Robert Y. Loo

Two-dimensional beam steering using an electrically tunable impedance surface

By covering a metal ground plane with a periodic surface texture, we can alter its electromagnetic properties. The impedance of this metasurface can be modeled as a parallel resonant circuit, with sheet inductance L, and sheet capacitance C. The reflection phase varies with frequency from +/spl pi/ to -/spl pi/, and crosses through 0 at the LC resonance frequency, where the surface behaves as an artificial magnetic conductor. By incorporating varactor diodes into the texture, we have built a tunable impedance surface, in which an applied bias voltage controls the resonance frequency, and the reflection phase. We can program the surface to create a tunable phase gradient, which can electronically steer a reflected beam over +/- 40/spl deg/ in two dimensions, for both polarizations. We have also found that this type of resonant surface texture can provide greater bandwidth than conventional reflectarray structures. This new electronically steerable reflector offers a low-cost alternative to a conventional phased array.

A tunable impedance surface performing as a reconfigurable beam steering reflector

We describe a reconfigurable microwave surface that performs as a new kind of beam steering reflector. The surface is textured with an array of tiny resonators, which provide a frequency-dependent surface impedance. By tuning the individual resonators, the surface impedance, and thus the reflection coefficient phase, can be varied as a function of position across the reflector. Using a reflection phase gradient, the surface can steer a reflected beam. As an example, we have built a simple mechanically tuned surface in which physical motion of only 1/100 wavelength generates a sufficient phase gradient to steer a reflected beam by /spl plusmn/16 degrees. To steer to greater angles, the surface can be configured as an artificial microwave grating, capable of /spl plusmn/38 degrees of beam steering. The concept of the tunable impedance surface demonstrated here can be extended to electrically controlled structures, which would permit more elaborate reflection phase patterns, and provide more capabilities, such as the ability to focus or steer multiple beams.

Reconfigurable aperture antennas using RF MEMS switches for multi-octave tunability and beam steering

The requirements for increased functionality within a confined volume will place greater burdens on electromagnetic platforms for air, space, and sea over the next few decades. An important piece of the any solution to these new requirements are transmitting and receiving apertures that can handle multi-octave bandwidths with beam steering capability. The ability of an aperture to be reconfigured for a particular mission will become essential. New types of devices are being developed which will enable the realization of these reconfigurable apertures. This paper presents a discussion of how one of these new devices, the RF MEMS switch, can be utilized to change the phase and frequency characteristics of conventional antenna elements to perform beam steering over a wide range of microwave frequencies.

A wideband beam switching antenna using RF MEMS switches

Reconfigurable aperture antenna technology has been enabled by the advent of RF MEMS switches. Typical measured insertion losses of less than 0.25 dB up to 40 GHz with superb broadband isolation means that reconfiguration of the antenna for multi-band operation can now occur with a few MEMS switches rather than multiple PIN diode or FET switch circuits. In this paper, we describe a reflectarray antenna approach that can perform true time delay beam steering by using cascaded RF MEMS switches/coplanar strip transmission line sections. Measured RF MEMS switch characteristics in coplanar strip transmission lines are presented and the modeled reflected phase of five cascaded switch/transmission line sections is presented.

Metal contact RF MEMS switch elements for ultra wideband RF front-end systems

Metal contact RF MEMS switch circuit elements have been developed to be used as building blocks for wideband signal routing. A switch element can have up to four switches fabricated monolithically on a single GaAs chip. More complicated switch circuits can be constructed by assembling multiple basic switch elements on a hybrid RF interconnection printed circuit board. In this paper, two basic types of switch functions are presented, single-pole multiple-throw switch elements and matrix switch elements. Measured insertion loss and isolation performances of these circuits are presented.

MEMS-based switched diversity antenna at 2.3 GHz for automotive applications

Our previous work is presented on diversity antenna experiments in the 2.3 GHz band, specifically for applications in the automotive environment. We describe the case of two-element position diversity, and we study the effects of the antenna positions on the amount of diversity gain. The presentation consists primarily of measurement data on a moving vehicle, with software post-processing to determine the effects of diversity combining. We present several diversity combining schemes, and describe a switched diversity circuit that we have built to address one of these schemes. An in-house developed SPDT MEMS switch, which has desirable insertion loss and isolation characteristics for the diversity switch, is shown. Also, a discussion on using a switch for the switched diversity antenna is included.

System design and performance of a wideband photonic array antenna

Presented in this paper is an overview of the development of a wideband photonic array antenna. The presentation will focus on the performance of a unique L-band 24 X 4 element conformal array, supported by a photonic true-time-delay beamforming network. A 2-ns pulse was injected into the system and the round trip impulse response was measured to demonstrate the arrays 550 MHz instantaneous bandwidth.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Photonics for phased-array antennas

In this paper, we discuss the applications and also several important system issues: insertion loss, noise figure, dynamic range and cost relating to photonics for wideband phased array antennas. This discussion is based on the work that we did on an L-band Optical Control of Phased Array Project funded by DARPA/Rome Lab. The antenna has been delivered to Rome Lab for further demonstration.

Microwave Beam Steering Reflector Based on a Tunable Impedance Surface

We describe a new kind of beam steering reflector consisting of a textured surface covered with an array of tiny resonators. The surface is mechanically tuned using a motion of only 0.01 wavelength. By individually tuning the resonators, the effective impedance of the surface, and thus the microwave reflection phase, can be varied. By configuring the surface with a reflection phase gradient, it can steer a reflected beam by as much as +/-38°. The concept is also amenable to electrically tunable structures, which can provide faster steering, and greater functionality.

Reconfigurable low power, light weight wireless system based on the RF MEM switches

We present novel wireless transceiver architecture based on microelectromechanical (MEM) microwave switches. The measurements and calculated results of a reconfigurable module with a switched antenna, tunable filter, and shared components for a low weight, low power compact wireless transceiver are discussed.