Radar-absorbing materials are used in stealth technologies for concealment of an object from radar detection. Resistive and/or magnetic composite materials are used to reduce the backscattered microwave signals. Inability to control electrical properties of these materials, however, hinders the realization of active camouflage systems. Here, using large-area graphene electrodes, we demonstrate active surfaces that enable electrical control of reflection, transmission and absorption of microwaves. Instead of tuning bulk material property, our strategy relies on electrostatic tuning of the charge density on an atomically thin electrode, which operates as a tunable metal in microwave frequencies. Notably, we report large-area adaptive radar-absorbing surfaces with tunable reflection suppression ratio up to 50 dB with operation voltages <5 V. Using the developed surfaces, we demonstrate various device architectures including pixelated and curved surfaces. Our results provide a significant step in realization of active camouflage systems in microwave frequencies.

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Graphene-enabled electrically switchable radar-absorbing surfaces

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Graphene-enabled electrically switchable radar absorbing surfaces

A region in a metallic panel that facilitates the transmission of radio frequency signals. The metallic panel may be included in a window such as the window of a vehicle or building. For example, the metallic panel may be used for heating or to reflect infrared radiation. An aperture is formed in the metallic panel to enable radio frequency signals to be transmitted through the metallic panel. The design of the aperture may be selected to enable the transmission of the desired frequency band. Furthermore, the aperture is designed such that there is a taper in the transmission amplitude and/or the phase to suppress lobing effects on the other side of the aperture. In an embodiment in which the metallic panel is used to conduct electric current, the aperture may be oriented such that the current may flow between the openings of the aperture. Accordingly, there may be uniform heating across the metallic panel without blocking the transmission of radio frequency signals in the desired frequency band.

Sidelobe controlled radio transmission region in metallic panel

This paper describes the design of a reconfigurable reflectarray element using commercially available radio frequency micro-electromechanical system (RF MEMS) switches. The element consists of a microstrip patch on the top surface and a slot with an actuated variable length in the ground plane. RF MEMS switches are mounted on the slot to electronically vary the slot length by actuating the switches and thus obtaining the desired phase response. Waveguide measurements and high frequency structure simulator (HFSS) simulations are used to characterize the reflectarray element. The four MEMS switches element gives 10 independent states with a phase swing of 150 deg and a loss variation from 0.4 dB to 1.5 dB at 2 GHz (more switches can provide larger phase shift). The loss is mainly attributed to the dielectric loss and the conductor loss, which occur due to the relatively strong electric fields in the substrate region below the patch and the large currents on the top surface of the patch, respectively, close to the patch resonance. Detailed analysis is performed to characterize the effect of the switches by taking into consideration the switch model and wire bonding effects.

RF MEMS Actuated Reconfigurable Reflectarray Patch-Slot Element

A method for determining at least one characteristic of an antenna requiring: a) positioning an antenna in a space surrounded by a waveguide; b) feeding an electric excitation signal (u tx (t)) into a feed connection of the waveguide; c) receiving the electric response signal (u rx (t)) emitted by the antenna resulting from the excitation signal (u tx (t)); d) determining at least one characteristic of the antenna from a portion of the response signal (u rx (t)) and a corresponding portion of the excitation signal (u tx (t)), where the portion of the response signal (u rx (t)) is evaluated in the time domain and satisfies the following conditions: (i) only one or more waves of the electromagnetic field caused by the excitation signal (u tx (t)) and running from the feed connection towards the antenna exist at the location of the antenna; and (ii) the electromagnetic field at the location of the antenna is a TEM field.

Antenna characterization in a waveguide

The work in this study develops the framework for placement and actuation of novel reconfigurable dual-offset contour beam reflector antennas (DCBRA). Toward that end, the methodology for the antennas' design is defined. In addition, two separate optimization problems are stated and solved: actuator position optimization and actuation value optimization. For the former, a method termed as greatest error suppression method is proposed where the position of each actuator is decided one by one after each evaluation of the error between the desired subreflector shape and the actual subreflector shape. For the second problem, a mathematical analysis shows that there exists only one optimal configuration. Two optimization techniques are used for the second problem: the simulated annealing algorithm and a simple univariate optimization technique. The univariate technique always generates the same optimal configuration for different initial configurations and it gives the low bound in the evaluation of the error. The simulated annealing algorithm is a stochastic technique used to search for global optimum point. Finally, as an example the results of the proposed optimization techniques are presented for the generation of a subreflector shape corresponding to the geographical outline of Brazil.

Design, modeling, and optimization of mechanically reconfigurable aperture antennas

A synthesis technique based on a mechanical finite-element (FEM) surface description of dual-offset reflector (DOSR) surfaces is described. This technique is used to implement a reconfigurable contour beam for a geostationary satellite application. The mechanical finite-element description of the surface takes into account the mechanical properties of the reflector surfaces and the effect of a finite number of forces applied at control points on the surface. Mechanical adjustment is achieved using high-deflection stacked piezoelectric actuators attached to the rear of a lightweight flexible subreflector surface. Both the main and subreflector surfaces are shaped for an initial required geographical coverage zone and then the main reflector surface is assumed to be fixed in shape. The variable shape subreflector is then synthesized to achieve the desired illumination for different geographical regions and different geostationary positions. Based on a study of the effect of the mechanical properties of the subreflector surface and actuator number and placement, a practical design has been completed and the ability to reconfigure the beam has been verified.

Reconfigurable contour beam-reflector antenna synthesis using a mechanical finite-element description of the adjustable surface

The present invention includes tapered anechoic chambers and chamber systems. This invention also includes machines or electronic apparatus using these aspects of the invention. The present invention also includes methods and processes for using these devices and systems. In a preferred embodiment, a TEM antenna is utilized that is terminated by a resistive card, or R-card. This makes for a very broadband antenna that may be used to properly illuminate the test zone in a tapered chamber without changing the feed antenna. A preferred chamber utilizes an absorber layout that has never been applied to a tapered chamber, called a Chebyshev absorber layout. The concept is to place the wedge absorber tips at different heights relative to the mounting side wall and pyramidal absorber tips at different heights relative to the back wall. In this pattern, wave reflections off the pyramid tips and valleys behave very much like reflections in a multi-section transmission line. Using this Chebyshev layout around the test zone, the test zone fields will be very clean of stray signals and the polarization will be dictated by the feed horn. Also, it is then not necessary to use very thick absorber for low frequency applications, which tend to deteriorate over time. A preferred chamber also comprises a design wherein an entire tip portion of the tapered section is a separate unit. This unit is preferably designed to rotate on a system of rollers positioned between the unit and the main chamber. The result is that the polarization of the chamber can be easily changed from vertical to horizontal by simply rotating the feed structure, which can be done without disconnecting any cables to the feed.

Tapered anechoic chamber

Wireless coverage in urban, suburban, and light industrial areas is a challenging problem for cellular network planners. This is mainly due to scattering from buildings and structures but also because area specific demographic features such as parks, streets, and sports arenas require specialized coverage. A contoured beam reflector antenna is a simple and effective solution for this problem, providing excellent control in both planes of the radiation pattern and a very sharp taper at the edge of coverage. The problem of wireless coverage planning using existing base-station antenna types is discussed, and a potential solution for these problems using contoured beam reflector antennas is demonstrated by examples. The implementation of this solution is made possible by using an inexpensive manufacturing technique involving a reconfigurable mould and a foam extrusion process.

Contoured beam reflector antenna for wireless applications

A novel way to design, synthesize and adjust the reconfigurable dual offset contour beam reflector antenna employing an adjustable subreflector is presented. The work also presents a graphical user interface based computer code that connects the electro-magnetic effects to the mechanical surface deflections. The subreflector surface is described by using the finite element method and the far-field radiation pattern is calculated by reflector diffraction synthesis. The reflector surface shape is adjusted using a set of linear piezoelectric point actuators attached to its back surface, from which the diffraction synthesis code calculates the radiation pattern. An example of this method applied to the contiguous US is also presented. As a future work, a software package will be built where the finite element code and the diffraction synthesis code are combined, and it will be used for advanced actuator placement and reflector design problem.

W.H. Theunissen

Papers

Design, modeling, and optimization of mechanically reconfigurable aperture antennas

Reconfigurable contour beam-reflector antenna synthesis using a mechanical finite-element description of the adjustable surface

Tapered anechoic chamber

Contoured beam reflector antenna for wireless applications

Reconfigurable contour beam generation using a smart subreflector antenna system