Fig. 1. Configuration of the proposed beam-steering cavity-backed patch array antenna.

A Novel Frequency Beam-Steering Antenna Array at Y-band

Armin Jam and Kamal Sarabandi Radiation Laboratory, Department of Electrical

Engineering and Computer Science University of Michigan

Ann Arbor, MI, 48109-2122 USA {arminjam, saraband}@umich.edu

Mehrnoosh Vahidpour Agilent Technologies

Santa Clara, CA, 95051 USA mehrnoosh.vahidpour@agilent.com

Abstract—In this article, a novel frequency beam-scanning phased array antenna operating at Y-band is presented. The proposed antenna is a travelling-wave antenna, with cavity-backed patch arrays as the radiating elements. The array generates a very narrow beam (less than 2.5˚) at the broadside of the antenna with ±25˚ steering capability. A prototype of the antenna is fabricated using silicon micromachining with high level of accuracy and low mass and cost. Moreover, a novel waveguide-probe near-field measurement setup is introduced and utilized to characterize the radiation characteristics of the micromachined beam-steering antenna and an excellent agreement between the measurement and full-wave simulation results is shown.

I. INTRODUCTION In recent years, there has been a growing interest in the

development of high millimeterwave (MMW) and terahertz (THz) devices and systems. The recent advances in the MMIC technology have opened up new applications in imaging and collision avoidance systems, ultra-fast wireless communication as well as biomedical applications [1]. Although the compact size and low mass of MMIC technology makes it suitable for most of the above-mentioned applications, this technology is mainly limited to short-range applications at the sub-MMW and THz frequency range, primarily due to low efficiency of the MMIC sources and radiators. Motivated by the need for compact, low mass and highly efficient radiators, MMW antennas have been widely investigated for applications ranging from collision avoidance systems to high data-rate communication [2]. This paper presents a novel architecture for the development of a scalable sub-THz antenna array with beam-steering capability with radiation efficiency in excess of 55% with a total mass of less than 5g. A prototype of the antenna array with 588 patch elements is fabricated using a convenient micromaching approach and tested at Y-band. The performance of this prototype is evaluated and an excellent agreement with full-wave numerical simulations is demonstrated.

II. ANTENNA GEOMETRY AND PRINCIPLE OF OPERATION The design of the proposed frequency scanning antenna is

based on using hollow rectangular waveguides with transverse slot cuts placed on the H-plane of the waveguide wall as the

radiating elements. The length of the waveguide between two consecutive slots provides the desired phase shift as a function of frequency that provides the desired beam-steering. In order to achieve a broadside radiation and a satisfactory amount of phase shift between the elements without the need for a large bandwidth, it is required to meander the waveguide. Fig.1 illustrates the configuration of the proposed array. The length of the waveguide segment between the consecutive elements, i.e. l, is chosen to provide the broadside radiation at the center frequency, i.e. 237.5 GHz. This requires l to be an odd multiple of the guided wavelength with the extra half-wavelength segment required to compensate the reversal in the field distribution over the slot, imposed by the geometry of the meandered waveguide structure. As the frequency changes from 230 to 245 GHz, the propagation constant in the waveguide changes, and provides the progressive phase change that is responsible for beam-scanning from 25˚ to -25˚.

864 um

580 um

100 um l

Cavity-backed patch array

Slot

1371978-1-4799-3540-6/14/$31.00 ©2014 IEEE AP-S 2014

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Fig. 2. Normalized pattern of the proposed antenna, (a) simulated, and (b) measured.

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The one-dimensional array discussed above forms a narrow beamwidth in the plane of the slots, while generating a wide beam in the plane perpendicular to that, i.e. elevation. In order to confine the beam in the elevation direction, the antenna aperture is widened by using slot-coupled cavity-backed patch arrays. This two-dimensional structure increases the effective length of the antenna in the elevation direction and provides a narrower beam, in elevation. In this design the center patch is fed by the slot extended over the top wall of the waveguide, while the rest of patches are series-fed by the excited center patch. The cavity around the patch elements offers a good mechanical support for the suspended membrane and has the advantage of increasing the isolation between the adjacent elements.

III. MICROFABRICTAION AND MEASUREMENT RESULTS The proposed frequency scanning antenna is fabricated

using silicon micromachining technology for low cost and high level of fabrication accuracy (<4um). The micro-fabrication is accomplished using three silicon wafers. The first wafer has a thickness of 1000um and contains the

waveguide trenches etched in silicon using the DRIE Bosch process [3]. The second thin wafer (250um) contains the cavity-backed trench of the patch array etched on the top side and has through-wafer slot etched from the reverse side. The wafers are next metalized by sputtered gold and are bonded to each other using thermo-compression gold-to-gold bonding. The micro-fabrication process of the patch array is similar to the process described in [4]. The process involves patterning gold on a thin layer of Parylene deposited on a carrier wafer as a membrane layer. The Parylene is next released from the carrier wafer and attached to the bonded wafers using a spun adhesive material.

The radiation characteristic of the fabricated antenna is obtained experimentally using a novel measurement method based on waveguide probe near-field approach. The measurement setup consists of an Agilent N5245 4-port network analyzer along with OML MMW frequency extending modules to perform near-filed measurement at Y-band. The radiation pattern of the antenna is illustrated in Fig. 2, where comparison between the simulated [Fig. 2 (a)] and measured [Fig. 2 (b)] pattern of the antenna in the azimuth plane is presented. It is noticeable that the measured and simulated responses are in excellent agreement. It is shown that the beam of the antenna steers from +25 to -24˚ as the frequency sweeps from 230 to 245 GHz. It is also shown that the antenna has a very narrow half-power beamwidth of less than 3˚ over the entire scanning range.

IV. CONCLUSION A novel frequency scanned array antenna operating at Y-

band is introduced in this paper. Full-wave simulation of the antenna is carried out, showing a beam-steering range of 50˚ with a HPBW of less than 3˚ over the entire frequency range. Additionally, a radiation efficiency of higher than 55% is achieved over the entire frequency range. The antenna is designed to be compatible with silicon micromachinging technology to achieve most accurate and low-cost fabrication of the antenna. A prototype of the antenna is microfabricated and tested using a Y-band near-field measurement setup, where it shows a good agreement between the measurement and simulated results.

REFERENCES

[1] I. Kallfass, J. Antens, T. Scheinder, F. Kurz, D. Lopez-Diaz, S. Diebold, H. Massler, A. Leuther, and A. Tessmann. “All active MMIC-based wireless communication at 220 GHz,” IEEE Trans. THz Sci. Tech., vol. 1, no. 2, pp. 477-487, Nov. 2011.

[2] N. Ghassemi, K. Wu., S. Claude, X. Zhang, J. Bornemann, “Low-Cost and High-Efficient W-Band Substrate Integrated Waveguide Antenna Array Made of Printed Circuit Board Process,” IEEE Trans. Antennas Propag., vol. 60, no. 3, pp. 1648-1653, Mar. 2012.

[3] M. Vahidpour, and K. Sarabandi, “2.5D Micromachined 240 GHz Cavity-Backed Coplanar Waveguide to Rectangular Waveguide Transition,” IEEE Trans. THz Sci. Tech., vol. 2, no. 3, pp. 315-322, May 2012.

[4] M. Moallem, and K. Sarabandi, “Miniaturized-Element Frequency Selective Surfaces for Millimeter-Wave to Terahertz Applications,” IEEE Trans. THz Sci. Tech., vol. 2, no. 3, pp. 333-339, May 2012.

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