Decoupling stub-loaded parallel dipole array with orthogonal polarization

Transcription

1 Decoupling stub-loaded parallel dipole arra with orthogonal polariation Kohei Omote a), Kauhiro Honda, and Kun Li Graduate School of Engineering, Toama Universit, 319 Gofuku, Toama-shi, Toama , Japan a) Abstract: This paper presents a novel design for a two-element stub-loaded dipole arra antenna. The purpose is to obtain the decoupling and orthogonal polariation characteristics using a small antenna configuration. A quarterwavelength stub loaded on one side of the two elements is operated as a phase shifter to create the opposite phase of current distribution. Moreover, the quarter-wavelength stub works as an orthogonal dipole antenna because the in-phase currents are enhanced on the stub, leading to orthogonal directivit. The results show that well-defined isolation and orthogonal polariation directivit can be achieved using the proposed antenna. Kewords: orthogonal polariation, decoupling, stub, dipole arra Classification: Antennas and Propagation References [1] D. Gesbert, M. Shafi, D. S. Shiu, P. J. Smith, and A. Naguib, From theor to practice: An overview of MIMO space-time coded wireless sstems, IEEE J. Sel. Areas Commun., vol. 21, no. 3, pp , Apr. 23. DOI:1.119/ JSAC [2] M. Nakano and H. Arai, Orthogonal polariation base station antenna technolog at cellular sstems and sstem evaluation, IEICE Trans. Commun., vol. J96-B, no. 1, pp. 1 15, Jan [3] D. T. Le, M. Shinoawa, and Y. Karasawa, Wideband MIMO compact antennas with tri-polariation, IEICE Trans. Commun., vol. E94-B, no. 7, pp , Jul 211. DOI:1.1587/transcom.E94.B.1982 [4] K. Omote, H. Sato, K. Li, K. Honda, Y. Koanagi, and K. Ogawa, Three-ais decoupling stub-loaded parallel dipole arra with tri-orthogonal polariation directivit, IEEE iwem 215, Hsinchu, pp. 1 2, Nov DOI:1.119/ iwem [5] K. Nishiawa, H. Okegawa, H. Ohmine, Y. Sunahara, and T. Katagi, A linear dipole antenna with crank sections suitable for an element operating at a lower band in dual-band arra antennas, IEICE Trans. Commun., vol. J85-B, no. 6, pp , June 22. [6] D. M. Poar, Microwave Engineering, 3rd ed., p. 187, John Wile & Sons, 25. [7] FEKO, Copedited Februar 1,

2 1 Introduction The multiple-input multiple-output (MIMO) technique was investigated to realie ultra-high-speed and high-capacit mobile communications [1]. The polaried- MIMO enhances the channel capacit more effectivel compared with the space- MIMO because of the low correlation between the antenna elements owing to space orthogonalit [2]. However, an orthogonall arranged antenna configuration is disadvantageous for multi-element compactness because of its large sie. Therefore, miniaturiing the antenna is indispensable [3]. This paper presents a novel design for a two-element stub-loaded dipole arra antenna (SLDA). The feature of this proposal is that it realies the orthogonal polariation and decoupling characteristics simultaneousl using a small antenna configuration [4]. The current distribution on the stub generates the radiation pattern using the common mode, whereas the nonradiation element of the stubloaded dipole produces well-defined isolation b using the opposite phase. The analtical and measured results confirm that well-defined isolation and orthogonal polariation directivit of SLDA can be achieved. 2 Operating principle of two-element stub-loaded dipole arra Copedited Februar 1, 217 The stub structure is generall used to eliminate the undesired current component as the parallel elements of the stub are nonradiation elements because of the opposite phase mode [5]. We propose operating a stub as a radiation element with a common mode in this paper in order to obtain orthogonal directivit. Fig. 1 shows the operating principle of the two-element SLDA antenna for realiing the orthogonal polariation and decoupling characteristics simultaneousl. Fig. 1(a) shows the current distribution of a two-element SLDA to illustrate the orthogonal polariation directivit. The SLDA comprises two parallel dipole antennas, in which one of the elements is loaded with a quarter-wavelength stub for creating the opposite phase of the current distribution as a phase shifter. When the stub-loaded element (Port 1) is ecited, the opposite phase of the current distribution is created with the same amplitude (I 1 and I 1 ) on Element 1. Thus, Element 1 is a nonradiation element because the currents cancel each other. In this case, the radiation pattern is similar to that generated b a horiontal dipole antenna because the in-phase currents I 3 flowing on the Element Stub are enhanced b each other. On the other hand, when the dipole element (Port 2) is ecited, the current of Element 2 (I 2 ) is created. Consequentl, the SLDA in Fig. 1(a) can be treated as two orthogonall arranged dipoles, resulting in orthogonal polariation directivit contributed b the current I 3 on Element Stub and I 2 on Element 2. Fig. 1(b) shows the decoupling mechanism based on the current distribution of the SLDA in Fig. 1(a). In Fig. 1(b), the separated dipole units on the right side correspond to Element 1, whereas the single dipole on the left side represents Element 2. Z12 a and Zb 12 show the mutual impedance between Elements 1 and 2 in the up-side and down-side, respectivel. Eqs. (1) and (2) show the matrices of the two dipoles (Element 1 and Element 2) between the voltage V 1 to V 2,or V 1 to V 2. 98

3 " # " #" # V 1 ¼ Za 11 Z12 a I1 ð1þ V 2 Z21 a Z 22 I 2 " # " #" # V 1 ¼ Zb 11 Z12 b I1 ð2þ V 2 Z21 b Z 22 I 2 Eq. (3) is obtained b transforming the plus and minus signs in Eq. (2). " # " #" # V 1 ¼ Zb 11 Z12 b I1 ð3þ V 2 Z21 b Z 22 I 2 The self-impedance and mutual impedance can be obtained b comparing Eqs. (1) and (3) as shown below, respectivel. Z11 b ¼ Za 11 ð4þ Z12 b ¼ Za 12 ð5þ The self-impedance in Eq. (4) has the same magnitude. The mutual impedance in Eq. (5) has the same magnitude and opposite sign. Therefore, the total mutual impedance Z12 T is ero, as epressed b the following equation. Z T 12 ¼ Za 12 þ Zb 12 ¼ Za 12 Za 12 ¼ The isolation S 12 depending on the Z parameter is defined in [6] as follows: 2Z12 T S 12 ¼ Z ðz 11 þ Z ÞðZ 22 þ Z Þ Z12 T ð7þ ZT 21 where Z is the load impedance, and Z 11 and Z 22 are the self-impedances. S 12 can be calculated as ero because Z12 T in Eq. (7) is ero, resulting in a high isolation in theor. ð6þ Element 2 I 2 Ε θ Port 2 I 2 Element 1 I 1 I 1 I 3 I 3 θ φ Element Stub Ε φ V 2 I 2 + Z 22 a Z 12 a Z 11 b Z 11 I 1 + I 1 V 1 I 1 I 2 + V 1 b Z 12 I 1 (a) Current distribution (b) Decoupling mechanism Fig. 1. Operating principle of two-element SLDA. The orthogonal polariation directivit and decoupling characteristics can be simultaneousl realied based on the two above-mentioned principles using two linear elements instead of a complicated structure. Copedited Februar 1, Design process of two-element stub-loaded dipole arra The simulation of a two-element SLDA was performed on the commercial EM solver FEKO [7] based on the principles illustrated in Fig. 1. Fig. 2(a) shows the 99

4 analtical model. It is designed to operate at a frequenc of 2 GH. The length of the dipole antenna is 7.5 cm (=2 at 2 GH). The purpose of this simulation is to determine the optimum dimension of the SLDA, including the stub length L and element separation d, as shown in Fig. 2(a). The separation of Element Stub is 2 mm. The element radius is.5 mm. Fig. 2(b) shows the isolation characteristics between Ports 1 and 2 for different element separations d as a function of the phase difference. The phase difference, which depends on the stub length L, can be calculated b the following equation. ¼ 2L 2 ð8þ In Fig. 2(b), the red, blue, and black lines show the cases in which the values of element separation d are 5 mm, 1 mm, and 2 mm, respectivel. In Fig. 2(b), the current flowing on the stub-loaded element has a distribution opposite to that on the dipole element when the length of the stub is L ¼ =4 ( ¼ ), which is equal to that on an orthogonal dipole arra antenna. Therefore, an isolation greater than 3 db can be achieved at 2 GH regardless of the antenna separation d. As shown in Fig. 2(b), a high isolation of 32.5 db is achieved when the separation d is 5 mm (:3 at 2 GH). On the other hand, the current flowing on the stub-loaded element has the same distribution as that on the dipole element when L ¼ ( ¼ ) or L ¼ =2 ( ¼ 2), which equals that on the parallel dipole arra antenna. Thus, the currents of the two elements enhance each other, which eventuall degrades the isolation performance compared with that when L ¼ =4. The optimum dimension of the SLDA is determined at L ¼ =4 and d ¼ 5 mm based on the simulation result shown in Fig. 2(b). Element 2 Element 1 θ 7.5cm Port 2 φ Element Stub L d (a) Antenna configuration 2mm S 21 [db] d=λ/32=5mm d=λ/16=1mm d=λ/8=2mm π 2π Δφ [rad] λ/4 λ/2 Length of Stub (b) Isolation characteristics Fig. 2. Design process of two-element stub-loaded dipole arra. 4 Measurement results Copedited Februar 1, 217 Fig. 3(a) shows the measurement setup of the two-element SLDA. As a preliminar eperiment, the dipole antenna with a feed structure of balun is used. Since the balun is the additional part compared with the analtical model, the are orthog- 1

5 θ Element 2 Port2 Balun d=5mm Element 1 L=3.75cm 2mm 1.8GH 2GH 2GH 1.8GH 2.2GH Port2 2.2GH Port2 (a) Measurement setup (b) Impedance characteristics 3dB dipole arra 23dB 26dB SLDA Solid Line: Measured Result Broken Line: Analtical Result (c) S 21 characteristics -plane -plane 1 -plane Port [dbd] 1 Port 2 E θ E φ f=2gh [dbd] (d) Orthogonal polariation directivit Fig. 3. Measurement results of the proposed two-element stub-loaded dipole arra. Copedited Februar 1, 217 onall-arranged along and -ais, respectivel, in order to reduce the undesired electromagnetic impact on radiation elements. The length of the stub is 3.75 cm (¼ =4 at 2 GH). The element separation d between two elements is 5 mm. Fig. 3(b) shows the measured impedance characteristics of the SLDA. When the specific port is ecited, the other port is terminated with 5 Ω. In Fig. 3(b), the red line shows the result of the stub-loaded element (Port 1) and the blue line shows that of the dipole (Port 2). As shown in Fig. 3(b), the impedance of Ports 1 and 2 at 11

6 2 GH is 79:4 þ j:2 Ω and 92:6 j21:9 Ω, respectivel. Therefore, the VSWR of the two elements are less than 2.5, indicating that there is no significant impedance mismatch when the stub is loaded. Fig. 3(c) shows the S 21 characteristic of the SLDA. The red lines show the proposed SLDA whereas the blue lines show the two-element vertical dipole arra designed at 2 GH. The solid and broken lines indicate the measured and analtical results, respectivel. As shown in Fig. 3(c), the SLDA can achieve an improvement of 23 db in the isolation at 2 GH compared with the result obtained using the dipole arra antenna. Fig. 3(d) shows the radiation characteristics considering the mismatch loss. The solid and broken lines indicate the measured and analtical results, respectivel. The blue line shows the E -component and the red line indicates E -component. In Fig. 3(d), an eight-figure radiation pattern with the E - and E -components in the - and -planes, respectivel, can be observed when Port 1 is ecited, whereas an omnidirectional radiation pattern with the E -component is obtained in the -plane. Therefore, the radiation pattern of Port 1 is similar to a horiontal dipole arranged along the -ais. When Port 2 is ecited, the radiation pattern is similar to that of a vertical dipole arranged along the -ais. These features indicate that the proposed SLDA can realie the orthogonal polariation directivit. In Fig. 3(d), the measurement results of the main polariation agree well with the analtical results in all the planes. However, undesired polariation directivit is observed. This phenomenon can be attributed to the electromagnetic coupling between the element and the balun, and the radiation of the unnecessar current caused b the difference in element length. 5 Conclusion This paper presents a design eample of two-element SLDA. The measured and analtical results show that well-defined isolation and orthogonal polariation directivit can be simultaneousl realied with a small antenna configuration using the proposed SLDA. As a future work, we are developing a practical antenna with low-profile based on the design principle presented in this paper. Copedited Februar 1,