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Flexible Antennas Design and Test for Human Body
Applications Scenarios
João Vicente Faria
Instituto Superior Técnico
Universidade de Lisboa
Lisbon, Portugal
Abstract— This work presents a flexible Coplanar Waveguide
(CPW)-fed UWB monopole antenna used for applications in the
proximity of the human body. The antenna structure consist of a
leaf-shape radiating patch and a symmetric ground plane both
printed on the same side of the dielectric substrate. The proposed
antenna has been designed, and subsequently fabricated on two
different substrate materials (standard commercial paper and
Kapton®) using two distinct fabrication processes. The
simulated and measured results for the return loss (|S11|),
radiation patterns and total and radiation efficiencies are in good
agreement, showing that the antenna has a very good
performance in flat at condition. The antenna achieves an
impedance bandwidth from 2.9 GHz to beyond 12 GHz,
satisfying the UWB standards (3.1-10.6 GHz).
In order to easily conform to the body surface, the
suggested UWB antenna was designed to meet the requirements
of wearable devices, such as being flexible, compact and
mechanically robust. Thus, the influence of bending and
crumpling deformations on the antenna Radio-frequency (RF)
performance has been investigated. The simulated and measured
results show that the proposed antenna functions satisfactorily
under smooth bending and crumpling conditions. However, for
more severe deformation scenarios the antenna presents a
considerable performance degradation.
Finally, the Electromagnetic (EM) behavior of this
antenna has been examined in the vicinity of a human upper arm.
Regarding the computational simulations, four human arm
models with different geometric and dielectric properties have
been presented. The antenna return loss, total efficiency, and
radiation patterns have been analyzed for different antenna-body
distances. Measurements with a real human arm have also been
carried out with the antenna placed 3 mm and 6 mm away from
the human arm. Furthermore, the antenna Specific Absorption
Rate (SAR) was evaluated, showing values considerably below
the regulatory standards.
Keywords—Flexible antennas, Coplanar Waveguide
(CPW), Ultra-Wideband (UWB), wearable applications,
return loss, impedance bandwidth, bending, crumpling,
Specific Absorption Rate (SAR).
I. INTRODUCTION AND STATE OF THE ART
Over the last years, our World has been in a paradigm
transformation related to the emergence of a new technological
trend based on new information and communication systems [1].
Therefore, telecommunications have become a significant part
of everyday societies life-style, promoting the share of
information (voice, data, photos, videos) between people
through computers and other mobile devices. More recently,
there have been an increasing demand and progress in Wireless
Communication Systems. This segment of communications
industry are currently established in many developing countries,
replacing gradually the traditional Wired Communication
Systems [1].
In these types of communication systems, there are certain
issues like channel characterization, antenna transmitted power,
and the behavior of the EM fields near the human body which
have to be well-known by the antenna designers. Once the
human body has a high dielectric permittivity and low
conductivity at microwave frequencies, body tissues are an
uninviting and often hostile environment for the wireless signal
propagation. In general, antennas placed near the human body
suffer from reduced efficiency, due to electromagnetic
absorption in human tissues, radiation pattern deformation, and
variations in impedance at the feeding point.
Flexible antennas are developed making use of substrates that
can be easily integrated in irregular surfaces without losing its
functional features. The emerging of new materials, with very
specifics chemical properties, and new manufacturing processes
has led to the development of dielectric substrates that promise
good results regarding flexibility, efficiency, weight, and
reproducibility of the antennas.
The choice of the dielectric on which the conductor is built
influences the antenna's behavior regarding impedance
bandwidth, radiation efficiency and gain. Thickness, dielectric
permittivity of the material and the loss tangent must be
measured and controlled so as to reach the specifications of the
different applications.
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The main objective of this work is to study and analyze a
flexible antenna for applications in the close proximity of the
human body. Since the antenna is designed to be mechanically
flexible, its capability to flex and function properly under
bending and crumpling conditions must be evaluated. The
interaction between the human body and the proposed flexible
antenna, and the influence on its performance, is also an issue to
be studied along this work. The proposed antenna is based on
[2], and its structure consists on a leaf-shaped monopole antenna
designed to operate at the entire UWB (3.1 GHz to 10.6 GHz)
frequency range. This work aims to investigate the possibility of
designing the proposed antenna structure using different
substrates. The designed antenna is optimized using CST
Microwave StudioTM and then fabricated through distinct
fabrication processes.
II. DESIGN OF A PRINTED CPW-FED UWB ANTENNA
A. Substrate Material Selection
Since the antenna is supposed to be mechanically flexible,
the suggested materials must present high malleability and
robustness levels, in order to tolerate certain deformation
scenarios, such as bending, crumpling, twisting and others. In
addition, these flexible materials should exhibit favorable
electromagnetic properties (loss tangent and relative
permittivity) to provide an easy integration with RF circuits and
to achieve the performance requirements of modern flexible and
wearable electronics. After reviewing a wide range of flexible
antennas reported on the literature, organic paper and Kapton®
polyimide film were selected as the candidates for the UWB
monopole antenna dielectric substrate materials. A comparative
study of the dielectric properties of the standard office paper and
Kapton® polyimide is presented in Table 1.
Table 1 - Kapton® and paper properties.
B. Antenna Geometry
The printed CPW-fed UWB monopole antenna proposed in
this work has an overall size of 37.5 mm x 33 mm (L x W). The
antenna geometry consists of a four-step monopole radiator and
the symmetrical rectangular ground plane (13 mm x 14.5 mm),
both placed on the same side of the substrate. The radiating patch
has a leaf shape, composed by a semielliptical base with a
diameter of 30 mm added to a centered-circle and two sided-
semicircles on its top. Moreover, a 50Ω coplanar waveguide of
3.2 mm width and 12.8 mm length is used to feed the radiating
patch. After performing an optimization process with CST
Microwave Studio™ software, the final antenna design
parameters are illustrated in Figure 1.
Figure 1- Geometry and design parameters of the proposed
UWB antenna in mm.
C. Fabrication Method
The fabrication process of flexible and wearable antennas
should guarantee good agreement with the design and simulation
results. The final antenna design was fabricated by two different
fabrication methods. The first step of the process was shared by
both methods and consists on exporting the antenna layout from
CST to AutoCAD software and design the production mask with
an appropriate scale factor (1:4 mm).
In the first fabrication procedure the metal patterns of the
antenna are photolithographically printed on the Kapton® film
substrate using the technology available in Instituto de
Telecomunicações (IT) RF laboratory.
The second fabrication method consists on attaching the
radiating patterns of the antenna directly on the paper substrate.
A CAMM-1 Servo X-24 Vinyl Cutter from Roland [3] available
in (EDP) FabLAB was used to cut off the antenna metal layer on
a flexible adhesive copper tape. The copper that is not part of the
antenna circuit was removed and the resultant radiating pattern
was then manually glued on a commercial sheet of paper. Both
prototypes were connected with a 50 Ω SMA connector, which
was carefully soldered on the antenna final prototype. Figure 2
show both fabricated antenna prototypes.
Figure 2 - Prototype I - Kapton® and Prototype II - Standard
office paper.
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D. Final Prototype Selection
After obtaining the different prototypes for the proposed UWB monopole antenna, the next step is to measure the free-space return loss for both designs.
From the measured data presented in Figure 3, the prototype I
(Kapton®) shows a -10 dB impedance bandwidth that clearly
covers the targeted 3.1-10.6 GHz UWB standard, while the
prototype II presents an upshift from 2.9 to 4.15 GHz in the
lower -10 dB frequency edge. These unexpected results may be
due to the lack of precision in the substrate material (paper)
characterization, the low mechanical resolution of the cutter, and
the poor accuracy of the fabrication process. Taking into account
the presented results, prototype I was selected as the final
prototype of the monopole UWB antenna.
Figure 3 - Measured antenna |S11| using paper and Kapton®
substrates.
E. Free-Space Performance
In this subsection, the performance of the antenna presented
in Figure 4 is evaluated in free-space scenario. The results for
the return loss, |S11|, operating band, radiation patterns and
efficiencies of the antenna are fully studied.
Figure 4 - Antenna simulation model (left) and final antenna
prototype (right).
1) Return Loss and Operation Band
After the designing stage, the performance of the adopted
antenna was evaluated in free-space scenario. From Figure 5, it
can be seen that the UWB band was fully covered for both
simulated and measured cases. The lower edge of the operation
band corresponds to the same frequency point in the simulated
and measured |S11| curves. As can be seen from measurements,
some ripples occur particularly at the lower frequencies, due to
the RF cable effect [4].
Figure 5 - Simulated and measured |S11| curves in free-space
2) Radiation Patterns
Figure 6 presents the 2D radiation patterns of the proposed
antenna in yz and xz planes. The radiation patterns presented
below have been computed at three frequency points (3.1 GHz,
6.05 GHz and 10 GHz) using the CST Microwave Studio™.
Figure 6 - 2D simulated radiation patterns at 3.1, 6.05, and
10GHz (yz-plane on the left and xz-plane on the right).
It is observed that the yz-plane patterns are almost
bidirectional with large back lobes and looking like a doughnut
or a slightly pinched doughnut at the three selected frequency
points. In its turn, the xz-plane radiation pattern is
omnidirectional at lower frequencies (3.1 GHz). Moreover, the
xz radiation pattern is close to a multi-lobe shape at the higher
frequency of the band (10 GHz).
3) Radiation and Total Efficiency
The antenna radiation and total efficiencies were simulated in free-space, Figure 7.
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Figure 7 - Simulated radiation and total efficiencies
The proposed antenna presents a very high efficiency at the
whole UWB band. The total efficiency is at least 95%, while the
radiation efficiency is very close to 100%. At some frequency
points the radiation efficiency slightly exceeds 100%, which is a
recurrent problem when the simulations are performed using
Computational Electromagnetics (CEM) tools [5].
The total and radiation efficiency of the proposed coplanar-
fed UWB antenna (Figure 8) was measured using a cavity-based
method. The adopted measurement procedure was based on an
extension of the generalized Wheeler Cap method [6] for
evaluating the efficiency of small UWB antennas.
Figure 8 - Estimated radiation and total efficiency.
Despite some missing data points (especially at higher
frequencies) that correspond to the invalid efficiency values
caused by cavity resonances, there is a reasonable agreement
between the simulations and measurements results of the
antenna efficiencies.
III. DEFORMATION EFFECTS ON THE ANTENNA
PERFORMANCE
In case of wearable antennas it is quite difficult to maintain a flat
antenna surface since these antennas should conform their shape
to the surface on which they are placed, without hindering the
user's movements and comfort. Moreover, the analysis of
antenna performance parameters under certain deformation
scenarios is crucial to guarantee a reasonable operation in the
close proximity of the human body. In this section, the proposed
UWB antenna is tested under various bending and crumpling
scenarios in order to investigate its potential behavior in
wearable applications.
A. Bending
The bending of the proposed antenna structure was
performed in two different directions: bending along the x–axis
(horizontal) and bending along the y-axis (vertical). In order to
mimic these bending deformations in CST Microwave StudioTM,
a vacuum cylindrical structure of radius R = 10 mm and R = 6
mm was directly attached on the antenna back-side substrate.
Then, the substrate layer and the radiating elements of the
antenna were separately bent around the cylinder. Figure 9
shows the simulation models for horizontal (x-axis) and vertical
(y-axis) bending of the proposed UWB antenna.
Figure 9 – Simulation models for horizontal (left) and vertical
(right) bending.
To recreate the bending scenario performed in CST
Microwave StudioTM, a 10 mm radius manufactured foam
cylinder was used. The antenna structure was directly bent
around the cylinder with the aid of two foam rings, as shown in
Figure 10.
Figure 10 - Bending measurements setup – horizontal bending
(left) and vertical bending (right).
1) Return Loss and Operation Band
Since the performance parameters are likely to be affected
during the bending operation, the return loss and the -10 dB
operation band of the proposed UWB antenna must be
evaluated under these deformations scenarios. The following
subsections present the comparison between the simulated and
measured |S11| when the proposed UWB antenna was
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horizontally and vertically bent. This comparative analysis was
performed using a 10 mm bending radius.
a) Horizontal Bending (x-axis)
Figure 11 - Comparison between the simulated and measured
return loss for the horizontal bending scenario (R=10mm).
As it can be observed from Figure 11, the return loss of the
CPW-fed monopole antenna is slightly affected under x–axis
bending. For both simulated and measured results, some marked
resonances become evident on the return loss curves. Moreover,
the lower edge of the -10 dB operation band is shifted towards
higher frequencies.
b) Vertical Bending (y-aixs)
Figure 12 - Comparison between the simulated and measured
return loss for the horizontal bending scenario (R=10mm).
The simulated and measured return losses of the antenna in
the y-axis bending direction are shown in Figure 12 exhibiting
good matching over the whole UWB band. In this case, only
small resonance shifts and minor bandwidth variations can be
observed. In simulations, an accentuated resonance is observed
at approximately 7.7 GHz. For both simulations and
measurements a wide -10 dB bandwidth is achieved and thus the
UWB demands are perfectly reached.
2) Total Efficiency
The simulated total efficiency of the antenna was also
evaluated when the antenna is bent around the x-axis and around
y-axis. For this, two curvature radius are considered: R = 6 mm
and R = 10 mm.
a) Horizontal Bending (x-axis)
Figure 13 illustrates the simulated total efficiency when the proposed UWB is bent around its x–axis at two different curvature radius. According to the results, it is obvious that this bending scenario has a strong influence on the simulated total efficiency of the proposed CPW-fed antenna.
For a smooth bending deformation (R = 10 mm), the total efficiency remains acceptable (between 80% and 100%) for the entire UWB frequency range. Otherwise, when the antenna is subjected to a bending radius of 6 mm, the simulated total efficiency is considerably reduced
b) Vertical Bending (y-aixs)
In Figure 14, which is presented below, the comparison of
total efficiencies is presented for a vertical bending deformation.
It can be attended that, regardless of the bending radius adopted,
the total efficiency of the proposed UWB antenna does not
change.
Figure 14 - Simulated total efficiency for the vertical bending
scenario.
Figure 13 - Simulated total efficiency for the horizontal
bending scenario.
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B. Crumpling
After analyzing the bending effects on the antenna
performance, the next step is to test the UWB monopole antenna
under crumpling conditions. Taking into account the small size
of the proposed antenna, crumpling deformations are not
expected to occur in a realistic usage scenario. This particular
deformation was only considered for the simulations.
The simulation method used to reproduce the crumpling
deformation is very similar to the one used for bending cases.
The antenna was laid over a cosine-type surface with different
crumpling amplitudes (CA = 1 mm; CA = 2 mm and CA = 4
mm), Figure 15.
This section describes the entire simulation procedure, and
presents the simulated results for the return loss and total
efficiency of the crumpled antenna.
Figure 15 - Antenna crumpling.
1) Return Loss and Operation Band
The simulated reflection losses of the antenna under various
crumpling cases are shown in Figure 16. It can be seen that
modifications on the antenna's impedance performance are more
pronounced, as the crumpling amplitude gets higher. Thus, the
bigger the crumpling amplitude is, the narrower is the antenna's
operation band. Among all of the considered crumpling
situations, only the CA = 1 mm case covers satisfactorily the
targeted 3.1-10.6 GHz UWB standard.
Figure 16 - Simulated return loss curves for crumpling
scenarios.
2) Total Efficiency
Figure 17 depicts the effect of the crumpling deformations on
the antenna simulated total efficiency. It can be seen that when
the crumpling amplitude increases from CA= 1 mm to CA= 4
mm, the antenna becomes less efficient.
Figure 17 - Simulated total efficiency for crumpling scenarios.
IV. ANTENNA PERFORMANCE IN THE HUMAN ARM
PROXIMITY
With the recent development of body-centric applications,
wearable antennas have become an essential part of WBAN
communication systems. Their design process and performance
evaluation are quite difficult, due to the presence of the user's
body [7]. The electrical characteristics of different body tissues
have a considerable effect on the performance of antennas that
operate in close proximity of the human body. Thus, one of the
main objectives of this section is to understand the interactions
between the human body and the EM waves radiated from
flexible antennas used in a wearable scenario. Studying the
nature of dispersive human tissues and their electrical and
geometrical properties, and how the proposed UWB monopole
antenna performance parameters are changed by introducing an
antenna-body distance (d) are the general guidelines of this
section.
1) Human Arm Models
Since the human body is a non-homogenous and multi-layer
medium, it is mandatory to study the dielectric properties of each
tissue comprised in the models presented above. Accurate
modeling of a real human arm requires the dispersion
characterization of each body tissue, on the operating frequency
range of the suggested antenna. In Figure 18, three four-layered
simplified models, and a homogenous arm model are shown.
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Figure 18 - Human arm models: elliptical, rectangular, flat,
and homogenous (from left to right).
The three human equivalent arm models have four dispersive
tissue layers which are skin with 2 mm thickness, fat with 4 mm
thickness, muscle with 35 mm thickness and bone with 35 mm
thickness. The electric properties (relative permittivity and loss
tangent) of each human arm tissue were defined in the whole
UWB frequency range.
The proposed homogenous human arm model consists of a
single dispersive material with an electrical conductivity (σ) of
1 S/m and a relative permittivity (εr) of 42. The electrical
properties of this arm model were obtained from [8].
2) Return Loss and Operation Band
In order to investigate the impedance performance of the
antenna near the human body, simulations were carried out using
the four simplified human arm models previously introduced.
Furthermore, in a laboratory environment, the proposed antenna
was tested in the presence of a real human arm.
For a comparative analysis, the simulated |S11| curves using
the four models are compared with the measured on-body return
loss curves considering a separation gap of 3 and 6 mm. The
free-space return loss curve is also plotted as a reference for
human arm results.
a) Antenna placed 3 mm off the models (d = 3 mm)
Figure 19 - Simulated and measured |S11| curves in the
proximity of the human arm at d = 3 mm.
For a distance between the antenna and the skin layer equals
to 3 mm, the elliptical model results show that the lower -10 dB
band edge is shifted downwards from 2.9 to 2.5 GHz in relation
to the free-space curve. Therefore, the simulation for this human
arm model can exceptionally cover a much wider frequency
band starting from 2.5 GHz to 10.6 GHz and beyond. For all the
other adopted arm models the lower edge of the -10 dB operation
band is slightly shifted to the right. It is also observed that, when
the antenna-model space (d) is 3 mm, some accentuated
resonances appear in the return loss curves, e.g. 5.5 GHz and 9.2
GHz. Although reasonable good agreement between the
simulated and measured results was achieved, some
discrepancies between the results were detected.
a) Antenna placed 6 mm off the models (d = 6 mm)
Figure 20 - Simulated and measured |S11| curves in the
proximity of the human arm at d = 6 mm.
By increasing the distance between the UWB antenna and the
human arm models to d = 6 mm the frequency shift on the lower
-10 dB band edge diminishes. As was expected, the antenna
impedance bandwidth starts to widen due to the less influence of
the human body tissues. It can also be seen that the magnitude
of the resonances becomes lower as the antenna-model distance
increases. Just as the d = 3mm case, the measured results are in
accordance with the simulated results.
3) Total Efficiency
The simulated total efficiency of the antenna is also presented
assuming the four human arm models kept at distance d of 3 and
6 mm from the antenna. The results were obtained in CST
Microwave StudioTM for 2-12 GHz frequency range.
a) Antenna placed 3 mm off the models (d = 3 mm)
Figure 21 - Simulated antenna total efficiency for d= 3 mm.
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As it apparent from the simulated results, the total antenna
efficiency is significantly deteriorated in close proximity of the
human body tissues. In this case (d = 3 mm), the simulated total
efficiency remains below 70% for the entire UWB frequency
range.
b) Antenna placed 6 mm off the models (d = 6 mm)
Figure 22 - Simulated antenna total efficiency for d= 6 mm.
When the antenna-body space is 6 mm, the presented UWB
monopole antenna still achieves a good total efficiency
(ranging within 70% and 90%) for almost the whole UWB
band. Comparing the results for each human arm model, it is
observed that the shape of the model has an insignificant
influence on the antenna total efficiency results. However, the
simulated results obtained for the homogenous model are
slightly different especially for the lower frequencies.
4) SAR (Specific Absortion Rate)
The whole evaluation of the SAR was performed in CST
Microwave Studio™ with the proposed compact UWB antenna
placed 3 mm off to the four-layered flat arm model. The flat
model was chosen for its simplicity and due to its simulation
time which is much shorter when compared with the other
suggested human arm models. The total mass of the chosen
model was then defined as 0.82 kg considering the density
values assigned to each tissue that comprises the arm model.
Since the quantification of SAR depends on the operating
frequency of the antenna, three frequency points (3.1 GHz, 6.05
GHz and 10 GHz) were chosen for this simulation over the entire
UWB band.
Furthermore, for 3.1-10.6 GHz UWB applications, the FCC
limits the spectral power density to -41.3 dBm/MHz, which
roughly corresponds to 8 x 10-5 mW of total radiated power per
MHz. Considering the antenna impedance bandwidth
approximately 9 GHz (3 GHz to 12 GHz), the input power is
normalized as 1 mW for all the SAR simulations.
Assuming the ICNIRP limit for the antenna SAR on human
limbs (4 W/kg) as a reference, only the maximum SAR results
for 10 g of tissue are considered for this analysis. The computed
values (Table 2) show that the maximum SAR of the proposed
UWB remains below this standard, for every case. When the
antenna is located 3 mm far from the model, the highest
maximum SAR value (10 g of tissue) occurs at 3.1 GHz
(0.0250931 W/kg). Thus, it can be noticed that, for a fixed
distance d, the maximum SAR of the antenna decreases as the
frequency increases.
V. CONCLUSIONS AND FUTURE WORK
A. Main Conclusions
In this paper a CPW-fed UWB antenna was totally designed
and developed. The antenna structure was simulated with
standard commercial paper and Kapton® as dielectric substrates
showing, in both cases, a -10 dB simulated impedance
bandwidth that totally covers the UWB frequency range. After
designing the optimized antenna geometry, two prototypes were
produced, through two distinct fabrication processes. After
testing these two prototypes in free-space, it was found that the
lower limit of the operation band was slightly shifted and thus
the measured return loss of prototype II (paper-based antenna)
does not comply |S11| < -10dB criterion for the entire UWB
operation band.
The free-space performance of the proposed UWB antenna
was analyzed and investigated by using numerical simulations
and measurements. The simulated results were divided in four
categories: the return loss curves (|S11|), operation bands,
efficiency (radiation and total) and radiation patterns (gain).
Otherwise, measurements concerned only the return loss,
operation bands and efficiency. Generally, the measured results
were in agreement with the simulated results in terms of return
loss and the UWB operation band was fully covered for both
simulated and measured |S11| curves.
Since one of the main goals of this work is to design a
flexible antenna to be integrated within flexible electronic
devices, bending deformations were also considered. Simulated
and measured results have shown good RF performance under
bending scenarios. However, for more sever bending conditions
the antenna parameters were strongly affected. It was noticed
from the results that vertical bending deformations have no
influence in the obtained results.
In order to evaluate the performance of the antenna under
crumpling scenarios, a cosine-type surface was built in CST
Microwave StudioTM. Three different crumpling amplitudes
were considered for the simulations (CA=1/2/4mm). In all
crumpling cases, the reflection coefficient and hence impedance
Table 2 - Simulated results for the Maximum SAR, Total SAR, and
Average Power for 1g and 10g of tissue considering a
distance between the antenna and the at arm model of 3
mm.
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bandwidth of the antenna has changed. As the crumpling
amplitude increases, the more noticeable are the effects on the
antenna's performance. The antenna total efficiency under
crumpling was reduced with the increasing of the crumpling
amplitude.
Finally, the proposed UWB antenna design was measured
and simulated near the human body. Four human arm models
were fully characterized for this purpose. In order to understand
how the body tissues influence the antenna parameters,
simulations were carried out using these human arm models. In
a real usage scenario, the proposed antenna was tested in the
close proximity of a real arm. It was found that the human arm
proximity strongly influences the antenna impedance matching.
As was expected, the antenna total efficiency was also reduced
due to the absorption by the human arm tissues.
B. Future Work
Although the antenna design process was carefully studied
during this work, there are still some aspects about the substrate
material selection and fabrication methods that need to be
discussed in more detail. There are a large amount of new
available substrate materials, such as silicone based polymers
(PDMS and LCP), e-Textiles and microfluidics (fluid metal
alloy injected into microfluidic channels comprising a silicone
polymers) [9] that can be used as dielectric substrates for the
proposed antenna design.
Moreover, some new fabrication processes should be
adopted for future work. The 3D printing is a relatively recent
and high technological method, which allows to create complex
objects and cavities of various shapes and sizes from a digital
model [10]. This layer-by-layer approach to produce 3D printed
antennas can be proposed in a future study as a promising
solution to fabricate the proposed UWB antenna structure.
Furthermore, the inkjet printing process is also a good candidate
for the antenna's fabrication method due to its capability of
printing flexible circuits quickly and with a minimal cost. The
method used for the fabrication of the prototype II is still a
poorly developed technique. This proceeding allows the antenna
designers to cut out copper circuits from adhesive backed copper
foil using a vinyl cutter. The desired metallic patterns can be
attach into different substrate materials. Therefore, the research
on this area should also be carried out for future work.
As future work, the antenna design presented during this
work can be tested in other relevant scenarios, such as new
deformations (twisting), under the movement of the human body
and under different environmental conditions (humidity, heat).
REFERENCES
[1] M. Fransman, “Evolution of the telecommunications industry into the internet age," Jets Paper - University of Edinburgh Institute for Japanese Eurpean
Technology Studies, 2000.
[2] M. Koohestani, N. Pires, A. Skrivervik, and A. Moreira, “Performance study
of a uwb antenna in proximity to a human arm," Antennas and Wireless
Propagation Letters, IEEE, vol. 12, pp. 555-558, 2013.
[3] Roland, “Camm-1 servo gx-24 vinyl cutter." [Online]. Available:
http://www.rolanddga.com/products/cutters/gx24/.
[4] T. Hertel, “Cable-current effects of miniature uwb antennas," in Antennas
and Propagation Society International Symposium, 2005 IEEE, vol. 3A, July 2005, pp. 524-527 vol. 3A.
[5] N. Pires, “Small antennas as wireless system components," Ph.D. dissertation, L’Ecole Polytechnique Féderale de Lausanne and Instituto Superior
Técnico, August 2014.
[6] N. Pires, C. Mendes, M. Koohestani, A. Skrivervik, and A. Moreira, “Novel
approach to the measurement of ultrawideband antenna efficiency," Antennas
and Wireless Propagation Letters, IEEE, vol. 12, pp. 1512-1515, 2013.
[7] P. Hall and Y. Hao, “Antennas and Propagation for Body-centric Wireless Communications”, ser. Artech House antennas and propagation library. Artech
House, 2006.
[8] M. Rutschlin, “Body wearable antennas," CST EUROPEAN USER
Conference, April 2013.
[9] G. Hayes, J.-H. So, A. Qusba, M. Dickey, and G. Lazzi, “Flexible liquid metal
alloy (egain) microstrip patch antenna," Antennas and Propagation, IEEE
Transactions on, vol. 60, no. 5, pp. 2151-2156, May 2012.
[10] S. S. Bukhari and W. G. Whittow, “Flexible 3-d printed substrates for
antenna applications," Progress in Electromagnetic Research Symposium (PIERS), p. 1-5, 2013.