Reconfigurable Low Profile Patch Antenna

Transcription

1 Bradley University Department of Electrical and Computer Engineering Reconfigurable Low Profile Patch Antenna Mr. James H. Soon Advisor: Dr. Prasad Shastry May 13, 2005

2 Abstract The objective of this project was to design, simulate, manufacture and analyze a polarization diverse microstrip line fed patch antenna capable of operating selectively with either linear or circular polarization. Acknowledgments I am grateful for Dr. Prasad Shastry s patient and expert guidance throughout my educational experience at Bradley University, and especially during this project. I would also like to thank my tutors - two graduate students - Mr. Balamurugan Sundaram and Mr. Krishna Katragadda, who were also very patient and gave of their time and knowledge unselfishly. Thanks guys - I wouldn t have been able to do this without your help.

3 Table of Contents Introduction... 4 Polarization... 4 Patch Radiation Characteristics... 6 Circular Patch Construction... 6 System Description... 7 Design... 8 Specifications... 8 Process... 8 Equations... 9 Simulation Results...11 Fabrication...15 Analysis...17 Conclusions...18 Project Summary...18 Recommendations for Future Projects...18 References...19 Appendix A: ADS-Momentum Simulations...20 Appendix B: Antenna Measurements...68 Page 3 of 84

4 Introduction Today s digital communications systems employ a variety of protocols to accurately transfer data. The overall quality of a communications path can be quantified by measuring its bit error rate. Diversity, in the communications world, refers to transmitting multiple copies of the same information. The probability of receiving errored bits can be reduced by employing different diversity methods. Polarization is one of those methods. The primary objective of this project was to test the viability of a polarization diverse device by designing, simulating, fabricating and analyzing a polarization diverse low profile patch antenna. Since ADS-Momentum simulation software was relatively new to the Bradley University RF area, a secondary objective of this project was to verify the dependability of simulation results from ADS- Momentum compared to measured results of several fabricated antennas. If simulation results were found to be sufficiently accurate, the trial and error element of the antenna design process could be moved from the fabrication and analysis cycle into the simulation cycle. The advantages of not fabricating until a design meets the intended specifications include reduced material costs and development time. Polarization Polarization refers to the orientation of the electric field vector. The specific type of polarization is defined by the geometric figure traced by the sum of the electric field vector as an observer looks directly into the direction of travel (Poynting vector) of the electromagnetic wave. Figure 1 shows an example of the components of the electric field vector (E x & E y ), phase difference ( ) and relative direction of travel. Figure 1: Electric Field Components Page 4 of 84

5 The two primary polarization senses are linear - either vertical or horizontal; and circular - either right hand or left hand polarization. Figure 2 illustrates the relationship between relative magnitude, phase and polarization sense. Figure 2: Polarization Relationships As Figure 2 illustrates, when all the electric field energy resides in the y plane, vertical polarization results. Conversely, when the electric field is in the x plane, horizontal polarization results. Circular polarization occurs when the electric field magnitude in the x and y planes are equal (their ratio = 1) and the signals are 90 out of phase with each other. Right hand circular polarization (RHCP) occurs when E x lags E y by 90, left hand circular polarization results when E x leads E y by 90. Figure 3 illustrates RHCP; the magnitudes are equal in x and y plane, and Ex lags Ey by 90. Figure 3: Right Hand Circular Polarization Page 5 of 84

6 Patch Radiation Characteristics Figures 4a and b show the manner in which patch antennas radiate. Figure 4b: Radiating Patch Figure 4a: Radiating Patch Figure 4a shows a microstrip line fed patch - note that the direction of radiation is normal to surface of the patch. Figure 4b shows a cut-away view of the fields radiating into the substrate material. The fields at the edge of the patch create a fringing effect - extending beyond the actual physical edge of the patch. This effect is accounted for in initial design calculations and is described later in this document. Circular Patch Construction As seen in Figure 2, two conditions for obtaining circular polarization are equal amplitudes and a 90 phase difference between the electric field x and y planes. A number of schemes are possible to cause an essentially square patch to radiate circularly. Figures 5a and 5b illustrate four of those methods. Figure 5a: Truncated and Nearly Square Patches Page 6 of 84

7 Figure 5b: Hybrid Fed and Reactive Splitter Fed Patches The truncated and nearly square patches alter the radiation pattern on the patches causing circular polarization, while the hybrid fed and reactive splitter patches feed the signal into the patch with a 90 offset, causing circular polarization. This project used a truncated patch - the simulation patch antenna is shown in Figure A-59. System Description The objective of this project was to use a microstrip line fed patch antenna capable of polarization diverse operation. An illustration of a dual polarization antenna is illustrated in Figure 6. Figure 6: Polarization Diverse Antenna Page 7 of 84

8 As Figure 6 illustrates, energy fed into the left port will result in horizontal polarization and energy fed into the bottom port will result in vertical polarization. While this antenna would be interesting to design, simulate, fabricate and analyze, this project s objective was to experiment with circular polarization as well. Figure 7 illustrates a circular and linear polarization diverse antenna. Figure 7: Switchable Polarization Antenna The two channels in Figure 7 are exaggerated to be able to show the diode operation. When the diodes are forward biased, the triangles are connected to the patch, which radiates linearly. When the diodes are reverse biased, the patch is truncated, resulting in circular (left circular) polarization. That s the objective of this project. The idea for this type of antenna came from A Reconfigurable Antenna For Switchable Polarization an IEEE paper by Mr. Y.J. Sung, T.U. Jang, and Y.S. Kim. 1 Specifications Gain > 5dBi Frequency Range: GHz range Return Loss < -15 db Radiation pattern Polarization: Linear, LHCP Bandwidth: approx. 2% VSWR 2.0:1 Process Design The project process consisted of an iterative approach - design, simulate, optimize design, simulate, optimize design, simulate - until satisfactory results are reached; then fabricate the final design, and lastly analyze the resulting fabrication. Then repeat the design - simulate - fabricate - analyze cycle for each antenna. Page 8 of 84

9 The plan called for a total of six antennas to be fabricated; one baseline linear polarization, one baseline circular polarization, and the final, switchable polarization antenna. Two antennae of each type were necessary for gain evaluation in the anechoic chamber. Design began with some fundamental patch antenna equations from Antenna Theory by Constantine Balanis 2. (See below) A short C++ program was written to obtain numerical convergence. PCAAD (Personal Computer Aided Antenna Design) was then used to further optimize the physical dimensions of the patch, and obtain an initial input impedance value. Those dimensions were then used to construct the first patch simulation in ADS-Momentum (Advanced Design System), and the resonant frequency, return loss, VSWR and input impedance were verified. After optimizing those values by adjusting the physical size in several simulations, final physical patch dimensions were obtained. Values for a quarter wave impedance matching network were obtained using the input impedance values from PCAAD and Momentum and the software programs Match and Mstrip. The quarter wave matching network was then constructed on the patch and the complete antenna, with a short 50 length for a connector was simulated using Momentum. After a final design was obtained, a post script file containing a mask was created, the file was delivered to Technicraft Display & Graphics, who manufactured a black linotype film. This film was then used during the fabrication process in the Microwave Integrated Circuit Fabrication Laboratory. Antenna analysis was performed by using a Network Analyzer and Anechoic Chamber to measure return loss, bandwidth, resonant frequency, VSWR, gain, the radiation pattern and axial ratio. Equations The following fundamental design equations from the Balanis text were used: (Equation 9.1) V 0 is the speed of light in a vacuum, fr is the resonant frequency desired, and r is the dielectric constant of the laminate material. The width was obtained and plugged into the next equation. For this project, fr = 2.4 GHz, r = 3.0, h = 120 mil ( cm). (Equation 9.2) Page 9 of 84

10 h, the height of the laminate material is known, and r effective is then found and used in equation for L, to account for the fringing effect of the patch, see Patch Radiation Characteristics on page 6. (Equation 10.1) Once the fringing effect is known, the length of the patch can be found. (Equation 10.2) Since this is a square patch, the length = the width, and the length becomes the new width. The calculations are repeated until the length and width converge. This becomes the value used in PCAAD. One other set of equations was used - from Robert Sainati s book, CAD of Microstrip Antennas for Wireless Applications 3, to calculate c - the length of the truncated segment (see Figure 5a). Where: s = area of the perturbation segment S = are of the patch c = length of a side of the cut out triangle L = length of the patch Q = patch quality factor (Equation 10.3) (Equation 10.4) (Equation 10.5) Page 10 of 84

11 Simulation Results A total of 20 Momentum simulations were performed, ranging in processing time of 30 minutes to around 2 hours. Simulation results of Reconfig 01, Reconfig 14, 16 and 20 designs can be found in Appendix A. The initial Momentum simulation showed a patch input impedance of around 1600, compared to PCAAD s projected input impedance of 284. At this point, simulation results from Momentum did not appear to be reliable. A quarter wave matching network based on the input impedance results from PCAAD was constructed and simulated in Momentum. The resulting 3D simulation (Figure 8, also Figure A-23) revealed a distorted radiation pattern, that was initially not expected. Further analysis of the situation suggested that although the quarter wave matching network was indeed correct, it was physically large and was creating another radiating surface which caused the pattern distortion. Figure 8: Reconfig 14 3D Top View It was decided to proceed with the fabrication of this antenna (Fab14), take the opportunity to analyze it, and verify the accuracy of the simulated results. A second linearly polarized baseline antenna (Reconfig 16) would also be designed, simulated, fabricated and analyzed using the second set of quarter wave matching network dimensions. Page 11 of 84

12 Figure 9 shows Reconfig 14 - linear baseline antenna #1 and Figure 10 shows Reconfig 16, linear baseline antenna #2. Figure 9: Reconfig 14 Figure 10: Reconfig 16 Page 12 of 84

13 11). Simulation of Reconfig 16 did show an improvement in the projected radiation pattern (Figure Figure 11: Reconfig 16 3D Top View One other Momentum simulation, Reconfig 20, is of interest. Figure 12 shows the antenna, using the first quarter wave matching design (3 additional designs remain un-simulated), Figure 13 shows the radiation pattern results - distortion is clearly apparent. Figure 12: Reconfig 20 Page 13 of 84

14 Figure 12: Reconfig20 Top View Simulated results of the axial ratio of Reconfig 20 (Figure 13), show non-linear but noncircular polarization sense. In other words, this design is moving toward circular polarization, but it s not there yet. Figure 13: Reconfig 20 Polarization Page 14 of 84

15 Fabrication Figure 14 shows the mask for Fab14. Fabrication of both antennas (Fab14_1 & 2) was attempted at the same time - using an entire 6 inch laminate board. Since much photo resist seemed to be wasted using such a large board, it was determined to cut the board in half and fabricate one antenna at a time for the next set of antennas. Figure 15 shows Fab14_1. Two changes were recommended for the next fabrication round due to the complications arising from attempting to fabricate a large board. change the developer solution to 100 ml developer /400 ml of water allow the antenna to remain in the oven for the full 15 minutes during the drying cycles. This information is recorded in Lab notebook #2, pages Figure 14: Fab14 Mask Figure 15: Fab14_1 Page 15 of 84

16 Note the silver marks (solder) on both the main patch and quarter wave matching network. Dust on the mask during the fabrication process left holes etched into both surfaces. This antenna was first measured in its original condition, then the holes were filled in with solder and measured again. No noticeable improvement was observed. Figures 16 & 17 show the Reconfig 16 mask and antennas. Note the lack of holes in the antennas - implying an increasing measure of fabrication skill. Figure 16: Fab16 Mask Figure 17: Fab16_1 & _2 Page 16 of 84

17 Analysis Appendix B contains the measured results for Fab14 & 16 antennas. Images of the Network Analyzer and Anechoic chamber used to analyze these antennas can be found in Lab Notebook #2 pages and 145. Additional images can be found on the Bradley University ECE Department Web Site under Research, Projects, 2005, Reconfigurable Antenna. A summary of antenna performance vs design specifications is in Table 17-1 Figure of Merit Design Spec Fab14 Fab16 Gain > 5 dbi 5.57 dbi Not measured Resonant Freq 2.40 GHz 2.42 GHz 2.42 GHz Return Loss < -15 db -25 db db Polarization Linear/Circular Linear Linear Bandwidth 2.0 % 2.7 % 1.2 % VSWR 2.0:1 1.11:1 1.48:1 Table 17-1 A summary of antenna performance vs simulated results is in Table 17-2 Antenna Resonant Freq Return Loss VSWR Zin Fab 14 Simulation 2.40 GHz db Fab 14 Measured 2.42 GHz -25 db Fab 16 Simulation 2.41 GHz db Fab 16 Measured 2.42 GHz db Table 17-2 Page 17 of 84

18 Conclusions Project Summary This was, for me, a fascinating project - I wish I could have completed it. The fabricated antennas met the design specifications fairly well - sensitivity analysis of both antennas should be performed to determine more precisely how well. Simulated results were also fairly close to measured results. Again, sensitivity analysis would reveal how well they match. It appears that the simulations can be trusted - this could significantly decrease total antenna design time. Antenna design will still be iterative however, and require much patience. Recommendations for Future Projects Complete the analysis of Fab 16 and continue design and implementation of the circularly polarized antennas; simulate an ideal short and open on the switchable polarization antenna, then finish the project! Implement the use of a self-leveling 360 laser level for accurate vertical positioning of the antennas in the anechoic chamber Obtain 3D anechoic chamber measurements Convert ADS-Momentum 3D simulation results to compare with the 2D results from the anechoic chamber Investigate quarter wave matching network for both circular and linear polarization Page 18 of 84

19 References 1. SUNG, Y.J., JANG, T.U., KIM, Y.S., A Reconfigurable Antenna for Switchable Polarization, IEEE Microwave and Wireless Letters, Volume 14, Number 11, November 2004, pp BALANIS, C.A., Antenna Theory - Analysis & Design, second Ed., John Wiley, 1997, Chapter 14 - Microstrip Antennas, pp SAINATI, ROBERT A., CAD of Microstrip Antennas for Wireless Applications, Artech House, 1996, p LEE, K.F. & CHEN, W., Advances in Microstrip and Printed Antennas, John Wiley, NAVARU, J.A., & CHANG, K, Integrated Active Antennas and Spatial Power Combining, John Wiley, Page 19 of 84

20 Appendix A: ADS-Momentum Simulations Page 20 of 84

21 Reconfig 01 Figure A-1 Figure A-2 Page 21 of 84

22 Figure A-3 Figure A-4 Page 22 of 84

23 Reconfig 14 Figure A-5 Figure A-6 Page 23 of 84

24 Figure A-7 Figure A-8 Page 24 of 84

25 Figure A-9 Figure A-10 Page 25 of 84

26 Figure A-11 Figure A-12 Page 26 of 84

27 Figure A-13 Figure A-14 Page 27 of 84

28 Figure A-15 Figure A-16 Page 28 of 84

29 Figure A-17: 3D Current Distribution Figure A-18: 3D Back Angle View Page 29 of 84

30 Figure A-19: 3D Front Angle View Figure A-20: 3D Front View Page 30 of 84

31 Figure A-21: 3D Bottom View Figure A-22: 3D Side View Page 31 of 84

32 Figure A-23: 3D Top View Figure A-24 Page 32 of 84

33 Figure A-25 Figure A-26 Page 33 of 84

34 Figure A -27 Figure A-28 Page 34 of 84

35 Figure A-29 Figure A-30 Page 35 of 84

36 Figure A-31 Figure A-32 Page 36 of 84

37 Figure A-33 Figure A-34 Page 37 of 84

38 Reconfig 16 Figure A-35 Figure A-36 Page 38 of 84

39 Figure A-37 Figure A-38 Page 39 of 84

40 Figure A-39 Figure A-40 Page 40 of 84

41 Figure A-41 Figure A-42 Page 41 of 84

42 Figure A-43 Figure A-44 Page 42 of 84

43 Figure A-45 Figure A-46 Page 43 of 84

44 Figure A-47 Figure A-48 Page 44 of 84

45 Figure A-49 Figure A-50 Page 45 of 84

46 Figure A-51 Figure A-52: 3D Back Angle View Page 46 of 84

47 Figure A-53: 3D Bottom View Figure A-54: 3D Current Distribution Page 47 of 84

48 Figure A-55: 3D Front Angle View Figure A-56: 3D Front View Page 48 of 84

49 Figure A-57: 3D Side View Figure A-58: 3D Top View Page 49 of 84

50 Reconfig 20 Figure A-59 Page 50 of 84

51 Figure A-60 Page 51 of 84

52 Figure A-61 Figure A-62 Page 52 of 84

53 Figure A-63 Figure A-64: Reconfig20 Top View Page 53 of 84

54 Figure A-65: Reconfig20 Side View Figure A-66: Reconfig20 Front View Page 54 of 84

55 Figure A-67: Reconfig20 Front Angle View Figure A-68: Reconfig20 Back Angle View Page 55 of 84

56 Figure A-69: Reconfig20 Bottom View Figure A-70: Reconfig20 Circular Axial Top View Page 56 of 84

57 Figure A-71: Reconfig20 Circular Axial Side View Figure A-72: Reconfig20 Circular Axial Front View Page 57 of 84

58 Figure A-73: Reconfig20 Circular Axial Back Angle View Figure A-74: Reconfig20 Circular Axial Bottom View Page 58 of 84

59 Figure A-75: Reconfig20 Linear Axial Top View Figure A-76: Reconfig20 Linear Axial Side View Page 59 of 84

60 Figure A-77: Reconfig20 Linear Axial Front View Figure A-78: Reconfig20 Linear Axial Front Angle View Page 60 of 84

61 Figure A-79: Reconfig20 Linear Axial Back Angle View Figure A-80 Page 61 of 84

62 Figure A-81 Figure A-82 Page 62 of 84

63 Figure A-83 Figure A-84 Page 63 of 84

64 Figure A-85 Figure A-86 Page 64 of 84

65 Figure A-87 Figure A-88 Page 65 of 84

66 Figure A-89 Figure A-90 Page 66 of 84

67 Figure A-91 Figure A-92 Page 67 of 84

68 Appendix B: Antenna Measurements Page 68 of 84

69 Fab 14_1 Figure B-1 Figure B-2 Page 69 of 84

70 Figure B-3 Figure B-4 Page 70 of 84

71 Figure B-5 Figure B-6 Page 71 of 84

72 Figure B-7 Figure B-8 Page 72 of 84

73 Figure B-9 Figure B-10With laser level Page 73 of 84

74 Figure B-11: With laser level Figure B-12: With laser level Page 74 of 84

75 Fab 14_2 Figure B-13 Figure B-14 Page 75 of 84

76 Figure B-15 Bandwidth = GHz GHz = GHz = 45 MHz Figure B-16 Bandwidth = GHz GHz = GHz = 66.6 MHz Page 76 of 84

77 Figure B-17 Figure B-18 Page 77 of 84

78 Figure B-19 Figure B-20 Page 78 of 84

79 Figure B-21 Figure B-22: Commercial Antenna to Fab14_1 Page 79 of 84

80 Figure B-23: Fab14_2 (Tx) to Fab14_1 (Rx) Page 80 of 84

81 Fab 16 Figure B-24 Figure B-25 Page 81 of 84

82 Figure B-26 Figure B-27 Page 82 of 84

83 Figure B-28 Figure B-29 Page 83 of 84

84 Figure B-30 Page 84 of 84