CHAPTER 5 HELICAL ANTENNAS 螺旋天線 C. A. Balanis, Antenna Theory, Ch. 1 W. Stutzman, Antenna Theory, Ch. 6, P 231 J. D. Kraus, Antennas, Ch. 7 Helical Antenna Array Wrist Radiophone Helical Antenna TDRSS Satellite Antenna "Farm" with Axial-mode Helical Antenna Array (? band) Philips DECT handset (1.89 GHz) with a normal-mode helical antenna 27 H.-R. Chuang, EE NCKU
5-2 The first helical antenna (1964) by J. D. Kraus.
5-3 Helical Antenna of Different Radiation Modes
5-4 Iridium Handset: Helical antenna for LEO satellite link?
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Helical antenna with a ground plane and connected to the center conductor of a coaxial line 5-6 N = helical turns D = diameter C= πd S = spacing between each turn L = S2+ C2 Total length of the helical wire = NL Total length of the helical antenna = NS Pitch angle α= tan 1( SC) = tan 1( SπD) Axial Mode: 34λ< C< 43λ, S λ 4, 12o< α< 18o( 14o near optimum ) (End-fire mode) Empirical formula: ( 12o< α < 18o & N > 3) CP Wave Input impedance R 14( C λ ) (+ 2%) (pure resistive) Half-power beamwidth 52λ32 HPBW = ( degree ) C NS 115λ32 Beamwidth between nulls FNBW = ( degree ) C NS Directivity D 15N CS 2 λ 3 Axial ratio 2N + 1 AR = 2N Normalized far-field pattern [ N 2 ψ] [ ψ 2 ] π S f () θ = sin cosθ sin ( ) ψ π ( cos θ) N sin ( ) = + 1 2 1 2 λ 2N
5-7 Normal Mode: (for wireless communication handset) jkr ki = η Se Eθ j sin θ 4πr (vertical pol. from vertical wire) v E = θˆ E + φˆ E θ φ (total field : CP?) jkr ( kd / 2) I φ = η e E sinθ 4r (horizontal pol. from horizontal 2 loop) E E θ φ ki = jη = η Se 4πr jkr 2 ( kd / 2) I 4r sin θ vertical polarization e jkr sin θ horizontal polarization 2λS If = 1 or C( = πd) = 2λS 2 ( πd) v E = je & Total field E = θˆ E + φˆ E θ φ θ φ circular polarization (CP) * Axial ratio: E AR = E θ φ = 2λS ( πd) 2 * If the helix height h ( = NS) < λ / 8 => radiation resistance 64( h / λ) 2 R r
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5-1 IE3D IE/MoM Numerical Computation
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5-13 Stutzman and Thiele, Antenna Theory and Design, 2nd Edition
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5-17 Multi-layer Chip Helix Antenna 將細小的金屬線包覆在介質中, 而此介質可能是鐵磁性物質 (35>μ>7) 陶瓷物質 ( ε r > 1), 或任何高介電係數的物質 包覆物質除了對細小金屬線產生支撐作用外, 也有縮小天線長度的功能 尺寸比較 : 對於 42.7MHz 時的操作頻率, 若為普通的 dipole 則至少需 158 公分長 作成晶片型天線則只有 5 8 2.5mm 3 設計手機用的雙頻晶片型天線 GSM 通用頻帶如下 ( 北美地區除外 ): GSM 9 GSM 18 傳送 89MHz~915MHz 171MHz~1785MHz 接收 935MHz~96MHz 185MHz~188MHz 所需頻寬 925MHz±35MHz 1795MHz±85MHz 所需頻寬 (%) 7.57% 9.47% Chip antenna 1 2 3 4 5 6 7 8 9 * 2 # Helix-monopole Dual-band Antenna: 894 MHz & 175 MHz
DESIGN OF A 9/18 MHZ DUAL-BAND LTCC CHIP ANTENNA FOR MOBILE COMMUNICATIONS APPLICATIONS This article presents the design simulation, implementation and measurement of a miniaturized 9/18 MHz dual-band low temperature co-fired ceramic (LTCC) chip antenna for mobile communication applications. The use of a helix-monopole type dual-band antenna is realized in a multi-layer printed LTCC structure. The Ansoft HFSS 3-D EM simulator, based on the finite-element method (FEM), is employed for design simulation. In the simulated structure, the PCB board of the RF circuit, connected to the chip antenna, is also included. A helix/three-monopole structure chip antenna is designed to achieve enough bandwidth at the 9 and 18 MHz bands. Although the measured antenna performance is shifted to a higher frequency for both the 9 and 18 MHz bands, the antenna patterns of the realized chip antenna are very close to the omnidirectional pattern of a dipole antenna at 9 and 18 MHz. Recently, multi-layer LTCC technology has been used extensively for the miniaturization of RF passive components and antennas. In addition to the high integration by embedding components and interconnects, high Q passive elements have been demonstrated with LTCC technology. 1 It is suitable to integrate antennas and other passive components, even with active devices. Higher integration and more compact structures make LTCC technology popular in RF applications. A chip antenna using LTCC technology for 2.45 GHz applications has been reported. 2 Another chip antenna, using meander lines mounted on LTCC chip, has a wide bandwidth (19.1 percent at VSWR<2) at 18 MHz. 3 Because these planar meander lines are deposited on the surface of the LTCC chip, their size is larger than that with embedded meander lines. In this article, LTCC technology is used to develop a 9/18 MHz CHI-CHANG LIN, YU-JUI CHANG AND HUEY-RU CHUANG National Cheng Kung University Tainan, Taiwan, ROC Reprinted with permission of MICROWAVE JOURNAL from the January 24 issue. 24 Horizon House Publications, Inc.
TECHNICAL FEATURE 3.6 MIL 3.6 MIL 25.2 MIL 26.8 MIL 3.6 MIL 8 LAYERS 8. MIL 2 LAYERS 3.6 MIL 2 LAYERS Fig. 2 Cross section of the LTCC chip antenna. S 11 (a) L m S f 1 f 3 f 2 BW FREQUENCY Fig. 3 Triple resonances from the monopoles. D dual-band chip antenna. The multilayer printed helical-monopole antenna structure is designed to obtain the dual-band function. The HFSS 3-D EM simulator, based on the finite-element method, is employed for design simulation. Simulation results are compared with the measured performance and discussed. L h L S (b) (c) FR-4 CIRCUIT BOARD GROUND PLANE WIDTH MICROSTRIP GROUND PLANE LENGTH LTCC NO GROUND BELOW LTCC Fig. 1 A dual-band helical-monopole antenna; (a) schematic, (b) LTCC chip and (c) chip antenna mounted on a FR-4 board. DUAL-BAND ANTENNA DESIGN A helical-monopole combination structure for GSM 9/18 MHz has been proposed for dual-band antenna designs. 4 Figure 1 shows a dual-band helical-monopole combination antenna, a multi-layer printed helical-monopole LTCC chip antenna and the geometry of a LTCC chip antenna mounted on a FR-4 substrate board. A cross section of the printed helical structure, realized by using via holes to connect upper- and lower-layer metals, is shown in Figure 2. The printed monopole is located on the middle layer. The dielectric constant and loss tangent of the LTCC ceramic are 7.8 and.47, respectively. The 3.6 mil thick top-layer ceramic is placed above the upper-layer metal. The thicknesses of the other layers are also shown. The dimensions of the LTCC chip and each layer are converted from the freespace helical-monopole antenna to the multi-layer ceramic based on its dielectric constant. The first monopole antenna, as shown in Appendix A, contributes to the higher frequency (18 MHz) band. The helical antenna is tuned to the lower frequency (9 MHz) band. In the initial design, the input impedance of the LTCC chip antenna is capacitive, hence several closer turns of helical lines are used as an inductor for input impedance matching. In order to increase the bandwidth, three printed monopoles TABLE I PARAMETERS OF A DUAL-BAND LTCC CHIP ANTENNA Length of first monopole (mil) 28 (7 mm) Length of second monopole (mil) 17 Length of third monopole (mil) 11 Length of helical antennas (L 1 ) (mil) 14 Width of helical antennas (W 1 ) (mil) 4 Width of meander lines (W 2 ) (mil) 1 Length of helical inductors (mil) 14 Width of helical inductors (mil) 2 LTCC package (mil) FR4 substrate (mil) Ground plane (mil) 21 515 (5.4 mm 13.1 mm) 14 2265 (35.6 mm 57.5 mm) 14 176 (35.6 mm 44.7 mm) with different lengths are used, integrated with the helix. Figure 3 illustrates the triple resonant frequencies used to achieve a wider bandwidth. The dimensions of the LTCC chip antenna are 21 515 44.8 mil (5.4 13.1 1.14 mm). The widths of the embedded meander lines and printed monopole antennas inside the LTCC chip are all 1 mil. The dimensions of the FR-4 circuit board are 1.4 2.265.39 in. (35.6 57.5 1 mm) and the size of the ground plane is 1.4 1.76 in. (35.6 44.7 mm). Additional dimensions are listed in Table 1. SIMULATION AND MEASURED RESULTS Figure 4 shows the simulated current distributions of the dual-band LTCC chip antenna at 9 MHz with different phase at, 45, 9 and 135. The current flowing along the metal strips inside the LTCC package can be observed. Figure 5 shows the photograph of a fabricated dual-band LTCC chip antenna on a FR-4 PCB board and the simulated and measured return losses. It is observed that the measured resonant frequencies are shifted from the simulated 9 and 18 MHz to
TECHNICAL FEATURE (a) (c) (a) (b) (d) Fig. 4 Simulated current distribution at 9 MHz; phase φ =(a), (b) 45, (c) 9 and (d) 135. Z θ φ Y (b) Fig. 7 Set-up for radiation pattern measurement in an anechoic chamber. (a) S 11 (db) 5 1 15 SIMULATED MEASURED 18 21 24 27 (a) 3 12 33 9 X 6 3 MEASUREMENT SIMULATION 9 135 1 2 3 45 18 4 2 2 25.5 1. 1.5 2. 2.5 (b) FREQUENCY (GHz) Fig. 5 Fabricated dual-band LTCC chip antenna on a FR-4 PCB board; (a) photograph and (b) simulated and measured return losses. TABLE II COMPARISONS OF SIMULATED AND MEASURED ANTENNA RETURN LOSS (S 11 ) AND RADIATION CHARACTERISTICS HFSS Lower Frequency Band Measurement S 11 (db) 7.73 21.8 Directivity (dbi) 2.1 2 Power gain (dbi).75 4 Efficiency (db/%) 2.85/51.4 6./24.3 Higher Frequency Band S 11 (db) 9.9 2.9 Directivity (dbi) 2.2 2 Power gain (dbi) 4 6 Efficiency (db/%) 6.2/2.6 8./13. (b) 21 24 18 27 3 12 33 9 6 3 Fig. 6 Simulated 3-D antenna patterns at (a) 9 and (b) 18 MHz. higher frequencies, 14 and 19 MHz. The measured return loss, S 11, is 21.8 and 2.9 db at 14 and 19 MHz, respectively. The simulated and measured return losses are also listed in Table 2. Figure 6 shows the simulated 3-D antenna patterns at 9 and 18 MHz. They are very close to those of an ideal dipole antenna. Figure 7 shows photographs of the setup for radiation pattern measurement of the chip antenna in an anechoic chamber. Figure 8 shows the simulated and measured 2-D antenna patterns in (a) 225 27 315 9 135 1 2 3 45 18 4 2 (b) 225 27 315 Fig. 8 Simulated and measured radiation patterns in the E-plane (XZ-plane) at (a) 14 and (b) 19 MHz. the E-plane (XZ-plane) for the lower and higher frequency bands. Satisfactory agreements between the simulation and measurement are observed.
VERTICAL HORIZONTAL 9 135 1 2 3 45 18 4 2 (a) (b) 225 27 TECHNICAL FEATURE 315 9 135 1 2 3 45 18 4 2 225 27 315 Fig. 9 Measured vertical and horizontal polarization patterns in the H-plane (XY-plane) at (a) 14 and (b) 19 MHz. Figure 9 shows the measured vertical and horizontal polarization patterns in the H-plane (XY-plane). The vertical-polarization patterns (Hplane) are very close to the omni-directional patterns of a dipole antenna. The measured antenna directivity is approximately 2 dbi at both the lower and higher frequency bands. The measured antenna power gains are approximately 4 and 6 db at 14 and 19 MHz, respectively, which are lower than the simulated results by approximately 2 to 3 db. The radiation efficiency (η r ) of the chip antenna can be obtained from the relation between the directivity (D) and power gain (G P ) in accordance with η r (db) = G P D η r (db) = 1 log[η r (linear scale)] As shown by the data, the simulated and measured antenna efficiencies are 51.4 and 24.3 percent at the lower frequency band, and 2.6 and 13 percent at the higher frequency band, respectively. The difference between the measurement and simulation results for the radiation efficiency of the LTCC chip antenna, which is about 2 db lower, may be due to the loss in the manufacturing process of the LTCC chip. CONCLUSION The design simulation, fabrication and measurement of a 9/18 MHz dual-band chip antenna using LTCC multi-layer technology is presented. The helix-monopole structure is used to obtain dual-band characteristics. The Ansoft HFSS 3-D EM simulator is used for the design simulation. Satisfactory agreement between simulated and measured results on antenna patterns has been achieved. The measured return losses (S 11 ) are 21.8 and 2.9 db at 14 and 19 MHz, respectively. The vertical-polarization patterns (H-plane) are very close to the omni-directional patterns of a dipole antenna. The measured antenna power gains are approximately 4 and 6 db at 14 and 19 MHz, respectively, which are lower than the simulated results by about 3 db. The lower gain may be due to the loss in the manufacturing process of the LTCC chip. The measured antenna efficiencies are 24.3 and 13 percent at the lower and higher frequency bands, respectively. In order to meet the requirements for GSM/DCS applications, correcting the frequency response and increasing the bandwidth are the goals for continuing the study. ACKNOWLEDGMENT The authors would like to thank the Computer and Communication Laboratory (CCL) of the Industrial Technology of Research Institute (ITRI), Taiwan, ROC, for its support of this project. References 1. J.W. Sheen, LTCC-MLC Duplexer for DCS-18, IEEE Transactions on Microwave Theory and Techniques, Vol. 47, No. 9, September 1999, pp. 1883 189. 2. Y. Dakeya, T. Suesada, K. Asakura, N. Nakajima and H. Mandai, Chip Multi-layer Antenna for 2.45 GHz Band Application Using LTCC Technology, IEEE International Microwave Symposium Digest, 2, pp. 1693 1696. 3. W. Choi, S. Kwon and B. Lee, Ceramic Chip Antenna Using Meander Conductor Lines, Electronics Letters, Vol. 37, 21, pp. 933 934. 4. P. Haapala, P. Vainikainen and P. Eratuuli, Dual Frequency Helical Antennas for Handsets, IEEE 46 th Vehicular Technology Conference, Vol. 1, 1996, pp. 336 338. Chi-Chang Lin received his BSEE degree from the Tatung Institute of Technology, Taipei, Taiwan, and his MSEE degree from Tatung University, Taipei, Taiwan, in 1999 and 21, respectively. He is currently working toward his PhD degree in electrical engineering at National Cheng Kung University, Tainan, Taiwan. His research interests include EM simulation and microwave antenna design. Yu-Jui Chang received his MSEE degree from National Cheng Kung University, Tainan, Taiwan, in 22. He worked on chip antenna design and HBT MMIC for his master study. He is currently with ELANsat Technologies Inc. as an R&D engineer. His research interests include RF circuit and antenna design for wireless communication systems. Huey-Ru Chuang received his BSEE and MSEE degrees from National Taiwan University, Taipei, Taiwan, in 1977 and 198, respectively, and his PhD degree in electrical engineering from Michigan State University, East Lansing, MI, in 1987. From 1987 to 1988, he was a post-doctoral research associate at the Engineering Research Center, Michigan State University. From 1988 to 199, he was with the Portable Communication Division of Motorola Inc., Ft. Lauderdale, FL. He joined the department of electrical engineering at National Cheng Kung University, Tainan, Taiwan, in 1991, where he is currently a professor. His research interests include antenna and RFIC/microwave circuit design for wireless communications, computational electromagnetics and applications, EMI/EMC, microwave communication and detection systems.
TECHNICAL FEATURE APPENDIX A LTCC CHIP OF THE HELICAL-MONOPOLE ANTENNA; (A) THREE-DIMENSIONAL AND (B) UPPER VIEWS. Y h Z L W X (a) W 1 W 2 FEED 2nd MONOPOLE Y L 1 1st MONOPOLE (b) X MATCHING NETWORK 3rd MONOPOLE HELICAL ANTENNA