On the Design of Uwb Circular Fractal Antenna with Notched-band Characteristics Using W-shape Slot

This paper presents the design of notched-band antenna with CPW-feed. The notched – band has been achieved by using the W-shape slot in ground plane. The antenna has been simulated using a 3D – Electromagnetic simulator. The experimental results of proposed antenna exhibit ultra wideband characteristics from 2.0-15.0 GHz except the notched-band frequency from 5.0 – 6.0 GHz. The slot is optimized with respect to width, length and position for a detailed study of notched-band characteristics. It is observed that the centre frequency and bandwidth of notched-band depend on the length and width of slot respectively. The experimental radiation patterns of this antenna are omni-directional in H-plane and bidirectional in E-plane. This antenna can be useful for UWB system, medical imaging and vehicular radar.


I. INTRODUCTION
With the release of the 3.1 -10.6 GHz band for ultra-wideband (UWB) operation, a variety of typical UWB applications evolved are indoor/outdoor communication systems, ground penetrating and vehicular radars, wall and through-wall imaging, medical imaging and surveillance.Many future modern UWB communication systems will be utilized to transmit /receive the high data rate with low power consumption at short range communication [1][2].Therefore, the realization of UWB antennas in planar technologies with small size is the primary requirement.The antenna should have omnidirectional radiation patterns throughout the band.In UWB operating band, the WLAN signal is also present which interferes the UWB signal.To stop the interference by external WLAN signal (5.0 to 6.0 GHz), a band notched filter is required.Using a band notched filter separately along with antenna increases the system size as well as system complexity.To overcome this complexity, many researchers have reported antennas with notched-band characteristics using various shapes of slots in ground plane, feed line and on the radiating patch [3][4][5][6][7][8][9][10]. The most common technique in open literature is a slot loading in the radiation patch [3][4][5].This gives poor cross-polarization characteristics because the distribution of surface electricity is changed.Other designing methods include incorporating a L-shaped slits in the ground plane to achieve notched-band function [6][7][8].
Beside these methods, another method is to design a resonant circuit on the feed line [9,10].The Lshape slot reported in [6][7][8] is large in size.There is a need to have a compact slot in size.This paper reports the compact W-shape slot in ground to achieve the notched-band characteristic.
Several researchers have also reported the UWB characteristics of antenna using fractal geometry.
Fractal geometry is useful for size reduction and good impedance matching throughout the band.Some researchers have reported the UWB fractal antenna with CPW -feed [10][11][12][13].Fractal geometry in monopole antennas with CPW -feed offers the advantages of compactness, UWB characteristics throughout the band, good radiation pattern and low dispersion.
In this article, a new UWB fractal antenna with notched-band characteristics is presented.The notched-band characteristic is achieved by cutting a W-shape slot in ground plane.This antenna is simple, easy to fabricate and integrate with MIC/MMICs devices.The notched-band characteristic is experimentally validated and simulated with respect to various parameters of the slot in the ground plane.
This paper has been divided into six sections.In Section -I, the introductory part of article is discussed.The second section of the paper describes the antenna geometry and slot in ground plane responsible for notched-band characteristics.In Section -III, the approximate design expressions for notched-band frequency is given.The Section -IV concentrates on the parametric analysis of slot dimensions responsible for notched-band characteristics.The section -V discusses the experimental and simulated results of antenna with and without notched-band and in last section the conclusions of the article is given.

II. ANTENNA GEOMETRY
The monopole antenna with CPW -feed is convenient to integrate with MIC/MMICs circuits.The fractal antenna with CPW -feed is advantageous to achieve the UWB characteristic without any additional matching circuitry.This paper proposes a new fractal antenna to achieve UWB characteristics.The geometry of proposed fractal antenna has been constructed inside the circular disc of 30 mm diameter with metallization thickness of 0.1 mm with the centre as origin (0,0).This is called zeroth iteration.The 1 st iterative structure has been constructed by creating 10 different triangles.First triangle with side length of 16.63 mm is made inside the circular disc.This triangle is moved along coordinate (-4.5 mm, 2.76 mm).The second triangle of side length 18.72 mm is made and rotated by an angle 40 0 .This triangle is moved along (-3.5 mm, -2.1 mm) coordinate.The third triangle with side length of 16.64 mm is rotated by 65 0 and moved along (-0.7 mm, -6.2 mm) coordinate.The fourth triangle with side length of 18.02 mm is rotated by 90 0 and moved along (3.3 mm, 5.1 mm) coordinate.The fifth triangle with side length of 10.1 mm is rotated by an angle 76 0 and moved along (5.79 mm, 8.75 mm) coordinate.The sixth triangle with side length of 16.64 mm is rotated by an angle 90 0 and moved along (-0.7 mm, 5.7 mm) coordinate.The seventh triangle with side length of 18.02 mm is rotated by an angle 110 0 and moved along (3.7 mm, -5.2 mm) coordinated.
The eighth triangle with side length of 10.1 mm is rotated by an angle 106 0 and moved along (6.15 mm, -8.4 mm) coordinate.Similarly, ninth & tenth triangles each of side length of 7.348 mm are rotated by an angle 100 0 and are moved along (9.15 mm, -7.75 mm) and (9 mm, 7.8mm) coordinate respectively.After placing these triangles, the geometry is obtained as shown in Figure 1 (a).Then these triangles are united and subtracted from the circular disc of 30 mm diameter.Now, one more circular disc with diameter of 23.4 mm and metallization thickness of 0.1 mm is made with centre at -3.3 mm as shown in Figure 1(b).Then this circular disc is subtracted as shown in Figure 1(c).This geometry is fed by CPW -feed.This is called 1 st iteration as shown in Figure 1(d).
In 2 nd iteration, geometry of 1 st iterative structure is scaled down by a factor of 0.79, copied in a circle of 23.4 mm diameter with centre at (0,-3.3mm) and subtracted.We get the geometry similar to first iterative geometry but smaller in size.This geometry is made to intersect with the original circle near the feed line.This is called second iteration.In 3 rd iteration, the second iterative fractal geometry is copied and scaled down with factor of 0.8.This geometry is copied in the circle of 18.72 mm diameter with centre (0, -5.64mm).This copied geometry is made to intersect at the original circle near feed line.In 4 th iteration, the third iterative fractal geometry is copied and scaled down by factor of 0.495.This geometry is copied in circle of 9.2664 mm with centre at (0, -10.3668 mm) and subtracted from circular disc.This geometry is made to intersect with the original circle near feed line in similar manner as earlier.This is the final proposed fractal antenna with optimized dimensions as shown in Figure 2a.The W-shape slot is created in the ground plane on both sides of the feed line as shown in Figure 2a.The dimensions of the W-shape slot are shown in Figure 2b separately to have the clarity in dimensional.The W-shape slot is made 2.516 mm away from ground edge at feed side.The total angle of the W-shape slot has been taken as 75 degree.One side of the W-shape slot cuts the upper edge of ground plane.Another side of W-shape slot is at 0.516 mm below the upper edge of ground plane.The total length of the slot cut in ground plane is 9.0 mm.(2) This expression can be used for calculating the rough slot length of slot in ground plane.This estimated length is optimized using 3D -Electromagnetic software.This antenna with notched-band characteristic has been designed on FR4 substrate of dielectric constant r = 4.3 and thickness of substrate h = 1.53 mm.

IV. PARAMETRIC STUDY OF SLOT
The parametric study of slot in ground for creating the notched-band characteristic has been carried out using HFSS software.The W-shape slot width, length and its position are the critical parameters to achieve the required notched -band characteristics.This is because, current distribution is stronger      The gain and radiation efficiency of the proposed antenna have also been calculated and tabulated in table 1.It is revealed from tabulated results that the gain and radiation efficiency drop drastically at notched-band frequency.This indicates that external signal like WLAN will not interfere in the operating band of this antenna.It is also observed at higher frequency that the radiation efficiency reduces which may be because of loss tangent of FR4 substrate.
(a) (b) (c) (d) Fig. 1.Basic Geometry of the proposed antenna (a) (b) Fig. 2 a) Proposed fractal antenna with optimized dimensions, b) Slot dimensions III.DESIGN EXPRESSIONS The notched -band has been created by cutting the slots in the ground plane.A W-shape slot is chosen to be the slot cut on both sides of the ground plane with its dimensions optimized systematically on HFSS simulation software.The optimized dimensions are slot width w = 0.21 mm and length of the slot L = 9.0 mm.These dimensions provide exact notched-band characteristics from 5.0 GHz to 6.0 GHz.The formulation for the calculation of notched-band centre frequency (f notch ) can be as follows: ε r = relative permittivity of the substrate b = thickness of the substrate w = width of the notch ε eff = effective permittivity of the substrate The total length of the slot is equal to L notch =L 1 +L 2 +L 3 +L 4 , where L 1 =2.0 mm, L 2 =2.15 mm, L 3 =2.15mm and L 4 =2.70 mm.Therefore, total length is L=9.0 mm.Since the slot is cut through the ground at upper edge at one end and other end in the ground plane.We consider it is to be open circuited at one end and short circuited at the other.The resultant length of slot is equal to λ/4 at notched center frequency.So, length of the slot i.e.L notch = λ/4.This can be substituted in equation (1); at notched-band centre frequency 5.5 GHz.The current distribution on the W-shape slot antenna is shown at frequencies 3.5 GHz and 5.5 GHz in Figure3.It is seen that the current density is stronger around slot at frequency 5.5 GHz i.e. at band-notched centre frequency.The current density is negligible at other than the notched -band frequency i.e. at 3.5 GHz as shown in Figure3.Due to stronger current density at frequency 5.5 GHz around W-shape slot, the W-shape slot length, width and position affect the notched -band characteristics.The proposed antenna has been optimized for the notched -band from 5.0 -6.0 GHz.The proposed antenna with optimized dimension is shown in

Fig. 5 . 6 .Fig. 6 .
Fig. 5. Simulated results of proposed antenna with respect to various width of slot

Table 1 :Frequency
Gain and Radiation Efficiency of the proposed antennaThe new proposed fractal antenna has been designed and experimentally validated.The antenna exhibits UWB characteristics beyond the FCC recommended band (3.1-10.6GHz).The notchedband from 5.0 GHz to 6.0 GHz is achieved using W-shape slot in ground plane.The effects on the notched -band characteristic have been studied with respect to length, width and position of slot.The length and width of slot affect the centre frequency and bandwidth of notched -band respectively.Measured radiation pattern of the proposed antenna is omni-directional in H-plane and bidirectional in E-plane.The gain and radiation efficiency of this antenna fall down drastically at notched-band centre frequency.This antenna is conformal, lightweight, small in size, low cost and easy to integrate with MIC/MMICs devices.Such type of antenna can be used for UWB system, Microwave medical imaging, vehicular radar and precision positioning.