I. INTRODUCTION
Nowadays multimedia applications are extended from narrow bandwidth audio application to wide bandwidth application such as video conferencing, online gaming, streaming videos, high-speed data transfer etc. In recent years, next generation wireless devices have been introduced, which support the multimedia applications given above in single device only. As devices are being miniaturized day by day it drives demand for small size of devices. Antenna is said to be a bottleneck in reduction of size and data rate enhancement of these devices. Antenna is a frequency-dependent device because its length is defined in wavelength. Antenna of specific length resonates at specific frequency. Hence, reducing size of the antenna is challenged if the frequency is defined. Another requirement in antenna design is to improve data rates in next generation wireless devices which require large bandwidth support of antenna. Furthermore, nowadays, single wireless devices include more than one technology, for example, cellphone mobiles in use nowadays contain different technologies such as GSM/CDMA, WiFi, WiMAX, Bluetooth, etc. in single device and drives the antenna's ability to operate on multibands. Hence, the major requirements of antennas are broadband and multiband designs to support multimedia applications as well as to support different technologies in a single device.
Microstrip patch antenna was introduced as planar antenna as alternate to high-profile wire antennas that were difficult to mount on any dynamic and moving surfaces as air pressure, etc. can affect reliability of wire antennas. However, microstrip antennas suffer from narrow bandwidth which is again problematic to support high-speed media applications as data rates are directly proportional to bandwidth. In the literature on Wire antennas, Rumsey's principle claims that frequency independence of antenna can be achieved by making dimensions of antenna dependent on angle. There are large numbers of shapes that are proposed in the literature that are following Rumsey's Principle hence Frequency independence. Examples of these types of shapes in Wire antennas are Log Periodic, Biconical Antenna, etc while in context of Planar antennas Fractal shapes are said to be satisfying Rumsey's Principle.
Fractal shapes were introduced to achieve multiband as well as broadband operation. There are large numbers of Fractal shapes that are proposed in the literature such as Sierpenski carpet, Sierpenski Gasket, Koch Monopole, Parany, etc. [Reference Maza, Cook, Jabbour and Shamim1, Reference Saluja and Khanna2]. In the literature, a large number of designs and analysis are present for Sierpenski Gasket fractals and its variant [Reference Mishra, Ghatak and Poddar3]. Modification in design of Sierpenski Gasket allows uniform current density over metallic patch, which improves radiation property of antenna [Reference Tsachtsiris, Soras, Karaboikis and Makios4, Reference Saluja and Khanna5].
II. PROPOSED DESIGN
In this paper, hybrid fractal shape is introduced to achieve optimized broadband operation of antenna while maintaining radiation pattern to be good. Hybrid of Sierpenski Gasket and Sierpenski carpet is considered as patch of antenna. Sierpenski Gasket is used as radiating fractal-shaped patch as well as impedance transformer for Sierpenski carpet. Major problem faced by the Sierpenski carpet shape is its feed. As there is no close-loop expression to find edge impedance of Sierpenski carpet patches, hence, it is difficult to find the length and width of feed to match this type of patch efficiently. Also, edge impedance for Sierpenski carpet comes out to be quite high therefore while feeding it deteriorates radiation pattern.
Coplanar type of feed is used to feed the proposed antenna. Coplanar waveguide (CPW) feed enhances bandwidth further as well as is advantageous in ease of integration with PCB of receivers or transmitters. In the literature, researchers are using CPW feed extensively due to its advantage over other feeding techniques. In [Reference Saluja and Khanna5], Saluja and Khanna proposed design using CPW-fed antenna with unidirectional radiation. In [Reference Wang, Wen, Huang and Sun6], the author claims design of CPW-fed Microstrip antenna to radiate at triple band of frequencies. In [Reference Wang, Wen, Huang and Sun6], U slot and I notch with CPW feeding are used.
A novel design of Microstrip antenna is presented in this paper. Hybrid fractals are used to enhance bandwidth of Microstrip antenna by tuning both fractals to closely spaced resonances. By Rumsey's principle, fractal shapes can produce wide bandwidth if designed such that every fractal generates closely spaced resonance. In [Reference Siakavara7], Saikavara presents design of hybrid fractal arrays by combining fractal array by different generators for wide band operations but with a large size. In the proposed design, CPW feed is used to feed the hybrid fractal patch. The proposed design is optimized to work for improved ultrawideband (UWB) operation. Figure 1 shows the design of antenna with dimensions of proposed antenna.
Fig. 1. Proposed antenna for UWB application: (a) front view of antenna, (b) patch of proposed antenna (c) feed for proposed antenna
III. RESULTS AND DISCUSSION
The proposed antenna is optimized with parameter‘s’, i.e. spacing between feed and ground plane. The s 11 graph for different values of ‘s’ is shown in Fig. 2. Optimum value of ‘s = 2 mm’ is selected and corresponding return loss is given in Fig. 3. As can be seen from Fig. 3 that antenna is properly matched in frequency range 2.1684 to 5.3287 GHz with notch at 2.74 GHz. It is evident from the literature that UWB support to antenna deteriorates gain performance of antenna. The proposed antenna presents good bandwidth gain product therefore that antenna performs well in terms of gain as well as bandwidth. In this novel design, Sierpenski Gasket is used as impedance transformer design to match feed with edge of Sierpenski carpet and also Sierpenski Gasket is radiating in close frequency bands to Sierpesnki Gasket bands. In Figs 4 and 5, current density at frequencies 2.38 and 4.925 GHz is shown, which depicts that at lower frequency, i.e. at GHz, larger part of patch is radiating. At higher frequencies, current density depicts that radiation is due to smaller slot of patch. Since both Sierpenski carpet and Sierpenski Gasket patches are resonating in close proximity in frequency results in merging of two bands, so that UWB is obtained while maintaining good gain bandwidth product (Fig. 6).
Fig. 2. Parameter s 11 versus frequency for different values of spacing ‘s’ between feed and ground plane.
Fig. 3. Parameter s 11 for proposed antenna at optimum value of spacing (s) = 2 mm.
Fig. 4. Current density of proposed wideband antenna at f = 2.38 GHz.
Fig. 5. Current density of proposed wideband antenna at f = 4.925 GHz.
Fig. 6. Radiation pattern of the proposed antenna at 4.32 and 2.8 GHz.
IV. CONCLUSION
A CPW-fed hybrid fractal-based antenna is considered for broadband applications. Two fractals are resonating in close proximity, therefore, overall broadband response of antenna is observed. The antenna works well in terms of s 11 in ranges 2–5 GHz and 7–9 GHz. The proposed design shows good response in terms of gain as well. The antenna can be efficiently used for UWB applications as well as WiFi and WiMAX applications as the gain provided by the antenna is quite good. Hybrid fractal approach increases the bandwidth of the antenna. The Sierpenski carpet type of antenna suffers from edge impedance matching problem while feeding and non-uniform density over its surface. The proposed antenna design offers solution to both the problems by using hybrid fractal given in this paper. Sierpenski Gasket is used as impedance transformer to Sierpenski carpet type of antenna.
Nitin Saluja received a Masters in Engineering from Thapar University in 2010 and is doing his Ph.D. degree. He is an Assistant Professor at Lingaya's University, Faridabad. His main research interests are design and optimization of Microstrip antenna design, Microwave Design, Fractal Antennas, Multiband and wideband antenna, and Wireless networks. He has published many papers in referred journals and conferences on Microstrip antenna and wireless networks.
Rajesh Khanna received a B.Sc. (Engg.) Degree in ECE in 1988 from REC, Kurukshetra and M.E. degree in 1998 from IISc., Bangalore. He was with Hartron R&D center till 1993. Until 1999, he was in AIR as AS Engg. Presently, he is working as Professor and HOD in the ECED at Thapar University, Patiala. He has published 80 papers in National and International Journal/Conferences. He has worth Rs 1.5 crore projects to his credit. His main research interest includes antennas, Wireless Communication, MIMO, and FFT.
