1,721,088 research outputs found
Fabry-Perot Cavity Antennas: The Leaky-Wave Perspective [Electromagnetic Perspectives]
No full textA tutorial overview is presented on the principles of operation, physical modeling, and design procedures for the class of planar radiators known as Fabry-Perot cavity antennas (FPCAs), with an emphasis on their characterization as 2D leaky-wave antennas (LWAs). After recalling their principal radiative features, alternative electromagnetic (EM) models are discussed, helping us to gain physical insight into the involved radiation mechanisms from different viewpoints. The leaky-wave approach appears particularly useful in this connection; it provides the framework for the subsequent review of design guidelines. Supplementary material that accompanies this article on IEEE Xplore addresses the main recent advances in the field, including wideband, reconfigurable, and array-based designs
The transition between reactive and radiative regimes for leaky modes in planar waveguides based on homogenized partially reflecting surfaces
Full text linkThe gradual transition from a reactive to a radiative regime is studied for leaky modes supported by multilayered planar open waveguides. The so-called leaky cutoff condition, i.e., the frequency for which the leaky phase constant equals the leaky attenuation constant , originally introduced for microstrip lines and other printed structures, is investigated here with the aim of providing detailed information on the relative amount of reactive and radiative attenuation for leaky modes excited by finite sources and propagating as cylindrical waves along general planar waveguides. Analytical results are derived on the basis of a lossy parallel-plate-waveguide model and are validated through full-wave numerical simulations of 2-D leaky-wave structures based on grounded slabs covered with lossless or lossy partially reflecting surfaces (including, e.g., graphene layers) that can be treated as homogenized sheets. An analysis of the complex wave impedance of the considered leaky modes is also provided, in order to assess the frequency ranges where a good input matching can be expected for practical sources. In this regard, an ad hoc impedance matching network is designed and full-wave validated for a specific case to show that is indeed possible to achieve a good impedance matching below the cutoff in practical designs
The transition between radiative and reactive region for leaky waves in planar waveguiding structures
No full textThe gradual transition from a radiative to a reactive regime is studied for leaky modes supported by planar waveguiding structures such as those used in Fabry-Perot cavity leaky-wave antennas (FPC-LWAs). These are typically studied above the so-called leaky cutoff, i.e., the frequency for which the leaky phase constant β equals the leaky attenuation constant α. However, an FPC-LWA can efficiently radiate even below the leaky cutoff, the condition for minimizing the beamwdith at broadside occurring for β < α. Nevertheless, well below the leaky cutoff, reactive phenomena may dominate radiative phenomena. These aspects are investigated here, providing a criterion to determine whether the relevant leaky waves are responsible for radiation rather than reactive phenomena. It is seen that the minimum-beamwidth condition occurs in a region where reactive effects can prevail over radiative effects. It is concluded that, although directivity is maximized, the gain may be adversely affected by impedance matching issues. These results are useful for the efficient design of LWAs
Genealogy of Leaky, Surface, and Plasmonic Modes in Partially Open Waveguides
Full text linkThe presence of a partially reflecting screen on top of a grounded dielectric slab strongly perturbs its modal spectrum. The modes supported by such structures have been investigated in the literature for some specific, yet relevant cases. Here, a comprehensive modal analysis is presented, which emphasizes the inductive or capacitive nature of the partially reflecting sheet. The results of this analysis not only provide a clear overview of a wide class of structures of relevance in applied electromagnetics, but also unveil the presence of some useful propagating regimes so far barely explored
A comparative analysis of Bessel and Gaussian beams beyond the paraxial approximation
No full textThe use of Bessel and Gaussian beams for focusing applications has long been debated. The former are attractive for their diffraction-resistant and self-healing character, whereas the latter notably exhibit higher efficiency. In previous works, efforts have been made to clarify whether Bessel or Gaussian beams are more suitable for wireless power transfer. However, the theoretical discussions proposed so far did not attract a general consensus from the community, the arguments often being based on restrictive, arbitrary hypotheses under the frame of the ray-optics and paraxial approximations. On the other hand, especially at microwaves, nonparaxial Bessel beams have found important applications. This work aims at providing an engineering perspective on the focusing properties of both Bessel and Gaussian beams by introducing significant figures of merit. The latter are defined taking into account the beam features that are of greatest interest in focusing applications, namely the beam efficiency, the maximum cover distance, and the beam resolution. Analytical expressions are provided, and results are shown for comparing the Bessel and Gaussian beam features when their beam waists are allowed to freely vary in a range of physical realizability
Fabry–Perot cavity antennas based on thick partially reflecting sheets: innovative design criteria
No full textFabry–Perot cavity antennas (FPCAs) are constituted by a grounded dielectric slab with a partially reflecting surface (PRS) on top, commonly implemented by an isotropic metallic thin sheet acting as a metasurface. These devices can be designed to radiate directive beams at broadside through a low-profile and compact structure at the expense of a typically narrowband behavior. The latter issue has recently been mitigated by placing a dielectric superstrate over the metasurface, generating a so-called thick PRS (see Fig. 1). This additional layer can suitably be designed to obtain a positive slope on the phase of the reflection coefficient, i.e., the thick PRS can show a non-Foster behavior. It is therefore possible to design the PRS in order to fulfill the cavity resonance condition to have a broadside beam over a relatively large frequency range
2-D Beam Scanning with Cylindrical-Leaky-Wave-Enhanced Phased Arrays
No full textDirective pencil beams scannable in both elevation and azimuth are obtained through a planar phased array placed inside a Fabry-Perot cavity. The key element of the proposed approach is the exploitation of a conical element pattern (EP) with high directivity in elevation, obtained through the excitation of a dominant, weakly attenuated cylindrical TM leaky wave of azimuthal order n = 0 by means of a simple coaxial probe. Then, a highly reduced number N of such sources are arranged to form a phased array radiating directive pencil beams. Beam-angle reconfigurability with continuous scanning both in azimuth and elevation inside a wide solid angular range is achieved by varying the array phasing and the operating frequency. An investigation on the features of truncated cylindrical leaky waves is first developed to properly characterize the EP. Then, conventional array theory is exploited to calculate the pattern of the entire array. The radiation efficiency is also evaluated accounting for the spurious surface wave related to the undesired excitation of the quasi-TEM mode. The proposed array design provides a simple and inexpensive innovative solution for obtaining a high-gain pencil beam continuously scanning in the 3-D space without suffering gain losses. In the presented implementation, the elevation angular scan, which is generally constrained by the wideband capability of the feeding system, by the requirements on the sidelobe level, and by the cutoff of the relevant leaky mode, ranges from about 21° to about 68°. Possible applications are envisaged for the next generation of wireless power transfer devices, for advanced radar and surveillance systems, earth observation, as well as for ceiling-mounted indoor localization and tracking
A Leaky-Wave Analysis of Resonant Bessel-Beam Launchers: Design Criteria, Practical Examples, and Potential Applicationsat Microwave and Millimeter-Wave Frequencies
Full text linkResonant 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
Reconfigurable THz leaky-wave antennas based on innovative metal-graphene metasurfaces
Full text linkGraphene ohmic losses notably hinder the efficiency of graphene-based terahertz (THz) devices. Hybrid metal-graphene structures have recently been proposed to mitigate this issue in a few passive devices, namely waveguide and Vivaldi antennas, as well as frequency selective surfaces. In this work, such a technique is extensively investigated to optimize the radiation performance of a THz Fabry-Perot cavity leaky-wave antenna based on a hybrid metal-graphene metasurface consisting of a lattice of square metallic patches interleaved with a complementary graphene strip grating. Theoretical, numerical, and full-wave results demonstrate that, by properly selecting the unit-cell features, a satisfactory trade-off among range of reconfigurability, antenna directivity, and losses can be achieved. The proposed antenna can find application in future wireless THz communications
Leaky-Wave Synthesis of Bessel Beams at Millimeter-Wave Frequencies
No full textBessel beams are focusing shaped beams that have recently attracted much interest for several practical applications thanks to their limited-diffracting and self-healing properties. Bessel-beam launchers are planar devices capable of generating Bessel beams. In this contribution, we show how to effectively model and design various types of such devices working at millimeter waves through a straightforward, accurate, and rigorous leaky-wave approach
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