Santi C. Pavone

On the Near-Field Shaping and Focusing Capability of a Radial Line Slot Array

We describe the design of a radial line slot array antenna with a shaped and focused near field. The antenna is designed in such a way to control the side lobe level and beamwidth of the normal component of the electric field with respect to the radiating aperture. The design procedure consists of two steps. In the first step, the requirements on the near-field pattern are provided over a focusing plane at a given distance from the radiating aperture. A set theoretic approach is then used to derive the aperture field distribution fitting the requirements over the near field. In the second step, the aperture field distribution is synthesized by accurately placing and sizing the slots of the antenna. The spillover efficiency is maximized during the design process. The antenna is centrally fed by a simple coaxial probe. The antenna design is validated by a prototype and measurements at 12.5 GHz.

Generation of non-diffractive Bessel beams by inward cylindrical traveling wave aperture distributions

The focusing capabilities of an inward cylindrical traveling wave aperture distribution and the non-diffractive behaviour of its radiated field are analyzed. The wave dynamics of the infinite aperture radiated field is clearly unveiled by means of closed form expressions, based on incomplete Hankel functions, and their ray interpretation. The non-diffractive behaviour is also confirmed for finite apertures up to a defined limited range. A radial waveguide made by metallic gratings over a ground plane and fed by a coaxial feed is used to validate numerically the analytical results. The proposed system and accurate analysis of non-diffractive Bessel beams launched by inward waves opens new opportunities for planar, low profile beam generators at microwaves, Terahertz and optics.

Experimental Validation of Bessel Beam Generation Using an Inward Hankel Aperture Distribution

In this paper, we prove experimentally that nondiffractive Bessel beams can be generated by using inward cylindrical traveling wave aperture distributions. An azimuthally invariant inward traveling wave distribution is defined over the aperture of a radial line slot array (RLSA) to launch a Bessel beam whose normal electric-field component assumes a truncated, zeroth-order Bessel function. An optimization procedure based on a holographic approach is used for tuning the position and size of the slots of the RLSA. The antenna is centrally fed by a coaxial probe transition. The final structure operates at 12.5 GHz. Full-wave simulations and measurements of the vertical component of the electric field show that a non-diffractive radiation is obtained within a range larger than 12 wavelengths in front of the antenna. The generated Bessel beam presents a stable half-power beamwidth of about 20 mm within this range. The proposed system may open new opportunities for planar, low-profile Bessel beam generators at millimeter waves, Terahertz, and optics.

Transverse circular-polarized Bessel beam generation by inward cylindrical aperture distribution

In this paper the focusing capability of a radiating aperture implementing an inward cylindrical traveling wave tangential electric field distribution directed along a fixed polarization unit vector is investigated. In particular, it is shown that such an aperture distribution generates a non-diffractive Bessel beam whose transverse component (with respect to the normal of the radiating aperture) of the electric field takes the form of a zero-th order Bessel function. As a practical implementation of the theoretical analysis, a circular-polarized Bessel beam launcher, made by a radial parallel plate waveguide loaded with several slot pairs, arranged on a spiral pattern, is designed and optimized. The proposed launcher performance agrees with the theoretical model and exhibits an excellent polarization purity.

Analysis and Design of Bessel Beam Launchers: Longitudinal Polarization

The paper presents the analysis and design of Bessel beam launchers using a finite inward cylindrical traveling wave aperture field distribution. The launcher radiates an electric field whose normal or longitudinal component takes the form of a zeroth-order Bessel function. The nondiffractive behavior of the structure in a well-defined area close to the radiating aperture is analyzed by decomposing the radiated field in its geometrical optics (GO) and diffractive (D) contributions. A closed-form expression is provided for the GO contribution whereas an asymptotic approximation is provided for the diffractive part. Such theoretical analysis allows a precise definition of the nondiffractive region for the generated Bessel beam. At the same time, it also highlights and predicts the oscillating behavior of the longitudinal component of the electric field along the z-axis due to the diffraction from the edges of the aperture. The proposed analysis is validated by a prototype at 30 GHz made by a radial waveguide loaded with metallic gratings and centrally fed by a coaxial probe. Measurement results for the longitudinal component of the electric field are in excellent agreement with full-wave results. In addition, the nondiffractive behavior for the radiated beam is reported over a bandwidth larger than 6.5% around 30 GHz. This behavior is peculiar of the nonresonant first kind Hankel aperture field distribution used for the generation of the Bessel beam.

Surface Wave Dispersion for a Tunable Grounded Liquid Crystal Substrate Without and With Metasurface on Top

The surface wave dispersion of a grounded layer of liquid crystals (LCs) is investigated by taking into account the inherent electrical reconfigurability of such a material. The spectral dyadic impedance Green’s function of the tunable LC grounded slab is calculated and the dispersion curve of the fundamental mode supported by the structure is presented, showing that the orientation of the optical axis of the LCs modifies the surface wave dispersion curve significantly enough to be applied for surface wave propagation control. Furthermore, it is demonstrated that the presence of an inductive metasurface on top of the LC layer impressively reduces the resonance frequency and increases the sensitivity to the continuous voltage biasing.

Propagation of nondiffracting pulses carrying orbital angular momentum at microwave frequencies

We discuss the generation and propagation of nondiffracting twisted pulses at microwaves, obtained through polychromatic spectral superposition of higher-order Bessel beams. The inherent vectorial structure of Maxwells equations has been considered to generalize the nondiffracting solution of the scalar wave equation with azimuthal phase variation. Since a wide frequency bandwidth is necessary to synthesize time-limited pulses, the non-negligible wavenumber frequency dispersion, which commonly affects the propagation in the microwave range, has been taken into account. For this purpose, a higher-order Bessel beam is generated by enforcing an inward cylindrical traveling-wave distribution over a finite aperture. We present and discuss the main aspects of the generation of twisted pulses in the microwave range, showing the promising possibility to carry orbital angular momentum through highly focused X-shaped pulses up to the nondiffractive range.

Comparison between broadband Bessel beam launchers based on either Bessel or Hankel aperture distribution for millimeter wave short pulse generation

In this paper, a comparison is presented between Bessel beam launchers at millimeter waves based on either a cylindrical standing wave (CSW) or a cylindrical inward traveling wave (CITW) aperture distribution. It is theoretically shown that CITW launchers are better suited for the generation of electromagnetic short pulses because they maintain their performances over a larger bandwidth than those realizing a CSW aperture distribution. Moreover, the wavenumber dispersion of both the launchers is evaluated both theoretically and numerically. To this end, two planar Bessel beam launchers, one enforcing a CSW and the other enforcing a CITW aperture distribution, are designed at millimeter waves with a center operating frequency of f¯=60GHz and analyzed in the bandwidth 50 - 70 GHz by using an in-house developed numerical code to solve Maxwells equations based on the method of moments. It is shown that a monochromatic Bessel beam can be efficiently generated by both the launchers over a wide fractional bandwidth. Finally, we investigate the generation of limited-diffractive electromagnetic pulses at millimeter waves, up to a certain non-diffractive range. Namely, it is shown that by feeding the launcher with a Gaussian short pulse, a spatially confined electromagnetic pulse can be efficiently generated in front of the launcher.

A novel approach to low profile scanning antenna design using reconfigurable metasurfaces

A novel approach for the design of scanning antennas is proposed. It relies on the use of a reconfigurable metasurface (MTS) whose modulation can be adjusted so as to vary the pointing angle of the supported leaky wave. The building block of the MTS is an electrically very small bistable elementary cell, or “MTS bit”; proper combinations of a number of these MTS bits lead to “MTS bytes” exhibiting different equivalent surface impedances. A leaky wave antenna can be realized by periodically arranging the MTS bytes. By acting on the switches controlling the single bits it is possible to modify the global modulation of the surface impedance, and hence to vary the leaky wave antenna pointing angle. This leads to an extremely light, low profile and potentially low cost scanning antenna.

Near-Field Focusing by Non-diffracting Bessel Beams

This chapter illustrates the capabilities of non-diffractive Bessel beams for near-field focusing. After a brief introduction of the non-diffractive phenomenon and its origin, the generation of non-diffractive Bessel beams by inward cylindrical traveling waves is analyzed in detail. A ray interpretation is proposed for such beams for infinite and finite radiating apertures. Their main radiation capabilities and limitations in terms of focusing, bandwidth, and operating range are discussed. In particular, it is shown that inward cylindrical traveling wave aperture field distributions can generate non-diffractive Bessel beams over a large bandwidth. As a practical implementation of the discussed theory, a class of launchers based on radial waveguides loaded by metallic gratings or slots is proposed, for which an efficient design and optimization technique is described. In addition, a different and unconventional leaky-wave approach is also adopted for the design of radial waveguides loaded by metasurfaces. The chapter ends by outlining the future research activities and possible applications of non-diffractive beams.