Fractal Antenna

1. A Compact Peano-Type Fractal Based Printed Slot
Antenna for Dual-band Wireless Applications
S. F. Abdulkarim, A. J. Salim, J. K. Ali, A. I. Hammoodi, M. T. Yassen, and M. R. Hassan
Microwave Research Group, Department of Electrical Engineering,
University of Technology, Iraq
Abstract—This paper presents the design of a new Co-Planar
Waveguide (CPW) fed dual band printed slot antenna with
enhanced bandwidths for use in wireless applications. The fractal
based slot structure of the proposed antenna is in the form of
Peano fractal curve of the second iteration as being applied to the
two sides of a rectangular slot. The proposed antenna is to be
etched using a substrate with relative permittivity of 4.4 and
thickness of 1.6 mm. The resulting antenna has overall
dimensions of 36 mm × 45 mm which is suitable for mobile
terminal applications. Modeling and performance evaluation of
the proposed antenna have been carried out using the CST
Microwave Studio. Simulation results shows that the proposed
antenna offers dual –10 dB impedance bandwidths of more than
1.2 GHz and 1.4 GHz for the lower and the upper bands
respectively with corresponding average gains of about 2.5 dBi
and 4 dBi throughout these bands. The first resonant band,
centered at 2.50 GHz, extends from 1.91 to 3.11 GHz. This band
covers the 2.4 GHz WLAN band (2.4–2.483 GHz) and the 2.5
GHz mobile WiMAX operating band (2.5–2.7 GHz), while the
second resonant band, centered at 5.20 GHz; extends from 4.51
to 5.91 GHz. This band covers the U-NII mid-band (5.47–5.725
GHz) and U-NII high-band (5.725 –5.875 GHz). Besides the
compact size, the antenna offers reasonable radiation
characteristics with omnidirectional radiation patterns in the two
bands.
Keywords—fractal geometry; slot antenna; dual-band antenna;
printed antenna; Peano fractal curve
I. INTRODUCTION
Recently, many communication services, such as PCS,
WiBro, WiMAX, and wireless LAN, has became available
below 6 GHz and multiple frequencies have been allocated for
the development of high-speed mobile information and
communication systems. These developments have triggered
the research to design compact and multiband antennas that
can transmit and receive of more than one frequency signal
[1]. Microstrip and printed antennas are promising candidates
for this design due to their low profile, low-weight, and ease
of fabrication [2]. In this respect, various fractal geometries
have found their way to be used in the antenna design to
produce compact and multiband antennas benefiting from their
unique properties; space filling and self similarity
respectively. On the other hand, slot structures are widely used
in the design of printed antennas with enhanced bandwidths.
Conventional fractal geometries such as Koch, Cantor,
Hilbert, Sierpinski, Minkowski and other fractal curves have
been successfully used to produce dual-band and multiband
printed slot antennas for various wireless applications [3-16].
In this context, the applications of fractal geometries in the
design of slot printed antennas can be classified into two
categories. In the first category, direct application of fractal
geometries has been adopted [3-11]. In such a case, the fractal
geometries constitute the whole antenna slot structures. The
multiband behavior of such antennas has been extracted
almost directly without the need of any tuning elements or slot
shape modification. However, in the second category, the slot
structure is combination of Euclidian structures, such as
triangle, square, rectangle and other polygons, and fractal
geometries superimposed on these structures, where each line
segment is replaced by fractal curve with certain iteration level
[12-17]. In this case, the multiband behavior has been reached
in different way. These include the addition of tuning stubs to
the feed line and modification of the slot structures by rotating
it around the antenna axis.
In this paper, a printed slot antenna with Peano-type slot
structure, has been presented as a candidate for use in dual-
band wireless applications. The slot structure of the proposed
antenna is essentially rectangular with two of its non-adjacent
side are modified in the form of Peano-type fractal geometry
of the second iteration. The antenna has been fed with a 50 Ω
CPW line etched on the same ground plane. The dual band
behavior of the proposed antenna has been reached by adding
two symmetrical tuning stubs to the slot structure. This results
in an antenna response with enhanced dual-band behavior
covering a wide variety of wireless communication
applications below 6 GHz frequency range.
II. THE PROPOSED ANTENNA STRUCTURE
In this paper, a modified version of the conventional Peano
fractal curve is proposed to modify a rectangular slot structure.
This Peano-type fractal geometry, as depicted in Fig. 1, has
been suggested to design an internal dual-band printed
monopole antenna for WLAN USB dongle [18]. In this
structure, the conventional Peano pre-fractal curve generator
has been stretched by a factor of 0.5 in only one dimension, so
the resulting fractal structure will no longer be with a square
outline.
2013 IEEE International RF and Microwave Conference (RFM2013), December 09-11, 2013 - Penang, Malaysia
978-1-4799-2214-7/13/$31.00 ©2013 IEEE 329

2. Figure 1 demonstrates the generation process of the
modified Peano pre-fractal curve up to the 2nd iteration. The
straight line in Fig. 1(a), (the initiator), has been replaced by
the nine segment structure in Fig. 1(b), the generator. Then, in
certain iteration n, each line segment has to be replaced by the
whole structure of its preceding iteration, taking into account
segment scaling and orientation. Hence, the 1st iteration
consists of 9 segments, and the 2nd iteration has 81 segments,
and so on.
Fig. 1. The steps of growth of the proposed Peano pre-fractal curve upto the
second iteration.
If the length of the initiator line is L, the length enclosed
by any pre-fractal structure at the nth iteration n, Ln is [19]:
12 −= n
n
n LL for n ≥ 1 (1)
The slot structure of the proposed printed antenna is
essentially of a rectangular shape. Two sides of the slot
structure have been modified in the form of the second
iteration of the modified Peano fractal curve shown in Fig.
1(c).
III. THE ANTENNA DESIGN
A CPW fed dual band printed fractal based slot antenna,
with slot structure dimensions depicted in Fig. 2, has been
initially designed. The slot structure is supposed to be etched
on the ground plane of an FR4 substrate with relative
dielectric constant of εr =4.4 and h =1.6 mm. By appropriate
dimension scaling, the resulting antenna has been found to
have a rectangular ground plane with dimensions of 45 mm ×
36 mm. The 50 Ω CPW feed has a length of 20 mm with a line
width of 2.0 mm and gap width of 1.85 mm. Other dimensions
of the antenna are: w1 = 9.31 mm, L1= 31.35 mm, and L2=
13.13 mm. Numerical analysis of the antenna performance is
carried out using the commercially available EM simulator,
the CST Microwave StudioTM
[19].
IV. PERFORMANCE EVALUATION
Simulation results, depicted in Fig. 3, imply that this
antenna offers a single-band resonance within the swept
frequency range of 1–7 GHz with a resonant frequency of
about 5.0 GHz. This does not prevent the possibility of the
existence of other resonances outside this rang. In addition,
there is no effect of the feed line length variation on the
antenna resonant frequency as demonstrated in return loss
response is shown in Fig. 3. The effect is only to enhance the
coupling of the resonant band.
As an attempt to enhance the coupling of the antenna
resonant bands, the antenna with the layout depicted in Fig. 2
has been modeled with prescribed substrate, with a coupling
stub added to the slot structure as shown in Fig. 4. The
addition of the tuning stubs to the slot structure will provide an
additional radiating path and thus lowering the antenna
resonant frequency. Simulation results, depicted in Fig. 5,
reveal that the antenna offers a dual-band response within the
sweep frequency of 1-7 GHz.
Fig. 2. The layout of the modelled antenna with respct to the coordinate
system.
Observing the influence of the various parameters on the
antenna performance, it has been found that the dominant
factor in the antenna is the slot length L1 in terms of the guided
wavelength λg:
eff
g
ε
λ
λ = (2)
where εeff is the effective dielectric constant. In terms of the
slot length L1 and the guided wavelength λg, the lower
resonant frequency, f1, is given by:
effL
c
f
ε1
1
2
= (3)
where c is the speed of light in free space.
Fig. 3. The return loss response of the modeled antenna depicted in Fig. 2
with the feed line length as a parameter.
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3. The modification of the slot structure by adding a tuning
stub at the lower end in the direction of the feed line will
contribute in providing longer radiating path. A parametric
study of the effect of the stub length on the antenna return loss
response is demonstrated in Fig. 5. The results imply that as
the stub length varies, both resonant bands are affected. In
summary, as the stub length is increased, the antenna starts to
acquire a dual band behavior. With a stub length of about 13
mm, an optimal antenna return loss response has been
obtained. As it is clear from Fig, 5, the decrease of the stub
length makes the antenna having a single resonating band
within the swept frequency range.
Fig. 4. The layout of the modelled antenna depicted in Fig. 2 with a coupling
stub in the slot structure.
Simulation results shown in Fig. 5, implies that the
proposed antenna offers dual –10 dB impedance bandwidths
of more than 1.2 GHz and 1.4 GHz for the lower and the upper
bands respectively. The first resonant band, centered at about
2.50 GHz, extends from 1.91 to 3.11 GHz. This band covers
the 2.4 GHz WLAN band (2.4–2.483 GHz) and the 2.50 GHz
mobile WiMAX operating band (2.5–2.7 GHz), while the
second resonant band, centered at 5.20 GHz, extends from
4.51 to 5.91 GHz. This band covers the U-NII mid-band
(5.47–5.725 GHz) and U-NII high-band (5.725 –5.875 GHz).
This makes the proposed antenna a suitable candidate for use
in a wide variety of communication services.
Fig. 5. Simulated return loss responses of the modelled stub loaded slot
structure antenna, depicted in Fig. 4, with the stub length ws as a paramerer.
Figure 6 demonstrates the gain responses throughout the
two resonant bands. The antenna offers an average gain of
about 2.5 dBi throughout the lower resonant band, and about
4.0 dBi throughout the upper band. These values are sufficient
for the operations of the most of the communication services
operating within the frequency range.
Fig. 6. The simulated gain responses throughhout: (a) the lower and (b) the
upper resonant bands of the antenna depicted in Fig. 4 with a stub length of 13
mm.
Fig. 7. The simulated far field radiation patterns of the proposed antenna at
(a). 2.50 GHz, and (b). 5.20 GHz.
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4. The far field radiation pattern characteristics of proposed
fractal slot antenna, for stub length of 13 mm, have been
numerically calculated as shown in Fig. 7. In this case, the
proposed antenna resonates at 2.50 GHz and 5.20 GHz in
broad side direction at φ=0° and φ=90°. The results show very
monopole like radiation patterns with omnidirectional
radiation.
Fig. 8. The simulated current distributons on the surface of the modelled
antenna antenna at (a). 2.50 GHz, and (b). 5.20 GHz.
To get more insight about the EM characteristics of the
proposed antenna, the current distributions generated in the
antenna have been simulated at 2.50 and 5.20 GHz, as shown
in Fig. 8. It is worth to note that the same color scale has been
adopted for the simulated current distributions at the two
frequencies. As the results of Fig. 8(a) implies the resonance
at 2.50 GHz is attributed to the larger surface current
distribution concentrated around the stub. However, the fractal
slot contributes less at this resonance. At the 5.20 GHz
resonance, it is clear from Fig. 8(b) that both the slot structure
and the tuning stub contribute to this resonance, in spite of the
effect of the tuning stub is apparently less.
V. CONCLUSIONS
A fractal based printed slot antenna has been introduced in
this paper, as a candidate for use in dual-band wireless
applications. It has been found that, the addition of a stub to
the antenna fractal based slot structure makes it resonating
with multiple bands within the 1---6 GHz frequency range.
The antenna has offered dual-band behavior with resonant
bands centered at 2.50 and 5.20 GHz with fractional
bandwidths of about 48% and 27% respectively. These
resonant bandwidths cover most of the recently operating
communication services below 6 GHz. Furthermore, the
proposed antenna offers good gain and reasonable radiation
characteristics. The compact size of the proposed antenna
makes it suitable for a wide variety of dual-band wireless
applications within the specified frequency range.
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