Jacob J. Adams

A reconfigurable liquid metal antenna driven by electrochemically controlled capillarity

We describe a new electrochemical method for reversible, pump-free control of liquid eutectic gallium and indium (EGaIn) in a capillary. Electrochemical deposition (or removal) of a surface oxide on the EGaIn significantly lowers (or increases) its interfacial tension as a means to induce the liquid metal in (or out) of the capillary. A fabricated prototype demonstrates this method in a reconfigurable antenna application in which EGaIn forms the radiating element. By inducing a change in the physical length of the EGaIn, the operating frequency of the antenna tunes over a large bandwidth. This purely electrochemical mechanism uses low, DC voltages to tune the antenna continuously and reversibly between 0.66 GHz and 3.4 GHz resulting in a 5:1 tuning range. Gain and radiation pattern measurements agree with electromagnetic simulations of the device, and its measured radiation efficiency varies from 41% to 70% over its tuning range.

Handwritten, Soft Circuit Boards and Antennas Using Liquid Metal Nanoparticles.

Soft conductors are created by embedding liquid metal nanoparticles between two elastomeric sheets. Initially, the particles form an electrically insulating composite. Soft circuit boards can be handwritten by a stylus, which sinters the particles into conductive traces by applying localized mechanical pressure to the elastomeric sheets. Antennas with tunable frequencies are formed by sintering nanoparticles in microchannels.

Broadband Equivalent Circuit Models for Antenna Impedances and Fields Using Characteristic Modes

An approach for modeling antenna impedances and radiation fields in terms of fundamental eigenmodes is presented. Our method utilizes the simple frequency behavior of the characteristic modes to develop fundamental building blocks that superimpose to create the total response. In this paper, we study the modes of a dipole, but the method may be applied to more complicated structures as the modes retain many of their characteristics. We show that the eigenmode-based approach results in a more accurate model for the same complexity compared to a typical series RLC resonator model. Higher order modes can be more accurately modeled with added circuit complexity, but we show that this may not always be necessary. Because this method is based on the physical behavior of the fundamental modes, it also accurately connects circuit models to radiation patterns and other field behavior. To demonstrate this, we show that far field patterns, gain, and beam width of a dipole can be accurately extrapolated over a decade of bandwidth using data at two frequency points.

Systematic Shape Optimization of Symmetric MIMO Antennas Using Characteristic Modes

We introduce a systematic approach to the shape optimization of compact, single-aperture MIMO antennas. Because the characteristic modes of a radiator represent its complete set of possible responses to an excitation, any port on the antenna must display the properties of a combination of one or more of these characteristic modes. By restricting our consideration to a class of symmetric antennas, the lowest order characteristic modes of a structure can be separated with practical decoupling networks, studied, and excited independently. We show that the quality factor of each characteristic mode effectively bounds the performance of any individual port excitation, and can be used to evaluate the fitness of the antenna for multiport excitation. Under this framework, we apply a genetic algorithm (GA) to synthesize low Q MIMO antennas while minimizing conductor area. Feed locations are specified on the optimized shape based on the weighted excitation strength of the desired modes, and a two-port MIMO antenna is implemented and measured, verifying the proposed theory.

Computing and Visualizing the Input Parameters of Arbitrary Planar Antennas via Eigenfunctions

We propose a method for modeling planar multiport antennas of arbitrary shape using characteristic mode theory (CMT) without physically including the feeds. The characteristic modes of the feed-free structure are expanded to form a basis for the eigenfields, and a virtual probe is introduced to excite the antenna. We develop a broadband multiport circuit model for the antenna impedance based on the excitation of each mode, where the feed locations only affect transformer ratios in the model, enabling design and analysis of arbitrary feed combinations over a wide frequency range. Because a CMT expansion can be computed for any planar geometry, the shape of the radiating element can also be arbitrary. While this approach is approximate, several examples are presented to demonstrate that its accuracy and flexibility make it suitable for various planar antenna design applications. With the rapid evaluation of input impedance at multiple excitation points, input parameters, such as the multiport S, Y, or Z parameters, can be plotted as a heat map on the antenna structure, facilitating planar multiport antenna optimization and feed selection.

A Compound Frequency- and Polarization- Reconfigurable Crossed Dipole Using Multidirectional Spreading of Liquid Metal

We present a crossed dipole with frequency and polarization agility using electrochemically actuated liquid metal. For the first time, this antenna uses multidirectional displacement of liquid metal to enable frequency and polarization reconfiguration without the need for mechanical pumps or semiconductor devices. The dipole arms are composed of liquid metal that can be shortened and lengthened within the capillaries by applying DC voltages to each arm. Varying the lengths of the dipole arms generates two independently tuned, linearly polarized resonances from 0.8 to 3 GHz and polarization that can be switched from linear to circular over a portion of this band (0.89–1.63 GHz). Moreover, a circuit model predicts the circular polarization frequency from the input impedance. Simulation and experimental results validate the antenna concept and analysis techniques.

Pump-free feedback control of a frequency reconfigurable liquid metal monopole

We demonstrate a pump-free method to control the length of liquid metal in a capillary as a means to change the operating frequency of a monopole antenna. An applied DC voltage controls the surface tension of the liquid metal filament, causing it to lengthen or contract, varying the antennas resonant length. A closed-loop feedback system tracks the antennas operating frequency and adjusts the applied voltage to shape the liquid metal towards the desired response. Measurements show that the process is controlled and fully reversible, dynamically adjusting to a programmed frequency.

Physics-Based circuit models for MIMO antennas using characteristic modes

Using characteristic mode theory, we develop a general broadband circuit model for the admittance of a multi-input, multi-output antenna. Each accessible port couples into radiation modes and to the other ports in a way that can be represented analytically or as a circuit block. This physics-based approach offers a systematic method to build a model of a complicated multiport antenna using simple building blocks. An example of a strip dipole antenna shows that a circuit model made from simple building blocks can model the multi-port S-parameters of the antenna within a few percent over a 10:1 frequency range.

Accelerated frequency interpolation of antenna impedances using characteristic modes

Characteristic mode theory can be a powerful tool in antenna analysis. Recent results have shown that each characteristic modes frequency response can be approximated by a template function related to the spherical mode that couples most of the power. By choosing the appropriate template function, the broadband frequency behavior of the characteristic modes can be modeled. Here we briefly outline an approach that uses this property in order to accelerate the interpolation of antenna impedances over a wide bandwidth.

Modeling of dense metallic grids for transparent transmission lines and antennas

Dense metallic grids can create surfaces that are conductive at microwave frequencies yet optically transparent. However, fullwave solutions of such grids are computationally expensive. To accelerate simulations of complex structures we consider three approximate models for a wire grid. Circuit-based models provide estimates of radiation efficiency for simple antennas without simulating the complex gridded structure. However their accuracy is limited, typically yielding efficiency 13% from the actual value. As an alternative, an effective surface impedance can be assigned to the grid. In this case, the error in efficiency averages only 3.4%, but a fullwave simulation is required, albeit less costly than simulation of the grid itself.