Tunable THz antennas based on a single unpatterned graphene sheet placed inside a grounded dielectric multilayer are studied with the aim of characterizing their performance in terms of pattern reconfigurability, directivity, and radiation efficiency. The considered structures belong to the class of Fabry-Perot cavity (FPC) antennas, whose radiation mechanism relies on the excitation of cylindrical leaky waves with an ordinary (i.e., nonplasmonic) sinusoidal transverse modal profile. This allows for achieving radiation efficiencies considerably higher than those of alternative graphene-based radiators based on the excitation of surface-plasmon polaritons (SPPs) either in bound or leaky propagation regimes. A customized efficient circuit model has been employed in order to obtain all the radiation characteristics of such graphene FPC antennas, which have also been fully validated by means of a CAD tool. The role of the graphene quality is explicitly taken into account in this comprehensive investigation, proving that it plays a remarkable role in establishing the antenna performance. In particular, it is expected that the standard quality of graphene allows for designing low-efficiency reconfigurable THz antennas based on SPPs and, conversely, high-efficiency FPC antennas with slightly reduced reconfigurability.

We present a Bessel-beam launcher based on a leaky radial waveguide consisting of a capacitive sheet over a ground plane that supports higher-order leaky modes. A propagating Bessel beam is generated above the radiating waveguide. The Bessel beam is transverse-magnetic (TM) polarized with a vertical component of electric field that is a zeroth-order Bessel function of the first kind. A higher-order leaky-wave mode is used to reduce losses at millimeter waves and, at the same time, avoid the thin dielectric layers used in previously proposed lower order leakywave Bessel launchers. Closed-form design equations are provided for the proposed structure. In addition, the operating bandwidth of the launcher is defined using dispersion analysis. Near-field measurements of a prototype operating in the frequency range 38- 39.5 GHz validate the concept. The measured launcher generates a Bessel beam with a stable spot size of about 4.3 mm (0.57 λ) over a nondiffractive range of about 16.4 mm (2.2 λ), within about a 4% fractional bandwidth.

In this paper, a systematic design of Fabry-Perot cavity antennas based on leaky waves is proposed in the THz range. The use of different topologies for the synthesis of homogenized metasurfaces shows that a specific fishnetlike unit cell is particularly suitable for the design of efficient THz radiating devices. Accurate full-wave simulations highlight the advantages and disadvantages of the proposed geometries, thoroughly considering the bounds dictated by technological constraints and the homogenization limit as well. The radiative performance of different designs for achieving theoretical directivities ranging from 15 to 30 dB is evaluated with reliable analytical and numerical methods, and completely validated with full-wave simulations. The relevant results corroborate the proposed systematic design, consolidating the validity and the usefulness of the leaky-wave approach, well established at microwave frequencies, to the more challenging and still unexplored THz range.

A radially periodic 2-D leaky-wave (LW) antenna is studied for the generation of zeroth-order Bessel beams within a limited spatial region and over a wide-frequency range. The antenna design is wideband and based on an annular metal strip grating placed on the top of a grounded dielectric slab, supporting a cylindrical leaky wave (CLW) with a fast backward spatial harmonic. The focusing capabilities of the relevant LW aperture fields are investigated over the considered frequency range (15-21 GHz), in conjunction with the dispersion analysis of the optimized structure, which is developed by means of an efficient in-house method of moments code. Full-wave simulations using a commercial tool including a simple coaxial feeder are presented and discussed, demonstrating the desired wideband operation. The antenna design is validated by means of measurements performed on a manufactured prototype, considering different frequencies and components of the electric field within the nondiffracting range of the system. The proposed design represents an attractive simple and low-cost solution potentially able to generate arbitrary-order Bessel beams at microwaves as well as in the millimeter-wave and terahertz frequency regions.

A planar substrate-superstrate configuration loaded by a graphene sheet is proposed as a novel reconfigurable antenna for terahertz applications. The effects of the introduction of graphene on the dispersion and radiation properties of the fundamental leaky modes supported by the structure are analyzed in detail, with the aim of investigating the possibility of beam scanning at a fixed frequency by varying the graphene chemical potential via an electrostatic bias. After optimizing the graphene position inside the antenna substrate for achieving maximum directivity at broadside, large scanning ranges are obtained in which the normalized attenuation constants of the fundamental TE/TM leaky-mode pair exhibit small values and reduced variation, thus producing a scanning process with an essentially constant narrow beamwidth. In the same ranges, the phase constants of the TE/TM-mode pair are shown to be equalized, thus opening the possibility of designing fixed-frequency scanning antennas with conical beams and reconfigurable dual polarization.