Quasi-Optical Discrete Beam Steering Grids

1999 International Microwave Symposium, THF3-1 Quasi-Optical Discrete Beam Steering Grids

John Mazotta, Michael DeLisio, Jung-Chih Chiao

Department of Electrical Engineering, University of Hawaii – Manoa

A BSTRACT

Quasi-optical discrete beam-steering grids were presented using PIN-diode switch arrays to change reactances across the transmission aper-ture. E-plane and H-plane beam-steering angles of (+10o, ?12.5o) and ±20o were demonstrated at 3GHz, respectively, with transmission losses less than 6dB. The insertion loss is due to the series resistance of the PIN-diodes and can be reduced by reducing the series resistance of the switches. The theory was verified with a 5-GHz passive grid, which had E-plane and H-plane beam-steering angles of (+12.5o, ?15o) and ±12.5o, respectively, compared with the theoretical angles of ±12o. The total transmission losses were less than 2dB. Millimeterwave discrete beam-steering grids are proposed using new MEMS see-saw-bar switches to reduce losses.

I NTRODUCTION

Quasi-optical power combining techniques offer a promising approach to realize compact, reliable, higher-power and economical systems at milli-meter and submillimeter wavelengths [1,2]. A complete quasi-optical transmitter or receiver requires monolithic beam-controllers for beam steering, focussing and switching in applications such as radar for aircraft-guiding, missile-seeking, and automobile collision avoidance systems as well as millimeterwave imaging cameras that see through smoke and fog. Electronic scanning systems are more flexible and have higher scan-ning speeds than mechanical scanning systems. An electronic scanning system allows beams to shift rapidly, so it can track more targets simulta-neously. Conventional waveguide beam-steering systems are heavy, bulky and usually require complicated control circuits and expensive phase shifters.

Previous efforts toward eliminating phase shifters considered periodic structures loaded with varac-tor diodes for millimeterwave beam-steering sys-tems. By controlling the biases on varactors, a linearly progressive phase shift can be provided across the aperture to steer the beam. Chekroun proposed RADANT [3], a three-dimensional array of varactor diodes for steering a beam. The advantages, including lower losses, high-speed control and simple control circuits, have been demonstrated with this approach on reflection beam-steering grids. Lam [4] demonstrated a phase shift of 70o with a 7-dB loss at 93GHz using a diode grid, mounted on a metal mirror, with 1600 Schottky-barrier varactor diodes as a reflec-tion beam steerer. Using the same approach, Sjogren et al. have demonstrated electronic beam-steering on a Schottky diode-grid with 7168 diodes [5]. A phase shift of 70o was achieved with a reflection loss of 3.5dB at 120GHz. Qin et al. reported a 130o reflection phase-shift with a 2.7-dB loss at 60GHz [6].

Extending the quasi-optical diode-grid method for beam-control to higher frequencies or higher sen-sitivity faces the technical challenge to reduce the losses caused by the series resistances of the Schottky diodes. The idea of using reconfigurable passive elements to provide phase shifts instead of varactor diodes was proposed to reduce insertion losses [7]. By stacking several layers of binary phase shifter arrays, linearly progressive phase shifts can be provided across the transmission aperture. Each unit cell provides either a capaci-tive phase shift or an inductive phase shift using

the switch to reconfigure the metal patterns. In

this paper, we investigated this discrete beam-steering grid architecture with different metal and switch configurations.

D ESIGN AND C ONSTRUCTION

Figure 1 shows the concept of transmission-type beam-steering grids. The incident waves enter the grid from the right side, pass through several layers of arrays, and re-radiate from the left side into the free space. The switches change the shunt reactances for the propagation waves from induc-tive, when the switches are closed by forward biasing, to capacitive, when the switches are open by reverse biasing. By changing the settings of switches in different layers to reach different re-actances, different discrete phase-shifts across the aperture can be achieved. The adjacent grids are spaced in quarter wavelengths and controlled in pairs to reduce the reflection losses. A phase variation across the transmitting aperture sets the direction of the beam. Two-dimensional beam steering can be achieved by arranging the bias lines to control the respective switch settings for E-plane and H-plane phase shifts.

Two designs were fabricated and tested: 4x 4 3-GHz beam-steering grids using PIN diodes as switches and 6x 6 5-GHz passive beam-steering grids using conductive tape to imitate microelec-

tromechanical system (MEMS) switches. The unit-cell dimensions are shown in Fig. 2(a) and the equivalent circuit for the unit cell is shown in Fig. 2(b). The shunt capacitance, C , and the shunt inductance, L , are determined by the metal patterns. The parasitic parameters of the diode are the series resistance, R , when the switch is closed,and the parasitic capacitance, Cp , when the switch is open. The diodes used are M-Pulse Microwave MP5084 PIN diodes with a nominal series resis-tance of 2? and a parasitic junction capacitance of 80fF. The substrate has a thickness of 250μm and a dielectric constant of 2.33. For the 3-GHz design, the unit cell size, a , is 30mm, the metal width for inductive reactance, w , is 2.8mm, and the gap for capacitive reactance, g , is 9.2mm. A shunt capacitance of 117fF and a shunt inductance of 12.1nH are expected, which should provide a phase shift of 45o when the switches change states

in one layer.

M EASUREMENTS

The beam-steering measurement setup is shown in Fig. 3. The incident waves came from a transmit-ting horn connected to an HP83592C sweeper with an output power of 16dBm. The steered pat-tern was measured by rotating a receiving horn, with the grid at the center of the circle. The receiving horn was connected to an HP8564E spectrum analyzer. The grid was surrounded by wave absorbers to eliminate reflections in the environment. The transmitting and receiving horns were cross-polarized in order to reduce diffraction from the absorber frame. The calibra-tion for losses was performed without the grid in the path.

Figure 4(a) shows the E-plane beam-steering results with the diode switches biased in parallel, and Fig. 4(b) shows the H-plane beam-steering with the diode switches biased in series. The forward and reverse biases on each diode are +0.72v and –3v, respectively. The figures also show the no-steering (normal) patterns when all the diodes are reverse biased. The E-plane beam-steering angles are +10o and –12.5o, while the H-plane beam-steering angles are ±20o. The theoretical steering angles are ±12o. The total losses for normal and steered beams are less than 6dB in both cases.

The transmission losses are due to the series resistance of the PIN diodes and the diffraction loss from the array size. To reduce the diffraction, a 5-GHz 6x6 beam-steering grid was designed. Due to the parasitic package capacitance of the PIN-diode, the on/off isolation of the diode switch is not enough to demonstrate beam steering. Therefore, 5-GHz passive grids were used to verify the theory and feasibility of using MEMS microswitches for switching reactances. The unit-cell dimensions are a =20mm, w =2.3mm and g = 6.7mm. A phase shift of 45o and a steering angle of 12o are expected when the switches change states. The MEMS switches were imitated by replacing the diodes with conductive tape. Figure 5(a) and (b) show the E-plane and H-plane steering for the 5-GHz grid. The E-plane beam-steering angles are +12.5o and –15o while the H-plane beam-steering angles are ±12.5o. The theoretical steered pattern in the E-plane, pre-dicted by the phased-array theory and the unit-cell antenna pattern, is compared with the measured pattern in Fig. 6.

MEMS M ILLIMETERWAVE S TEERERS

For millimeterwave applications, the PIN-diode beam-steering grids should be monolithically fab-ricated. Stephan [8] demonstrated a quasi-optical monolithic PIN-diode beam-switching array with a 6-dB transmission loss at 70GHz. The diode resistance is estimated to be 39?. Similar mono-lithic PIN-diodes could be used in our beam-steering grids. However, the series resistance of diodes increases when the operating frequencies increase. Therefore, it is important to reduce the series resistance of switches for millimeterwave applications. One option is to use microelectro-mechanical (MEMS) switches. Using metal-to-metal contacts, MEMS switches may provide less

tions [7] using SiO x N y membrane-supported can-

tilever MEMS switches faced technical challenges such as stress control of the membrane, thermal dependence of actuation voltages and “stiction” of membrane contacts. To address these issues, a novel MEMS switch architecture, the see-saw-bar switch, has been designed and fabricated using multi-layer polysilicon surface micromachining techniques. Fig. 7 shows the concept and a photo of the see-saw type switch. A see-saw bar, with bias electrodes on both ends, is held by moveable hinges on the sides of the bar. One end of the bar is attached to a metal contact pad to make con-nection between two separated metal pads. With biases on different bias electrodes, the metal con-tact can be pulled up or pushed down to open or close the connection. The rigid polysilicon bar can help prevent the stiction and degradation associ-ated with thin-film membranes. The performance of the new MEMS switches is currently under investigation.

C ONCLUSIONS

The concept of quasi-optical discrete beam-steer-ing grids was demonstrated at 3GHz using PIN-diode switch arrays and at 5GHz using passive grids. The measured patterns agree with theory. Millimeterwave beam-steering grids are proposed using MEMS see-saw-bar switches in order to reduce transmission losses.

A CKNOWLEDGMENTS

We appreciate very much the support for John Mazotta by the NASA Undergraduate Space Grant Fellowship and the current support for him by the U.S. Army Research Office under Grant No. DAAG55-98-1-0475.

R EFERENCES

[1] J. Mink and D. R utledge, “Guest Editor's Overview of the Special Issue on Quasi-Optical Techniques,”IEEE Trans. Microwave Theory Tech., pp.1661, Oct. 1993.

[2] R. York, “Quasi-Optical Power Combining Tech-niques,” Millimeter and Microwave Engineering for Communications and Radar, J. Wiltse, Ed., Critical Rev. of Opt. Science and Tech., Vol.54, pp.63, 1994.

[3] C. Chekroun, D. Herrick, Y. Michel, R. Pauchard and P. Vidal, “Radant: New Method of Electronic Scanning,” Microwave J., pp.45, Feb. 1981.

[4] W. Lam, C. Jou, H. Chen, K. Stolt, N. Luhmann and D. R utledge, “Millimeter-Wave Diode-Grid Phase-Shifters,” IEEE Trans. Microwave Theory Tech., pp.902, May 1988.

[5] L. Sjogren, H. Liu, F. Wang, T. Liu, X. Qin, W. Wu, E. Chung, C. Domier and N. Luhmann, “A Monolithic Diode Array Millimeter-Wave Beam Transmittance Controller,” IEEE Trans. Microwave Theory Tech., pp.1782, Oct. 1993.

[6] X. Q in, W. Z hang, C. D omier, N. L uhmann, W. B erk, S. Duncan and D. Tu, “Monolithic Millimeter-Wave Beam Control Array,” Digest of IEEE Microwave Theory Tech. Symposium, pp.1669, 1995.

[7] J.-C. Chiao and D. Rutledge, “Microswitch Beam-Steering Grid,” The Intl. Conf. on Millimeter Submilli-meter Waves Applications, San Diego, CA, Jan. 1994.

[8] K. Stephan, F. Spooner and P. Goldsmith, “Quasi-optical Millimeter-Wave Hybrid and Monolithic PIN Diode Switches,” IEEE Trans. Microwave Theory Tech., pp.1791, Oct. 1993.

(a)

50μm

Actuation Bias

Contact

Hinge

(b)

Figure 7: (a) The concept and (b) a photo of the see-saw-bar MEMS switch (top view).