Academic journal article Journal of Digital Information Management

Novel Scanned Beam Millimeter-Wave Antenna Using Micromachined Variable Capacitors on Coplanar Structures

Academic journal article Journal of Digital Information Management

Novel Scanned Beam Millimeter-Wave Antenna Using Micromachined Variable Capacitors on Coplanar Structures

Article excerpt

1. Introduction

Over the last decade, RF-MEMS research has been strongly concentrated on developing robust, reliable MEMS switches with shunt capacitance and phase shifters for cell phone, Alters, and wideband radar applications [1]-[7]. This paper focuses on the modeling, design and implementation of tunable serial RF-MEMS capacitor on coplanar waveguide (CPW) with very low insertion loss from 85 to 105 GHz. This capacitor will be integrated in LWA to obtain a scanning-beam from broadside to endfire direction. This capacitor can be easily built using surface micromachining in CPW center microstrip conductor by having two plates facing each other separated by an air-gap. Recently, much work has concentrated on the development of scanning-beam antennas based on simple microstrip metamaterials structures [8], [9] and MEMS technology [10], [11] due to their high quality factor Q.

The design of LWA with variable serial capacitance obtained with RF-MEMS technology creates a variation of the propagation constant (S) permitting the scanning-beam of the antenna from the broadside to close endfire direction. To understand the exact mechanisms, a multiphysics model that captures the coupling among electromagnetic and mechanical domains is required. An electrostatic force caused by the DC bias voltage ([V.sub.DC]) is then translated into motion. This motion ends when electrostatic force and mechanical force are equal. In this effort, many techniques have been published to push the tuning range of MEMS variable capacitors [1], [3], [7]. A variation of the propagation constant caused by the motion of the capacitor movable plate permits the variation of the antenna beam direction. In this work, we will propose a new LWA with a regularly number of RF-MEMS serial capacitors (N=10) which is fabricated by surface micromachining technology, for millimeter-wave range. The dimensions of this capacitor are fully compatible with PolyMUMPS process. Sophisticated and accurate full-wave analyses such as Method of Moment (MoM) and Differential Quadratic Method (DQM) are given for analyzing, in general, any particular geometry instance of the antenna under consideration.

This paper is structured as follows: In Section II, the topology and concept of the proposed MEMS capacitor in CPW structure is presented. Section III shows the theory of how MEMS capacitors move. Section IV, presents the fabrication process of unit cell of the antenna with MEMS capacitors followed by sections V, presenting simulations resultants of this antenna.

2. Antenna Unit-Cell Design

2.1 Topology of RF-MEMES capacitor and principle

Most RF-MEMS devices are actuated using electrostatic forces. Among the most commonly used types of actuators are the parallel or lateral plate actuators. Nevertheless, electrostatic actuation has some limitations due to its nonlinear nature. This section presents a novel control design that regulates the response of a unit cell (UC) of the CPW with serial MEMS capacitor that schematically illustrated in the Fig. 1a.

The RF-MEMS capacitor consists of a metal movable plate, suspended on the gap of CPW center strip conductor. This movable plate is supported at either ends by metal (anchors, Fig. 1(b) attached to the center conductor. The capacitor fixed plate is connected to the bottom ground plane through simple microstrip as shown in Fig. 1(b-c). Since the top plate is free to move, the serial capacitance can be increased by decreasing the distance of separation. This is done by applying a [V.sub.DC] voltage between the top and the bottom capacitor plates as shown in Fig. 2. This [V.sub.DC] voltage difference sets up an electrostatic force of attraction and pulls the movable plate toward the fixed plate, thereby decreasing the distance of separation and increasing the capacitance. The pull-in voltage Can be calculated from the effective spring constant of the support of capacitor movable plate, as is found to be [1]:

[V. …

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