Locher, I., Klemm, M., Kirstein, T., & Troester, G. (2006). Design and
characterization of purely textile patch antennas.
IEEE Transactions
on Advanced Packaging
,
29
(4), 777 - 788.
https://doi.org/10.1109/TADVP.2006.884780
Peer reviewed version
Link to published version (if available):
10.1109/TADVP.2006.884780
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IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 29, NO. 4, NOVEMBER 2006 777
Design and Characterization of Purely
Textile Patch Antennas
Ivo Locher, Student Member, IEEE, Maciej Klemm, Student Member, IEEE, Tünde Kirstein, and
Gerhard Tröster, Senior Member, IEEE
Abstract—In this paper, we present four purely textile patch an-
tennas for Bluetooth applications in wearable computing using the
frequency range around 2.4 GHz. The textile materials and the
planar antenna shape provide a smooth integration into clothing
while preserving the typical properties of textiles. The four an-
tennas differ in the deployed materials and in the antenna polar-
ization, but all of them feature a microstrip line as antenna feed.
We have developed a manufacturing process that guarantees un-
affected electrical behavior of the individual materials when com-
posed to an antenna. Thus, the conductive textiles possess a sheet
resistance of less than
1
in order to keep losses at a min-
imum. The process also satisfies our requirements in terms of ac-
curacy meeting the Bluetooth specifications. Our investigations not
only characterize the performance of the antennas in planar shape,
but also under defined bending conditions that resemble those of a
worn garment. We show that the antennas can withstand clothing
bends down to a radius of 37.5 mm without violating specifications.
Index Terms—Fabric antennas, conductive textiles, fabric sub-
strates, wearable computing.
I. INTRODUCTION
W
EARABLE computing is a fast growing field in appli-
cation-oriented research. Steadily progressing miniatur-
ization in microelectronics along with other new technologies
enables integration of functionality in clothing allowing entirely
new applications. The vision of wearable computing describes
future electronic systems as an integral part of our everyday
clothing serving as intelligent personal assistants. A wearable
computer is always on, does not restrict the user’s activities and
is aware of the user’s situation. It features easy-to-use inter-
faces and supports him unobtrusively with
in situ information
[1]. An important ingredient of such a system is the connection
to a wireless personal area network (WPAN). For this purpose,
we propose the use of purely textile antennas that guarantees
flexible and comfortable embedding into clothing as depicted in
Fig 1(a). A flexible embedding is important since bending radii
as small as 10 mm can occur in garments along the body, espe-
cially around joints. Fig. 1(b) qualitatively shows the curvature
Manuscript received January 12, 2006; revised April 14, 2005 and May 22,
2006.
I. Locher was with the Electronics Laboratory and the Wearable Computing
Laboratory, Department of Information Technology and Electrical Engineering,
Swiss Federal Institute of Technology (ETH) Zürich, 8092 Zürich, Switzer-
land. He is now with Sefar, Inc., 8803 Rüschlikon, Switzerland (e-mail: ivo.
locher@sefar.ch).
M. Klemm, T. Kirstein, and G. Tröster are with the Electronics Laboratory
and the Wearable Computing Laboratory, Department of Information Tech-
nology and Electrical Engineering, Swiss Federal Institute of Technology
(ETH) Zürich, 8092 Zürich, Switzerland .
Digital Object Identifier 10.1109/TADVP.2006.884780
Fig. 1. Textile patch antenna on the human body. (a) Mounted textile patch
antenna for a Bluetooth WPAN. (b) Curvature radii along the human body.
distribution along the human body. The darker a region appears,
the smaller the radius is. A curvature on a human body consists
of a superposition of bends in arbitrary directions. Only textiles
can follow these exposures. This property is called Drapability.
In contrast, a flexible PCB substrate such as polyamides allows
bending only in a single direction at a time. The material pro-
hibits a superposition of bends. Our textile antennas for wear-
able applications withstand such bending stresses while main-
taining their radiation specifications.
In this paper, we present four textile patch antennas for Blue-
tooth [2] in the frequency range from 2400 to 2483.5 MHz.
Our antennas feature a 10-dB bandwidth of 200 MHz on av-
erage. Even when bent around a radius of 37.5 mm resembling
a mounting on a human upper arm, Bluetooth specifications can
be assured. The planar structure with a maximal thickness of 6
mm maintains wearing comfort when integrated into clothing.
In contrast to a probe feed, our microstrip feedline does not in-
crease the height of the patch antennas. The fundamental com-
position of a patch antenna is shown in Fig. 2. More information
about antennas including an overview of antenna specific terms
is given in [3].
Using a microstrip feed not only guarantees a flat structure,
but also allows the assembly of electronic components directly
on the fabric in antenna proximity. By applying a similar tech-
nology as described in [4], autonomous systems with only few
nontextile components are feasible.
Prior to the antenna design, we carried out systematic inves-
tigations regarding the electrical performance of the deployed
materials, i.e., the conductive textiles and the fabric substrates.
1521-3323/$20.00 © 2006 IEEE
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778 IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 29, NO. 4, NOVEMBER 2006
Fig. 2. Composition of a patch antenna with microstrip feedline.
We were interested in the electrical resistance of the conduc-
tive textiles, its homogeneity, and its behavior under elongation
stress as well as the skin depth in the frequency range around
2.4 GHz. Second, we determined the permittivity of the fabric
substrates in the same frequency range with regard to the hu-
midity level.
Two of the four antennas feature linear polarization and two
of them possess left-handed circular polarizations (LHCP). Our
goal was to explore the tradeoffs between electrical performance
and maintaining textile properties. Because of the flexible na-
ture of textiles, we not only characterized the antennas in planar
shape, but also under defined bending conditions that resemble
those of a worn garment. In the following sections, we will also
comment on interesting textile-specific problems we encoun-
tered during the design process.
II. R
ELATED WORK
Most existing wearable computers still consist of bulky and
rigid boxes and are portable rather than wearable. Some ap-
proaches have been made to integrate electronic components
into clothing, but usually the textile itself just serves as a carrier
of conventional electronics. Another approach is to use textiles
for electrical functions such as transmission lines and sensors.
First, systematic studies of electrical properties of textile trans-
mission lines were carried out by Cottet et al. [5]. They proved
that signals with a bandwidth of more than 100 MHz can be
transmitted over textile transmission lines using wire pair con-
figuration.
The next step was the development of packaging and as-
sembly technologies for electronic components in order to
enable electrical circuits in textiles [4].
Wearable antennas presented by Salonen [6] and Massey [7]
are partially based on textiles possessing an inverted-F shape
that results in a stiff structure. Other textile antennas described
by Tanaka et al. [8] and Salonen et al. [9] are designed as rect-
angular patches with probe feed and linear polarization. An-
tennas such as presented in [10] only utilize fabrics as substrate,
whereas the patches and ground planes are copper foils.
In this paper, we advanced our work presented in [11] by ap-
plying materials with better electrical performances and by new
methods for fabrication of the antennas. All antennas provide
a microstrip feedline, whereas two antennas feature linear po-
larization and two antennas feature left-handed circular polar-
ization. Furthermore, we conducted more rigorous simulations
and measurements on the antennas regarding their performance
in planar shape as well as under bending.
Fig. 3. (a) Nickel-plated fabric. (b) Cross-section drawing.
III. T
EXTILE MATERIALS
Planar antenna structures are generally favored in wearable
applications since they can easily be integrated in clothing. In-
spired by the simple buildup of printed microstrip antennas, we
adapted this technology to textiles. Therefore, we needed an
electrically conductive fabric for the ground planes as well as
for the antenna patches. Second, we required a fabric substrate
with constant thickness and stable permittivity. An accurate de-
termination of the electrical parameters for the fabrics (dielec-
tric substrate and conductive textile) is crucial for correct an-
tenna simulations and agreement with measurements.
A. Electrically Conductive Fabric
For the purpose of a textile antenna design, a conductive
fabric needs to satisfy the listed requirements below. The closer
a fabric meets the requirements the better performs the antenna.
• A low and stable electrical resistance
of the
fabric is desired to minimize losses
• The (sheet) resistance must be homogeneous over the an-
tenna area. In other words, the variance of the resistance
must be small.
• The fabric should be flexible such that the antenna can be
deformed.
• A stretchable fabric supports the deformation behavior of
an antenna.
An electrical resistance of
is a reasonable choice for
conductive fabrics. From an electrical point of view, we would
recommend using copper foils. However, such foils lack of dra-
pability and elasticity that limits their use in clothing.
Among the many fabrics, we chose three variants for further
investigations concerning the stated requirements
1) a no-name nickel-plated woven fabric (the plating thick-
ness is about 250 nm);
2) a silver plated knitted fabric [12];
3) a silver–copper–nickel plated woven fabric [13].
1) Nickel-Plated Fabric 1): Although nickel shows excellent
resistance against corrosion, the nickel-plated fabric 1) turned
out to be not suitable for antenna applications since the plating
was applied after weaving process. Thus, the woven fibers are
not entirely plated where they cross each other. Fig. 3 shows
the fabric with removed weft fiber (vertical yarn). The unplated
sections can be recognized easily. As a result, a single fiber is
not continuously conductive. The electrical current cannot flow
along a single fiber, but instead must “hop” from wrap fiber to
weft fiber and vice versa over a very small overlapping plating
of crossing fibers. These transitions from fibers to fibers are the
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LOCHER et al.: DESIGN AND CHARACTERIZATION OF PURELY TEXTILE PATCH ANTENNAS 779
Fig. 4. (a) Conductive, knitted P130
"
y;
!
x
and (b) woven Nora fabric.
TABLE I
P
ROPERTIES OF
CONDUCTIVE
KNITTED AND
WOVEN FABRIC
main reason for the sheet resistance of about . Addition-
ally, measurements showed that the sheet resistance is inhomo-
geneous over the area.
As a consequence, we focused on the other two conductive
fabrics (2 and 3) for the antenna design. The fibers of these
fabrics were plated before weaving and knitting, respectively.
Therefore, their sheet resistance is much better as shown in
Table I, though still caused by the fiber-to-fiber transition resis-
tance mainly. For comparison, Table I also gives the equivalent
thickness of a pure silver foil to achieve equal conductivity as
the fabric. Pictures of the fabrics can be seen in Fig. 4.
2) Silver-Plated Knitted Fabric 2): The fabric consists of
entirely plated polyamide fibers. Given by the nature of knit-
ting, fabric 2) is viscoelastic featuring an approximated Young’s
modulus of
kPa at elongations of more than 30%.
Regarding mechanical properties, an antenna composed of this
fabric is bendable and, therefore, it can comfortably be inte-
grated into clothing since the fabric elongates where necessary.
From a manufacturing point of view, the elasticity is a draw-
back because precise shaping as well as assembly of the antenna
without warpage is difficult. The antenna shapes manufactured
finally achieved a geometrical accuracy of about
mm. The
warpage and bending have, of course, influence on the antenna
characteristics, i.e.,
. Second, they affect the sheet resistance
due to strain stress. We measured this resistance change de-
pending on elongation (and strain stress) in the
- and -di-
rection, which is shown in Fig. 5. Since the knitting possesses
a column-like structure [see Fig. 4(a)], the resistance behavior
differs in the orthogonal axes (inhomogeneity). Elongation in
the
-axis has only a minor effect on the resistance, whereas the
resistance increases significantly when elongated in the
-axis.
Both curves progress linearly for small elongations and flatten
out for larger elongations. This behavior is not yet entirely un-
derstood. It will supplementary perturb the antenna character-
istic in case of bending, especially when bending results in a
Fig. 5. Sheet resistance change due to elongation of the knitted fabric 2).
stretch in the -axis. In fact, the value given in Table I is valid
for a zero-force strain. Furthermore, relaxation after a stretching
of 10% lasts about 20 s. Thus, fast cycles between strain stress
and release result in a hysteresis regarding geometrical dimen-
sions as well as sheet resistance.
This material satisfies all the stated requirements but homo-
geneity in resistance when bent.
3) Silver-Copper–Nickel-Plated Woven Fabric 3): Com-
pared to the knitted fabric 2), fabric 3) features low elasticity
due to its woven structure [see Fig. 4(b)]. In contrast to fabric
1), the fibers of fabric 3) are plated before weaving resulting
in a low electrical resistance (Table I). The Young’s modulus
of about 2.4 GPa is mainly determined by the polyamide
material of the thread in fabric 3). Therefore, its shape can be
manufactured precisely, but bending of such an antenna is then
limited. Additionally, the edges of this fabric tend to fray easily
due to the nature of woven fabric. This effect can be minimized
by using manufacturing techniques explained in Section V. In
conclusion, the woven fabric possesses the best electrical prop-
erties among the three fabrics to build well-behaved antennas
with geometrical accuracies of about
mm.
Shape Precision of the Conductive Fabrics: The woven an-
tenna patch can only be shaped with the finite precision given
by the thread thickness. In fact, the thread pitch virtually “dis-
cretizes” the possible sizes of the conductive patches. Assuming
an antenna patch consists of
threads in one direction. The
patch width then corresponds to
times thread pitch. Notice
that the next possible width of the patch is either
times
thread pitch or
times thread pitch.
In case of the knitted antenna patch, instable dimensions due
to elasticity of the fabric limit precision.
Plating Thickness: The targeted frequency range
(2.4 GHz) features a theoretical skin depth of about 1.3
m
given by (1), where Ag plating is assumed. Thus, the skin effect
increases electrical resistance that causes additional losses.
Moreover, the plating thickness of the fabrics is restricted to
several hundred nanometers in order to maintain textile prop-
erties. This fact again increases damping in the final antenna
occluding the skin effect. The impact of this effect is illustrated
in Section VI:
(1)
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780 IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 29, NO. 4, NOVEMBER 2006
Fig. 6. Cross section of spacer fabric substrate.
where is skin depth, is frequency, is magnetic perme-
ability, and
is electrical conductivity.
B. Textile Substrates
The textile substrate provides the dielectric between the
antenna patch and the ground plane (see Fig. 2). It requires
constant thickness of few millimeters and low permittivity such
that the antenna patch becomes large and effects of absolute
tolerances in geometry diminish. Thus, we target a relative
tolerance of about 3% corresponding to the accuracy given in
Section III-A.
We chose a woolen felt with a thickness of 3.5 mm and a
polyamide spacer fabric with a thickness of 6 mm as substrates.
The felt with a density of 1050 g/m
is dimensionally more
stable and harder to bend, whereas the spacer fabric with
530 g/m
is lighter and elastic due to its knitting-based struc-
ture. Furthermore, the spacer fabric can easily be compressed as
might intuitively be guessed by looking at Fig. 6. Nevertheless,
the fabric totally recovers after release of the load. Such a fabric
can be integrated in jackets as depicted in Fig. 1(a) and function
as thermal insulation simultaneously.
We used techniques explained by Grzyb et al. in [14] in order
to extract the permittivity of the textile substrates. This method
utilizes scattering parameter measurements of two microstrip
transmission lines of different lengths. As a result, connection
discontinuities in the measurement setup can be eliminated by
computation. Finally, a permittivity
for the felt
and
for the spacer fabric was extracted at a
frequency of 2.4 GHz. The loss tangent of the felt is
, whereas the loss tangent of the spacer fabric is negli-
gible. Extensive humidity measurements covering a range from
20%RH to 80%RH within a temperature range of 25
Cto80 C
showed that permittivity variations are negligible compared to
the measurement uncertainty. This result closely corresponds to
the numbers given in [15] for the humidity effect on the permit-
tivity of air since both substrate permittivities are close to 1.
In contrast, flexible substrates such as polyamide and liquid
crystalline polymers (LCP) are foils. Therefore, they lack dra-
pabilty and are only bendable in one direction at a time. This
textile-atypical behavior of foils is a major drawback for inte-
gration into clothing.
C. Clothing Deformation
We defined two bending radii for the antennas aiming the
mounting on arms and legs in wearable applications. The
effects of bending on the electrical performance of antennas
can be seen in Section VI. Geometrically, the elongation
is connected with the bending radius according to (2) and
Fig. 7. Patch elongation due to bending.
Fig. 8. Patch antenna sketch.
Fig. 7 assuming constant material thickness and a centered
neutral phase. These assumptions are approximately valid for
the knitted fabric, whereas the inelasticity of the woven fabric
merely causes a small dent in the substrate. In other words, the
knitted fabric follows the curve of the bending while the woven
fabric stays straight:
(2)
IV. A
NTENNA DESIGN
A. Theory and Simulation
Targeting wearable applications, a planar structure of the an-
tenna is important, but also is a planar antenna feed. Thus, we
discarded probe feeds, which actually eases the antenna design.
Instead, we use microstrip feed lines. In the design of the mi-
crostrip antenna patches, transmission line model equations (3)
and (4), such as given in Balanis [16], were applied for a rough
estimate. The correspondence of the formula symbols to the ac-
tual antenna shape is shown in Fig. 8.
is the resonance fre-
quency of the antenna, and
is the extended distance of the
principal E-plane. For further information, please consult Bal-
anis [16]:
(3)
(4)
We chose a 75-
microstrip feed line for the spacer fabric
since a 50-
line would become about 28 mm wide, leading to
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![TABLE III DIMENSIONS OF THE FOUR ANTENNAS INCLUDING TOLERANCES [MILLIMETERS]](/figures/table-iii-dimensions-of-the-four-antennas-including-22yg7wve.webp)
