A beam-forming technique for leaky-wave antennas

Indeed, circular polarization seems to be better for improving w x

the quality of indoor communication 10 .

ACKNOWLEDGMENTS

The authors would like to thank the IDRIS for their com-puter facilities, and Ms. Fabienne Blanchet for her mechani-cal support.

REFERENCES

1. D.M. Pozar, A reciprocity method of analysis for printed slot and slot-coupled microstrip antennas, IEEE Trans Antennas

Propa-Ž .

gat AP-34 1986 , 1439᎐1446.

2. H. Ohmine, T. Kashiwa, T. Ishikawa, A. Iida, and M. Matsunaga, An MMIC aperture coupled microstrip antenna in the 40 GHz band, Proc ISAP, 1992, pp. 1105᎐1108.

3. M. Stotz, G. Gottwald, H. Haspeklo, and J. Wenger, Planar millimeter wave antennas using SiNx membranes on GaAs, IEEE

Ž .

Trans Microwave Theory Tech 44 1996 .

4. A. Iida, M. Hieda, K. Itoh, H. Ohmine, K. Matsuo, T. Furuya, and T. Kashiwa, A 100 GHz-band antenna stacked monolithic receiver using multilayered configuration, Proc ISAP’96, Chiba, Japan.

5. Avi & Peschard, ZAC de la Goulgatiere, 35000 Chateaubourg,` ˆ France.

6. O. Lafond, M. Himdi, and J.P. Daniel, Aperture coupled mi-crostrip patch antenna with a thick ground plane in millimeter

Ž .

waves, Electron Lett 35 1999 , 1394᎐1396.

7. M. Himdi, O. Lafond, S. Laigner, and J.P. Daniel, Extension of cavity method to analyse aperture coupled microstrip patch

an-Ž .

tenna with thick ground plane, Electron Lett 34 1998 , 1534᎐ 1536.

8. O. Lafond, M. Himdi, and J.P. Daniel, Slot coupled millimeter

Ž .

printed antenna arrays 60 GHz , Millenium Conf Antennas Propagat AP2000, Davos, Switzerland, Apr. 2000.

9. Ensemble 5.1, Ansoft Corp.

10. C. Loyez, N. Haese, O. Lafond, P. Lefevre, G. Lewandowski, and P.A. Rolland, Indoor propagation channel considerations in 60 GHz high data rate communications, EUMC Wireless 2000 Conf, Paris, France, Oct. 2000.

䊚 2001 John Wiley & Sons, Inc.

A BEAM-FORMING TECHNIQUE FOR

LEAKY-WAVE ANTENNAS

I-Yu Chen,1Chien-Jen Wang,1Shih-Jeng Liu,1and Christina F. Jou1

1_{Institute of Communication Engineering}

National Chiao-Tung University Hsinchu, Taiwan, R.O.C. Recei¨ed 18 July 2000

ABSTRACT: A beam-forming technique for leaky-wa¨e antennas (LWAs which can switch electronically in three different radiation) patterns is de¨eloped in this letter. A simple two-terminal feeding mi-crostrip-line LWA works as a radiating element. By¨arying the phase of the two injection signals from 0 to 180T, the emitting power from the LWA can produce three different radiation modes: the normal mode (one sharp beam , the difference mode) ( )⌬ , and the sum mode ⌺ . The( ) antenna displays large-bandwidth characteristics, and can be useful near crossroads for collision a¨oidance or in the radio location of¨ehicles. 䊚 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 28: 108_{᎐110, 2001.}

Key words: leaky-wa¨e antennas; switched-beam antennas; beam steering

I. INTRODUCTION

Ž .

Recently, leaky-wave antennas LWAs have become popular, and there is a growing interest in various types of leaky-wave

w x antennas used as frequency-scanning elements 1᎐2 . Leaky-wave antennas possess the advantages of low profile, fabri-cation simplicity, easy matching, narrow beamwidth, and frequency-scanning capability. As is known, the one-beam

w x w x

scanning LWA 1 and the dual-beam scanning LWA 2 have been studied intensively. However, the above-mentioned re-search cannot change the beam mode electronically, such as from a one-beam mode jumping into a dual-beam mode. Lin

w x

et al. 3 proposed a class of short LWAs with coaxial-to-mi-crostrip transitions on which the differential signals propa-gate. The sum and difference radiation patterns must be shown by using two different circuit structures, such as the end-fed and center-fed LWAs.

In this study, we present an alternative approach to achieve ⌬᎐⌺ radiation patterns using a simple two-terminal feeding

Ž .

microstrip-line leaky-wave antenna design see Fig. 1 . When a short LWA is excited in phase from the two-terminal feeds, a great amount of electromagnetic energy of the leaky mode will survive when it travels down to the other end of the antenna. The remaining power reflects and offsets against the other injection signal, creating the sum pattern. It is a broad-side pattern. When the LWA is excited out of phase, the reflected wave will be added into the other injection signal, creating the difference pattern. It is a dual-beam pattern. The advantage of this design is not only that the ⌬᎐⌺ patterns can be chosen electronically, not mechanically, but the antenna can also perform as a traditional beam mode by controlling the phase of the input signals.

II. ANTENNA DESIGN

The geometry and coordinate system for this leaky-wave antenna design are shown in Figure 2. Each slot radiates the

w x

same field as the magnetic dipole 4 , with the equivalent magnetic current density M_RSfor the right injection and M_LS for the left injection. The RTrDuroid substrate used has a

Figure 1 Simple two-feeding microstrip-line leaky-wave antenna

Figure 2 Geometry and coordinate system for the topology of this designed leaky-wave antenna

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 28, No. 2, January 20 2001 108

thickness of 0.635 mm and a dielectric constant of ␧ s 2.2.r

To excite the first higher order mode within the operating range of frequency, this microstrip leaky-wave antenna is fed asymmetrically, and the width W and length L of the

radiat-Ž .

ing element are empirically chosen to be 0.61 cm 240 mil

Ž .

and 2.794 cm 1100 mil , respectively.

III. THEORETICAL AND EXPERIMENTAL RESULTS

The microstrip leaky-wave antenna is operated at 15.5 GHz. Figure 3 shows a comparison between the measured and theoretical H-plane patterns of a broadside sum pattern, as

Ž

the two feeds have in-phase current excitation the phase .

difference is 0⬚ with a maximum directivity of 8.3 dBi. Figure 4 shows the pattern of a difference pattern, as the two feeds

Figure 3 Measured and theoretical broadside sum pattern as the excited injection signals are in phase

Figure 4 Measured and theoretical dual-beam difference pattern as the excited injection signals are out of phase

Ž

have out-of-phase current excitation the phase difference is .

180⬚ with a maximum directivity of 8.5 dBi. Note that the main beam positions of the out-of-phase mode are at ␪ s "26⬚. Figure 5 shows a comparison between the measured and theoretical prediction of the traditional one-beam radia-tion pattern. The antenna radiates a pencil beam for a phase difference of 90⬚ between the two injection signals. It can be observed that reasonable agreement between the simulation and experimental results is obtained. The measured results prove that this beam-forming technique allows one to choose the sum pattern, the difference pattern, or the traditional one-beam radiation pattern electronically. The radiation pat-tern of this antenna design depends on the phase difference of the two injection signals, instead of different circuit struc-tures. Figure 6 shows a comparison of the measured and theoretical S-parameters of this designed antenna, and the results show that the bandwidth is at least 2 GHz.

IV. CONCLUSIONS

It is demonstrated that the sum, the difference, and the traditional one-beam radiation patterns can be chosen

elec-Figure 5 Measured and theoretical one-beam traditional pattern as the excited injection signals have a phase difference of 90⬚

Figure 6 Comparison of the measured and theoretical S-parame-ters of this designed antenna

tronically by using a simple two-terminal feeding leaky-wave antenna structure. This beam-forming technique for leaky-wave antennas exhibits the properties of flexible beam switch-ing and large bandwidth, and will be a suitable candidate for applications such as mobile communication and satellite com-munication.

REFERENCES

1. W. Menzel, A new traveling wave antenna, Proc 8th European Microwave Conf, 1978, pp. 302᎐306.

2. C.J. Wang, C.F. Jou, and J.J. Wu, A novel two-beam scanning

Ž .

leaky-wave antenna, IEEE Trans Antennas Propagat 47 1999 , 1314᎐1317.

3. J.M. Lin, C.K. Tzuang, G.J. Chou, and S. Su, Short leaky wave

Ž .

antenna of sum and difference patterns, Electron Lett 32 1996 , 1247᎐1249.

4. C.A. Balanis, Antenna theory analysis and design, Wiley, New York, 1997, 2nd ed.

䊚 2001 John Wiley & Sons, Inc.

MODELING OF A TWIN-RIB

NONRECIPROCAL PHASE SHIFTER BY

THE SPECTRAL-INDEX METHOD

Arokiaswami Alphones1

1_{Centre for Wireless Communications}

National University of Singapore Singapore 117674

Recei¨ed 20 July 2000

ABSTRACT: The design of a phase shifter with minimum de¨ice length that could be easily integrated into the isolator is in¨estigated in this work. A twin-rib-wa¨eguide nonreciprocal phase shifter ha¨ing composite magnetic garnet film is characterized here using the spectral-index method. The proposed phase shifter has a de¨ice length of 22.54 mm at 1.3␮m wa¨elength, which is the length of the entire isolator.䊚 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 28: 110᎐113, 2001. Key words: optical isolator; nonreciprocal phase shift; gyrotopic; twin rib; spectral index

INTRODUCTION

The nonreciprocal phase shifter is an important component of optical isolators that protect semiconductor lasers from reflected light. Concepts of integrated optical isolators are based on either nonreciprocal TE᎐TM mode conversion or nonreciprocal phase shifts of TM modes. The nonreciprocal

< phase shift of TM modes refers to the difference⌬␤ s ␤_{f w}

<

y␤bw between the forward and backward propagation con-stants ␤_{f w} and ␤ , respectively. Nonreciprocal effects in_bw passive linear materials are caused by the contributions of ␧ of odd order in the external, quasi-static magnetic field or for ferromagnetic or antiferromagnetic substances in magnetiza-tion. Typically, magnetic garnets such as yttrium iridium

Ž .

garnet YIG are used in the near infrared region between the 1.3 and 1.55 ␮m wavelength where optical communica-tion applicacommunica-tions are being developed. The magnetic garnets

are grown by liquid phase or sputter epitaxy on paramagnetic

Ž .

substances like gadolinium gallium garnet GGG . Each mag-neto-optic medium with an external dc magnetic field applied in the transverse plane along the y-axis has the permittivity tensor ␧_{x x} 0 j␦ 0 ␧ 0 Ž . ␧ s . 1

˜



0

The gyrotopy ␦ represents the magneto-optic effect due to the applied magnetic field, and affects only TM modes. The gyrotopy parameter changes its sign if the magnetization is reversed, and is related to Faraday rotation ⌰ by_F

2 n⌰_F

Ž .

␦ f 2

k0

where n is the refractive index and k0 is the free-space propagation constant.

THEORY

The optical isolator in Figure 1 comprises two nonreciprocal phase shifters and two symmetric Y-branch couplers

config-w x ured in the form of a Mach᎐Zehnder interferometer 1 . It functions as an isolator that routes forward- and backward-propagating signals differently. The isolator relies on the nonreciprocal phase shift between counterpropagating TM modes in a magneto-optic waveguide. An enhancement of the nonreciprocal effect is necessary to minimize the device length. At present, magnetic garnets are the only materials that have been studied for nonreciprocal devices in the infrared region. Because of their high Faraday rotation, mag-netic garnet films are suitable for realizing integrated optical

w x isolators 1 .

The nonreciprocal phase shifters in Figure 1 employing single-rib magneto-optic waveguides have been analyzed

us-w x

ing the effective-index method 2 , as well as the more accu-w x

rate spectral-index method 3 . It was found that, by loading a high-refractive-index material on the surface of the magneto-optic film, the nonreciprocal effect was enhanced. The thick-ness of this high-index cover can be adjusted in order to concentrate the field on the top side of the magneto-optic

Figure 1 Mach᎐Zehnder nonreciprocal phase shifter with twin-rib

Ž . Ž .

configuration. a Forward propagation. b Backward propagation

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 28, No. 2, January 20 2001 110