On-body semi-electrically-small tag antenna for ultra high frequency

IET Microwaves, Antennas & Propagation
Research Article
On-body semi-electrically-small tag antenna
for ultra high frequency radio-frequency
identification platform-tolerant applications
ISSN 1751-8725
Received on 23rd June 2015
Revised on 18th November 2015
Accepted on 7th December 2015
doi: 10.1049/iet-map.2015.0398
www.ietdl.org
Milan Svanda ✉, Milan Polivka
Faculty of Electrical Engineering, Czech Technical University in Prague, Technicka 2, 166 27, Prague 6, Czech Republic
✉ E-mail: [email protected]
Abstract: In this study, the authors proposed the impedance-flexible on-body semi-electrically-small tag antenna for the
European ultra-high frequency radio-frequency identification (UHF RFID) frequency band. The radiator is based on
differentially-fed coupled shorted-patches and vertical folding techniques, using a loop excitation. The proposed
excitation methods enable the input impedance to be tuned to the complex impedance of typical UHF RFID chips as
well as to 50 Ω impedance. In addition, the virtual electric short applied inside the structure simplifies the topology and
manufacture. The antenna dimensions are 40 × 50 × 3 mm3 (0.12 × 0.14 × 0.01 l0) and represent the electrical size ka =
0.58 at 866 MHz. The total efficiency measured in open space exceeds 60%. In case that the antenna is situated on a
human phantom, it accounts for 50%. Accordingly, the antenna provides the read range of 5.6 m (in open space) and
5.1 m (when located on human chest).
1
Introduction
On-body high-efficiency small antennas: this set of requirements draw
attention to the most fundamental features of modern planar antennas
for body-centric wireless communication [1, 2]. Unfortunately,
satisfactory parameters for all three features cannot be achieved
without a trade-off, due to the contradictory nature of these features.
Placing the antenna on a lossy human body causes power
absorption, and consequently reduces the radiation efficiency, due to
the interaction of the electromagnetic field with the surrounding
body. Making the antenna electrically small results in the same
effect, due to the principal limitation of electrically small antennas
[3]. Patch-type antennas are perfectly suited for operation directly
on the surface of the human body, with no significant impact on
their impedance and radiation properties. This is due to their
inherently present metallic ground plane, which serves as an electric
shield for the body. Other types, e.g. dipoles, may also have been
used to operate on the human body, but they require a spacing pad
with a minimum distance of at least 0.02 l0 [which represents
several millimetres at ultra high frequencies (UHFs).] for the
electrical parameters of the antenna to remain unaffected.
Typically, two sorts of materials, flexible or rigid, are available for
manufacturing patch-type antennas that are suitable for operation on
the human body, usually with a negligible change to their electrical
parameters due to the inherent presence of the ground plane. These
radiators, which are usually made from flexible foam and conductive
fabric, in most cases integrated into the wearer’s clothing, are usually
referred to as wearable or on-body antennas [4–11]. A conductive
fabric or sewn threads are usually applied for the conductive
pattern of the radiator [7, 8]. This is good for providing some
flexibility of the material and some conformity with the surface,
but at the same time it reduces the efficiency and the gain of fabric
antennas typically by as much as 3 dB, as has been demonstrated
in several studies [8, 9].
A number of small platform-tolerant tag antennas for on-body
applications made of rigid substrates have already been published,
e.g. in [12–23]. However, their main disadvantage is their very
small radiation efficiency, typically lower than 20%, which results
in negative gain and a short read range. A special differentially-fed
patch-type topology has been developed that enables the design of
an extremely low antenna profile (<0.01 l0, typically 1 mm or less
in thickness in the UHF band) with sufficient radiation efficiency
IET Microw. Antennas Propag., 2016, Vol. 10, Iss. 6, pp. 631–637
& The Institution of Engineering and Technology 2016
(typically 50% and more) [24–26]. Due to the resulting extremely
low profile and weight, these antennas may also be considered
wearable, and they can be integrated into a conference badge
without any disturbing effects.
Progress has been made in the design of wearable antennas using
the classical rigid high-frequency substrate applicable in the
European UHF radio-frequency identification (RFID) frequency
band (866–869 MHz), by combining the application of coupled
shorted-patches [25, 26] with vertically-folded patch techniques
[27], which the authors have developed and presented earlier. The
progress presented here, using rigid substrates, shows the
following features:
(a) A further reduction in the footprint size in exchange for just a tiny
increase in height, so that the overall electrical size of the antenna
approaches the limit of electrically small antennas (ka = 0.58).
(b) Feeding by a loop, enabling the input impedance to be matched to
the typical complex impedance of RFID chips, and as well as to 50 Ω
impedance for the direct measurement of the received signal
strength.
(c) Simplification of the structure topology using inner virtual
electric wall that does not affect the electrical parameters of the
radiator (i.e. sufficient total efficiency is maintained) and reduces
the manufacturing complexity
2
On-body antenna miniaturisation
2.1 Application of coupled shorted-patches and vertical
folding techniques
The basis for vertical folding of a patch cavity is the idea that the
conventional rectangular microstrip patch antenna can be described
as a resonating cavity with magnetic walls at the open side edges;
[28]. The cavity can be folded, while the electrical length of the
whole folded cavity and also the resonant frequency remain
constant, and the physical length of the antenna is reduced to
almost 50%.
If the vertical folded patch cavity technique is applied to the
low-profile coupled shorted-patches structure, we can achieve a
novel miniaturised radiating structure that retains good radiating
features in a free space and also in the close vicinity of a human
631
Fig. 1 Side cross section and electric field distribution of the folded antennas
a Vertically-folded shorted-patch antenna
b Proposed vertically-folded coupled shorted-patches antenna with inner metallic wall
c Proposed vertically-folded coupled shorted-patches antenna without inner metallic wall
body/phantom. This is due to the presence of one radiation slot in the
central part of the structure and thus avoiding the interaction of
fringing edge-fields with the body as in the case of the classical
patch antenna. The side cross section with the electric field
distribution of the vertically-folded quarter-wavelength patch
antenna and the proposed vertically-folded coupled shorted-patches
antenna is shown in Figs. 1a and b, respectively [27]. The coupled
shorted-patches tag antenna consists of two 1.52 mm thick layers of
low-permittivity GML 1000 woven-glass laminate with ɛr = 3.2, and
loss tangent tan δ = 0.003, as shown in Fig. 3a. The resulting
antenna size is 40 × 50 × 3 mm3 (0.12 × 0.14 × 0.01 l0), which
represents electrical size ka = 0.58 at 866 MHz.
2.2
Fig. 2 Comparison of real and imaginary part of input impedance of the
folded coupled-patches antenna with and without inner metallic wall
Application of the virtual electric wall
The inner metallic wall is present due to the connection of the
shorted edges of the two vertically-folded shorted patches, as
shown in Fig. 1b. The wall brings two disadvantages: increased
manufacturing complexity and a higher cost price. However, as
Fig. 3 Directly excited folded coupled-patches RFID tag
a Sketch of the antenna
b Photograph of antenna monopole arrangement for purpose of measuring impedance and efficiency
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IET Microw. Antennas Propag., 2016, Vol. 10, Iss. 6, pp. 631–637
& The Institution of Engineering and Technology 2016
3
3.1
Fig. 4 Simulation and measurement of parameters of the directly-excited
vertically-folded coupled-patches tag antenna
a Transmission coefficient
b Radiation efficiency
c Total efficiency
Effect of feeding on impedance matching
Direct excitation
Here, we designed the antenna to be a conjugate matched to NXP
G2XM chip with Zchip = 22 − j195 Ω. The feeding chip is located
in the centre of the metallic pattern upon the antenna upper layer,
as shown in Fig. 3a. The dimensions follow: w = 40 mm, l = 50
mm, lp = 44 mm, g = 2 mm and h1 = h2 = 1.52 mm. The changes
in structural parameters of folded coupled-patches enable the
antenna impedance to be tuned to complex conjugate values
with respect to the most of impedances of passive UHF RFID
chips [27]. The main drawback of this solution consists in the
restricted range of input impedance that is achievable by direct
excitation. Similar to Fig. 5a depicting the input impedance of
the loop-excited antenna presented in Section 3.2, the
impedance curve is situated in the vicinity of the Smith diagram
circumference. However, input impedance close to 50 Ω is
difficult to achieve applying the only parametric changes of
patch topology.
The performance properties of the directly-excited antenna were
verified in a monopole-type arrangement in order to avoid the use
of a balun situated between the antenna and the coaxial connector,
as shown in Fig. 3b. The monopole-type input impedance then
accounts for one half of the value for a dipole-type impedance.
Zmonopole = Zdipole/2 is therefore considered for further evaluation
∗
= 22 + j195 Ω).
(where Zdipole = Zchip
The transmission coefficient, as shown in Fig. 4a, between the
antenna and the chip input impedance was evaluated from the
standard reflection coefficient measurement. The measurement was
performed with and without a human body phantom (made of agar
boiled in salted water with the concentration of 3.45 g/l with
ɛr ∼ 55 and tanδ ∼ 0.5, with dimensions of 80 × 110 × 15 mm3),
which was enclosed in the back of the antenna.
Considering the RFID application, the antenna and chip
impedances are properly matched, provided that their input
impedances represent a complex conjugate. Consequently, the
power transmission coefficient t is calculated by means of the
following equations
t = 1 − |G|2 = can be seen from the electric field distribution in Figs. 1b and c, there
is no need for a real metallic short wall, as the zero electric field in
the middle of the patch enables the use of a virtual short with
negligible overall influence on the field distribution and the input
impedance, as shown in Fig. 2.
4Rant Rchip
2 2 ,
Rant + Rchip + Xant + Xchip
(1)
0≤t≤1
G=
Zchip − Zant
,
Zchip + Zant
0 , |G| , 1
(2)
Fig. 5 Simulated impedance curves of the loop-excited antenna
a Matched to complex RFID input impedance
b Matched to 50 Ω input impedance
IET Microw. Antennas Propag., 2016, Vol. 10, Iss. 6, pp. 631–637
& The Institution of Engineering and Technology 2016
633
Fig. 6 Loop-excited folded coupled-patches RFID tag
a Sketch of the antenna (3D view)
b Sketch of the antenna (side view)
c Photograph of antenna monopole arrangement for the purpose of measuring input impedance and efficiency
Table 1 Wearable UHF RFID tag antenna performance
Number
1
2
3
4
5
6
7
8
9
10
11
Ref.
Size, mm3
ka (–)
ηtot, %
G, dBi
[16]
[17]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
our design #1
our design #2
68 × 30 × 3.0
35 × 25 × 3.0
45 × 20 × 3.0
72 × 25.5 × 3.2
74.5 × 20 × 3.0
58 × 34 × 0.85
99.9 × 45 × 0.86
75 × 42 × 2
134 × 58 × 3
50 × 40 × 3.0
50 × 40 × 3.3
0.73
0.42
0.48
0.73
0.75
0.65
1.06
0.83
1.39
0.58
0.58
23.4
22.6
10.6
44.0
21.1
8.1
23.5
33.2
45.2
51.0
35.0
−1.7
−3.6
−6.9
0.6
−1.0
−9.0
−3.5
−1.8
1.8
−0.2
−2.0
This monopole-type arrangement enables doing measurements
of the radiation and the total efficiencies by the Wheeler cap
method [29]. A cap size of 122 × 122 × 122 mm3 was used. The
measurements were performed with and without the human
body phantom, as shown in Figs. 4b and c. Very good
immunity from the phantom and also sufficient total efficiency
(i.e. the radiation efficiency multiplied by the power
transmission coefficient t) can be observed at an operation
frequency of 866 MHz.
3.2
Loop excitation
where Γ is the reflection coefficient between the antenna and chip
impedances, Rant and Rchip represent the antenna and chip input
resistances, respectively, while Xant and Xchip stand for the antenna
and chip input reactances, respectively.
Unlike the directly-excited antenna, the loop-excited folded
coupled shorted-patches RFID tag antenna requires a third
substrate layer to be added; as shown in Fig. 6a. Thanks to the
extra degree of freedom (the excitation loop length), it is
possible to affect the antenna impedance due to the mutual
coupling of the excitation loop and radiation body so that 50 Ω
impedance can be achieved; as shown in Fig. 5. Rather than
building-up the semi-analytical equivalent model for the
calculation of input impedance, which might hardly be purely
analytical due to the complicated or even possible simply
analytical description of the coupling, we present in the
following section input impedance changes as a function of
structural parameter variations. The dimensions of the
loop-excited antenna depicted in Fig. 6a follow: w = 40 mm, l =
50 mm, lp = 42 mm, g = 2 mm, h1 = h2 = 1.52 mm, h3 = 0.25 mm,
lloop = 43 mm, wloop = 30 mm and wstrip = 1 mm.
Very similar as in the previous case, the performance properties of
the loop-excited antenna were verified in a monopole-type
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IET Microw. Antennas Propag., 2016, Vol. 10, Iss. 6, pp. 631–637
& The Institution of Engineering and Technology 2016
Fig. 7 Simulation and measurement of parameters of the loop-excited
vertically-folded coupled-patches tag antenna
a Transmission coefficient
b Radiation efficiency
c Total efficiency
Fig. 8 Zin of the loop-excited folded coupled shorted-patches antenna with
antenna length l as a parameter
Fig. 9 Zin of the loop-excited folded coupled shorted-patches antenna with
inner patches length lp as a parameter
a Real part
b Imaginary part
a Real part
b Imaginary part
arrangement in order to avoid the use of a balun situated between the
antenna and the coaxial connector; as shown in Fig. 6b.
The transmission coefficient, as shown in Fig. 7a, between the
antenna and the chip input impedance was evaluated from the
standard reflection coefficient measurements, performed with
and without the same human body phantom, which was
enclosed in the back of the antenna. The monopole-type
arrangement enabling measurements of efficiency by the
Wheeler cap method [29] was used to verify the properties of
the antenna; as shown in Figs. 7b and c. In comparison to the
directly excited version, the third added layer gives rise to
the decrease in the total efficiency (measured with the help of
the agar phantom) by about 20%. However, this value is still
sufficient considering the small size of the structure. Sufficient
immunity of the impedance detuning and the radiation
efficiency due to the presence of a phantom is retained at an
operating frequency of 866 MHz.
Table 1 summarises the comparison of the electrical size with
the total efficiency and gain of the proposed designs and several
other designs that have been published [16–19]. The efficiencies
of the reference designs have been evaluated from the gains
calculated from the Friis formula; see e.g. [30]. Directivities
have been calculated from the beamwidth of the radiation
patterns. It is clearly visible that in comparison to electrically
larger antennas (see rows 1, and 4–9 in Table 1), the tag
proposed here provides a larger or at least comparable total
efficiency and gain. At the same time, it shows a larger total
efficiency and gain than the electrically smaller antennas (see
rows 2 and 3 in Table 1).
4 Parametric study of the input impedance
retuning
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& The Institution of Engineering and Technology 2016
The flexibility to retune the input impedance is illustrated using a set
of parametric studies; as shown in Figs. 8–11.
The principle of setting the antenna impedance for the required
complex value or directly to 50 Ω at the specified frequency of the
European UHF RFID band is based on the change in the surface
currents and electromagnetic field distribution on the excitation
loop and in inner structures of the antenna. This change can be
controlled by modifying the loop geometry and, at the same
time, by modifying the size of folded coupled shorted-patches
structure. In order to tune the input impedance for a complex
conjugate value to the RFID chip impedance Zchip = 22 − j195 Ω
at the observation frequency of 866 MHz, the parametric study
shown in Figs. 8–11 employs the impedance-sensitive
dimensions, such as antenna length, inner patch length, antenna
width and excitation loop circumference length. The changes
are applied exclusively to this parameter, while the other
parameters remain unchanged.
5
Read range and identification test
To evaluate the performance of tag antennas in real operating
conditions, the read range tests were performed in 4 m wide
corridor with the fixed transmitted power of 30 dBm and the
standard 8 dBi reader antennas, generating 6.3 W of effective
635
Fig. 10 Zin of the loop-excited folded coupled shorted-patches antenna with
loop circumference C as a parameter
Fig. 11 Zin of the loop-excited folded coupled shorted-patches antenna with
width w as a parameter
a Real part
b Imaginary part
a Real part
b Imaginary part
isotropic radiated power (EIRP). The tag antenna with the NXP
G2XM UHF RFID GEN 2 chip (chip power sensitivity Pchip min
= −15 dBm) was fixed at the height of 1.3 m in free space and on
a person’s chest; as shown in Fig. 12. The tagged figurant was
standing on the corridor axis and near the corridor wall.
Table 2 indicates the read range evaluated for both, the directly
excited as well as the loop-excited folded coupled shorted-patches
RFID tag antennas located in a free space and also for the
antennas attached to a human chest. Read range in Table 2 was
Table 2 Identification tests of the folded coupled patches antennas in
free space as well as on the human chest in buildings corridor
(recalculated for EIRP = 4 W)
Antenna
Position in the
corridor
Read range, free
space, m
Read range,
human chest, m
directly
excited
loop
excited
on axis
at the wall
on axis
at the wall
6.0
5.6
5.5
5.0
5.5
5.1
4.7
4.3
recalculated for EIRP = 4 W conformable with the European RFID
specification.
Shorter read range of the loop-excited tag in comparison with the
directly-excited tag can be observed. The above mentioned
phenomenon can be explained by analysis of the total efficiency of
the structure which comprises multiple of the radiation efficiency
and the transmission coefficient. In the case of the loop-excited
antenna radiation efficiency is sufficiently lower than that of the
directly-excited antenna and thus causes a lower value of total
efficiency; see Figs. 4 and 7. The lower total efficiency results in
lower read range.
6
Fig. 12 Photograph of test configuration
a General view
b Detail of the person with chest-fixed TAG in 4 m wide corridor
636
Conclusions
We have proposed an improved input impedance-flexible on-body
semi-electrically small tag antenna with sufficient radiation
efficiency for the European UHF RFID frequency band. Loop
IET Microw. Antennas Propag., 2016, Vol. 10, Iss. 6, pp. 631–637
& The Institution of Engineering and Technology 2016
excitation of the antenna enables the impedance to be tuned to the
complex impedance of the RFID chip, and as well as to 50 Ω by
changing the structural parameters (loop length) at the expense of
small radiation efficiency decrease. Further, we have shown that
the simplification of the topology consisting through using a
virtual electric wall does not affect the electrical parameters of the
radiator.
The performance properties of the two feeding configurations
were verified with and without a human body phantom in the
monopole-type arrangement. The read range tests in corridor were
performed in order to evaluate the performance of TAG antennas
in real operating conditions. Sufficient radiation efficiency
independent on the presence of the phantom consequently read
range longer than 4 m were verified.
Besides the mentioned radiofrequency identification of people, the
presented antennas can be alternatively used for identification of
metallic objects (e.g. containers, pallets and racks in stores) or for
identification of liquids kept in bottles or canisters.
7
Acknowledgments
This work received support from Czech Science Foundation project
no. P102/12/P863 and from COST project no. LD 14122
WiPEELMAG, which forms a part of COST project no. IC 1301
WiPE.
8
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