A Novel Swastik Shaped Spidron Fractal Array Antenna for S-Band Applications
Swetha Amit 1, Chinmoy Kumar P R 2, Nayana Arvind Laxmeshwar3, Saurabh R Badenkal4
1Assistant Professor, Dept. of Telecommunication Engg. M S Ramaiah Institiue of Technology, Bangalore-54
2, 3, 4 B.E Students, Telecommunication Engg. M S Ramaiah Institiue of Technology, Bangalore-54
Abstract: Fractal geometries have shown to be attractive for antenna designers because of the unique features they offer. In this paper, a dual-resonant frequencies Swastik shaped Spidron arm fractal slot antenna array of four elements with two feed lines and a single port is developed for Satellite communication/ IEEE802.11band802.11g / WiMAX. The fractal antenna is fed by two micro strip lines at end of substrate for resonance at 2GHz and 3.8GHz. The antenna structure on the substrate has dimensions of 55mm*55mm*1mm. A conducting reflector is placed at a distance k=17mm under the substrate to reduce back radiation from the fractal antenna, thereby enhancing the antenna gain. The positioning of the micro strip feeder is done in such a way so as to reduce the coupling losses. The single element Spidron antenna is structured for 10 iterations. Results of 10 iteration Spidron arm fractal slot antenna for each array element indicate 10db reflection coefficient at all the above said resonant frequencies.
Index terms: Swastik-shape, Spidron arm Fractal, Koch-Like antenna, Slot antenna array, Dual-band, micro strip feed, coupling loss.
I.INTRODUCTION
In today world of wireless communications, there has been an increasing need for more compact and portable communications systems. Fractal geometry antennas are being studied in order to answer those requirements. Fractal antenna theory uses a modern (fractal) geometry that is a natural extension of Euclidian geometry. The project undertaken was to construct antennas using fractal patterns in order to obtain desired performance properties such as compact size and multi-band behavior. One of the prevailing trends in modern wireless mobile devices is a continuing decrease in physical size. In addition, as integration of multiple wireless technologies becomes possible, the wireless device will operate at multiple frequency bands. A reduction in physical size and multi-band capability are thus important design requirements for antennas in future wireless devices. The use of fractal patterns in antenna design provides a simple and efficient method for obtaining the desired compactness and multi-band properties. Fractals can be constructed using iterations; this procedure is normally called Iterated Function systems (IFS). Fractals are made up from the sum of copies from itself; each copy will be smaller copy from the previous iterations. The fractal antenna not only has a large effective length, but the contours of its shape can generate a capacitance or inductance that can help to match the antenna to the circuit.
Microstrip antennas were born of microstrip circuit technology and inherited many characteristics, such as low radiating efficiency and narrow bandwidth that are undesirable for a radiator [6] .Owing to the low profile, light weight, and the ease with which they can be integrated with active devices [5], microstrip antennas have become widely used over the past few decades. For dual-band operation, dual-port micro strip feeding lines are used [3]-[4]. The length of the microstrip lines is selected, so that the antenna resonates at a particular operating frequency (l/3 ≤ fs1 ≤ l/2). The length of the stripline needs to be tuned to account for the fringing fields at the edges of the patch. Finally, the width of the patch is used to adjust the input impedance of the antenna. The feedlines are arranged in a manner to reduce coupling losses.
In the paper, we propose a dual-resonant frequency antenna that utilizes an aperture-shaped Spidron fractal slot on a single substrate. Array of 4 elements is structured to form a shape of Swastik. The proposed antenna has only one radiating port which is fed by two conventional micro strip feeding lines. Positioning of the striplines was done to verify the performance of the antenna. Details on the proposed antenna with 10 iterations of Spidron slot fractal antenna and their performance evaluation is discussed in following sections. With array of 4 elements, dual resonant frequency in S-band are obtained which is used for applications in Satellite communication, IEEE802.11band802.11g / WiMAX.
The bandwidth is increased as compared to 7 iterations which has been discussed [7]. A conducting reflector at the bottom of the fractal antenna is used to enhance the gain. The reflector is placed at a distance of l/4 which is 17mm.
II.SINGLE SPIDRON FRACTAL SLOT ANTENNA
A. Single Antenna Configuration
Figure 1: Geometry of the antenna with front view and side view.
Figure 1 show the configuration and design parameters for the Swastik shaped Spidron Fractal array antenna proposed in this paper. A Spidron fractal is a plane figure that is iteratively constructed from a series of progressively smaller, contiguous right triangles using a common angular factor (α) as shown in figure 2. [1]
Iteration 10
Figure 2: Iterations of Spidron Fractal Slot Antenna.
Let the dimensions of the 1st triangle be ‘a’ mm x ‘b’ mm x ‘c’ mm as shown in the Figure 2.Then for the next iteration triangle the scaling factor ‘S’ is given by
S (scaling factor) = , for 0° 45°
Therefore the 3 sides of the new triangle are got by scaling down all the 3 sides by S.
Let a1, b1 and c1 be the dimensions of the new triangle such that a1=S*a, b1=S*b, c1=S*c, then the perpendicular a1 is placed on hypotenuse of the 1st triangle to get the 2nd figure. This is termed as 2nd iteration. The same process is continued up to 10 iterations.
Parameter / Value / Parameter / Valueα / 35.5° / sw1 / 1.8 mm
h / 36.5mm / sh1 / 20 mm
t / 1 mm / sd1 / 33.1 mm
mw / 55 mm / sw2 / 1.8 mm
mh / 55 mm / sh2 / 15.5 mm
k / 17 mm / sd2 / 19.6 mm
Table 1: Dimensions for the proposed antenna.
III.ANALYSIS OF ANTENNA ELEMENT
The antenna was simulated using an ANSYS high-frequency structure simulation (HFSS) based on a three-dimensional finite element method (FEM). [9]
The array configuration is shown below. It is formed by combining the base of the four single spidron structures to form a shape of Swastik.
Figure 3: Proposed Spidron arm Fractal array antenna
The Spidron fractal-shaped slot is etched on the ground plane (the upper side) of a FR-4 epoxy substrate with a permittivity constant of e =4.4 and a thickness of t=1 mm. In this antenna design, the sequence for the Spidron fractal generation is repeated ten times in the direction of increasingly smaller triangles. The antenna is built for a single element and then a array configuration is framed. Two 50Ω microstrip feeding lines are located on the bottom side of the substrate to excite the Spidron fractal array. The overall dimension of the proposed antenna is 55mm*55mm*1mm. In the proposed antenna design, a conducting reflector is adopted to block back radiation from the Spidron arm fractal slot. The distance of the conducting reflector is placed at a distance of 17mm, thus reducing back radiation and enhancing the gain. The reflector distance is chosen to be 17mm which is λ/4.[2]
IV.SIMULATED RESULTS
The proposed Swastik shaped Spidron arm Fractal slot array antenna is designed for 10 iterations. By constructing 10 iterations the S-parameters are improvised that with its lower iterations. The substrate with the slot and feeding lines is attached to reflector using four foam supports which have a relative permittivity of 1.06. With array configuration of 4 elements, the antenna is tuned to resonate for S-band frequency.
The simulation results show the VSWR plot, S parameters, Smith chart and radiation pattern. Depending on impedance matching, the standing wave ration can be evaluated. From the simulation results of the S11 parameters, their values are -33.25 dB and -23.32 dB for 2GHz and 3.8GHz respectively as shown in figure 4. Its corresponding values of VSWR are -1.04 and 1.14 for dual frequencies of 2GHz and 3.8GHz respectively as shown in figure 5. Thus the antenna is resonating at the above said frequencies to have applications on Satellite communication, IEEE802.11band802.11g / WiMAX.
Figure 4: S11 Parameters.
Figure 5: VSWR.
The input impedance is plotted using a Smith Chart. It would determine any of the antenna’s resonance frequencies. The impedance of the antenna is adjusted through the design process to be matched with the feed line and have less reflection to the source. The Smith chart is as shown in figure6 which tells that at the resonant frequencies 2GHz and 3.8GHz, the antenna is resonating at 50W impedance with its inductive and capacitive components going to zero.
Figure 6: Smith chart
Figure 7a and 7b shows the 3D radiation pattern for the proposed antenna.
Figure 7a: Radiation pattern.
Figure 7b: Radiation pattern.
V.CONCLUSION
A dual resonant frequency Swastik shaped Spidron fractal slotted array antenna is proposed, designed, and simulated in this study. A novel Spidron fractal slot was utilized with VSWR values very close to 1 and thus providing high impedance matching and dual band resonance and a favorable coupling level is obtained. The resonating frequency bands at 2GHz and 3.8GHz serves applications for Satellite communication, IEEE802.11band802.11g / WiMAX.
REFERENCES
[1] Swetha Amit, Chinmoy Kumar P R, Nayana Arvind Laxmeshwar, Saurabh R Badenkal, “A Spidron Fractal Antenna with Enhanced Impedance Matching for Wideband Applications”, ICMARS Dec 2013, International Center for Radio Sciences, Jodhpur,Rajasthan.
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