Dual-beam microstrip leaky-wave array excited by aperture-coupling method

2496 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 51, NO. 9, SEPTEMBER 2003

Dual-Beam Microstrip Leaky-Wave Array Excited by Aperture-Coupling Method

Tai-Lee Chen and Yu-De Lin

Abstract—This paper describes the design of the microstrip leaky-wave

array excited by the aperture-coupling technique. The microstrip first higher order leaky mode is employed as the radiation source. Separating the radiators from other components with the ground planes provides op-timal design of radiators and others so that the inherent broadband, high gain and frequency-scanning properties of the leaky-wave antenna can be exploited. Four kinds of feeding arrangements to excite dual beams are experimented in K band. Markedly reducing the required elements than patch-array for high gain design can simplify complexity of the feeding layout. Versatile designs combining the merits of multilayer technology offer simplicity and efficient design for practical wireless applications.

Index Terms—Aperture coupling, leaky wave, microstrip antenna.

I. INTRODUCTION

Broadband services such as broadband wireless access (BWA), local multipoint distribution service (LMDS), and multipoint multichannel distribution service (MMDS) are popular in wireless communications. The spectrum used in these applications is allocated in the millimeter wave frequency range. In addition, surveillance applications such as radar sensor, automobile collision avoidance system and toll and traffic management system are also operated in the microwave and millimeter wave frequency range. In these wireless applications, directional an-tenna and multibeam anan-tenna play important roles [1].

Printed-circuit type antenna is the trend in antenna development, which is compact and relatively inexpensive. In millimeter wave and wide band applications, resonant type antennas, such as patch and dipoles, are usually not suitable due to their narrow bandwidth, complexity in the matching network design for array applications, and serious tolerance requirement in fabrication. Printed circuit type leaky wave antenna [2]–[5] is a better candidate in millimeter wave applications owing to its merits such as simplicity in array design, broadband, and beam-scanning capability.

The dual-beam pattern can be easily implemented by leaky-wave an-tenna without complicated feeding networks. The feeding structures to excite the microstrip leaky mode usually require extra matching or transition circuit [2], [3]. These circuits, however, would generate un-desired fields that will affect the current distribution of the antenna mode and contaminate the radiation pattern. These extra circuits might also limit the bandwidth. First proposed by Pozar as the feeding for the patch antenna [6], aperture coupling can avert these drawbacks by sep-arating the radiators from other networks with a conductor plane. This technique can be used to design optimally for both antenna and other circuits [7].

II. DESIGN OF THEMICROSTRIPFIRSTHIGHERORDERLEAKY-WAVE ARRAYEXCITED BY THEAPERTURE-COUPLINGMETHOD Fig. 1 illustrates four kinds of feeding arrangements for dual-beam aperture-coupled microstrip leaky-wave array. The feeding microstrip

Manuscript received February 19, 1999; revised December 1, 2000. This work was supported in part by the National Science Council of the Republic of China under Contract NSC91-2213-E009-130 and in part by the MOE Program for Promoting Academic Excellent of Universities under Grant 89-E-FA06-2-4. T.-L. Chen is with the Department of Physics, National Central University, Taoyuan, Taiwan, R.O.C.

Y.-D. Lin is with the Institute of Communication Engineering, National Chiao Tung University, Hsinchu, Taiwan, R.O.C.

Digital Object Identifier 10.1109/TAP.2003.816312

(a)

(b)

(c)

(d)

Fig. 1(a). Parallel-end-fed dual-beam aperture-coupled microstrip leaky-wave array.L =80 mm,w =4.3 mm,lc =1.7 mm,lo =1.3 mm,ws =0.2 mm,"r1=2.2,"r2=10.2,h1=0.508 mm,h2=0.635 mm,wm =0.68 mm,lm =1 mm, element space=7.8 mm. (b) Parallel-center-fed dual-beam aperture-coupled microstrip leaky-wave array.L =100 mm, slot length= 3.4 mm. (c) Series-end-fed dual-beam aperture-coupled microstrip leaky-wave array. (d) Series-center-fed dual-beam aperture-coupled microstrip leaky-wave array.

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 51, NO. 9, SEPTEMBER 2003 2497

(a) (b)

(c) (d)

Fig. 2(a). (a) Measured H planeEantenna power gain patterns of the dual-beam microstrip leaky-wave array in Fig. 1(a). (b) Measured H planeEantenna power gain patterns of the dual-beam microstrip leaky-wave array in Fig. 1(b). (c) Measured H planeEantenna power gain pattern of the dual-beam microstrip leaky-wave array in Fig. 1(c). (d) Measured H planeEantenna power gain pattern of the dual-beam microstrip leaky-wave array in Fig. 1(d).

network denoted by a dashed line is arranged on the second layer. The spacings between elements are equal and optimized to fulfill the max-imal antenna gain, which is estimated by the waveguide model [8]. Two kinds of substrates are used here. Duroid laminate with lower dielec-tric constant ("r= 2:2, h = 0:508 mm) is adopted for the leaky-mode

microstrip antenna layer and the higher dielectric constant substrate ("r = 10:2, h = 0:635 mm) for the feeding networks. This

arrange-ment will raise the antenna efficiency and reduce the layout area of the feeding networks.

Four kinds of the feeding networks are (a) parallel-end fed, (b) parallel-center fed, (c) series-end fed, and (d) series-center fed. For (a) and (c), the dual beams are excited from both ends of the microstrips; while (b) and (d) excite the leaky mode from the center of the microstrip as the structure described in [5]. The parallel-feeding methods ((a), (b)) can easily achieve the desired power division and

the phase distribution. Here, equal power dividers with modified quarter-wave transformers are used to implement the equal-amplitude and the equal-phase distribution. The series-feeding methods ((c), (d)) has simpler feeding networks than the parallel-feeding methods ((a), (b)). And no power divider is even required for center-feeding method ((d)). Since the leaky microstrips are used as the same radiator for both feeding ends of the end-feeding methods ((a), (c)), they are shorter than the leaky microstrips of the center-feeding methods ((b), (d)). Nevertheless, the end-feeding structure needs more power dividers than the center-feeding structure.

The microstrip first higher order leaky mode travels along the mi-crostrip with an exponentially attenuating fieldE0e0jxe0x under the strip, where and  are the phase and attenuation constants of the leaky mode [9]. The far field can be calculated by the equivalence magnetic currentM = E 2 n, and the direction of the radiation beam

2498 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 51, NO. 9, SEPTEMBER 2003

can be roughly estimated in the direction of = cos01(=k0), where

 is the elevation angle from the microstrip, and k0 is the free space wavenumber. The propagation constants can be derived by solving the spectral domain integral equations with appropriate integral contour of the inverse Fourier transform and the moment method numerical technique [10]. The shape of the coupling aperture must be properly designed in order to effectively excite the leaky mode. On the inter-face between the two layers the distribution of the equivalence mag-netic current on the aperture should be consistent with the field of the boundary condition of the leaky mode. Narrow slot can satisfy this aperture boundary condition. The slot length is basically shorter than half wavelength of the slot mode to prevent resonance. The length of an open stub of the exciting microstrip is about one quarter of the guide wavelength and is capable of serving as another matching circuit to ad-just the imaginary part of the input impedance.

III. EXPERIMENTRESULTS

Fig. 2 shows the measured H planeEantenna power gain patterns of the arrays in Fig. 1.Eon H plane in this coordinate system is the dominant radiation field. The parallel-fed antennas ((a), (b)) have peak power gain of 15.4 dBi, which is roughly half of the gain of the end-fed single-beam array described in [8]. In general, to obtain high antenna gain by conventional patch array, a large number of patches with com-plicated power dividers and matching networks are required [11]. With the frequency varying from 21 GHz to 24.5 GHz, the angle between the peaks of the dual beams spreads from 38to 72for the parallel-fed array. The nulls of the pattern of the end-fed array ((a), (c)) are more pronounced than that of the center-fed array ((b), (d)). This is because of the field superposition effects from the different source pairs whose current distributions are in different exponentially decaying directions. In the series-fed experiments (c) and (d), the same coupling aper-tures are used. Power excited by every aperture is different so that the current distribution on every radiating element is different. This causes the measured antenna gains of (c) and (d) to be less than those in the parallel-fed cases (a), (b). For in-phase excitation in the series-fed cases (c) and (d), the length of the feeding microstrip between aper-tures must be taken to be the multiples of its guide wavelength. Under this condition, the bandwidth of the series-fed type is narrower than that of the parallel-fed type, and therefore limits the beam-scanning ca-pability. Comparing these measurements, some back radiation (to0z direction) resulting from the feeding circuits is found. This back radia-tion is smallest in the series-center fed array (d) that adopts the simplest feeding circuits.

IV. CONCLUSION

This work describes the designs of the microstrip leaky-wave array excited by the coupling technique. A merit of the aperture-coupling method is its extra degree of freedom in designing feeding net-works. By utilizing the versatile feeding arrangements on the different layers, center-, end- and parallel-, series-fed four-element dual-beam arrays are implemented. Experimental results confirm that the designs of the feeding structures can achieve practical wireless applications.

REFERENCES

[1] B. Zimmermann, W. Wiesbeck, and J. Kehrbeck, “24 GHz microwave closed-range sensors for industrial measurement applications,” Microw.

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[5] T.-L. Chen and Y.-D. Lin, “Aperture-coupled microstrip line leaky wave antenna with broadside mainbeam,” Electron. Lett., vol. 34, no. 14, pp. 1366–1367, Jul. 1998.

[6] D. M. Pozar, “Microstrip antenna aperture-coupled to a microstripline,”

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[7] D. M. Pozar and D. H. Schaubert, Microstrip Antennas, the Analysis and

Design of Microstrip Antennas and Arrays. New York: IEEE Press, ch. 5.

[8] T.-L. Chen and Y.-D. Lin, “A K-band aperture-coupled microstrip leaky-wave antenna,” IEICE Trans. Electron., pp. 1236–1241, July 1999.

[9] A. A. Oliner and K. S. Lee, “The nature of the leakage from higher modes on microstrip line,” in IEEE MTT-S. Dig., 1986, pp. 57–60. [10] Y.-D. Lin and J.-W. Sheen, “Mode distinction and radiation efficiency

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Improved Compensation for the Mutual Coupling Effect in a Dipole Array for Direction Finding

H. T. Hui

Abstract—A new and practical method is proposed to compensate for

the mutual coupling effect in a dipole array deployed for direction finding. This method does not require the known current distributions on the an-tenna elements or the known elevation angles of the incoming signals. A new definition of mutual impedance is introduced to characterize the effect due to mutual coupling between dipole elements. The new mutual imped-ances are calculated based on an estimated current distribution. It is shown that current method has a significantly better ability to compensate for the mutual coupling effect than previous methods. Computer simulations using the MUSIC algorithm are provided to demonstrate this method.

Index Terms—Dipole array, MUSIC algorithm, mutual coupling effect,

mutual impedance.

I. INTRODUCTION

Direction finding is an important function of an antenna array. How-ever, it was shown that typical eigenstructure-based direction finding algorithms, such as MUSIC, require accurate knowledge of the re-ceived signal voltages from antenna terminals as inputs [1]. This places a critical requirement on an antenna array for direction finding. This requirement is especially difficult to meet for a dipole antenna array due to the strong mutual coupling effect between the dipole elements. There have been many methods suggested to identify or to compensate for the mutual coupling effect in dipole antenna arrays. In [2], the au-thors used the concept of mutual impedance to derive the open-circuit voltages from the terminal voltages and showed that mutual coupling has a significant effect on the performance of adaptive arrays. This method was later used in [3] to compensate for the mutual coupling

Manuscript received November 13, 2001; revised September 21, 2002. The author is with the Electrical and Electronic Engineering Department, Nanyang Technological University, Singapore 639798, Singapore (e-mail: [email protected]).

Digital Object Identifier 10.1109/TAP.2003.816303 0018-926X/03$17.00 © 2003 IEEE