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A Novel Planar Fractal Multiband - UWB Antenna
Balakrishnan M., Gautam. P., Gilson Varghese, Boaz
Israel Joseph, Amit Upadhyay
B.Tech student
School of Engineering ,CUSAT
Kochi, India
Binu Paul
Associate Professor
Division of Electronics ,SOE
CUSAT
Kochi, India
Abstract—This paper presents the design of self-similar, space-
filling fractal antenna comprising of a modified circular patch
antenna inscribed with square shaped slot. The antenna is
microstrip fed with reduced ground dimensions. The substrate
chosen for fractal antenna is FR4 material with thickness of
1.6mm and relative permittivity 4.4. The fractal antenna show
multiband behavior due to self-similar iteration. The reduction in
size when compared to conventional circular patch antennas is
due to the slot loading. This structure yields three operating
bands with centre frequency around 2.1GHz,2.7GHz and the
upper UWB.
Index Terms—Fractal, UWB, multiband, Antenna.
INTRODUCTION
'A fractal is a rough or fragmented geometric
shape that can be split into parts, each of which is (at
least approximately) a reduced-size copy of the
whole'. The term is coined by Benoît Mandelbrot in
1975 and was derived from the Latin word fractus,
meaning 'broken' or 'fractured' [1].
The fractal antennas are widely popular due to
their multiband nature as compared to conventional
patch antennas. This results from the multiple
wavelengths current distributions achieved by self-
scaling within the same geometry. This has made
fractal antennas frequency independent. The
reduction in the size of antenna is due to space
filling fractal characteristics and increased electrical
length [2].
Ultra wideband (UWB) communication system is
one whose instantaneous bandwidth is many times
greater than the minimum required to deliver
particular information. They have
1. Large instantaneous bandwidth which enables
fine time resolution for network time
distribution, precision location capability.
2. Short duration pulses which are able to provide
robust performance in dense multipath
environments by exploiting more resolvable
paths.
3. Low power spectral density allows coexistence
with existing users and has a Low Probability
of Intercept (LPI).
4. Data rate may be traded for power spectral
density and multipath performance [3].
Among various types of UWB antennas, printed
monopole antennas have drawn great attention in
recent years since they are low-cost, low-profile, and
easy to fabricate. Moreover, monopole antennas
present suitable ultra wideband characteristics and
near-omni directional radiation patterns. A very long
length or a wide surface in a limited area can be
produced by using fractal geometries. This provides
a possibility to design miniaturized wideband
antennas having radiation patterns and input
impedance characteristics similar to larger antennas
[4]. Broadband (BW ≤ 2:1) and ultra wideband
(UWB) (BW ≥ 2:1 or 25% as per FCC definition)
wireless communication systems justify the demand
in designing the wideband fractal antennas. The slot
loaded antennas are popular for broadband
applications. The compact size and bandwidth
enhancement in a fractal antenna can be obtained by
optimized slot loading. UWB radiation can be
accomplished by perturbing electromagnetic
resonance to broaden the resonant dip, by
maintaining 'similar' radiation/scattering geometry
(shape and dimension) in terms of wavelength.
UWB antenna designs using these approaches are
referred to as type-I / type-III UWB antennas
respectively. Type-II UWB antennas offer superior
pulse response but may have low efficiency. Type-
III UWB antennas offer frequency-independent gain
and patterns are often dispersive[7-9].
DESIGN OF THE ANTENNA
The evolution of the geometry is shown in Fig. 1.
(a) ( b)
( c) (d)
Fig. 1.a-d The evolution of fractal geometry with two iterations
A circular metallic patch with 26 mm diameter is
the basic structure, within which a square slot of
side 16 mm is inscribed. The FR4 substrate used is
of thickness h=1.6 mm, relative dielectric constant
εr=4.4 and loss tangent tan δ=0.02. The antenna is
microstrip fed with feed width of 3 mm and length
11.5 mm. The reduced ground has width 40 mm and
length 11 mm as shown in Fig. 2. The overall
dimension of the antenna is 37.5mmx40mmx1.6mm.
Fig. 2. The proposed antenna geometry (all dimensions are in mm)
Modelling and performance evaluation of the
proposed antenna has been carried out using
commercially available EM simulator HFSS® from
Ansoft Corporation [6]. The basic monopole gives a
wideband characteristic and inscribing a slot within
the structure results in additional bands. Since the
current distribution in the conventional patch is
along the circumference, it is known that by
inserting an optimized slot additional desired
resonances can be excited. To ensure the space
filling aspect an additional perturbation was
introduced, as shown in Fig. 1.c. The final fractal
geometry depicted in Fig. 1.d, inscribes a scaled
down version of the Fig. 1.c. with the iteration ratio
of 2:1. There are only two iterations in the proposed
antenna.
The radius of the circular monopole estimated for
a resonance frequency of 2.5GHz as per [5] is
16.77mm. The return loss characteristic of the
circular monopole is shown in Fig. 3.a. The
geometry offered a wide 2:1 VSWR band from 2.66-
5.81GHz and from 6.57GHz - >10GHz. It was then
modified by inscribing a square slot of side 16mm
with the diagonals of the square coinciding with
centre of the circle as depicted in Fig. 1.b. The
simulated reflection characteristic of this geometry
shows an improvement in the lower band but
perturbed the UWB response. It was seen that, with
this slot insertion the lower cut off frequency
shifted towards lower side. Based on this
observation the radius of the circular patch was
finally optimized to 13mm. A smaller circular
perturbation of radius 3mm centered at an offset of
1.2mm from circumference of the circular monopole
was then incorporated. This geometry was repeated
inside the square so as to get second iteration of the
fractal structure. It has a concentric circular region
of radius 6 mm with an inscribed square of side
7mm and smaller tuning circle with radius 1.5mm.
The smaller circle acts as the inductive path between
the two iteration geometries and is important in
offering good radiation characteristic in the higher
band as can be seen in Fig. 4.b.
RESULTS AND DISCUSSIONS
Return Loss Characteristics
The simulated response of the circular monopole
showed wide band behavior with a lower cutoff of
2.6GHz as shown in Fig. 3.a. The modified
monopole of Fig. 1.b resulted in improved
performance in the lower spectrum as depicted in
Fig. 3.b. The modification shown in Fig. 1.c.
resulted in slight lowering of lower cutoff
frequencies of second and third bands. By
incorporating a second iteration as in Fig. 1.d, an
additional lower resonance was excited with centre
frequency at 2.1GHz. The 2:1 VSWR bands of the
proposed antenna are 2.01-2.18GHz (centered at
2.12GHz), 2.42-3.28GHz (centered at 2.83GHz),
and 6.96-9.83GHz (upper UWB) as shown in Fig.
3.d. . However this resulted in a third unmatched
resonance around 4.63GHz which need to be
properly tuned if 3.1-10.3GHz UWB band is
desired.
By further optimizing the scale factor, band 1 can
be shifted to GSM 1900 application band. These
results show a considerable mismatch in the lower
spectrum of UWB which need to be studied further
Frequency, GHz
2 4 6 8 10
Return loss S11,dB
-40
-30
-20
-10
0
Circular Monopole
Proposed geometry
Fig. 3.a. Return loss characteristics of circular monopole
Frequency, GHz
2 4 6 8 10
Return loss S11,dB
-40
-30
-20
-10
0
Circular Monopole with inscribed square
Proposed geometry
Fig. 3.b. Return loss characteristics of modified circular monopole with
square slot
Frequency, GHz
2 4 6 8 10
Return loss S11,dB
-40
-30
-20
-10
0
Iteration 1
Proposed geometry
Fig. 3.c. Return loss characteristics of fractal geometry after first
iteration
Frequency, GHz
2 4 6 8 10
Return loss S11,dB
-40
-30
-20
-10
0Band1
Band 2 Band 3
Fig. 3.d. Return loss characteristics of proposed fractal geometry
Thus the reflection characteristic shows that first
resonant frequency falls at 2.1GHz UMTS
downlink, the second one is in 2.4GHz WLAN band
and third band is upper ultrawideband. This
multiband operation is due to the fractal iteration in
the antenna geometry.
The proposed fractal antenna resonates at 2.1GHz
in contrast with the conventional circular microstrip
patch antenna of the same broader dimension
resonating at 2.6GHz. This clearly reveals the
miniaturization achieved in the overall dimension of
the antenna due to multiple iteration fractal loading.
Parametric Analysis
By utilizing the optimetrics feature of HFSS®,
parametric study was conducted on the dimensions
of the fractal geometry. A variable 'a' was defined as
extension to the radius of outer most circle and 'b' as
the extension given to the side of the square
inscribed. Fig 4 illustrates the effect of one of the
parametric variations on return loss. It is evident that
on varying 'a', lower resonance of the UWB
response changes.
Radiation Pattern
One of the main challenges when developing
UWB antennas is the instability of the antenna
boresight gain[10]. The main beam of the monopole
antenna squints at higher frequencies, due to far
field cancellations caused by out of phase surface
current distributions resulting in the instability of the
antenna patterns.
Frequency, GHz
2 4 6 8 10
Return loss S11,dB
-40
-30
-20
-10
0
r=13,s=16.5
r=13,s=17
r=13,s=17.5
r=13,s=18
r=13,s=16
(a) Effect of varying b, with a = 0(radius of outer circle=13mm)
Frequency, GHz
2 4 6 8 10
Return loss S11,dB
-40
-30
-20
-10
0
r=12.5,s=16
r=13.5,s=16
r=14,s=16
r=14.5,s=16
r=13,s=16
(b) Effect of varying a, with b = 0(side of square =16mm)
Fig.4.a-b Result of Parametric study performed
(c)
(b)
(a)
(d)
Fig.5. 3D Radiation pattern at (a)2.1GHz (b)2.5GHz (c)7.5GHz (d)9GHz
The simulated 3D and 2D radiation pattern is
obtained using HFSS. The 3D radiation pattern of
Fig. 5 shows that antenna offers omni directional
behavior, at 2.1GHz and 2.5GHz, like that of a
monopole. The 2D co-polar radiation patterns in the
E plane and H plane (for =0° and 90°) are shownϕ
in Fig. 6.a-d.
-20
-15
-10
-5
0
0
30
60
90
120
150
180
210
240
270
300
330
Eco
Hco
2.1GHz
(a)
-20
-15
-10
-5
0
0
30
60
90
120
150
180
210
240
270
300
330
2.5GHz,phi=0 vs Theta
2.5GHz,phi=90 vs Theta
2.5GHz
(b)
7.5GHz
-20
-15
-10
-5
0
0
30
60
90
120
150
180
210
240
270
300
330
7GHz,phi=0 vs Theta
7GHz,phi=90 vs Theta
7GHz
(c)
-20
-15
-10
-5
0
0
30
60
90
120
150
180
210
240
270
300
330
9GHz
(d)
Fig. 6.a-d.Simulated 2D Radiation patterns at
(a)2.1GHz (b)2.5GHz (c)7.5GHz (d) 9GHz
It is seen that from Fig 5 and 6, that antenna
offers linearly polarised omni directional radiation
pattern for the lower bands. At higher frequencies,
the y-z pattern becomes distorted but the omni-
directional radiation performance is a highlight
which makes this antenna suitable for application in
a UWB enabled wireless device.
A slot hampers the radiation characteristics due to
removal of considerable metallization. The fractal
geometry introduced additional current paths within
the square slot. The scaled down self-similar
iteration not only improves metallization but
improves radiation characteristics and also make the
antenna multi-band. Fig 7 illustrate the simulated
gain plot of antenna in the proposed bands. The
antenna offers reasonably good gain in both the
lower bands and moderate gain in upper bands for
UWB applications.
The surface current distribution on the geometry
is shown in the Fig. 8. Figure 8.a depicts that lower
resonance at 2.1GHz can be attributed to the
extended resonant element offered by insertion of
square within circle. The resonance at 2.5GHz is the
fundamental mode of circular patch as seen from
Fig. 8.b.
A time domain study of the proposed geometry is
underway. Table 1 shows a comparison of work
reported in literature with the proposed antenna. The
proposed antenna is under fabrication and
verification of the results is pending
Frequency, GHz
1.8 1.9 2.0 2.1 2.2 2.3
Gain ,dB
-10
0
10
20
Band 2
2.4 2.6 2.8 3.0 3.2
Band 3
6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5
Band 1
Band 2
Band 3
Band 1
Fig. 7 Gain of the proposed antenna in various bands
(a) ( b)
( c) (d)
Fig.8.a-d Current distribution of the proposed antenna at (a)2.1GHz
(b)2.5GHz (c)7.5GHz (d) 9GHz
COMPARISON BETWEEN REPORTED ANTENNAS AND THE PROPOSED ANTENNA
Feature Overall
dimension
Bands
supported
Average gain
in bands
Ref
[10]
Wide
band
56.32x
38.72x 3 1.35-9GHz 5dB upto 6GHz,
3dB for 6-9GHz
Ref
[7]
Triple
bands 38x25x 1.6
2.4-2.5GHz,3-
4GHz, 5.4-
5.9GHz
1.89dB,
1.9dB,1.7dB
Ref
[11]
Four
bands
with
CCSR
75x75x 1.6
Centered
around 1.GHz,
2.45GHz,
3.96GHz,
5.25GHz
6.0dB,2.3dB,
4.0dB,4.0dB
Ref
[12]
Metal
plate
monopol
e with
trident
feed
40x40
metal sheet
over
150x150
ground
1.376-
11.448GHz
4-7dB upto
6GHz, ~7dB
for 6-11.5GHz
Prop
osed
anten
na
Planar
and
compact
with
good
radiation
characteri
stics
37.5x40x
1.6
2.01-2.18GHz ,
2.42-3.28GHz
and 6.96-
9.83GHz.
9.4dB,7.5dB,
~1dB
CONCLUSION
A new planar fractal antenna with two iterations
has been introduced. The antenna exhibits desirable
performance in multiple bands around
2.1GHz/2.6GHz and the upper UWB system. The
simulated antenna is compact, easy to fabricate,
exhibits moderate gain and stable radiation patterns
that render it suitable for multiband wireless
applications.
ACKNOWLEDGMENT
The authors wish to acknowledge CREMA,DOE,
CUSAT for all the support extended in performing
the measurements.
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