In advanced communication systems large bandwidth and smaller dimensions are required
In advanced communication systems large bandwidth and smaller dimensions are required. There have been growing demands for antenna design that possess features such as low size, low profile, low cost, multiband and broadband etc 11, 34, 35.
There is immense requirement of antennas in the field of communication e.g. in personal communication, WIMAX, Wi-Fi, WLAN, missile technology, satellites, spacecrafts and aircrafts etc 20,25. Antenna is a transducer that is used at transmitter side and receiver side for transmission and reception of electromagnetic waves into free space. It is a structure which intermediates between guiding device and free space. It is suitable for microwave frequencies from 300 MHz to 300 GHz 1, 25, 32.
With the development in communication systems engineering, there is requirement of miniaturized antennas. Deschamps offered idea of microstrip patch antenna (MPA) in 1953. But first practical microstrip antenna was fabricated by Munson and Howell in 1970 6, 24. Microstrip Antennas are suitable for communication due to its small size, low profile, light weight, inexpensive to fabricate using printed circuit technology 1,10,20,14. A patch antenna is a narrowband and wide beam antenna. It can be fabricated by engraving the antenna element pattern in metal outline tied to an insulating dielectric substrate such as a printed circuit board with a continuous metal layer attached to the opposite side of the substrate which acts as a ground plane, its disadvantages are low gain, low efficiency, low power handling capability, poor polarization purity, requirement of superior substrate and high temperature stability and its major disadvantage is narrow bandwidth 1,33,21.
Also Fractal antennas are very useful in cellular phone and microwave communications. Fractal antennas have attractive way out to use a single small antenna operating in several frequency bands. 28, 34, 11.
A fractal is a rough configuration which can be subdivided in many parts, each part is a reduced copy of the whole. Fractals are generally self-correlated and are scale independent and they increase the perimeter of material 4,15,16. Mandelbrot is a person who is associated with the mathematics of fractals. In 1977, He added a definition of fractal dimension 6,11. Fractals can also be in form of mathematical structures e.g. Fractals also describe many real-world objects in form of clouds, mountains, turbulence, and coastlines 12,14.
Fractal antennas can also be designed by repeating the scaled version of basic antenna element iteratively 15, 16. The complexity of antenna increases with increasing number of iterations. Also number of frequency bands is directly proportional to the number of slots etched on patch means as number of slots increases, number of resonances increases so fractal antenna can be used as multiband antenna 28,31.
There are many shapes of fractals which can be used in antenna designing. These shapes can be described mathematically as: Sierpinski carpet and gasket, Minkowski gasket, Koch snowflake, Mandelbrot set, crossbar etc 16. These are the deterministic fractals which can be designed using a recursive algorithm.
The major advantage of fractal structure is to increase the electrical length of antenna as compared to conventional microstrip antenna. This property offers two advantages. One is reduction of the overall size of the antenna and the other is introduction of multiple resonant frequencies 35. it is also included in the concept of fractal antenna that with etched multiple slots antenna gain is reduced because maximum portion of the conducting patch is etched out. There are many characteristics of fractal antennas such as self-similarity, gap filling, fractional ratios and unlimited complexity which are advantageousof fractal antennas over conventional microstrip patch antennas34 .
Fig 1: stages in the construction of a Sierpinski gasket Fractal antenna 24
There are many versions of present antennas that are created by fractal antennas such as dipole, monopole, patch, biconical, conformal, spiral, helical and others 35.
In order to increase bandwidth and for multiband operation, many techniques are followed such as different iterations, different substrate materials and varying thickness of substrate. Different parameters of antenna like return loss, gain, bandwidth and directivity of proposed structures are considered.Major purpose of fractal antenna is to increase the electrical length of antenna without influencing the radiation characteristics of the conventional antenna 3,16.
In order to increase the performance of antenna in electromagnetic applications, designing of new substrate textures is required.7. Because fabrication of inhomogeneous materials for fabrication of antenna substrate is a difficult task and due to shortage of analysis tools, Due to challenges associated in the fabrication of inhomogeneous materials and short access to analysis tools, material optimization has not been pursued 8,9. An important advantage of this optimization approach is that it is possible to solve many problems and objectives. 4,9.
High-permittivity material of antenna substrate suffers from narrow bandwidth and low efficiency 20,26.
There are three methods to improve the bandwidth such as increasing the size, increasing the profile or using different feeding networks. It is difficult to find best method of these 1,10.
Among microwave frequency range, X-band is greatly required in various new wireless applications 2,18. X-band is used for point to point communication such as traffic control, radar and satellite communications 22. There is a major requirement of X band technology because of its high data transmission rate and large bandwidth 27,29. Many researchers are attracted to design X-band antennas and these antennas are still a foremost challenge to equalize these applications 30.
Thus a high gain and broad band, and highly efficient microstrip antenna is greatly desirable in order to make a trustworthy and cost effective radio link for wireless applications 23, 19.
