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.

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.

The intermodulation distortion produced by passive and tunable liquid metal (LM) antennas is investigated. Four kinds of monopoles, including a passive copper monopole, a varactor-tuned copper monopole, a passive LM monopole, and a tunable LM monopole using electrochemically controlled capillarity (ECC), are compared for their linearity and power-handling capabilities. Linearity is assessed using a two-tone distortion test, in which two fundamental tones close in frequency result in third-order intermodulation tones at frequencies above and below the original tones separated by the frequency difference. The passive LM monopole has comparable linearity to the passive copper monopole, while the linearity of the ECC-tuned LM monopole is at least 40 dB better than that of the active varactor monopole. The reconfigurable LM antenna also handles higher power (31 dBm) before failure than does the active varactor-tuned antenna (24 dBm).

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.

We demonstrate a single-feed planar antenna capable of independently reconfiguring its operating frequency, radiation pattern and polarization using stretchable, encapsulated liquid metal parasitic elements. The liquid metal is contained within elastomeric fibers that can be mechanically translated, stretched or relaxed to alter the position or length of each conducting element on a 2D surface, physically reshaping the metal on the antenna aperture. This eliminates several practical challenges associated with fluidic actuation of liquid metal and makes the actuation scheme much faster and more reliable than other recent approaches. Using this scheme, the reshapable aperture (RESHAPE) design supports continuously reconfigurable operating frequencies from 2.45 GHz to 6.5 GHz, while supporting either linear ŷ and ẑ polarizations. At the same time, the radiation pattern can also be reconfigured in all planes, over a continuous range, to a maximum of ±45° away from the broadside direction for most frequencies. For all these possible states, the antenna maintains a 2:1 VSWR and a total efficiency of >60%. We explain the operation of the design at several frequencies, analyze the coverage area, and present measurements of a fabricated prototype.