where the input resistance and reactance are related to the variation of relevant geometrical parameters. A matching scheme is as more agile as the iso-lines for resistance and reactance are mutually parallel. The basic matching schemes are illustrated with the help of some non-commercial examples taken from the recent open scientific literature.
2.1 T-match
With reference to Figure 1, the input impedance of a (planar) dipole of length l can be changed by introducing a centered short-circuit stub, as detailed explained in the old book [14], and more recentl
y resumed in [15]. The antenna source is connected to a cond dipole of length a ≤ l , placed at a clo distance b from the first and larger one. The electric current distributes along the two main radiators according to the size of their transver ctions.
Figure 1: T-match configuration for planar dipoles and equivalent circuit where the impedance step-up ratio (1+α) is related to the conductors’ cross-ctions.
It can be proved, [14], [15] that the impedance at the source point is given by
a
t a t in Z Z Z Z Z 22)1(2)1(2αα+++= (3)
where 2/tan 0ka jZ Z t = is the input impedance of the short-circuit stub formed by the T-match conductors and part of the dipole; )/(log 276100e e r r b Z ′≅ is the characteristic impedance of the b-spaced two-conductors transmission line; Z A is the dipole impedance taken in its centre in the abnce of the T-match connection; r e =0.25w and '25.8w r e =′ are the equivalent radii of the dipole and of the matching stub, suppod to be planar traces, and )/ln(/)/ln(e e r b r b ′=α is the current division factor between the two
conductors.
The geometrical parameters a , b and the trace’s width w’ can be adjusted to match the complex chip impedance Z chip . The T-match acts as an impedance transformer (Figure 1). In ca of half a wavelength
dipoles, the resulting input impedance at the T-match port is inductive, while for smaller dipoles, the t
otal input impedance can be both capacitive and inductive.
Figure 2: Matching chart for the T-match in Figure 1, in the ca l=λ/2, w= λ/100, w’= w/3 and Z A=75Ω.
For example, Figure 2 shows a matching chart for the T-match layout having fixed the ratio between the dipoles cross-ctions to w/w’=3. The input resistance and inductance depend on both the stub size a and b, but with different rules. It is known that the cross-ction of the cond conductor plays a considerable effect on the resulting antenna impedance. In particular, it can be easily verified from (3) that the increa of the w/w’ ratio will rai the impedance values, and the iso-lines for resistance and reactance become nearly vertical and mutually parallel (strong dependence on b size) resulting in a reduced matching agility. Even with small values of a and b, high values of input resistance are generally found, making difficult the impedance matching to real microchip transmitters unless some shape modification of the main radiator are considered. A single T-match la
yout could be therefore not completely adequate to match high impedance-pha-angle microchips. In such cas, further degrees of freedom are added by means of multiple T-match stages. The T-match geometry can be also embedded within the main radiator yielding a compact structure as in Figure 3.
Figure 3: Example of embedded T-match feed. This antenna has been propod in [16] to be rolled around cardboard reels.
2.2 Inductively coupled loop
材质报告Instead of the T-match, the radiating dipole may be sourced via an inductively coupled small loop [1
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7] placed at a clo proximity to the radiating body (Figure 4). The terminals of the loop are directly connected to the microchip. This arrangement adds an equivalent inductance in the antenna. The strength of the coupling, and therefore of the added reactance, is controlled by the distance between the loop and the radiating body as well as by the shape factor of the loop.
Figure 4: Layout of the inductively coupled feed and equivalent circuit. Parameters R A , C A , L A gives the circuit model of the radiating body near its (ries) resonance.
The inductive coupling can be modeled by a transformer and the resulting input impedance en from the loop’s terminals is
A
loop in Z M f Z Z 2)2(π+= (4)
where loop loop L f j Z π2= is the loop input impedance. Whether the dipole is at resonance, the total input
reactance depends only on the loop inductance L loop , while the resistance is related to the sole transformer mutual inductance M :so翻译
loop in A in L f f X f R M f f R 0002002)()
(/)2()(ππ== (5)
Under the assumption that the radiating body is infinitely long, the loop inductance and mutual coupling M can be expresd in terms of the loop size and of its distance from the dipole through analytical formulas [18]. It is important to note that the mutual coupling, and therefore the total input r
esistance is dependent on both the loop shape and on the dipole-loop distance, while the reactance L loop is mainly affected by the only loop’s aspect ratio.
Figure 5 shows an example of matching chart, computed by Method of Moments [19], for the loop-driven dipole, in the particular ca of resonant dipole and square loop (a =b ). As expected from (3), the input reactance is nearly un-affected by the loop-dipole distance (d ) and the corresponding iso-lines are vertical. For a fixed size of the loop, the resistance reduces when the loop-dipole distance increas. A design procedure could therefore initially lect the loop size with the purpo to cancel the chip capacitive reactance, and further on a proper loop-dipole distance d is chon to match the chip resistance. This layout is particularly effective for microchips having high impedance pha angle.
