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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.

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The absorbing performance of a nonlinear waveform-dependent metasurface with pulsed signals is demonstrated. The metasurface is a periodic structure containing diodes, capacitors and resistors. These enable us to first rectify a high-power incoming signal to a static field, then store the energy during the illumination and dissipate it before the next pulse comes in. The incident pulses contain a finite width of spectrum of around 4.2 GHz. By using the nonlinear metasurface, absorption that depends on both the power and the duty cycle of the incoming signals is measured. These measurements demonstrate the first waveform-dependent absorbing metasurface.Introduction: A surface current propagates on a conducting surface and, with sufficiently high power, can disrupt electronic devices deployed there, leading to microwave interference issues. One difficulty lies in handling different levels of signals simultaneously. This is because widely used linear materials, such as lossy materials [1], Salisbury screens [2] or high-impedance metasurfaces [3, 4], either simply absorb or reflect any incoming signals, including low-power signals used for wireless communications. Therefore, it is important to decouple the high-power surface impedance from its low-power response by introducing nonlinearity. Such an idea was proposed in [5], where circuit-based nonlinear metasurfaces absorbed high-power pulsed sine waves, while allowing transmission of low power continuous waves. Interestingly, these structures exhibited waveform dependence, which distinguishes even the same frequency in accordance with the waveform, i.e. pulsed or continuous signals. Moreover, the absorbing performance was demonstrated with a Gaussian pulse including a finite width of frequency spectrum. Such a pulse is assumed to be a more realistic threat to modern electronics. However, this demonstration was limited to only numerical simulations. Therefore, in this Letter, we experimentally investigate the absorbing performance of the nonlinear metasurface with short pulses containing a finite width of frequency spectrum.

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