On the Design of Nano-arm Fractal Antenna for UWB Wireless Applications

This paper presents the design of a nano-arm fractal antenna suitable for ultra wide band applications. The CPW-feed and fractal concept have been used to achieve the ultra wide bandwidth. The shape of the fractal geometry, the number of iterations and the number of nano-arms are the deciding factors for achieving wider impedance bandwidth. The experimental result of the fractal antenna exhibits ultra wideband characteristics in the frequency range of 2.55 GHz to 11.84 GHz corresponding to an impedance bandwidth of 131.77%. The measured radiation patterns of the proposed antenna are nearly omni-directional in the H–plane and bidirectional in the E-plane. The antenna can be useful for modern wireless communication, medical imaging and ground penetrating radar. Index Terms – Planar Monopole antenna, Multiband antenna, Fractal Geometry, CPW Feed and UWB System


I. INTRODUCTION
The recent progress in UWB wireless communication applications has remarkably increased the demand for wideband antennas with smaller dimensions than conventionally possible [1].The antenna size with respect to the wavelength is the parameter that will have an influence on the radiation characteristics, gain and efficiency.Conventional microstrip antenna has limitations of narrow bandwidth, low gain and size of λ/2 [1][2].There are several techniques reported in the open literature to improve the bandwidth of the microstrip patch antenna such as insertion of air gap, stacking, ground coupling etc.The coupling effect is possible by changing the feed type and selecting proper values for its parameters.CPW feed offers better resonant characteristics and higher impedance bandwidth.[3][4].
The fractal geometry along with the CPW feed can be used for achieving ultra wide bandwidth [4].
The fractal geometry is supported by its two properties i.e. self similarity and space filling [5].The self-similarity property is useful for multiband or ultra wideband (UWB) feature while space filling property is useful for antenna miniaturization.Using these properties, several UWB monopole antennas have been reported in the literature [6][7][8][9][10][11].This paper presents an UWB antenna with andwidth beyond the required FCC band.Several modifications in the antenna structure like adding arms, increasing the number of iterations and introducing slots in the ground plane have been made to achieve the desired impedance bandwidth.A detailed parametric analysis has been presented and discussed.The simulated results of this antenna have been validated with experimental results.
The fractal antenna structure taken for investigation is shown in Figure .1.This fractal antenna is designed on a substrate of dielectric constant ε r = 4.3, thickness 1.53 mm and with dimensions 63.5 mm x 65 mm.The fractal antenna is constructed from a solid circular disc of radius 12.5 mm as shown in Fig. 1.This fractal antenna is constructed by using a set of scaled versions of the same shape which is similar to the concept of Sierpinski Gasket.In the first iteration, the perimeter of the circular patch is divided into twelve equal arcs, on the inner side of each arc; an isosceles triangle is drawn with an angle of 40 degrees from the perpendicular bisector of each node (edge of each arc).The resultant shape appears to be a chain of twelve inward looking triangles.This portion is etched off from the base.As a second step in the first iteration, eleven circular slots of 1 mm diameter have been drawn having their centres at a distance of 3.75 mm on the inner side of each node (except for the node near the feed) on the line connecting the node and the centre.These circular slots are then etched off the base.The resultant geometry from here onwards is referred to as the generator.This constitutes the first iteration.
The 2 nd iteration is a reduced copy of the design of the first iteration, repeated in an inner circle of 8.125 mm radius (which is 0.65 times of 12.5mm).Likewise the third and the fourth iterations are repetitions of their previous ones at a reduced ratio of 0.62, within the concentric circles.The infinite iterative structure which is ideally perceivable is not practically possible because of fabrication constraints.So for the present investigation, a fourth iterative Fractal Antenna is finalized and extensively studied with respect to its characteristics.
The CPW feed and its parameters such as the gap between the feed and the ground and the width of the feed are the critical factors which decide proper impedance matching.For the initial stage, they are fixed as 0.4 mm and 3.2 mm respectively.CPW feed offers excellent impedance bandwidth along with better radiation characteristics.Every dimension of the proposed antenna has been optimized to give the most appropriate results.

II. CURRENT DISTRIBUTION
The surface current density distribution is simulated at 2.3 GHz for the proposed fractal antenna corresponding to the first, second, third and fourth iterations and is illustrated in Fig. 2. Maximum current distribution is observed near the gap between the feed and the ground, on the edges of the radiating patch, on the ground near to the patch, and along the x-axis of the ground plane.That is why the size of the patch, the width and the length of the ground plane, the gap between the ground and the patch, the gap between the ground and the feed, all are crucial parameters for achieving the wide impedance bandwidth and need to be optimized.Initially, the fractal monopole antenna is simulated with the simple circular patch of radius 12. 5mm with CPW feed.The ground plane length and width are taken to be 34.64 mm and 31.1 mm.But with this circular patch and rectangular ground plane, the required UWB impedance bandwidth is not achieved.The twelve nano -arm fractal antenna as shown in Fig. 2 is simulated with respect to the various parameters for achieving the UWB characteristics.The parametric study with respect to various parameters is discussed below.

A. Effect in the Number of Iterations
The parametric study with respect to the number of iterations has been performed.The iterative behaviour of the fractal antenna is carried out and shown in Fig. 3.The circular microstrip patch antenna of radius 12.5 mm resonates at a frequency of 2.45 GHz.After the application of fractal geometry, the resonant frequency is shifted to the lower frequency side at 2.2 GHz.This is due to the increase in the resonant length due to the application of fractal geometry.The application of fractal leads to the reduction in the size of the patch.There is a distinct shift in the fundamental resonant frequency dip as the number of iterations goes on increasing.As the number of iterations increases, the first resonant frequency dip on the reflection coefficient curve shifts to the lower frequency side as illustrated in Fig. 3.It is also observed that the reflection coefficient improves as iteration increases through out the band.As seen in Fig. 3, the reflection coefficient is poor for the first iteration but improves substantially at the fourth iteration through out the operating band.This is one of the importance characteristic of fractal geometry that impedance matching improves with proper number of iterations.It means no extra circuitry is required for the impedance matching of a fractal antenna.This behaviour of the antenna is observed till the fourth iteration.So, a fouth iterative twelve nano-arm fractal antenna is considered for further investigation.This antenna is said to exhibit -10 dB reflection coefficient bandwidth in the frequency range of 2.6 GHz to 11.8 GHz.

B. Effect of Ground Plane Length (G L )
The length of the ground plane plays a vital role in determining the UWB characteristics.The effect on the reflection coefficient characteristics of the variation in the ground length is shown in Fig. 4.
The ground plane length is varied in steps of 2 mm from 28.64 mm to 34.64 mm.As the length of the ground plane increases, the reflection coefficient curve is shifted to the lower frequency side.This is clearly observable near the second, third and fourth dips of the reflection coefficient curve shown in  It means that there is one value of ground width for optimum impedance matching.

D. Effect of Gap between Patch and Ground (G P-G )
The gap between the patch and the ground is also a critical factor in determining the impedance bandwidth of the antenna.Hence, a parametric analysis with respect to the gap between the patch and the ground is done with respect to the fourth iterative fractal antenna.The behaviour is observed by varying the gap (G P-G ) in steps of 0.1 mm.The gap between the patch and the ground is varied to 0.4 mm, 0.5 mm, 0.6 mm, and 0.7 mm and their corresponding reflection coefficient characteristics are shown in Fig. 6.
There is no one to one correspondence between the change in the reflection coefficient and the gap between the patch and the ground.As the ground plane is closer to the radiating patch, most of the fields will be radiated and less fringing fields reside in the substrate.

E. Effect of the Gap between the Feed and the Ground
This twelve nano-arm fractal antenna is fed with CPW feed and it is well known that for better impedance matching, the gap between the feed and the ground, and the width of the feed are the deciding parameters.Here the width of the feed is kept constant at 3.2 mm.The gap between the feed and the ground is selected so as to provide best matching for the 50 ohm line.The separation between the feed and the ground was optimized over various values to yield wider bandwidth.The optimum gap between the ground and the feed is obtained as 0.5 mm.At this gap, the impedance is around 50 ohms.increasing.This may be due to the higher magnitude of higher modes at higher frequencies and the fractal geometry of the antenna.At lower frequencies omni directional nature of radiation patterns in H -plane and bidirectional nature of radiation pattern in E -plane are evident.

Fig. 1 .
Fig. 1.Four iterative Concentric Nano-arm fractal antenna and its side view

Fig. 3 .
Fig. 3. Effect on the return loss characteristics due to increase in iterations

Fig. 4 .
Fig. 4. It is observed that better performance is achieved at 34.64 mm ground length throughout the band.

Fig. 4 .
Fig. 4. Effect of variation in length of the ground plane

Fig. 5 .
Fig. 5. Effect of variation in width of the ground plane

Fig. 6 .
Fig. 6.Variation of gap between patch and the ground

Fig. 7 .
Fig. 7. Effect of the separation between feed and ground

Fig. 9 .
Fig. 9. Simulated reflection coefficient with and without modified ground