IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 49, NO. 6, JUNE 2001 855
Microstrip-Fed Microstrip Second Higher Order
Leaky-Mode Antenna
Tai-Lee Chen, Yu-De Lin, and Jyh-Wen Sheen
Abstract—With the appropriate placement of the slots and via holes, the microstrip second higher order leaky-mode antenna fed by a microstrip is presented. Two main beams with titled angles from the strip are measured in the predicted radiation leaky band. The leaky band of the microstrip second higher order mode is deduced from the characteristics of the propagation constants that are calculated by the spectral domain analysis. The proposed feeding method provides a direct connection with the circuits based on the microstrip line.
Index Terms—Antenna feeds, leaky-wave antennas, microstrip.
I. INTRODUCTION
B
ASED on the investigation of the leaky mode of the planar transmission structures, the applications of printed leaky-wave antenna have gradually attracted attention recently [1]–[5]. The planar leaky-wave antenna is a better candidate in microwave and millimeter-wave applications owing to its advantages such as broad-band, higher gain, and fre-quency-scanning properties. Especially in the applications of multi-beam requirement, such as multipoint communications and surveillance systems, the leaky-wave antenna can reduce the complexity of the feeding network design [4], [5].Based on the first higher order leaky mode of the microstrip, several microstrip leaky-wave antennas were developed. How-ever, there is little literature utilizing the characteristics of the second higher order leaky mode as the radiation source [6], [7]. A microstrip line combined with a coplanar waveguide (CPW) underneath the strip as a leaky-wave antenna was proposed in [6]. With the CPW running under the strip, the microstrip second higher order leaky mode would not be purely excited because the guiding wavelength of the leaky mode is not necessarily the same as that of the equivalent conductor-backed CPW. In [7], two kinds of short-end CPW feeding structures of the microstrip second higher order leaky mode antenna were developed, one of which arranges the short-end CPW on the strip and another on the ground plane. However, to connect with circuits based on the microstrip line, extra CPW-to-microstrip transition circuits must be applied to these CPW feeding structures. These transi-tion circuits are usually narrow-band in nature and might affect
Manuscript received July 20, 1999; revised October 19, 2000. This paper was supported in part by the Ministry of Education of Republic of China under the Academic Excellence Grant.
T.-L. Chen and Y.-D. Lin are with the Institute of Communication Engi-neering, National Chiao Tung University, Hsinchu, Taiwan, R.O.C. (e-mail: [email protected]).
J.-W. Sheen is with the Computer and Communication Research Laborato-ries, Industrial Technology Research Institute, Hsinchu, Taiwan, R.O.C.
Publisher Item Identifier S 0018-926X(01)03595-5.
Fig. 1. Microstrip-fed microstrip second higher order leaky-wave antenna and the coordinate system." = 10:2, h = 0:635, L = 65, w = 5:2, ws = 0:2,
wm = 0:62, ls = 9:1, lp = 1:3, lt = 7:4 (mm).
the radiation pattern of the antenna. To overcome these defects, the microstrip line feeding structure for the microstrip second higher order leaky mode is proposed in this paper.
II. MICROSTRIPFEEDINGSTRUCTURE FOR THEMICROSTRIP SECONDHIGHERORDERLEAKY-MODEANTENNA The microstrip second higher order leaky-mode antenna with the microstrip feeding structure on both ends of the antenna is depicted in Fig. 1. To design the leaky-wave antenna, the radi-ation region of the microstrip must be identified first. The nor-malized propagation constants of the second higher order leaky mode are shown in Fig. 2. They are obtained by the spectral domain analysis (SDA) with the appropriate inverse transform integral path and the moment method technique [8]. The inverse transform integral path includes part of the real axis, the surface wave poles, and a leaky portion that locates on the second Rie-mann sheet with a vertically exponentially growing wave [8], [9]. The phase constant along the strip is smaller than the freespace wavenumber , which indicates the possibility of ra-diating into space. This is different from the covered stripline structure of which the phase constant is larger than and the wave radiates laterally when the leakage occurs [10], [11]. The fast-wave radiation region is located before the frequency point
and after [9].
The slots are etched where the longitudinal currents along the strip are small. The slot length is chosen to be about a quarter of the guided wavelength of the leaky mode. Two via holes are placed at about a quarter width from the edge of the strip (the fringing effect is taken into account) near the slots to match the boundary condition of the second higher order leaky mode on the feeding interface. In this way, the vertical electric fields 0018–926X/01$10.00 © 2001 IEEE
856 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 49, NO. 6, JUNE 2001
Fig. 2. The normalized phase constant and attenuation constant of the microstrip second higher order leaky mode with the specification in Fig. 1.
Fig. 3. MeasuredS-parameters and the RPA of the second higher order leaky-wave antenna in Fig. 1. under the strip at the via holes are shorted and the microstrip
dominant mode can be suppressed.
III. EXPERIMENTALRESULTS
The measured scattering parameters and the relative power
absorbed (RPA ) which indicates how
much power is dissipated in the circuit are shown in Fig. 3. The frequency band where both and are small (RPA in-creases) is in the radiation region in Fig. 2. The dips of are caused by the bound modes resonating forward and backward on the strip. These bound modes (dominant mode and second
higher order bound mode) are not matched by the feeding struc-ture out of the leaky band. The measured three-dimensional power gain pattern of the antenna fed by one port (with another end cut flatly) is depicted in Fig. 4. The pattern is measured in an anechoic chamber with the HP85301 antenna measure system. The magnitude of the radial component is the measured power gain in dBi. In the chosen coordinate system, is the dominant polarization of the radiated fields. This can be deduced from the waveguide (cavity) model that assumes two magnetic side walls under the edges of the strip. The peak power gain of the tilted
dual mainbeam is about 7.3 dBi ( , ) at 16
CHEN et al.: MICROSTRIP-FED MICROSTRIP SECOND HIGHER ORDER LEAKY-MODE ANTENNA 857
Fig. 4. Measured power gain patterns (3 dB/contour) of the truncated open end of the antenna in Fig. 1 at 16 GHz. Peak(; ) = (65 ; 130 ), peak power gain
= 7:3 dBi.
angle along above the strip. It could be attributed to the radia-tion caused by the survival power on the truncated end.
IV. CONCLUSION
An effective exciting technique for the microstrip second higher order leaky mode is devised in this paper. To satisfy the current and field distributions of the leaky mode, the slots and the via holes are combined around the feeding structure. Antenna fed directly by the microstrip offers the advantage of easy connecting with the circuits based on the microstrip line. Measurement of scattering parameters confirms the expected leaky band that was deduced by the spectral domain analysis. Measured two tilted main beams reveals the possibility of use in multi-beam applications.
REFERENCES
[1] W. Menzel, “A new traveling-wave antenna in microstrip,” Archie fur
Electronik und Ubertranungstechnik, vol. 33, pp. 137–140, Apr. 1979.
[2] A. A. Oliner and K. S. Lee, “The nature of the leakage from higher-order modes on microstrip line,” in 1986 IEEE MTT-S Int. Microwave Symp.
Dig., Baltimore, 1986, pp. 57–60.
[3] Y.-D. Lin, J.-W. Sheen, and C.-K. C. Tzuang, “Analysis and design of feeding structures for microstrip leaky wave antenna,” IEEE Trans.
Mi-crowave Theory Tech., vol. 44, pp. 1540–1547, Sep. 1996.
[4] C. Luxey and J.-M. Laheurte, “Simple design of dual-beam leaky-wave antennas in microstrips,” IEE Proc.—,Microw. Antennas Propagat., vol. 144, no. 6, Dec. 1997.
[5] T.-L. Chen and Y.-D. Lin, “Microstrip leaky-wave antenna fed by short-end CPW-to-slot transition,” Electron. Lett., vol. 35, no. 6, Jan. 1999.
[6] C.-K. C. Tzuang and C.-C. Lin, “Millimeter wave micro-CPW integrated antenna,” in SPIE 1996, Denver, CO, Aug. 1996, pp. 513–518. [7] Y.-D. Lin, P.-M. Chi, and T.-L. Chen, “Design of the feeding
struc-tures for the excitation of the microstrip line second higher order mode leaky-wave antenna,” in 1997 IEEE Antenna and Propagation Society
Int. Symp. Digest, pp. 1142–1143.
[8] J. S. Bagby, C. H. Lee, D. P. Nyquist, and Y. Yuan, “Identification of propagation regimes on integrated microstrip transmission lines,” IEEE
Trans. Microwave Theory Tech., vol. 41, pp. 1887–1894, Nov. 1993.
[9] Y.-D. Lin and J.-W. Sheen, “Mode distinction and radiation-efficiency analysis of planar leaky-wave line source,” IEEE Trans. Microwave
Theory Tech., vol. 45, pp. 1672–1680, Oct. 1997.
[10] D. R. Jackson et al., “An excitation theory for bound modes, leaky modes and residual-wave currents on stripline structures,” Radio Sci., vol. 35, no. 2, pp. 495–510, March–April 2000.
[11] R. Mesa and R. Marques, “Integral representation of spatial Green’s function and spectral domain analysis of leaky covered strip-like lines,”
IEEE Trans. Microwave Theory Tech., vol. 43, pp. 828–837, 1995.
Tai-Lee Chen received the B.S. degree in mathe-matics from the National Taiwan University, Taipei, Taiwan, R.O.C., in 1989, and the M.S. and Ph.D. degrees in communication engineering from the National Chiao Tung University, Hsinchu, Taiwan, R.O.C., in 1991 and 1999, respectively.
His research interests include analytical and com-putational electromagnetics, and antenna design.
Yu-De Lin received the B.S. degree in electrical en-gineering from National Taiwan University, Taipei, Taiwan, R.O.C., in 1985 and the M.S. and Ph.D. de-grees in electrical engineering from the University of Texas at Austin in 1987 and 1990, respectively.
In 1990, he joined the faculty of the Department of Communication Engineering, National Chiao Tung University, Hsinchu, Taiwan, R.O.C., where he is currently a Professor. His current research interests include numerical methods in electromagnetics, characterization and design of microwave and millimeter-wave planar circuits, and analysis and design of microwave and millimeter-wave antennas.
Jyh-Wen Sheen received the B.S. degree in control engineering and the M.S. and Ph.D. degrees in communication engineering from the National Chiao Tung University, Hsinchu, Taiwan, R.O.C., in 1991, 1993, and 1996, respectively.
In 1996, he joined the RF Communication Systems Technology Department, Computer and Communi-cation Laboratories, Industrial Technology Research Institute (ITRI), Hsinchu, Taiwan, R.O.C., and is an RF Engineer and is currently developing a miniatur-ized RF filter and high-gain antenna. His research in-terests include the analysis and design of various planar-type leaky-wave an-tennas and the investigation of surface-wave leakage phenomena of uniplanar transmission lines.
