2013 23rd Int. Crimean Conference “Microwave & Telecommunication Technology” (CriMiCo’2013). 9—13 September, Sevastopol, Crimea, Ukraine 2013: CriMiCo’2013 Organizing Committee; CrSTC. ISBN: 978-966-335-395-1. IEEE Catalog Number: CFP13788 559

FOUR-BEAM ANTENNA ARRAY FOR 24 GHz APPLICATIONS FED BY 4 x 4 BUTLER MATRIX

Pablo Sanz, Izabela Slomian, Ilona Piekarz, Jakub Sorocki, Piotr Kaminski, Krzysztof Wincza, Slawomir Gruszczynski

AGH University of Science and Technology, Krakow, Poland slomian@agh.edu.pl, ilona.piekarz@gmail.com, jakub.sorocki@gmail.com, piotr.kaminski@agh.edu.pl,

krzysztof.wincza@agh.edu.pl, slawomir.gruszczynski@agh.edu.pl

Abstract — A four-beam antenna array operating within 24 GHz frequency range has been developed. As a feeding network of such antenna a 4 x 4 Butler matrix has been designed utilizing a low-cost microwave laminate having permittivity 3.38 and thickness 0.203 mm. The developed Butler matrix consists of an appropriate connection of six directional couplers, two of which have been used to form a transmission-line crossover. The designed matrix has been integrated with the four-element linear antenna array confirming the usability of the utilized Butler matrix in beam-scanning applications.

I. Introduction

Smart antenna systems have been introduced to im-prove wireless performance and to increase system ca-pacity by spatial filtering, which is capable to separate spectrally and temporally overlapping signals received from multiple users. Switched beam systems are referred as antenna array systems that form multiple fixed beams with enhanced sensitivity in a specific area. Such an an-tenna system detects signal strength, selects one of the several predetermined fixed beams, and switches from one beam to another as the user moves. However, re-cently there can be noticed a huge interest in antenna systems that perform scanning functions. For instance, there is a great scientific and commercial interest in car radars for anti-crash and pre-crash systems, for which beamformer networks with antenna arrays are becoming a standard. Generally, automotive radars for anti-crash systems integrate a beamformer network and an antenna array having the capability of scanning the generated beam by a simple switching of the input. A Butler matrix [1]-[3] is a beamformer that contains the least number of couplers compared to other beamformer networks.

The aim of this work is to present a four-beam anten-na array fed by a single 4 x 4 Butler matrix operating at 24 GHz. A Butler matrix designed in a microstrip technique has been used as a basic element of a feeding network. As a basic element of the Butler matrix a 3 dB branch-line coupler is utilized. All designed elements of the feeding network have been integrated together with a linear 4 x 1 antenna array.

II. Multibeam Antenna Design The general concept of the Butler matrix is presented

in Fig. 1. [1], and consists of four 3dB/90° directional couplers and two -45° phase shifters. In order to appro-priately form the Butler matrix and to connect the outputs to the linear antenna array two transmission line crosso-vers are required. In the proposed design branch-line directional couplers have been used and the single layer structure on a RO4003 laminate having thickness of 0.203 mm and permittivity of 3.38 has been utilized. This type of directional coupler has been used due to the fact that branch-line coupler is easy to design and manufac-ture and 3 dB coupling is achievable in single-layer mi-crostrip technology. However, the branch-line coupler features narrow bandwidth; nevertheless, due to the utilization of narrowband square patches as radiating elements, the broad bandwidth is not required. The choice of dielectric substrate was dictated by the opera-tional frequency range of the entire circuit. The transmis-sion line crossovers have been realized as tandem con-nections of two 3dB/90° directional couplers [2]. In order

to achieve appropriate phase properties of the networks, transmission-line sections of appropriate length have been inserted in the outer channels. The layout of the designed Butler matrix is presented in Fig. 2 showing specifically particular parts of antenna array feeding network. To verify the Butler matrix abilities of providing appropriate power distribution it has been integrated with 4 x 1 linear antenna array composed of basic line-fed square patch radiating elements with the insets for im-pedance matching improvement.

The radiating elements have been placed 6.25 mm apart, what equals a half of wavelength for the center frequency. Simulated frequency responses of the de-signed Butler matrix (before integration with the linear antenna array) are presented in Fig. 3 and Fig. 4, whereas Fig. 5 and Fig. 6 show the reflection coefficient of the proposed radiating element and the co-polar ra-diation patterns obtained in H-plane presenting the mul-tiple beams of the 4 x 1 linear antenna array. The ob-tained amplitude characteristics show good properties at the center frequency which are transmission coefficients between output and input ports equal 6 dB and isolation between particular inputs and particular outputs better than 25 dB. There are some noticeable discrepancies between differential phases obtained when ports #2 and #3 were fed; nevertheless, the results obtained when ports #1 and #4 were fed are in close vicinity. The 10 dB impedance bandwidth of the radiating element’s reflec-tion coefficient equals almost 800 MHz what gives 3.3% of relative bandwidth. It is a regular result for line-fed microstrip antenna elements etched on a thin dielectric layer. The reflection coefficient noted at 24 GHz is better than 20 dB. The co-polar radiation patterns of the multibeam antenna array show the arrangement of par-ticular radiation beams. It can be observed that they are placed at angles -36°, -12°, 12° and 36° with respect to the normal vector to antenna surface. The antenna array integrated with the Butler matrix has been manufactured in order to verify its design correctness. The photograph of the developed four-beam antenna array operating in 24 GHz frequency range is shown in Fig. 7. Reflection coefficient measured at antenna inputs have revealed, that the manufactured antenna array is not properly matched. Therefore, to reduce the impedance mismatch, the proposed antenna has been tuned by introduction of small pieces of metal plate at its input. Measurement results of the corrected reflection coefficient are present-ed in Fig. 8. It can be noted, that obtained reflection co-efficient is better than 16 dB at 24 GHz. The co-polar radiation patterns (Fig. 9) have been measured in H-plane and demonstrate similarities with the calculated ones in terms of beams inclination and the shapes of

2013 23rd Int. Crimean Conference “Microwave & Telecommunication Technology” (CriMiCo’2013). 9—13 September, Sevastopol, Crimea, Ukraine 2013: CriMiCo’2013 Organizing Committee; CrSTC. ISBN: 978-966-335-395-1. IEEE Catalog Number: CFP13788 560

outer beams. However, the two center beams are dis-torted. It is most likely caused by the transmission line’s radiation, which has deteriorative influence on radiation patterns. Nevertheless, the acquired results might be considered as satisfactory due to the relatively high fre-quency, inaccuracy of transmission lines arising from utilized technology and employed in the design single layer structure.

III. Conclusions The proposed four-beam antenna array integrated

with Butler matrix and operating within 24 GHz frequency range employs single layer structure, what significantly lowers the fabrication cost. The Butler matrix acting as beamforming network for 4 x 1 antenna array is composed of four 3dB/90° branch-line directional couplers and two 45° phase shifters. The Butler matrix as well as 4 x 1 an-tenna array were designed and analyzed electromagneti-cally. The four-beam antenna array utilizing 4 x 4 Butler matrix has been manufactured and measured showing good results and confirming antenna array’s usability.

IV. Acknowledgment This work was supported in part by the National Cen-

ter for Research and Development under Lider Program, contract no. LIDER/06/19/L-2/10/NCBiR/2011 and in part by statutory activity of the Department of Electronics, AGH University of Science and Technology.

V. References [1] Butler J., Lowe R. Beam-forming matrix simplifies design of

electronically scanned antennas, Electron. Des., 1961, vol. 9, pp. 170-173.

[2] Wincza K., Gruszczynski S. A broadband 4 x 4 Butler matrix for modern-day antennas. Proc. 35th European Microwave Conference, Paris, 2005, pp. 1331-1334.

[3] Wincza K., Gruszczynski S., Sachse K. Design of integrated stripline multibeam antenna arrays fed by compact Butler matrices. IEEE Antennas and Propagation Society Interna-tional Symposium, 2007, pp. 1385-1388.

Fig. 9. Measured co-polar radiation patterns of the manufactured four-beam microstrip antenna array

Fig. 1. Generic concept of the four beam antenna array

fed by 4 x 4 Butler matrix

Fig. 2. Layout of the designed 4 x 4 Butler matrix

(a)

(b)

Fig. 3. Amplitude characteristics of the designed 4 x 4 Butler matrix, when port #1 is fed (a), and when port #2

is fed (b).

2013 23rd Int. Crimean Conference “Microwave & Telecommunication Technology” (CriMiCo’2013). 9—13 September, Sevastopol, Crimea, Ukraine 2013: CriMiCo’2013 Organizing Committee; CrSTC. ISBN: 978-966-335-395-1. IEEE Catalog Number: CFP13788 561

(a)

(b)

(c)

(d)

Fig. 4. Differential phase characteristics of the designed 4 x 4 Butler matrix, when port #1 is fed (a),

when port #2 is fed (b), when port #3 is fed (c) and when port #4 is fed (d)

Fig. 5. Calculated reflection coefficient of

the designed radiating element

Fig. 6. Calculated radiation patterns of the multibeam

antenna array.

Fig. 7. Picture of the developed four beam antenna array

with 4 x 4 Butler matrix as a feeding network

Fig. 8. Measured reflection coefficients of the manufac-

tured four-beam microstrip antenna array