Impedance surfaces are artificially designed periodic structures that support surface waves. This paper presents a novel concept of a nonlinear impedance surface, whose impedance increases with the surface wave power. The proposed surface consists of a lattice of modified slotted mushroom-like cells loaded with varactor diodes and lumped circuits. The circuits are designed to sense the incoming surface wave power and feed back the signal to control the bias of the varactors, which eventually tunes the surface impedance. Such proposed power-dependent impedance surface has been demonstrated by a prototype consisting of 4 × 10 cells. It is observed from the experiments that as the power varies from 16 to 27 dBm, the surface exhibits a surface impedance range as much as j385 to j710 Ω. This characteristic enables the surface to be potentially useful in self-trapping surface-wave waveguide, power-dependent beam-steering antenna reflector, and other nonlinear applications.

Conventional artificial impedance surfaces are too thick for practical use at ultrahigh frequencies (UHFs). Thin structures with passive capacitive loading are possible in the UHF band, but only with narrow bandwidth. This paper presents a non-Foster loaded artificial impedance surface to achieve lowprofile and wide bandwidth simultaneously in the UHF band. The high resonant frequency associated with a thin structure is reduced by the synthesized non-Foster-type impedance load. For a demonstration, we designed a 5-mm-thick actively loaded surface for obtaining a high-impedance condition supporting slow transverse magnetic (TM) surface waves with a propagation index of 1.5, which corresponds to a 420jΩ impedance surface. The non-Foster impedance is synthesized to be equivalent to -2.8 pF || -3.6 nH. Both the non-Foster impedance and the surface are fabricated and tested with a near-field measurement. The retrieved refractive index shows a constant TM propagation index of 1.5 over a bandwidth of 450-800 MHz, a 56% fractional bandwidth. By varying the biasing voltage of the non-Foster circuit, the impedance can be tuned from 250 to 420 jΩ, while maintaining low dispersion over the same bandwidth.

In this paper, multi-channel capacitive sensor arrays based on microstrip band-stop filters are studied. The sensor arrays can be used to detect the proximity of objects at different positions and directions. Each capacitive sensing structure in the array is connected to an inductive element to form resonance at different frequencies. The resonances are designed to be isolated in the frequency spectrum, such that the change in one channel does not affect resonances at other channels. The inductive element associated with each capacitive sensor can be surface-mounted inductors, integrated microstrip inductors or metamaterial-inspired structures. We show that by using metamaterial split-ring structures coupled to a microstrip line, the quality factor of each resonance can be greatly improved compared to conventional surface-mounted or microstrip meander inductors. With such a microstrip-coupled split-ring design, more sensing elements can be integrated in the same frequency spectrum, and the sensitivity can be greatly improved.

Periodically loading non‐Foster circuits (NFCs) in a conventional waveguide has been proved to be possible for achieving broadband non‐dispersive fast‐wave (FW) propagation. However, the unconditional stability of such structures has been argued due to the multiple NFC loadings. Despite the potential instability, it is demonstrated that the NFC‐loaded waveguide can still be stable for specific terminations, in particular, with the 50 Ω load that is commonly used in radio frequency/microwave applications. For purpose of demonstration, a NFC‐loaded waveguide that contains substantially more unit cells than in prior work has been designed and measured, showing a stable and causal FW propagation with a 1.2 c phase velocity over the bandwidth of 80–120 MHz.

Superluminal pulse propagation has been observed in past investigations. Conventional methods rely on resonance-based gain/absorption line materials, inevitably inducing significant pulse distortion. Motivated by the negative reactance slope of the non-Foster circuits, we investigated superluminal propagation based on a non-Foster loaded waveguide. Superluminal square-wave pulse transmission with negligible distortion has been observed in time-domain measurements. In addition, the propagation distance limitation for a non-Foster-based superluminal waveguide is derived by analyzing the distortion of the transmitted waveform. We conclude that a distortion-limited superluminal output pulse cannot emerge more than the original pulse duration ahead of the same pulse transmitted in vacuum, in agreement with causality. This investigation provides an alternative solution to the gain/absorption approaches, and is directed toward a more comprehensive understanding of superluminal propagation.

A low-profile, directive, high-power, simple radio frequency source can play a crucial role in long-distance communication, sensing, and anti-interference research. Conventional approaches to achieving high-power radiation require a microwave source, an amplifier, and a separate radiating structure. In this paper, we transform a static voltage to electromagnetic radiation, which can be electronically steered, using only a thin sheet of metasurface. The proposed approach can be scaled both in power and frequency. The concept makes use of the k = 0 mode in a periodic resonant surface to excite a series of phase-locked individual sources on the surface. The phase-locked sources create an overall coherent mode in the far field toward the desired direction. The proposed idea is inspired by spark-gap transmitters, pulsed ring-down sources, and antenna arrays to provide a novel and highly scalable electromagnetic source. We envision that the proposed active metasurface for converting a static field to electromagnetic radiation can be potentially used in long-range communication, sensing devices, and radars.