Design of an Ultra Wideband (UWB) Circular Disc Monopole Antenna

Transcription

1 Degree project Design of an Ultra Wideband (UWB) Circular Disc Monopole Antenna Supervisor: Sven Erik Sandström School of Computer Science, Physics and Mathematics Submitted for the Degree of Master in Electrical Engineering Specialization in Signal Processing & Wave Propagation Author: Jamshaid Hussain Asif Date: Subject: Master Thesis Level: Second Level Course code: 5ED06E

2 i Acknowledgement All praises to Almighty Allah, whose enormous blessings give me strength and make me able to complete this thesis. I would like to express my heartfelt gratitude to my supervisor, Sven-Erik Sandström for his guidance, support and encouragement. He was quite instrumental to my study with all his invaluable view points and extra ordinary motivation. I have been higly inspired by the zeal and enormous depth of knowledge of his field. I hope that the study will comply with his optimum expectations. I cannot finish without mentioning my parents and my wife, who have been offering all round support during the period of my study.

3 ii Abstract My task was to design a circular disc monopole antenna in the Ultrawideband range i.e GHz using the ADS (Advanced Design System) package. In order to achieve the desired matching I simulated different sizes of the radiator, feed line and the ground plane of the antenna and observed the current flow in a circular monopole at different frequencies. I did some miniaturization (circular shape and chopping) of the antenna and observed the current flow and radiation pattern in 2D and 3D.

4 CONTENTS iii Contents 1 Introduction Purpose Specification Methodology Breakdown of report Literature review Bandwidth Radiation pattern Gain and Directivity Beamwidth and Efficiency Requirements for a UWB antenna Literature for the monopole antenna The circular disc monopole antenna Antenna design Substrate Design of the antenna Feed line Feed gap Ground plane Bandwidth Current flow Miniaturization Current flow on a circular disc Bandwidth of the disc antenna Chopping of the disc Bandwidth after chopping Final design and results Final design Current distribution Radiation pattern Gain and directivity Efficiency Conclusion 29

5 1 INTRODUCTION 1 Chapter 1 1 Introduction At present we are witnessing a very rapid growth of wireless communications and in this process antennas with extremely large bandwidth are in demand. A range of applications can then be covered with fewer, or preferably, just one antenna. Such an antenna would cover the band from 0.8 to 11 GHz, and include all the existing standards: AMPC 800, GSM 900, GSM 1800, PCS 1900, WCDMA/UMTS (3 G), U-NII, DECT, WLANs, European Hiper LAN I, II, and UWB ( GHz) [1]. For these systems, Ultrawideband (UWB) technology provides high data rates and short-range wireless communication systems, coding for security and low possibility of interception, as well as multipath rejection and radar applications. This technology uses an ultrawide bandwidth of 7.5 GHz, in the range 0.3 to 10.6 GHz. 1.1 Purpose First Design a circular disc monopole antenna in the Ultrawideband range i.e GHz in Advanced Design System (ADS) Second Achieve miniaturization and observe the current flow and radiation pattern in 2D and 3D. Third Compare performance in terms of the standard antenna parameters. 1.2 Specification The parameters used in the simulation: 1. Substrate : RO4350B 2. Permittivity : Loss tangent : Height : 254 µm 5. Conductivity : 5.8E+007 Siemens/m

6 1 INTRODUCTION Methodology The software that is used for UWB simulation is the Advanced Design System (ADS) Breakdown of report This report has 5 chapters and begins with an introduction and a statement of the purpose of the thesis. Chapter 2 gives a brief literature review and some antenna basics. Chapter 3 discusses the circular disc monopole antenna and miniaturization. Chapter 4 describes the final design and results for the UWB circular disc monopole antenna and Chapter 5 gives some concluding remarks.

7 2 LITERATURE REVIEW 3 Chapter 2 2 Literature review The basic antenna concepts that need to be considered in an antenna design are described in this chapter. At the same time, the primary requirements for a UWB antenna and some general approaches to achieve wide operating bandwidth are discussed. Also, some classic UWB antenna configurations are touched upon Bandwidth The bandwidth (BW) is the range of frequencies within which the performance of the antenna conforms to a specified standard. Generally, in wireless communications, the antenna is required to provide a return loss of less than -10 db over its bandwidth. In the band there is typically a standing wave ratio in the interval (1 < V SW R < 3) [2] Radiation pattern The radiation pattern (or antenna pattern) is the representation of the radiated field or intensity as a function of spherical angles. The most important radiation property is the two or three-dimensional distribution of radiated energy as a function of the angular direction. An isotropic antenna has equal intensity in all directions and cannot be realized physically. A linearly polarized antenna is often described in terms of its principle E-field and H-field patterns. The E-field is defined as the plane containing the electric-field vector and the direction of maximum radiation whilst the H-field is defined as the plane containing the magnetic-field vector and the direction of maximum radiation [2] Gain and Directivity The gain of an antenna is defined as the ratio of the intensity transmitted in a direction to the isotropic intensity. The isotropic intensity equals the absorbed power from the generator divided by 4π [2]. radiation intensity Gain = 4π total input (accepted) power = 4π U(θ, π) P in (1)

8 2 LITERATURE REVIEW 4 In the maximum direction one has the directivity, D 0 = U max U 0 = 4πU max P rad (2) Beamwidth and Efficiency The beamwidth of a pattern is defined as the angular separation between two points at the same level on opposite sides of the pattern maximum. There are a number of beamwidths. The most widely used beamwidth is the Half- Power Beamwidth (HPBW). There is a trade-off between the beamwidth of the main lobe and the size of the sidelobes. The beamwidth of the main lobe also describes the capability of the antenna to distinguish between two adjacent radiating sources or radar targets [2]. The efficiency relates gain and directivity Requirements for a UWB antenna The antenna has a critical function in UWB systems. Large bandwidth is vital to attain good time domain characteristics [3]. For the narrow band case, one has approximately the same performance over the whole bandwidth and the basic parameters, such as gain and return loss, are essentially constant. In contrast, UWB systems are often used for data transmission and this means that a large bandwidth is occupied. The width of the bandpass filter determines the impulse response of the channel. Distortion of the received waveform is one of the things that a UWB antenna should avoid [4] Literature for the monopole antenna The conventional monopole is a straight wire configuration above a ground plane. It is one of the most widely used antennas for wireless communication due to simplicity, low cost, omindirectional radiation and ease of matching to 50 Ω [2]. A more elaborate design is a monopole in the shape of a disc above a ground plane. The current is mainly distributed along the edge of the disc, which indicates that the first resonant frequency is associated with the dimension of the circular disc [5]. Fig. 1 shows the return loss and the bandwidth for an antenna of this type. This confirms the UWB characteristic of the proposed circular disc monopole antenna. It is seen in Fig. 2 that the resonant frequency decreases when the diameter of the disc is increased [5].

9 2 LITERATURE REVIEW 5 Figure 1: Simulated and measured return loss curves [5]. Figure 2: Simulated return loss curves for different dimensions [5]. Printed monopole antennas (PMAs) can be integrated with printed circuit boards and is an expanding UWB technology. Rectangular monopoles have narrow bandwidth and high impedance and circular (disc) monopoles are therefore preferable. When modified the PSMA to semicircular which gives the high bandwidth for the communication channel. The large bandwidth and low pulse dispersion makes it suitable for UWB applications [6].

10 2 LITERATURE REVIEW 6 Figure 3: a) Typical feed lenght and b) simulated VSWR for a disc monopole [6]. CDM stands for circular disc monopole. In CDM the special structure reduces the spatial volume and miniaturizes the antenna. This study shows that the CDM is suitable as an UWB antenna. The projected dimensions are two times larger than the reported CDM antenna while keeping the same ground plane. A height of 1 mm was found to give maximum bandwidth. The bandwidth corresponding to a V SW R < 2 ranges from 1.17 to 12 GHz [7].

11 3 THE CIRCULAR DISC MONOPOLE ANTENNA 7 Chapter 3 3 The circular disc monopole antenna When a circular disc with a hole is mounted over a ground plane it will act as a monopole antenna. An increase in the diameter of the disc causes an increase in the bandwidth of the monopole antenna. An antenna feedline is attached to the disc and passes through the ground plane. The gain of a monopole antenna is twice that of the corresponding dipole antenna. A circular disc monopole antenna is shown Fig. 4. Figure 4: Circular disc monopole antenna [8]. 3.1 Antenna design The task was to design a circular disc monopole antenna in the ultra wideband (UWB) range GHz Substrate In the ADS simulation the Roger substrate RO4350B was used. In the substrate layers the substrate has Free Space layers on both sides while in the Layout layers the substrate has metallization layers on both sides; cond is for the radiator and cond2 is for the ground Design of the antenna The UWB range from 3.1 to 10.6 GHz has the centre frequency 6.85 GHz (wavelength 43.8 mm).

12 3 THE CIRCULAR DISC MONOPOLE ANTENNA 8 The radius of the radiator is equal to λ/4 [8]. The disc has radius r= mm and Length L= mm Feed line The proposed CDM has feedline Width (W) =0.54 mm Length (L) =15.31 mm Height (H) =0.254 mm Impedance (Z)=50 Ω For the length and width, ADS Linecalc produced: Figure 5: Length and width of the feedline using linecalc.

13 3 THE CIRCULAR DISC MONOPOLE ANTENNA Feed gap The VSWR for the feed gaps 0.81, 0.71, 0.64 and 0.21 mm is shown in Figs. 6, 7, 8 and 9. Figure 6: VSWR for a feed gap of 0.81 mm. Figure 7: VSWR for a feed gap of 0.71 mm.

14 3 THE CIRCULAR DISC MONOPOLE ANTENNA 10 Figure 8: VSWR for a feed gap of 0.64 mm. Figs. VSWR. 6, 7, 8 and 9 show that the smallest feedgap produces the best Figure 9: The VSWR for a feed gap of 0.21 mm is at an acceptable level. Figs. 6, 7, 8 show a V SW R > 2 which is not acceptable. In Figure 9 the VSWR is well below 2 over a wide range and this is sufficient for the UWB antenna Ground plane The ground plane in Figure 10 is a cond2 layer with width 50 mm and height 25 mm.

15 3 THE CIRCULAR DISC MONOPOLE ANTENNA 11 Figure 10: Layout of the UWB circular disc monopole antenna. 3.2 Bandwidth The VSWR of the UWB circular disc monopole antenna is below 1.8 for a wide band as shown in Figure 9, and generally in the range 1 < V SW R < 3. The bandwidth of the UWB circular disc monopole antenna in this case lies between 3.1 and 10.6 GHz. 3.3 Current flow The current distribution on the UWB CDM at 4.6, 7.7 and 10 GHz is shown in Figs. 11, 12 and 13, respectively. The current is concentrated to the edges of the antenna.

16 3 THE CIRCULAR DISC MONOPOLE ANTENNA 12 Figure 11: Current at 4.6 GHz. Figure 12: Current at 7.7 GHz.

17 3 THE CIRCULAR DISC MONOPOLE ANTENNA 13 Figure 13: Current at 10 GHz. The current around the edges is high while the blue color indicates a low current and this implies that the centre of the disc could be removed. 3.4 Miniaturization In the first step of miniaturization, a hole with a diameter of 2.5 mm was introduced, as shown in the layout of the CDM in Figure 14. In the following figures the diameter is increased gradually. Figure 14: The antenna with a 2.5 mm hole.

18 3 THE CIRCULAR DISC MONOPOLE ANTENNA 14 Figure 15: The antenna with a 5 mm hole. Figure 16: The antenna with a 7.5 mm hole.

19 3 THE CIRCULAR DISC MONOPOLE ANTENNA 15 Figure 17: The antenna with a 10 mm hole Current flow on a circular disc The simulated currents for these geometries are shown in Figs. 18, 19, 20 and 21. The result in Figure 18 shows that the hole has little effect on the current distribution. Figure 18: Current flow for an antenna with a 2.5 mm hole.

20 3 THE CIRCULAR DISC MONOPOLE ANTENNA 16 Increasing the hole to 5 mm as in Figure 19 does not have any major effect. Figure 19: Current flow for an antenna with a 5 mm hole. Figure 20: Current flow for an antenna with a 7.5 mm hole.

21 3 THE CIRCULAR DISC MONOPOLE ANTENNA 17 Figure 21: Current flow for an antenna with a 10 mm hole. In Figs. 18 and 19 the current flow is not disturbed by the circular hole while in Figs. 20 and 21 there is an effect Bandwidth of the disc antenna The simulated VSWR for the CDM antenna with a radius of 12.5 mm and an inner circular hole of 2.5, 5, 7.5 and 10 mm is shown in Figs. 22, 23, 24 and 25, respectively. In Figure 22 a VSWR below 2 is maintained for the range 3.1 and 10.6 GHz. Figure 22: Bandwidth of the CDM with a circular hole of 2.5 mm.

22 3 THE CIRCULAR DISC MONOPOLE ANTENNA 18 The VSWR with a circular hole of 5 mm is shown in Figure 23. The bandwidth corresponding to V SW R < 2 is still the interval GHz. Figure 23: Bandwidth of the CDM with a circular hole of 5 mm. A minor deterioration is seen when the hole is increased to 5 mm. When the hole is increased further the VSWR extends well above 2 in the range of interest. Figure 24: VSWR for a 7.5 mm hole.

23 3 THE CIRCULAR DISC MONOPOLE ANTENNA 19 Figure 25: VSWR for a 10 mm hole. A circular disc antenna as shown in Figs. 22 and 23 has a VSWR that lies below the reference value Chopping of the disc The VSWR for the the 5 mm antenna was below 1.9 so this case was selected for a test where the disc is chopped from the top. A piece that is 7, 8, 9 and 12.5 mm is chopped off. Figure 26: The CDM chopped at 7 mm from the top.

24 3 THE CIRCULAR DISC MONOPOLE ANTENNA Figure 27: The CDM chopped at 8 mm from the top. Figure 28: The CDM chopped at 9 mm from the top. 20

25 3 THE CIRCULAR DISC MONOPOLE ANTENNA 21 Figure 29: The CDM chopped at 12.5 mm from the top Bandwidth after chopping The bandwidth of an antenna is the range of frequencies over which it is functional, usually centered on the resonant frequency. The design is based on the VSWR. The VSWR and bandwidth of the UWB of the circular disc monopole antenna is shown in Figs. 30, 31, 32 and 33. Figure 30: Bandwidth of the CDM chopped at 7 mm from the top.

26 3 THE CIRCULAR DISC MONOPOLE ANTENNA 22 Figure 31: Bandwidth of the CDM chopped at 8 mm from the top. Figure 32: Bandwidth of the CDM chopped at 9 mm from the top.

27 3 THE CIRCULAR DISC MONOPOLE ANTENNA 23 Figure 33: Bandwidth of the CDM chopped at 12.5 mm from the top. The bandwidth appears in the figures and it is clear that only Fig. 30 meets the requirement V SW R < 2.

28 4 FINAL DESIGN AND RESULTS 24 Chapter 4 4 Final design and results 4.1 Final design This part of the report explains the final layout of the UWB circular disc monopole antenna after chopping. The VSWR (5 mm hole, chopped at 7 mm) was below the reference value 2. Figure 34 shows that the CDM antenna is oriented along the x-y plane. Figure 34: Final layout of a UWB circular disc monopole antenna after miniaturization. 4.2 Current distribution The final design was simulated at 4.6 GHz. Figure 35 shows that the current distribution is not disturbed by the miniaturization.

29 4 FINAL DESIGN AND RESULTS Radiation pattern Figure 35: Current flow at 4.6 GHz. The simulated radiation patterns, in 3D polar plots, are presented in db in Figs. 36, 37 and 38. Figure 36: Radiation pattern at 4.6 GHz. Figure 36 shows the lobes in the θ = 90 plane.

30 4 FINAL DESIGN AND RESULTS 26 Figure 37: Radiation pattern in the plane θ = 90 at 7.2 GHz. Figure 38: Radiation pattern in the plane θ = 90 at 10 GHz. The radiation patterns are almost omnidirectional and there is some frequency dependence.

31 4 FINAL DESIGN AND RESULTS Gain and directivity The gain and directivity of the UWB circular disc monopole antenna at 4.6 GHz is shown in Figure 39. Figure 39: Gain and directivity at 4.6 GHz. The elevation diagram for the directivity and the gain is shown for φ = 0. The maximum CDM directivity at 4.6 GHz is 2.71 db and the maximum gain is 2.44 db. 4.5 Efficiency The efficiency is defined as the ratio between the radiated power and the power delivered to the antenna. Figure 40 shows that the efficiency of the UWB circular disc monopole antenna is around 90% which is acceptable for this type of antenna.

32 4 FINAL DESIGN AND RESULTS 28 Figure 40: Efficiency at 4.6 GHz. Figure 41: Efficiency as a function of frequency. Figure 41 shows that the efficiency decreases with frequency.

33 5 CONCLUSION 29 Chapter 5 5 Conclusion The design of a circular disc monopole as an ultra-wideband antenna for the 3.1 to 10.6 GHz band has been carried out with the ADS software tool. Designs that allow miniaturization were devised through simulation and observation of the current distribution. Geometries with a hole in the disc and chopped versions of geometries with a hole in the disc were studied. A VSWR below 2 is achieved for the UWB band. The proposed CDM antenna has an efficiency of about 90%. The radiation pattern of the proposed antenna is presented at 4.6, 7.2 and 10 GHz.

34 REFERENCES 30 References [1] Z. N. Chen and M. Y. W. Chia, Broadband Planar, John Wiley, Chichester, UK, [2] C. A. Balanis, Antenna theory, John Wiley, 3rd Edition, [3] E. Lim, Z. Wang, C.U. Lei, Y. Wang and K.L. Man, Ultra wideband antennas past and present, IAENG International journal of computer science, Vol 37, Pages , [4] S. Licul, J. A. N. Noronha, W. A. Davis, D. G. Sweeney, C. R. Anderson, and T.M. Bielawa, A parametric study of time-domain characteristics of possible UWB antenna architectures, In Vehicular Technology Conference, Blacksburg, 2003, Pages [5] J. Liang, C. C. Chiau, X. Chen, and C. G. Parini, Study of a printed circular disc monopole antenna for UWB systems, IEEE Transactions on Antennas and Propagation, Vol 53, Num 11, Pages , Nov., [6] K. P. Ray, Y. Ranga, and P. Gabhale, Printed square monopole antenna with semicircular base for ultra-wide bandwidth, Electronics Letters, Vol. 43, Num 5, Pages 13-14, [7] N. P. Agrawall, G. Kumar, and K. P. Ray, Wide-band planar monopole antennas, IEEE Transactions on Antennas and Propagation, Vol. 46, Num 2, Pages , [8] K.P. Ray, Design aspects of printed monopole antennas for ultra-wide band applications, Hindawi Publishing Corporation, International journal of antennas and propagation, Article ID , 2008.

35 REFERENCES 31 SE Kalmar / SE Växjö Tel +46 (0) dfm@lnu.se Lnu.se/dfm