05-11-2016, 11:57 AM
Bandwidth enhancement of modified square fractal microstrip patch antenna using gap-coupling
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abstract
Narrow bandwidth is a major constraint of microstrip antennas. This paper illustrates the design of a gap
coupled modified square fractal microstrip patch antenna which has been designed to overcome this
limitation. The intended design has an impedance bandwidth of 85.42% around the resonant frequency
of 1.844 GHz. This antenna can be simultaneously used for Bluetooth, WLAN and WiMAX applications.
IE3D Zeland simulation software has been used for the simulation of the proposed design.
Introduction
Microstrip Patch Antennas have always been a source of
attraction for the researchers due to their highly desirable attributes
such as low profile structure, light weight, conformal shape,
cost-effectiveness, high efficiency, ease of installation, small volume,
and compatibility with microwave integrated circuits (MIC)
and monolithic microwave integrated circuits (MMIC) [1,2]. These
qualities have resulted in wide applications of microstrip patch
antennas in radar, satellite and mobile communications. However
microstrip patch antennas suffer from a major limitation of very
low impedance bandwidth, typically about 5% bandwidth with
respect to central frequency.
Extensive research has been carried out in the past two to three
decades in an attempt to increase the bandwidth of patch antennas.
These bandwidth enhancement techniques include use of Frequency
Selective Surface [3,4], use of low dielectric substrate, use of
multiple resonators, use of thicker substrate [5], employing stacked
configuration [6] and use of slot antenna geometry [7,8]. Singh et al.
[9] proposed a T-slot rectangular patch antenna with an impedance bandwidth of 25.23%. Aneesh et al. [10] demonstrated that an Sshaped
Microstrip patch antenna can achieve a bandwidth of
21.62%. Mulgi et al. [11] proposed a wideband gap-coupled slot
rectangular microstrip array antenna with an impedance bandwidth
of 26.72%. Khanna and Srivastava [12] designed a square
patch antenna with modified edges and square fractal slots with a
bandwidth of 30%. Tyagi and Vyas [13] designed a slotted U-shaped
microstrip antenna with PBG structure which has an impedance
bandwidth of 35%. Kajla et al. [14] proposed a microstrip patch
antenna combining Crown and Sierpinski fractal slots which has a
bandwidth if 44%. Gupta et al. [15] showed that an impedance
bandwidth of 63.3%, 72.10% and 37.5% can be achieved using two,
three and six slit-slotted circular patch antennas respectively.
Numerous other geometries have been developed to improve the
bandwidth of conventional patch antennas, for instance, squarering
slot antenna [16], inverted and non-inverted V-shaped
slotted trapezoidal patch antenna [17], a U-shaped slot in an
equilateral triangular patch antenna [18], circular patch antenna
with a diamond shaped slot [19] and a transmission line fed crescent
patch antenna [20]. Various other fractal geometries that have
been explored in the previous works by researchers include the
Koch Snowflake fractal monopole antenna [21], a rhombic patch
monopole antenn awith modified Minkowski fractal geometry [22],
a novel broadband fractal Sierpinski shaped microstrip antenna
[23], a wideband Sierpinski shaped slot antenna [24] and the Giuseppe
Peano fractal antenna [25]. The Giuseppe Peano fractal is basically a space-filling curve, generally applied to the boundary or
edges of the patch to achieve antenna miniaturization and multiband
characteristics [26,27].
This paper elaborates the design of a gap-coupled modified
square fractal microstrip patch antenna using co-axial feeding
technique which operates in the frequency range of 1.68e4.16 GHz
i.e. 85.42% around the resonant frequency of 1.844 GHz. This antenna
has been designed for simultaneous use in Bluetooth, WLAN
and WiMAX applications.
2. Fractal antenna
A fractal antenna [28e30] can be described as an antenna that
uses a fractal, self-similar design to increase the perimeter (both
internal and external) of the material that is able to transmit or
receive electromagnetic radiation within a given total surface area
or volume. The term fractal means broken or irregular fragments.
Fractals are commonly made up of multiple copies of themselves at
varied scales. They possess the unique qualities of self-similarity
and space-filling property. Fractal antennas offer certain advantages
such as large bandwidth, improved VSWR, miniaturization of
antenna, multiband and wideband performance. It has been
observed that as the order of iteration increases, the resonant frequency
of the fractal antenna decreases. Such antennas suffer from
certain limitations such as complicated fabrication and designing as
well as low gain in some cases.
3. Gap coupling
The concept of gap coupling [11,31e34] is used to enhance the
bandwidth of patch antenna and also to achieve dual frequency
operation. In a gap-coupled structure, as shown in Fig. 1, a parasitic
patch and a feed patch are placed close to each other. The feed
patch is excited by a feeding technique where as the parasitic patch
gets excited through the gap-coupling between the two patches. If
the resonant frequencies of these two patches are close to each
other, then a broad bandwidth can be achieved. If the dimensions of
the feed patch and the parasitic patch are same, then the gap
coupled structure creates two different resonant frequencies.
4. Co-axial or probe feed
This is a very common technique used to feed a microstrip patch
antenna. As shown in Fig. 2, the inner conductor of the co-axial
connector extends through the dielectric and is soldered to the radiation
patch where as the outer conductor remains connected to the
ground plane. This feeding technique has a major advantage that the
feed can be applied at any desired location inside the patch to achieve
proper impedance matching. Co-axial feeding technique allows
easy fabrication and offers low spurious radiation.
Bandwidth improvement
The following two techniques have been employed to improve
the impedance of the conventional microstrip patch antenna:
5.1. Fractal geometry
A fractal antenna possesses the unique feature of self-similarity.
A self-similar set is a set that contains scaled down copies of itself.
This property is responsible for the multiband and wideband
characteristics of the fractal antenna. A basic square fractal antenna
has been designed to achieve wideband characteristics (Fig. 3).
5.2. Gap-coupled structure
In order to introduce gap-coupling for further bandwidth
enhancement, the basic square fractal antenna has been modified
by introducing parasitic square patches in each iteration (Fig. 4).
6. Description of the proposed antenna design
The proposed antenna has been designed on FR4 glass epoxy
substrate with dielectric constant of 4.4, loss tangent of 0.0013 and
substrate thickness of 1.6 mm. The simulation performance of the
suggested gap-coupled patch antenna has been analyzed by using
IE3D simulation software [35]. The design specifications are as
follows (Table 1):
6.1. Base shape of basic square fractal antenna
In the base shape, a square feed patch of side length 28 mm has
been taken on a finite ground plane of side 50 mm. A square slot of
side length 14 mm (half the size of basic square patch) has been
embedded in the center of this square feed patch. The Fig. 5(a)
shows the structure of the base shape of the basic square fractal
antenna. Fig. 5(b) demonstrates that the base shape has dual frequency
bands of 36.67% (1.67e2.42 GHz) and 24.7% (3.82e4.9 GHz)
with return losses 22.63 dB and 11.83 dB at resonant frequencies
1.864 GHz and 4.3 GHz respectively. A VSWR of 2.4 and
1.689 are available at 1.864 GHz and 4.3 GHz respectively.
Conclusion
In this paper a probe fed gap-coupled modified square fractal
microstrip patch antenna has been designed to overcome the
constraint of narrow bandwidth of the conventional patch antenna.
This technique has achieved much better results in terms of
bandwidth enhancement as compared to the geometries discussed
in the literature. The proposed design has an impedance bandwidth
as high as 85.42% around the resonant frequency of 1.844 GHz. This
antenna has a VSWR of 1.029 and a return loss of 36.89 which are
noticeable results. This antenna, with a gain of 3.31 dB and an antenna
efficiency of 97.56%, is simultaneously applicable for Bluetooth
(2.4e2.48 GHz), WLAN (2.4e2.484 GHz specified by IEEE
802.11 b/g standards), Mobile WiMAX (2.5e2.69 GHz specified by
IEEE 802.16e standards), and WiMAX (3.4e3.69 GHz specified by
IEEE 802.11a standards) applications.