Bessel beams can be generated at microwaves by means of compact, planar devices based on Fabry-Perot cavity leaky-wave resonators. Most designs are based on the excitation of a single transverse magnetic/electric (TM/TE) leaky mode by means of a vertical electric/magnetic dipole source. In this work, we analyze the important case of a horizontal magnetic dipole which excites both TM and TE modes, developing an original theoretical framework suitable for resonant radiators. Analytical expressions are provided for the dominant leaky-wave aperture field. The near-field distribution is then evaluated through an accurate numerical integration of the radiating currents, and validated through full-wave simulations for the relevant case of a Bessel-beam launcher when excited by a waveguide-fed slot. Different designs are presented and analyzed in order to obtain TM, TE, and hybrid polarization, finding in all cases an excellent agreement between simulations and theoretical results. Finally, we evaluate and compare the wireless power transfer efficiency of two coupled TM-polarized, TE-polarized, or hybrid-polarized Bessel-beam launchers when a wireless link is established in the radiative near-field region. Interestingly, full-wave results demonstrate the superior performance of resonant Bessel-beam launchers in TM or TE polarization with respect to the nonresonant hybrid case.

Resonant Bessel-beam launchers are low-cost, planar, miniaturized devices capable of focusing electromagnetic radiation in a very efficient way in various frequency ranges, with recent increasing interest for microwave and millimeter-wave applications (i.e., 3–300 GHz). In recent years, various kinds of launchers have appeared, with different feeding mechanisms (e.g., coaxial probes, resonant slots, or loop antennas), field polarization (radial, azimuthal, and longitudinal), and manufacturing technology (axicon lenses, radial waveguides, or diffraction gratings). In this paper, we review the various features of these launchers both from a general electromagnetic background and a more specific leaky-wave interpretation. The latter allows for deriving a useful set of design rules that we here show to be applicable to any type of launcher, regardless its specific realization. Practical examples are discussed, showing a typical application of the proposed design workflow, along with a possible use of the launchers in a modern context, such as that of wireless power transfer at 90 GHz.

The generation of focused beams in the millimeter- and submillimeter-wave ranges, with transverse-electric (TE) polarization, is investigated in the radiative near-field region. The desired field distribution is achieved through a leaky-wave beam launcher consisting of a grounded dielectric slab with an annular strip grating on top excited by a circular slot on the bottom ground plane. The latter is fed by a Marié transducer, which converts the input, fundamental TE10 mode of a standard rectangular waveguide into the higher-order TE01 mode propagating in the circular waveguide connected to the device. The generation of TE-polarized diffraction-limited Bessel and Bessel–Gauss distributions is achieved by suitably synthesizing the annular strip grating. Simulated results are in excellent agreement with those predicted by leaky-wave analysis providing a proof-of-concept for the generation of TE-polarized Bessel and Bessel–Gauss beams at 300 GHz with a beam size of 1.7 and 1.9 mm up to the nondiffractive range of about 25 and 15 mm from an aperture plane with radius of 12.75 mm, respectively.

Transverse-electric (TE) polarized Bessel-beam launchers working at millimeter-wave frequencies are theoretically described, simulated, realized, and experimentally validated. A limited-diffractive near-field distribution up to the so-called nondiffractive-range distance of about 30 mm has been obtained in this work. To generate a pure TE leaky mode, avoiding undesired transverse-magnetic (TM) field components, an innovative feeding scheme is designed consisting of a circular array of four radial slots. To enhance compactness, the four slots are excited through a single-port meandered microstrip feeding network, which has been optimized to ensure the same signal distribution exciting the slots while minimizing unwanted coupling among them. Full-wave simulations allowed for accurately designing a resonant cavity at 30 GHz able to generate a TE-polarized Bessel beam (BB). Two launchers of this kind are then exploited to create a wireless power transfer (WPT) link whose performance is first computed numerically and then verified by means of measurements for different transmitter-receiver distances, namely 25, 35, and 48 mm. These values respectively fall within, in proximity of, and away from the nondiffractive region of the transmitting (TX) device, showing an increasing WPT performance deterioration, as expected. For the case of a 25-mm distance, the evaluated $|S_{21}|$ is $-$ 12 dB, in excellent agreement between full-wave and measurement results.

Open stopband (OSB) mitigation techniques are commonly used to improve the far-field radiating properties of leaky-wave antennas based on periodic structures. Recently, leaky waves have been proposed to focus energy in the near field through Bessel beams. However, the focusing character of Bessel beams is notably limited to a maximum distance known as the nondiffractive range. In this work, an OSB mitigation technique is originally exploited to significantly extend the nondiffractive range of a Bessel beam generated by a leaky-wave launcher in the microwave/millimeter-wave range. A comprehensive analysis of this device is presented, comparing the performance of the proposed launcher with the typical structure of a leaky-wave Bessel-beam launcher where the OSB is not suppressed. Theoretical results, corroborated by full-wave simulations, demonstrate that the proposed device achieves an impressive nondiffractive range of about 25 m. The latter, at 30 GHz, approximately corresponds to 2500 wavelengths (in vacuum) and to 50 times the aperture diameter which is about 50 cm. These results look particularly attractive for, e.g., near-field communications and wireless power transfer applications, where focusing energy in narrow regions and over large distances is a key factor.

Two-dimensional leaky-wave antennas offer effective, compact, single-feeder, easy-to-fabricate solutions to the longstanding problem of realizing a simultaneously directive and low-profile radiating device. These traveling-wave antennas have been thus proposed as wideband, reconfigurable, or frequency-scanning radiating structures in different application contexts, spacing from the microwave to terahertz frequency range. These diverse contexts call for a comprehensive guide to characterizing and designing two-dimensional leaky-wave antennas. In this work, a review of numerical techniques for the analysis of either quasi-uniform or radially periodic leaky-wave antennas is proposed in order to provide the reader with straightforward yet effective design guidelines. Theoretical results are corroborated through full-wave simulations of realistic three-dimensional models of the considered devices, thus demonstrating the effectiveness of the proposed methods.