A Comparative Study between two Novel Fractal Monopole Antennas for UWB Applications

Transcription

1 A Comparative Study between two Novel Fractal Monopole Antennas for UWB Applications Indranil Acharya 1,A 1 Vellore Institute of Technology School of Electronics Engineering (SENSE) Chennai, Tamil Nadu, India A. indranil.acharya2013@vit.ac.in Abstract. In this paper, two novel fractal monopole antennas are analyzed in details. The basic generator structure for the two antennas is a hexagonal patch antenna. Different approaches are adopted for designing the two antennas. The first antenna exhibits a wide operational bandwidth of 7.41 GHz between 3.36 GHz to GHz. The second antenna on the other hand has a bandwidth of 6.35 GHz between 3.35 GHz to 9.7 GHz. A 50 ohm impedance matched microstrip feed having dimensions 12 x 3 mm 2 is used to excite the two antennas. Both the antennas are subjected to various parameter variations like change in the strip length and width, variations in the dielectric etc. and all these results are displayed in appropriate S 11 plots. Stable radiation patterns at different frequencies in the UWB spectrum can be observed from both the antennas. Moreover, the accumulation of surface current at different operating frequencies is also presented. All the design and analysis of the two novel antennas is carried out in HFSS 14. Keywords: Fractal, UWB Technology, return loss, gain, bandwidth, current density. 1 Introduction UWB antennas gained tremendous impetus since 2002 when FCC [1] authorized the unlicensed use of 7.5 GHz bandwidth in the frequency range of 3.5 GHz to 10.6 GHz as the official spectrum for ultra-wideband communication. UWB technology is wellknown for its signal robustness, high rate of data transmission, low power consumption and relatively simple hardware requirement.printed wide-slot antennas including co-planar waveguide feed and microstrip line feed prove to be suitable candidates for UWB applications mainly because they are compact and easily integrable and therefore various attempts are made to increase the bandwidth of the antennas using such methods [2-7]. However such printed antennas are usually

2 limited in terms of the operational bandwidth which is only found to be 4:1. In order to mitigate the cost and complexity of compact equipments like handsets and Personal Digital Assistants (PDAs), it is quintessential to design compact ultra-wideband antenna systems. Fractal antennas are indispensible when compactness is concerned. Use of the fractal concept in antennas has opened a new horizon in the designing of UWB antennas because of its self-similarity and space-filling properties [8-11]. The space filling property is used to increase the effective electrical path length within a compact area. Although most fractal antennas are narrowband like Sierpinski-gasket, Koch Curve, Minkowski etc. some wideband fractal antennas are also getting due recognition in recent times. In this paper, two novel UWB fractal antennas are analyzed in details. The basic generator structure is kept unaltered in order to ease the process of comparison. Various parameters of both the antennas are varied and the response of both of these antennas is noted and presented in a comparative manner in terms of the radiation patterns and the reflection co-efficient. Moreover a comparative study of the different iterations as well as the modification in the ground is also presented. Both the antennas exhibit stable radiation patterns at different frequencies in the UWB spectrum. 1.1 First UWB Monopole Antenna Design The antenna is mounted on FR4 dielectric substrate having dielectric constant value 4.4 and thickness 1.6 mm. Fig. 1 shows the first UWB monopole antenna. Fig. 1 First UWB Antenna (a) Front View (b) Back View The various dimensions of the above antenna is displayed in Table 1. TABLE 1: Dimensions of the antenna Dimensions of the antenna Optimized Value (in mm) L 31 W 28 R 9 t 12 d 3 z 11.1 p 5.99

3 h 2.59 A. Results Obtained Fig. 2 and Fig. 3 show the return loss and the gain of the antenna. It can be observed from the S 11 plot that the antenna resonates at frequencies 4.25 GHz, 5.8 GHz and 9.65 GHz having return loss of db, db and db respectively. A wide bandwidth of 7.41 GHz is obtained and can be effectively utilized for IEEE WiMAX (3.5 GHz) and IEEE a-WLAN (5.2 GHz and 5.8 GHz) applications. Fig. 2 Return Loss of the first UWB monopole antenna Fig. 3 Radiation patterns of the antenna at (a) 4 GHz (b) 6 GHz and (c) 7.5 GHz Radiation patterns are obtained by varying theta (ɵ) and phi (ɸ) angles [12]. The gain values at these different frequencies are displayed in Table 2. TABLE 2: GAIN VALUES AT DIFFERENT FREQUENCIES Frequency (in GHz) Gain Value (in db) Evolution of the antenna between different iterations Fig. 4 shows the gradual evolution of the antenna between various iterations.

4 Fig. 4 Diagrammatic Representation of (a) Basic Antenna (b) 1 st iteration (c) Final Antenna The basic antenna consists of a hexagonal patch having each side length, R= 9 mm. The 1 st iteration involves placing five small hexagons having each side length, x=2.5 mm along the sides of the basic hexagonal patch. The final iteration involves placing small circles each having radius 0.69 mm being placed along the sides of the smaller hexagon as well as on the basic hexagonal patch itself. The transition between the different iterations is displayed in Fig. 5. Fig. 5 Transition between the different iterations 1.2.a Combined S 11 plot of the three different iterations The combined S 11 plot of the antenna for three different iterations is shown in Fig. 6. As can be observed from the plot, increasing the number of iterations along with the application of the Koch geometry in the ground plane leads to the achievement of wide operational bandwidth compared to the other iterations. The lower operating frequency shifts slightly with the increase in the number of iterations. This modification enhances the return loss in the mid UWB spectrum. Increasing the number of iterations ensue multiple resonance modes which gets overlapped leading to wide bandwidth. The introduction of Koch geometry increases the electrical path which causes further excitation of additional modes and contributes in a way to the enhancement of the bandwidth.

5 Fig. 6 Combined S 11 plot of the various iterations of the antenna 1.2.b Effect of variation in microstrip feed width Fig. 7 shows the combined S 11 plot for various values of the feed strip width. The range of variation in this case is limited between 2.5 mm to 4 mm. As can be inferred from the graph, with increase in the feed width the lower operating frequencies get shifted towards the left along with a reduction in the obtained bandwidth. This is due to spurious radiations from the microstrip feed which starts radiating as an entity itself. Lowering the strip width although increases the bandwidth but reduced the reflection co-efficient values in the lower UWB spectrum. Fig. 7 Combined S 11 plot for various values of the microstrip feed width 1.2.c Effect of variation in the length of the microstrip feed Fig. 8 shows the combined S 11 plot for various values of the feed strip length. From the graph it can be observed that increasing the length of the feed shifts the lower operating frequency to the right along with a reduction in the obtained bandwidth. On the other hand, decreasing the length reduces the refection co-efficient for the lower frequencies with a reduction in the operational bandwidth. The reduction in bandwidth in this case is even more pronounced.

6 Fig. 8 Combined S 11 plot for various values of the microstrip feed length 1.2.d Effect of using different dielectrics as substrates Fig. 9 shows the effect of variation of the dielectric substrate on the reflection coefficient of the antenna. The different substrates which are used here include: FR4 epoxy having a dielectric constant of 4.4, Polyamide having a dielectric constant value 4.3, Taconic TLY and Arlon AD600, having dielectric constant values 2.2 and 6.15 respectively. Reduction in dielectric constant value increases the bandwidth of the antenna by a considerable amount whereas with higher dielectric value, the bandwidth gets reduced immensely as is observed in case of Arlon AD600. More dielectric constant value causes much of the radiations to get entrapped within the substrate, thereby diminishing the operational bandwidth. Fig. 10 Return Loss of the antenna for different substrates 1.3 Current Distribution of the First Monopole Antenna The surface current density of the final fractal antenna is shown in Fig.11. At 4 GHz most of the surface current is concentrated at the edges of the patch and the ground plane. This shows that placing the small circles has affected the impedance matching at lower frequencies. At 6 GHz it is found that the lower portion of the fractal monopole and the upper portion of the ground mainly in the vicinity of the Koch has more effect on the impedance matching. The current distribution at 7.5 GHz reveals that there is more current concentration at the junction of the feed and the monopole and also along the fractal edges of the ground and its lower portion. Thus

7 the application of Koch geometry in the ground plane has significant impact at both lower and upper frequencies. It can thus be inferred that the surface current distribution is more uniform at the lower frequencies than the higher frequencies owing to the current nature [13]. Fig. 11 Surface Current Distribution at (a) 4 GHz (b) 6 GHz and (c) 7.5 GHz 2.1 Second UWB Monopole Antenna Design The second antenna is mounted on FR4 dielectric substrate having dielectric constant value 4.4 and thickness 1.6 mm. Fig. 12 shows the second UWB monopole antenna. Fig. 12 Second UWB Monopole Antenna The detailed dimensions of the antenna are shown in Table 3. Two circles each having radius of 4 mm is removed from the two upper corners of the ground plane. TABLE 3: DIMENSIONS OF THE SECOND UWB ANTENNA Dimensions of the antenna Optimized Value (in mm) C 31 J 28 q 3 i 12 V 11.1 G 9

8 A. Results Obtained Fig. 13 and Fig. 14 show the return loss and the gain of the antenna. It can be observed from the S 11 plot that the antenna resonates at frequencies 4 GHz, and 7.4 GHz having return loss of db and db respectively. A wide bandwidth of 6.35 GHz is obtained and can be effectively utilized for IEEE WiMAX (3.5 GHz) and IEEE a-WLAN (5.2 GHz and 5.8 GHz) applications. Fig. 13 Return Loss of the second UWB monopole antenna Fig. 14 Radiation patterns of the antenna at (a) 4 GHz (b) 6 GHz and (c) 7.5 GHz The gain values at these different frequencies are displayed in Table 4. TABLE 4: GAIN VALUES AT DIFFERENT FREQUENCIES Frequency (in GHz) Gain Value (in db) Evolution of the antenna between different iterations The evolution of the antenna between different iterations is displayed in Fig. 15.

9 Fig. 15 Diagrammatic Representation of (a) Basic Antenna (b) 1 st iteration (c) Final Antenna The basic antenna consists of a hexagonal patch having each side length, G=9 mm. The 1 st iteration involves placing five rectangles having dimensions l=5 mm and n=2.19 mm along the sides of the hexagonal patch. The final iteration involves placing small hexagons each having sides length of 0.75 mm being placed on the smaller rectangles and on the patch itself. The transition between the different iterations is displayed in Fig. 16. Fig. 16 Transition between the different iterations 2.2.a Combined S 11 plot of the three different iterations The combined S 11 plot of the antenna for three different iterations is shown in Fig. 17. From the graph, it can be observed that increasing the number of iterations leads to a wider operational bandwidth. Moreover there is considerable improvement in the mid UWB spectrum. This is due to the excitation of extra resonance modes which gets overlapped and increases the operational bandwidth. The modification in the ground plane further contributes to the cause in terms of enhancing the electrical path length.

10 Fig. 17 Combined S 11 plot of the various iterations of the antenna 2.2.b Effect of variation in microstrip feed width The combined S 11 plot for different values of the strip width is shown in Fig. 18. Like the previous case, here also the strip width variation is limited between 2.5 mm to 4 mm. As can be seen, lowering the strip width shifts the mid UWB frequency to the right whereas increasing the strip width shifts the frequencies to the left. This behavior is unlike the first antenna in which the lower frequencies get affected the most. Most promising result is observed in case of 2.5 mm in which the operational bandwidth is more compared to others. Fig. 18 Combined S 11 plot for various values of the microstrip feed width 2.2.c Effect of variation in microstrip feed length Fig. 19 displays the combined reflection co-efficient plot for different values of the microstrip feed length. Close inspection reveals that decreasing the length of the feed diminishes the operational bandwidth to a great extent. Moreover the lower frequencies in the UWB spectrum gets shifted upwards denoting that much of the power is reflected back. This is due to the impedance mismatch at the junction of the monopole and the feed. On the other hand increasing the length of the feed also decreases the bandwidth although there is an improvement in the lower frequencies. Length of the feed affects the impedance matching and thus the bandwidth. These results are in unison with that of the first UWB antenna.

11 Fig. 19 Combined S 11 plot for various values of the microstrip feed length 2.2.d Effect of using different dielectrics as substrates Fig. 20 shows the effect of variation of the dielectric substrate on the reflection coefficient of the antenna. Like in case of the first antenna here also the same substrates are used. But unlike the first antenna, the increase or decrease in dielectric constant value reduces the operational bandwidth as compared to FR4 although there is some improvement in the return loss values of the lower frequency components in the UWB spectrum. These results are in contradiction with the first UWB antenna the main reason for which is the uneven radiation from the sharp edges of the patch. Fig. 20 Return Loss of the antenna for different substrates 2.3 Current Distribution of the First Monopole Antenna The surface current distribution for the second UWB antenna is shown in Fig. 21. As can be seen from the figure, at 4 GHz most of the surface current is concentrated at the edges of the patch and the ground plane. This is well in unison with the first antenna the reason being the placement of the small hexagons along the perimeter of the patch. At 6 GHz most the surface current gets accumulated at the lower portion of the patch and the upper portion of the ground. At 7.5 GHz the current density is significant at the fractal edges of the patch and the ground. This suggests that the ground plane has more impact at the higher frequencies as compared to the lower frequencies. The wavelength of the EM waves being long at the lower frequencies, the small segments of the patch contribute less in the overall radiation. At higher

12 frequencies, the segment dimensions become comparable to the wavelength which significantly enhances the radiation. Fig. 21 Surface Current Distribution at (a) 4 GHz (b) 6 GHz and (c) 7.5 GHz 3 Conclusion In this paper a comparative study is carried out between two novel UWB fractal monopole antennas. The first antenna exhibits a wide bandwidth of 7.41 GHz while the second antenna has a bandwidth of 6.35 GHz. Both the antennas exhibit stable radiation patterns at different frequencies in the UWB spectrum. The presence of Koch geometry in the ground plane of the first antenna is the reason for having a wider bandwidth compared to the second one as it contributes to the excitation of higher order modes. The effect of different parameter variations in both the antennas is analyzed in details and the results obtained are compared. Both the antennas thus serve as potential candidates for UWB Applications. References 1.FCC: FCC 1 st report and order on ultrawideband technology Washington, DC, Sze Jia Yi, Wong Kin Lu. Bandwidth enhancement of a microstripline-fed printed wideslot antenna. IEEE Transactions onantennas and Propagation, vol. 49, no. 7, pp , Chen Horng Dean. Broadband CPW-fed square slot antenna witha widened tuning stub. IEEE Transactions on Antennas andpropagation, vol.51, no.8, pp , Lee Haeng Lyul, Lee Hyun Jin, Yook Jong Gwan, et al. Broadband planar antenna having round corner rectangular wide slot,proceeding of the International Symposium of IEEE Antennas and Propagation Society,, San Antonio, TX,USA. Piscataway, NJ, USA, pp , Jun 16-21, Behdad N, Sarabandi K. A multiresonant single-element wideslot antenna. IEEE Antennas and Wireless Propagation Letters, vol.3, no. 1, pp.5-8, 2004.

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