A Defected Ground based Fractal Antenna for C and S Band Applications

In this paper, a modified ground based rectangular shaped fractal patch antenna will be presented. Since, the research community has witnessed a significant demand of both light weight and small sized micro-chip antenna in the domain of wireless communication the physical size of the communication antenna has been reduced manifold. The optimum performance of the wideband transmission antenna can be achieved while improving the certain antenna performance parameters that include: reflection coefficient (S11), radiation pattern, antenna gain, Voltage Standing Wave Ratio(VSWR) and available communication bandwidth. Our proposed antenna design and structure encompasses several frequency bands like satellite communication (2.3 GHz), Earth-to-Space communication band (2.17 – 2.2 GHz), Wi-Fi communication band (2.4 GHz for IEEE 802.11b and 802.11g standards) Wireless LAN (2.40 – 2.84 GHz), Bluetooth (2.45 GHz), Mobile Wi-Max (2.5 – 2.69 GHz). The proposed antenna has been designed, built and simulated by utilizing the available FR4 epoxy substrate material having a relative permittivity of 4.5. A line feeding technique has been implemented in the designing of antenna of the size of (34 x 32 x 1.8) mm. In order to realize the objective a ground plane has been made defective. The proposed antenna will successfully operate in the C and S band applications having a frequency range of 2.05 – 4.88 GHz. Keywords—Antenna gain; voltage standing wave ratio (VSWR); coefficient of reflection; antenna radiation pattern; defected ground


I. INTRODUCTION
Mobile communication system demands antenna with wider bandwidth and smaller size than traditional ones these days [1][2][3][4][5][6][7][8].Small size antennas are more demanding in wireless mobile communication.The increased demand has created the interest of researchers to work on micro strip antenna design [5].Delay Tolerant Networks (DTN), Mobile Ad-hoc Networks (MANET) and Wireless Sensor Networks include mobile wireless devices.In mobile communication, devices are not necessarily connected to each other.DTN works well in these types of situations.Broadcasting properties of Epidemic Routing Protocol in DTN equipped with shortrange communication antennas have been studied by researchers [6].When an array antenna is used in mobile communication, it improves the performance by increasing channel capacity and efficiency spectrum [8].Thus, micro strip antennas are used in mobile communication like DTN and MANET to improve channel bandwidth and range of devices.The proposed paper designs rectangular patch antenna in which the concept of fractal is applied by subtracting circles inscribed in rectangular slots from the patch.Specially defected ground concept has been used to enhance impedance bandwidth and performance of the proposed antenna.Self-assured geometries like snowflakes, trees, clouds, coastlines etc. were named fractal geometries due to their irregular shapes [9].Any shape or figure having same statistical characteristics during its whole length is known as fractal.Fractal antennas are widely used antenna structures due to their numerous advantages like small size also a wideband characteristics along with improved performance parameters.They are also used in a variety of applications like radar, missiles, aircraft, satellite communications, mobile communication base stations, handsets and biomedical telemetry services [10].Initially, every antenna operates with single frequency band which causes limited space problem.Earlier multiple antennas were used to obtain multiband operations but now a day's single fractal antenna can produce multiple bands [11].Therefore, the technique to develop wideband antenna by enforcing fractal concept has been used in the proposed antenna design.In the growth of wireless communication, antennas have to be less in weight, having more than one band and compact so that these can easily be fabricated [12].However, conventional Microstrip Patch Antennas have some limitations such as poor efficiency, narrow bandwidth and low gain [13].To avoid these limitations of antennas, several enhancement techniques have been developed.Some researchers have given the different fractal shapes like Sierpinski Carpet, Sierpinski Gasket, and Slotted Patch with thick substrates and low dielectric constant substrate materials [14].Various fractal geometries are used to design wideband fractal antennas due to properties like space filling, self-similarity etc. Various performance parameters of the antenna like Reflection Coefficient (S11), Radiation Patterns, Gain and VSWR are calculated using numerical simulation and measurements.
In this presented paper, Section II presents structure of the proposed antenna and design.Simulated and experimental verification of proposed antenna has been given place in Section III.Section IV describes conclusion and major findings.

II. DESIGN AND STRUCTURE OF ANTENNA
Fractal antenna methodology for the design of the basic structure of defected ground based rectangular fractal patch antenna has been used.Various equations (1)(2)(3)(4)(5)(6) are used for designing the proposed antenna [8][9][10][11].www.ijacsa.thesai.orgPractical width of patch is calculated using equation Where v0 is the free-space velocity of light The effective dielectric constant, εeff is given Where ɛr is dielectric constant of material, w is width of patch, h is height of substrate.
Prolonged electrical length, ΔL of the patch due to fringing effect can be calculated using The effective length, Leff is given The dimensions of ground plane are calculated as under (5) (6) The steps for designing the proposed antenna are as given.
Step 1: Design of antenna starts with a substrate sandwiched between the ground plane on the lower side and patch on the upper side.Antenna is designed with FR4 epoxy substrate having dielectric constant 4.4, loss tangent 0.002 and mass density 1900 kg/m3.The size of substrate is 32 mm×30 mm×1.6 mm.
Step 2: Rectangular patch of size 10.5 mm×9 mm is used above the substrate as shown in Figure 1(a).Circles inscribed in rectangular slots of various dimensions are subtracted from the patch in the various iterations.
Step 3: In the 2nd iteration, a circle of diameter 8.8 mm is subtracted from the patch along with a union of two rectangular slots of 1.4×9 mm2 and 9×10.5 mm2 as shown in Figure 1(b).
Step 4: In the 3rd iteration, one more circle of diameter 2.9 mm is inscribed in rectangular patch of size 4.4 mm×3.8 mm is subtracted as shown in Figure 1(c).
Step 5: In the 4th iteration, one more circle of diameter 1.8 mm is subtracted from rectangle patch of size 7 mm×6 mm as shown in Figure 1(d).
Step 6: The size of ground plane is 32 mm×30 mm.Defected ground concept is used to enhance the performance in terms of increased bandwidth, gain, Reflection Coefficient (S11).Rectangular slot of 8×21 mm2 is subtracted from the patch to reduce the ground plane.
Step 7: The performance of proposed antenna is checked using various feed techniques.50 ohm micro strip line feeding technique gives best results.The characteristics parameters are calculated by changing the position of the feed along all the sides of the proposed antenna.Line feed of size 2.6 mm×14 mm is used as a lumped port.Table 1 shows various design parameters of the proposed antenna.
Table 2 shows the substrate, patch, microstrip line feed, slots in various iterations, strip size and defected ground dimensions.Figure 1 shows all the four iterations of the proposed rectangular fractal patch antenna.
Five different substrates have been used to design the proposed antenna with different relative permittivities of materials.The proposed antenna is fabricated on FR4 epoxy material as it gives better performance characteristics and impedance bandwidth.Figure 2 illustrates that the proposed prototype is compact in size, cost effective and light in weight as the dimensions of patch and ground are very less as compared to ordinary fractal antenna.Fabricated prototype of the proposed antenna is shown in Figure 2.The size of antenna is compared with the size of coin.[15].A comparison of S 11 of the proposed antenna for all the iterations is shown in Figure 3. Figure shows that the antenna is capable of producing wideband frequency response.From Figure it is clear that 0 th iteration has acceptable reflection-coefficient from 3.25 GHz to 4.82 GHz, from 3.06 GHz to 4.93 GHz in 1 st iteration, from 2.93 GHz to 4.84 GHz in 2 nd iteration and from 2.05 to 4.88 GHz in 3 rd iteration.
The results are also analyzed with different values of D 1 , D 2 and D 3 as shown in Figure 4(a), 4(b) and 4(c) respectively.It is clear that the proposed system resonates at high frequencies with D 1 and D 2 .For D 3 = 4.4 mm, resonant frequency shifts to lower side and miniaturisation of antenna takes place.

B. S 11 with Different Defected Ground Surfaces
It is well known from various studies that when ground plane of antenna is reduced, it changes the performance characteristics of antenna.The performance parameters of the proposed antenna with defected ground become better than antenna with full ground plane.The proposed antenna with different defected ground slots at bottom side of substrate is analysed.Four rectangular slots are etched from the ground surface as shown in Figure 5.
It is quite evident that a slot with size 9×24 mm 2 gives greater bandwidth as compared to other sizes of slots.

C. Effect of Feed Position
The proposed antenna is excited using microstrip line feed.Effect of two other positions at right and left side of patch is also analyzed and as shown in Figure 6.Both the feed positions (at left and right corners) do not provide sound energy as compared to the centre feeding point.

D. Radiation Pattern
This parameter provides information about the energy that is radiated by the antenna [20].Either a rectangular or a polar format is used to present these pattern [21].The two dimensional radiation patterns of 0 th , 1 st , 2 nd and 3 rd iterations of the proposed antenna at various resonating frequencies for ɸ = 0 o and ɸ = 90 o are displayed in Figure 7. 0 th iteration has a gain of 4.77 dB at 3.25 GHz, 1 st iteration has a gain of 5.92 dB at 3.06 GHz, 2 nd iteration has a gain of 5.56 dB at 2.93GHz and 3 rd iteration has achieved a gain up to 6.83 dB at 2.85 GHz frequency.
Table 4 shows the results of various iterations of the proposed antenna in terms of resonant frequency (Fr), Reflection coefficient S 11 (dB), Gain (G) and VSWR.Better results are obtained in terms of gain and bandwidth in final iteration.

E. VSWR
Voltage Standing Wave Ratio (VSWR) of antenna is used to get impedance matching [22].The Values of VSWR are 1.492, 1.328, 1.273 and 1.230 at 3.35 GHz, 3.90 GHz, 3.00 GHz and 2.85 GHz resonant frequencies in 0 th , 1 st , 2 nd and 3 rd iterations, respectively.A comparison between simulated and measured VSWR is shown in Figure 8.The proposed antenna has acceptable values of VSWR in all the iterations.
The effect of different substrate materials like Bakelite, Glass, Arlon and FR4 on the Reflection coefficient (S 11) of the proposed antenna is shown in Figure 9.It is found that Arlon material has minimum value of S 11 .Due to availability of FR4 material, the proposed antenna is fabricated using FR4.The proposed antenna is fabricated on FR4 material substrate.The fabrication of antenna is done with the help of screen printing technique.The start frequency is 1.0 GHz and stop frequency is 12 GHz.The antenna is connected with Vector Network Analyser (VNA) experimental kit after calibration of port .The analysis of S 11 is as shown in Figure 11.A little variation is occurred due to environmental noise.However, another frequency band is achieved in measured return loss.
The Proposed antenna has area of 960 mm 2 with bandwidth of 2830 MHz.It is compared with existing antennas [22][23][24][25][26][27][28][29].A comparison in terms of Gain (G), substrate size and material, bandwidth, number of frequency bands and resonant frequencies is shown in Table 5.The proposed antenna is better in terms of performance parameters like gain and bandwidth as shown in Table below.

IV. CONCLUSION
A defected ground based fractal antenna with wideband frequency response has been designed.The antenna operates from 2.05 GHz to 4.88 GHz covering S and C band applications.All the simulations are performed by finite element method based High Frequency Structural Simulator.These bands cover applications like earth to space communication, satellite communication, WLAN, WiFi, RFID, microwave oven, Bluetooth, wireless computer networking, mobile Wi-Max, direct to home services, satellite communication for downlink, wireless fidelity and satellite communication for uplink.The impedance matching of proposed antenna is done by microstrip line feeding technique.Various iterations of the proposed antenna geometry are performed to improve the characteristic parameters.The final iteration has a gain of 6.83 dB with S 11 value -19.72 dB, bandwidth of 2830 MHz and VSWR of 1.23 at 2.85 GHz frequency.Antenna size gets reduced by 38.73 % with deflective ground surface.The frequency is shifted towards the lower side.

Fig. 5 .
Fig. 5. S11 Plot of Defected Ground with Different Rectangular Slots in Ground.

Fig. 6 .
Fig. 6.Graph of Return Loss with Different Feeding Points.

Fig. 7 .
Fig. 7. Two Dimensional Radiation Pattern of the Proposed Antenna for (a) 3.92 GHz in 0 th Iteration (b) 3.82 GHz in 1 st Iteration (c) 3.70 GHz in 2 nd Iteration (d) 2.85 GHz in 3 rd Iteration.

Fig. 8 .
Fig. 8.Comparison between Simulated and Measured VSWR of the Proposed Antenna.

Fig. 9 .
Fig. 9. Effect of Substrate Material on Performance of Proposed Antenna.F.Current DistributionSurface current distribution is observed at magnitude J surface field.It represents the characteristics of radiation intensity and flow of current in the radiating element of antenna.The current distribution at resonant frequency 2.85 GHz is 401 ampere per meter and at 2.45 GHz, it is 328 ampere per meter as shown in Fig10 below.This parameter also depicts that a particular area of patch antenna is responsible for radiation resonance frequency.

TABLE III .
DISTRIBUTION OF FREQUENCY BANDS, ASSOCIATED RETURN LOSS, GAIN, VSWR, IMPEDANCE BANDWIDTH AND PERCENTAGE BANDWIDTH

TABLE V .
COMPARISON OF THE PROPOSED ANTENNA WITH SIMILAR ANTENNAS