Anisotropic impedance surfaces activated by incident waveform.

Electrical control of amplitude and phase of terahertz radiation (THz) is the key technological challenge for high resolution and noninvasive THz imaging. The lack of active materials and devices hinders the realization of these imaging systems. Here, we demonstrate an efficient terahertz phase and amplitude modulation using electrically tunable graphene devices. Our device structure consists of electrolyte-gated graphene placed at quarter wavelength distance from a reflecting metallic surface. In this geometry, graphene operates as a tunable impedance surface which yields electrically controlled reflection phase. Terahertz time domain reflection spectroscopy reveals the voltage controlled phase modulation of π and the reflection modulation of 50 dB. To show the promises of our approach, we demonstrate a multipixel phase modulator array which operates as a gradient impedance surface.

Recently a lot of attention has been devoted to the electromagnetic scattering from anisotropic impedance wedges. Specifically, rigorous spectral solutions for three-dimensional scattering from anisotropic impedance wedges with a PEC face have been presented, when the impedance tensor of the loaded face exhibits a vanishing surface impedance along an anisotropy axis and a finite value in the orthogonal direction. However, these solutions are valid only when the direction of vanishing impedance is either parallel or perpendicular to the edge. A new rigorous spectral solution is presented in this paper, which is valid when the vanishing impedance direction is arbitrarily oriented with respect to the diffracting edge. The solution applies to both the half-plane and the planar-junction configurations. It is worth noting that the anisotropic impedance model is also suitable to simulate corrugated surfaces or strip loaded grounded dielectric slabs.

Circular waveguides with anisotropic impedance surfaces can be used in gyro-devices in order to improve their performance. Among them are dielectric-loaded metallic waveguides [1–5], axially [6] and azimuthally [7] corrugated waveguides. Their eigenmodes are purely (or nearly) TE and TM modes only in a number of extreme cases. Such cases were considered previously in [1–7]. However, in the general case, when the surface impedance is arbitrary, waveguide modes are hybrid HE and EH waves. Derivation of the boundary-value problem for such waves is the main aim of the study.

Limited electronic steering can be accomplished using a focusing reflective surface by changing the feed location or direction of incidence. In this paper we describe a method of electronic beam steering by using a tunable impedance surface. Since there are no moving parts or complicated feed networks, this method of reflective beam steering can have great utility for low-cost and lightweight scanning antennas.

An anisotropic impedance surface based on electromagnetic bandgap (EBG) theory is proposed to produce reconfigurable dual-beam radiation through surface-wave (SW) lensing by controlling SW propagation across the surface-to-air interface. The reconfiguration of the lensing effect is triggered by switching between 4.0- and 4.5-GHz frequencies, utilizing the anisotropy of the impedance surface created from two mutually exclusive EBGs along the two orthogonal spaces. The bandgaps are formed by a periodic arrangement of two different split-ring resonators (SRRs) with bandgaps at around 4.0 and 4.5 GHz along the orthogonal y- and x-axes, respectively, producing a strong axially symmetric anisotropy. As a result, a surface-bound wave, i.e., both transverse electric (TE) and transverse magnetic (TM), faces asymmetric propagation reluctance toward these two directions one of which is very high compared to the other at frequencies around the individual bandgaps of the unit cells. The proposed impedance surface is designed and simulated using commercial electromagnetic simulation software and then fabricated and measured to validate the model.

The eigenvalue problems for step-index and dielectric-lined waveguides with anisotropic cladding are transformed to the similar form as for impedance waveguide. Both problems are then reduced to separate ones, each describing either HE or EH hybrid waves. The frequencies and the fields of the hybrid waves are also determined. It is proven that HE (EH) waves behave generally like TE (TM) waves inside the core regions of both step-index and dielectric-lined waveguides regardless of the wave frequency and the waveguide parameters.

Coupled first-order differential equations are obtained for the field amplitudes of a single wave in cylindrical weakly irregular waveguide with anisotropic impedance surface. The equations are valid for both types of hybrid waves. For closed uniform cavity with ideal side walls, they yield analytical solution to eigenvalue problem. For open gyrotron cavity, they reduce to known form as impedance approaches zero. Gyrotron cavity with anisotropic wall impedance of special form is considered as numerical example.

A broadband low-profile dual-circularly polarized (dual-CP) reflect-array (RA) cells covering both <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> -band and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ka</i> -band is presented in this paper. As an anisotropic impedance surface, the functional layer is designed based on multi-resonant structure and printed on substrate with a thickness of 0.762 mm. The circuit is composed of elliptical ring, arc-shaped and rectangular metal microstrip lines. The air layer between the functional layer and the metal plate can effectively extend the cells bandwidth (BW), and the overall profile is 1.8mm. The dynamic phase and Berry phase can be adjusted by changing the size of the arc-shaped microstrip lines and the rotation of the cells. The diffraction results show that the phase of left-handed (LH) CP and right-handed (RH) CP waves can be controlled independently and reflected by high purity with the cross-polarization BW of 64% (19.72 GHz ~ 38.44 GHz). A dual-CP RA with 1013 cells is designed to realize reflect beams at θ = -20° for LHCP wave and θ = 15° for RHCP wave at 30 GHz. Furthermore, the 3-dB gain BW can cover the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ka</i> -band and is expected to be used in wireless and satellite communication systems.

A novel beam steerable antenna employing tunable high impedance surface with liquid crystal is proposed. This antenna utilizes three microstrip patches as the radiators and a tunable high impedance surface based on liquid crystal as the ground plane. In this design, liquid crystal is deliberately disposed under the two parasitic microstrip patches to reduce the effect of relatively large dielectric loss and to improve the gain and radiation efficiency of the antenna. More importantly, this work explores a tunable high impedance surface based on liquid crystal, which has advantages of simple structure and biasing scheme as compared with other tunable high impedance surfaces based on solid-state devices. It is shown that by tuning the permittivity of liquid crystal, the high impedance surface becomes to support the propagation of TE surface waves to strengthen the mutual coupling between main and parasitic microstrip patches. Consequently, the main lobe can be steered to the desired direction and the scanning range of the antenna is enlarged. To prove this novel concept, a Ka-band prototype is fabricated and tested. Measured results show that the antenna not only has acceptable gain, but also keeps a satisfactory scanning range. In addition, this antenna consumes negligible DC power and thus is a strong antenna competitor for 5G access point applications.

Wireless power transfer (WPT), a technology full of promise, whose electromagnetic compatibility (EMC) has become a vital issue. In this study, we investigated the electro-magnetic emission generated by WPT systems using resonant magnetic field coupling, especially in applications with two-coil configurations. A novel finite element analysis based model for the two-coil WPT system is proposed. Useful information about the distribution of magnetic field can be acquired from the proposed model. The influences of coil distance, coil angle and probe distance on the magnetic field is also studied in detailed. Finally, a two-coil WPT testbed is built to provide an intensively investigation for the electromagnetic emission.

The quality factor $(Q -factor)$ of a coil can be improved by using a negative impedance converter (NIC) with transistors. Wireless power transfer (WPT) system with the high $Q$ -factor coil can have extremely enhanced power transfer efficiency (PTE) and received power at the receiver side. The NIC is composed of a bipolar junction transistor (BJT), a voltage divider, a resonance circuit, elements for impedance matching, and a tunable impedance. The negative impedance at 6.78 MHz can be obtained by using the tunable impedance in the NIC, and it can be adjusted as desired by the user. As a result, the loss of the transmitting coil is significantly reduced from 2 $\Omega$ to 0.1 $\Omega$ The $Q$ -factor of the coil is enhanced from 370 to 7,067, which is 19.1 times higher than that of the typical coil. The measured PTE is 96% and 79% at a transmission distance of 50 and 200 mm, respectively. Hence, the performance of a WPT system, which uses an NIC with a BJT, demonstrates that it can be applied to mobile applications.

Reflection of an acoustic line source by an impedance surface with uniform flow

The scattering problem of acoustic plane waves from comb-like impedance gratings on a rigid surface has been investigated in this paper. A rigorous analytic approach for homogeneous plane-wave incidence is presented based on the periodicity of the grating structure, in which the problem was solved as a mixed boundary value problem and the scattered field was represented by the tangent velocity difference across a partition wall of the grating. A singular integral equation has been derived for the tangent velocity difference, which can directly be solved with the Gauss-Chebyshev procedure. The resulting solution consists of a series of Bloch-Floquet waves (plane bulk wave and surface wave modes) with explicit expressions for the expansion coefficients. When the grating period is much less than the incident wavelength (ka << 1), the grating structure is equivalent to a plane impedance surface and no surface waves can be excited with homogeneous plane-wave incidence. When the grating period is comparable to the incident wavelength, resonance phenomena are predicted under certain conditions and surface waves can form, even with homogeneous plane-wave incidence. The dispersion relation for surface waves has also been examined. The impedance effects of the grating on the reflection and diffraction waves as well as on the dispersion and formation of surface waves have been studied, with the acoustically hard grating being the special case of the general impedance grating.

Wireless power transfer (WPT) based on magnetic resonance is the technology that might isolate humans from wires. In practical terms, the WPT utilizes the same fundamental principle researched for at least half a century under inductive power transfer. In recent years, WPT technology has undergone tremendous development, with a grid-to-load efficiency greater than 90% and the transmission distance rises from a few millimeters to several hundred millimeters at kilowatt power levels. The advancements make the WPT particularly desirable for static and dynamic electric vehicle (EV) charging applications. This work designs and analyzes multiple circular coil geometries appropriate to EV wireless charging for the WPT application. Using ANSYS Maxwell software to simulate, construct and evaluate the performance of various coil assemblies, including coupling coefficient (k), Mutual inductance (M), self-inductance (L), and magnetic flux density (B) for air gap distance of 50mm to 200 mm. The coil dimensions have been the same for all coils, and the air gap length and the copper wire specifications are uniform for equality. Using the proposed multiple coil-based WPT in EVs may easily overcome charging time, range, and cost challenges.

The self-focusing effect of optical beams has been a popular topic of study for quite a while, but such a nonlinear phenomenon at microwave frequencies has never been realized, partially due to the underdevelopment of nonlinear material. In this research, self-focused electromagnetic (EM) surface waves are demonstrated on a circuit-based, power-dependent impedance surface. The formation of a self-focused beam is investigated using a series of discrete-time simulations, and the result is further validated in measurement. It is experimentally observed that, in contrast to the normal scattering of low-power surface waves, high-power waves propagate through the surface while maintaining narrow beam width, and even converge extremely tightly to create a hot spot with higher power. The result is essentially a nonlinear effect of the surface that compensates for the natural tendency of surface waves to diffract. This intriguing experiment can be extended to various potential EM applications such as power-dependent beam steering antennas and nonlinear microwave propagation or dissipation.