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2016 International Conference on
Mathematical Methods in Electromagnetic Theory
Design of Fractal Loop Antenna with Integrated
Ground Plane for RF Energy Harvesting
Miaowang Zeng
School of Electronics and Information Technology
Sun Yat-sen University
No. 132 Waihuan East Road, Panyu, Guangzhou, China
zengmw@mail2.sysu.edu.cn
Andrey S. Andrenko, Xianluo Liu, Hong-Zhou Tan
and Bo Zhu
SYSU-CMU Shunde International Joint Research Institute
No. 9 Eastern Nanguo Road, Shunde, Guangdong, China
andrey_andrenko@sdjri.com
Abstract— In this paper, we present a novel impedance
matching method for loop antenna by integrating the in-loop
ground plane (ILGP). Compact Koch fractal loop is designed for
ambient radio frequency (RF) energy harvesting applications at
GSM1800 band. Comparing to the conventional matching
method for loop antenna, the proposed ILGP doesn’t require any
additional layout and can easily match the impedance to 50
Ohms. The in-loop ground plane makes antenna more compact
and provides high total efficiency. The performance of the
proposed antenna has been verified by simulation and
measurements. It should be noted that the ILGP matching
method is useful for various planar loop antennas utilized for
different wireless communication applications.
Keywords—Loop antenna; Koch fractal; impedance
matching; in-loop ground plane; RF energy harvesting
I. INTRODUCTION
Recently, RF energy harvesting has become a focus of
intense research for enabling the advances in Internet-of-
Things technologies. Ever-growing deployment of modern
wireless communication infrastructure provides various RF
energy sources in ambient environment, such as mobile
cellular towers, Wi-Fi, and TV broadcasting signals. Urban
and semi-urban environment RF energy density in London
was measured and reported in [1]. Outdoor ambient RF energy
spectral measurements were also carried out in Shunde,
Guangdong, China and presented in [2]. Both survey results
demonstrate that the GSM1800 bands have relatively high
power level and are promising for ambient RF energy
harvesting. Rich multi-path environment of the signals from
the cellular base stations combined with an uncertainty of their
position makes an omnidirectional antenna a preferred choice
for ambient RF energy harvesting. This paper presents the EM
analysis of a novel loop antenna integrated with a ground
plane to be specifically utilized in the GSM1800 band
rectenna at the next design stage.
Fractal geometries have been widely used in antenna
design because of their self-similarity and space-filling
characteristics leading to antenna miniaturization and multi-
frequency behavior [3, 4]. When the circumference of loop
antenna is about one wavelength ( ), its maximum radiation
direction is normal to the plane of the loop [5]. This feature
makes the one wavelength loop antenna suitable for many
applications, such as wireless communications, Yagi-Uda
arrays, and RF energy harvesting antennas. However, the loop
radiation resistance is about 200 Ohms [5, 6], which
introduces a severe insertion loss when fed with a 50-Ohm
port. The impedance can be matched by placing a large
reflection ground plane and selecting a proper substrate
thickness [5]. In such a case, the radiation pattern is made
unidirectional and reflection ground plane requires extra area
as compared with original loop antenna thus violating the
miniaturization requirement of the antenna design.
To solve these problems, an impedance matching method
is proposed by integrating in-loop ground plane in the fractal
loop antenna. With proper balanced port to microstrip line
transition design, the integrated ground plane adjusts the
impedance to about 50 Ohms. In contrast to the reflection
ground plane matching method, the integrated in-loop ground
plane loop antenna exhibits the same radiation pattern and
avoids larger layout. The proposed ILGP matching method has
been validated by the EM simulation and measurements. The
antenna has been designed and optimized in ANSYS HFSS
environment and fabricated on printed circuit board (PCB).
The organization of the paper is as follows. Section II
describes the configuration of a proposed Koch fractal loop
antenna. Section III discusses and analyzes the performance of
proposed ILGP impedance matching method. Simulation and
measurement results are presented in Section IV while Section
V draws a short conclusion.
II. PROPOSED KOCH FRACTAL LOOP ANTENNA
A. 2nd order Koch fractal loop antenna design
Based on the Koch fractal loop antenna proposed for RFID
applications [4], a 2nd order Koch fractal square loop antenna
has been modified and designed for RF energy harvesting at
GSM1800 band. Fig. 1 shows the initial layout of fractal loop
antenna with the size of 41mm X 41mm. The antenna is based
on square loop shape as it is divided into eight 2nd order Koch
fractal elements, three 2.5mm-long straight strips and two
balanced feed strips. 2nd order Koch fractal element is chosen
because of compactness of loop layout and PCB fabrication
precision limitation. Each fractal basic segment has 1.62mm
978-1-5090-1956-4/16/$31.00 ©2016 IEEE
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2016 International Conference on
Mathematical Methods in Electromagnetic Theory
length and 0.8mm strip width. Two balanced feed strips are
1.5mm-wide and directed inside the loop along Y axis. The
total electric length of the antenna printed on a 0.8mm-thick
Teflon substrate and shown in Fig. 1 is
1.3L
at the design
frequency 1.81GHz. Discrete balanced 50-Ohm port is used in
HFSS simulation.
Fig. 1. Layout of 2nd order Koch fractal loop antenna.
After optimization the simulation results show that the
return loss of the antenna shown in Fig.1 is worse than -10dB
at the resonant frequency. It means there would be an
impedance mismatch with a 50-Ohm rectifier circuit or feed
port causing energy loss. This phenomenon is predictable
taking into account the relationship between the wavelength
and loop perimeter. The antenna input impedance can be
decreased by placing a large ground reflector at a distance
h
below the loop. Theoretically, by selecting a proper distance
h
the impedance can be adjusted to 50 Ohms. However, if the
antenna is fabricated on the top surface of a dielectric
substrate and ground plane is on the bottom, the necessary
choice of a thickness of the commercially available substrates
is quite limited. Besides, the required large ground plane
increases the total dimensions of antenna structure and thus
makes the antenna prohibitively large.
B. Proposed in-loop ground plane impedance matching
Here, we propose an impedance matching method by
integrating an in-loop ground plane (i.e. occupying an area
inside the loop) on the bottom surface of thin dielectric
substrate. Fig. 2 shows the layout of fractal loop antenna
designed with the proposed ILGP. Compared to an initial
layout shown in Fig. 1, the balanced strips are now modified
to form a transition to a coaxial port used for prototype
measurements. The left strip is shortened (i.e. connected by
via) to the ground plane. The right strip is connected to an
inner cylinder of a coaxial port. The ILGP is printed on the
bottom surface of a substrate and composed of 25 X 25mm
square and triangle shape. The strips and triangle section form
a compact balun transition. By optimizing the side length of
ILGP square, input antenna impedance can be easily matched
to 50 Ohms.
To evaluate the effect of proposed ILGP, the return loss of
fractal loop antenna with and without ILGP has been
calculated and presented in Fig. 3. The dashed line curve
indicates the antenna depicted in Fig. 1 while and the solid
line curve corresponds to the antenna presented in Fig. 2. Note
that the two S11 curves are calculated for the same fractal loop
dimension with only difference being the ILGP and feed port
modification. The return loss of loop antenna without ILGP is
larger than -10dB at resonant frequency 1.9GHz. For the
proposed antenna shown in Fig.2, the S11 is below -30dB at
resonant frequency 1.81GHz. It turns out that ILGP helps
matching the input impedance of loop antenna to nearly 50
Ohms. In addition, the antenna with ILGP has lower resonance
frequency, which also illustrates how ILGP makes the antenna
layout more compact as compared to the initial fractal design.
Fig. 2. Layout of the proposed ILGP integrated antenna.
Fig. 3. Calculated return loss of proposed fractal loop antenna with and
without ILGP.
III. ANALYSIS OF PROPOSED ANTENNA
In order to analyze the ILGP effect on the input impedance
of fractal loop antenna, the S11 Smith chart data of antennas
with and without ILGP have been ploted in Fig. 4. The curves
represent the frequency sweep from 1.5 to 2.5GHz. The
resonant frequency of initial antenna without ILGP is 1.9GHz,
which is marked by m1. The impedance is 1.9228-i*0.452
with the 50-Ohm reference producing the 96.14-i*22.6 Ohm
value. It has a substantial capacitive reactance contributing to
the impedance mismatch Next, marker m2 indicates the
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2016 International Conference on
Mathematical Methods in Electromagnetic Theory
resonant frequency of the proposed antenna with ILGP. This
antenna resonates at 1.81GHz with an impedance value being
equal to 47.415-i*0.845 Ohms. Therefore, the ILGP produces
an inductive reactance and the antenna input impedance is
now matched well to 50 Ohms.
Fig. 5 shows the 0-degree phase instant surface current
distribution on the antenna at 1.81GHz. The surface current is
mostly concentrated on the two sides parallel to X-axis and the
feed strips. Very weak current is induced on the two segments
parallel to Y-axis. This feature leads to the phenomenon of
producing the major antenna radiation pattern in the YZ-plane.
Fig. 4. Smith chart of S-11 of the antennas with and without ILGP
Fig. 5. Phase instant surface current distribution on the proposed antenna at
1.81GHz
IV. MEASUREMENT RESULTS
The proposed Koch fractal loop antenna with ILGP
depicted in Fig. 2 has been fabricated using PCB technology
as shown in Fig. 6. The coaxial feed is directly connected to a
50-Ohm 3.5mm-SMA connector. The antenna layout is
printed on the low-cost 0.8mm-thick Teflon substrate with
2.55
r
and
tan 0.01
. The return loss of the proposed
antenna has been measured with Keysight N5247A network
analyzer while its radiation characteristics have been recorded
in the Satimo anechoic chamber.
(a)
(b)
Fig. 6. Top view (a) and bottom view (b) of the fabricated antenna prototype.
Fig. 7 presents the simulated and measured return loss of
the proposed antenna. It shows that the measured S11 is less
than -30dB at 1.78GHz confirming the required impedance
match of the antenna with ILGP. Simulation and measurement
results agree well, even though measured resonant frequency
is a little bit lower than 1.81GHz. The measured S11 < -10dB
level is from 1.73GHz to 1.84GHz, i.e. the impedance
bandwidth is 110MHz covering the GSM1800 bands.
Fig. 7. Measured and simulated return loss of the antenna shown in Fig. 6.
Fig. 8 presents the calculated and measured 3D total E-
field gain patterns of the proposed antenna at 1.81GHz. Fig. 9
shows the YZ-plane 2D radiation patterns of simulated and
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2016 International Conference on
Mathematical Methods in Electromagnetic Theory
measured gain also at 1.81GHz. In the set up, the coaxial feed
is oriented along the 180-degree line (-Z-axis). The
discrepancy between simulated and measured gain around
180-degree is attributed to the effect of SMA port. Fig. 10
presents the measured total efficiency and peak gain versus
frequency. Total efficiency reaches 85% while the gain is
3.2dBi. Both of the values are quite high considering the
compact dimensions of the antenna prototype.
(a)
(b)
Fig.8. Calculated (a) and measured (b) 3D radiation patterns of the proposed
antenna.
Fig. 9. YZ-plane 2D radiation patterns of simulated and measured gain.
Fig. 10. Measured efficiency and peak gain of the antenna shown in Fig. 6.
V. CONCLUSION
A compact Koch fractal loop antenna has been designed and
presented in this paper. Novel impedance matching method for
the loop antenna is proposed by integrating an in-loop ground
plane. The performance of this integrated antenna is improved
and verified by simulation and measurements. In the next
stage, an integrated fractal loop-microstrip RF rectifier circuit
has been designed with ILGP acting as its ground plane. It
should be noted that in addition to the RF energy harvesting
applications the proposed matching method can be easily
applied in the design of various planar loop antennas for
wireless communications and wireless sensor networks.
REFERENCES
[1] M. Pinuela, P.D. Mitcheson, and S. Lucyszyn, “Ambient RF energy
harvesting in urban and semi-urban environments,” IEEE Trans.
Microwave Theory and Techniques, vol. 61, no. 7, pp. 2715-2726, July
2013.
[2] A. S. Andrenko, X. Lin, and M. Zeng, “Outdoor RF spectral survey: a
roadmap for ambient RF energy harvesting,” IEEE TENCON 2015
Conference Proc., Nov. 2015.
[3] X. Yang, J. Chiochetti, D. Papadopoulos, and L. Susman, “Fractal
antenna elements and arrays,” Applied Microwave and Wireless, May
1999, pp. 34-46.
[4] A. S. Andrenko, “Conformal fractal loop antennas for RFID tag
applications,” IEEE Int. Conference of Applied Electromagnetics and
Communications Proc., ICECom 2005, Oct. 2005, pp. 1–6.
[5] C. A. Balanis, Antenna Theory: Analysis and Design, John Wiley and
Sons, 3rd Ed., New Jersey, 2005.
[6] J. D. Kraus and R. J. Marhefka, Antennas for All Applications, 3rd Ed.,
McGraw-Hill, New York, 2002.
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