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.

We present the design and experimental demonstration of a programmable subwavelength coil array, which can manipulate the evanescent magnetic field leading to a precisely controlled electric-field pattern at an image plane in the near-field zone. A synthesis methodology is outlined for 2-D near-field manipulations. Simulation based on a full-wave electromagnetic solver clearly demonstrates the focused electric-field pattern, and its implementation is discussed. An example of a three-layer subwavelength coil array with 48 units is fabricated and measured. The consensus of the measured results with the simulated and synthesized results verified the presented synthesis method. Such a device, capable of producing a programmable magnetic field and electric field, respectively, in orthogonal planes, will find applications in biomedical devices such as neural stimulation, and potentially other areas such as noncontact charging.

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. © The Institution of Engineering and Technology 2013.