Novel compact asymmetrical fractal aperture Notch band antenna
Duvuri Sri RAMKIRAN1, Boddapati Taraka Phani MADHAV1,*, Annam Manikanta PRASANTH1, Naga Sri HARSHA1, Vishnu VARDHAN1, Karimikonda AVINASH1,
Maddipati Nava CHAITANYA1, and Usirika Sharmila NAGASAI2
1
Department of ECE, K L University, AP, India 2
Sri Vasavi Institute of Engineering and Technology, Nandamuru
E-mails: ramkiran@kluniversity.in; *btpmadhav@kluniversity.in;
annammanikanta@gmail.com; nagasriharsha@gmail.com; vishnuvardhan@gmail.com;
karimikondaavinash@gmail.com; nagachaitanya@gmail.com;
sharmilanagasai@gmail.com *
Corresponding author, phone: +91-9908243452
Abstract
A compact novel fractal aperture co-planar waveguide fed monopole antenna
for multiband applications is proposed in this paper. The structure is
asymmetric along the principle axis and seems to be like amoeba shape of
radiating element. A band notch characteristic also achieved through this
design for communication band applications. The antenna parameters were
investigated to fully understand the behaviour and later for the optimisation
process. The simulated results through HFSS tool are giving satisfactory
results to notch particular band of frequencies and which is giving motivation
for the fabrication of the proposed model. All the antenna parameters
including S parameters and radiation patterns and current distributions are
studied through simulation and experimental validation is done through the
proto type modelling on FR4 substrate. Except the Notch band, the proposed
antenna model giving excellent radiation characteristics with VSWR less than
2. The prototyped antenna model is occupying a compact size of 18 X 14 X
1.6 mm on FR4 dielectric substrate material with dielectric constant 4.4.
Keywords
Compact; Fractal; Asymmetrical fractal; Amoeba shape; VSWR; Notch band;
VARDHAN, Karimikonda AVINASH, Maddipati N. CHAITANYA, and Usirika S. NAGASAI Introduction
Multiband antennas are gaining their applications in the mobile communication fields
with their compact nature [1-2]. High performance, compact size and low cost often meet
their requirements for the modern microwave communication systems with their numerous
advantages and applications antennas with different configurations like fractals, EBG
structures and defected ground structures are been used to enhance the antenna parameters
with multiband characteristics [3-8]. Miniature and low profile antennas in these categories
have undesirable intrinsic attributes such as narrow band width and inefficient radiation
characteristics resulting from reducing the antennas dimensions smaller than a quarter wave
length at operating frequency [9-10]. It has been demonstrated that fractal geometries, which
are based on space filling characteristics and self-similarity attributes, can be used to improve
performance of the antenna. Also fractal based antennas can effectively coupled energy to
free space. In addition, different feeding methodologies can be applied on fractal antennas
without decreasing their performance for example micro strip line feed and coplanar wave
line feeding methods [11-12].
Electromagnetic band gap structures are widely used to reject certain frequency band,
popularly known as photonic band gap structures. However it is difficult to use an EBG
structure for the design of the microwave or millimetre wave components due to the
difficulties of modelling [13]. There are so many design parameters that effect on the band
gap property such as number of lattice, lattice shapes, lattice spacing and relative volume
fraction. Another problem is caused by the radiation from the periodic etched defects in order
to minimize problems associated with EBG structures can be addressed and reduce by using
special defected ground structures [14]. The defected ground structures is one of the most
interesting areas of research in microwave integrated circuits with their advantages like
suppression of harmonics and making compact physical dimensions of the circuit.
In this article it is designed a compact asymmetrical fractal antenna with defected
ground structures to improve the performance with multiband characteristics in the operation.
The fractal structure seems to be like amoeba shape and the defected ground structures are
used on the ground plane to improve the wide band characteristics at multibands with
notching for certain frequency bands. The detailed description regarding antenna dimensional
characteristics and the corresponding parameters results are discussed in the subsequent
Material and method
The complete configuration and parameters of the coplanar wave guide fed
asymmetric fractal antenna with different iterations are shown in Figure 1. The structure
consisting of Fractal shaped tree like monopole with defected ground on the same side of the
substrate. The antenna is printed on commercial substrate material FR4 with dielectric
constant 4.4 and los tangent 0.24. The overall dimension of the antenna is around
18×14×1.6mm. The proposed coplanar wave guide fed asymmetric fractal antenna consists of
fractal patch with an array of fractal unit cells connected like branches to a tree.
a. b.
c. d.
Figure 1. Notch Band Antenna Iterations
The antenna’s rectangular ground plane is etched on the same side of the substrate for
a particular model and ground plane is etched with radiating element shaped slot in another
model and defected ground structure with periodic holes on another model is designed and
discussed in these configurations. To achieve band notch property for certain frequency bands
VARDHAN, Karimikonda AVINASH, Maddipati N. CHAITANYA, and Usirika S. NAGASAI Table 1. Antenna parameters
S.No Parameter Dimensions (mm)
1 Wsub 18
2 Lsub 14
3 Wf 3
4 Lf 5.5
5 Gap 0.3
6 Wn 2.2
7 g 1.79
8 Wg 7.2
9 Lg 6
10 L1 2.2
11 Ws 1
12 Ls 2.3
13 Lst1 5.1
14 Lst2 1.5
15 Wst1 0.2
16 Wst2 5.5
The antenna dimensions were optimized through parametric analysis using
electromagnetic tool HFSS and the final dimensional characteristics are presented in table 1.
Methodology:
1. Initially by using mathematical formulation, the dimensional characteristics of the antenna
are calculated.
2. Antenna simulation is done using FEM based HFSS tool and after that optimization is
done using parametric analysis.
3. Prototyped model is fabricated on FR4 substrate and tested using ZNB 20 VNA for
S-parameters and Radiation characteristics. A good agreement between simulated and
measured results are analyzed and presented in this work.
Results and discussion
The antenna S - parameters will give a first sight regarding its operating frequencies,
impedance and band width. Figure 2 shows the S11 parameter of the antenna models with
different iterations. Ground plane and radiating element are placed on single side with notch
in the ground resulting dual band characteristics with resonant frequencies at 5GHz and
width of 40% and at 10GHz it is giving an impedance band width of 35%. Radiating element
is taken on front side with full ground on another side resulting quad band characteristics at
higher frequencies between 15-30GHZ. When defected ground structure is taken on the
ground plane with the same shape of the radiating element then the antenna is resonating at
triple band with wide band width at the highest resonating frequency of 28GHz. At this
resonant frequency antenna is showing a band width of 4GHZ with impedance band width of
14%. When defected ground structure with photonic band gap periodic design is taken on the
ground plane the antenna is resonating at penta-band which covers different frequency bands
between 12-30GHz.
Figure 2. Return loss
The voltage standing wave ratio of all the iterations is shown in Figure 3 which also
indicating the same resonating frequency parameters like return loss curve. All these
iterations are maintaining a 2:1 ratio of VSWR at the corresponding resonant frequency.
Figure 3. VSWR plot
VARDHAN, Karimikonda AVINASH, Maddipati N. CHAITANYA, and Usirika S. NAGASAI
Figure 4. Antenna radiation pattern for Iteration 1
Figure 5. Antenna radiation pattern for Iteration 2
Figure 6. Antenna radiation pattern for Iteration 3
Figure 4 shows E and H plane radiation characteristics of the base model at 10 GHZ.
A low cross polarization of less than -32db can be observed from H-plane characteristics of
the base model. An omni directional radiation pattern also can be observed in co-polarization
for H-plane. The E-plane radiation characteristics are seems to be directive with nulling at 90
degrees and 270 degrees respectively. Figure 5 shows the radiation characteristics of the
second model with quasi omni directional co-polarization and low cross polarisation less than
-24db. Figure 6 and 7 also show the radiation characteristics for antenna model 3&4. These
models are giving directive radiation patterns in most of the cases instead of omni or quasi
omni directional radiation patterns.
The complete behaviour of the antenna with respect to the mode of propagations can
be analysed through current distribution characteristics results (Figure 8-11).
Figure 8. Current distribution for iteration 1 Figure 9. Current distribution for iteration 2
Figure 10. Current distribution for iteration 3 Figure 11. Current distribution for iteration 4
Figure 8 shows the current distribution plot of antenna1 at 10 GHz. The maximum
current density is focused at the edges of fed line and near by ground plane as shown in the
figure. Other than at the position nearer to fed line, the ground plane current distribution is
VARDHAN, Karimikonda AVINASH, Maddipati N. CHAITANYA, and Usirika S. NAGASAI structure also showing low current density with uni-directional flow towards feed line. Figure
9 shows the current distribution over the monopole antenna of model 2. On the feed line
current elements are moving in opposite direction and meeting at the centre of feed line which
results cancellation of lower operating band frequencies. Most of the current distribution with
maximum intensity is focused on the branches connected to the tree shaped radiating element
structure.
Figure 12 shows the peak directivity of the antenna models at their corresponding
resonant frequencies.
Figure 12. Frequency Vs Directivity
A maximum directivity of 5dB is attained for model 2 at 22 GHz resonant frequency.
An average directivity of 2.4 dB is attained for model 4. Figure 13 shows frequency vs gain
plot for antenna models. A maximum gain of 3.6 dB is attained for the model defected ground
without holes. An average gain of 2db is attained for defected ground with PBG structured
model. Figure 14 shows the prototyped antenna on FR4 substrate with thickness 1.5mm.
Figure 14. Prototyped antenna on FR4 substrate, (a) Top View, (b) Bottom view
VARDHAN, Karimikonda AVINASH, Maddipati N. CHAITANYA, and Usirika S. NAGASAI The bottom view of the antenna with slotted portion similar to the radiating element
and rectangular slots on either side can be observed. An SMA connector is connected at 50
ohms impedance point on the feed line can be observed in the Figure 14.
Figure 15 shows the measured reflection coefficient of the antenna with the help of
R&S ZNV 20 Vector Network Analyzer. The results obtained from the VNA are similar to
that of simulation results obtained from the HFFS Simulation tool.
Conclusions
A compact Fractal Aperture Notch Band Antenna is designed and prototyped in this
work. The proposed antenna is working at different frequency bands and notching certain
frequency bands in the communication systems frequency range. A maximum directivity of 5
dB and maximum gain of 3.6 dB is attained from the current design.
Antenna is showing excellent radiation characteristics with rejection of particular
frequency bands with the help of defected ground structure on the bottom surface gives
motivation to use the proposed antenna in the desired communication band of operations. The
prototyped antenna is measured with ZNB 20 vector network analyzer and obtained results on
the instrument are in good agreement with the simulation results obtained from the high
frequency structure simulation tool.
Acknowledgements
Authors would like to express their gratitude towards the department of ECE and
management of K L University for their support and encouragement during this work. Further
Madhav likes to express his gratitude to DST through FIST grant SR/FST/ETI-316/2012.
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