No abstract

The role of frequency is very important in electromagnetics since it may significantly change how a material interacts with an incident wave if the frequency spectrum varies. Here, a new kind of microwave window is demonstrated that has the unique property of controlling transmission and reflection based on not only the frequency of an incoming wave but also the waveform or pulse width. This is achieved by designing a planar periodic surface with circuit elements including diodes, which convert most of the incoming signal to zero frequency. This surface can preferentially pass or reject different kinds of signals, such as short pulses or continuous waves, even if they occur at the same frequency. Such a structure can be used, for example, to allow long communication signals to pass through, while rejecting short radar pulses in the same frequency band. It is related to the classic frequency selective surface, but adds the new dimension of waveform selectivity, which is possible only by introducing nonlinear electronics into the surface. Thus, the study is expected to provide new solutions to both fundamental and applied electromagnetic issues ranging from traditional antenna design and wireless communications to emerging areas such as cloaking, perfect lenses, and wavefront shaping.

High-power signals traveling along the surface of the shielding of transmitter systems may leak into the system through openings between connecting parts and cause damage to vulnerable electronic devices. This problem could be alleviated by implementing lossy coatings or recently developed passive power-dependent nonlinear surfaces. However, these solutions will either suppress the performance of the electromagnetic devices being shielded or be highly power-dependent. Applying transistors creates an active nonlinear metasurface that can allow the absorption of the surface to be directly controlled by the system, or tuned in response to the local power level using feedback control. This can provide a sharp absorption response with a wide range of controllable power threshold. Different absorption rates at the same power level can also be achieved by applying different biasing to the transistors. In this paper, the first transistor-based, thin, switchable, and tunable high-power surface wave absorber is proposed with full wave and circuit cosimulation analysis as well as waveguide and anechoic chamber measurement.

No abstract

No abstract

An electrically tunable metasurface that absorbs continuous electromagnetic (EM) surface waves is proposed by taking advantage of varactor diodes embedded in the surface. On the one hand, the varactors perform as the main dissipating components due to their parasitic series resistance; on the other hand, they function as the tuning elements because the dissipation is highly dependent on their capacitance. Therefore, the absorption of the surface can be tuned by the direct current biasing voltage across the varactors, which is validated numerically and experimentally in this letter. This absorbing mechanism of the surface differs from prior surface-wave absorbers and can lead to greater flexibility for absorbing metasurfaces. In this work, a power-dependent absorbing performance is achieved by loading microwave power sensors. If incorporated with other types of sensors, the absorption could potentially be controlled by corresponding physical variables such as light, pressure, or temperature, thus giving rise to various absorbing applications in a complex EM environment.

Abstract-Frequency independent fast-wave (FW) propagation with phase velocity greater than the speed of light can be ideally realized in a dielectric medium whose relative permittivity is positive, but less than 1. Conventionally, FW propagation is implemented by non-TEM waveguides or antiresonance-based metamaterials, which suffers from the narrow bandwidth due to the dispersion. In contrast, non-Foster circuits provide a brand new method for reducing the dispersion so as to broaden the bandwidth. This paper demonstrates broadband FW propagation in a microstrip line that is periodically loaded with non-Foster circuits. Discrete transistorbased non-Foster circuits functioning as negative capacitors are successfully designed with the novel modified negative impedance converter circuits. A 10-pF negative capacitor over a bandwidth of 10-150 MHz has been implemented. The fabricated circuits have been integrated into a microstrip line to form a FW waveguide. The retrieved phase velocity of the effective medium from the measured -parameters characterizes a stable and causal FW medium with constant phase velocity of from 60 to 120 MHz, and this has been further verified by Kramers-Kronig relations and the near-field measurements along the waveguide. In conclusion, a stable, causal, and broadband FW waveguide has been achieved by means of transistor-based non-Foster circuits. The implemented broadband FW propagation can potentially be applied in broadband leaky-wave antennas and cloaking techniques.Index Terms-Fast-wave (FW), metamaterials, non-Foster circuits, periodically loaded transmission line.

Two kinds of surface-wave waveguide (SWG) topologies are proposed in this paper with the objective to achieve the property of supporting both transverse magnetic (TM) and transverse electric (TE) modes with the same phase velocity. The first type is composed of two frequency-selective surfaces (FSSs) as layers whose dominant modes are TM mode and TE mode, respectively. For illustration'C the combination of loop-type FSS and wire-grid-type FSS is analyzed and its dispersion characteristics are examined as well. The second class also consists of two layers. For the top layer, there are gaps in one direction and continuous conducting strips in the orthogonal direction. The bottom layer is created from a 90°rotation of the top layer. As a particular illustration, a modified bow-tie-like SWG structure is investigated. The simulated results show that the two proposed SWG structures exhibit the property of supporting both TM mode and TE mode with the same phase velocity over a broad bandwidth. In addition, the effects of lattice types on dispersion diagrams are discussed in this paper. Near field measurements are also carried out to validate the simulations and good agreements are achieved.