806 research outputs found
Directive EBG Antennas Based on Lattice Modes
No full textThe emission of radiators can be shaped by means
of Electromagnetic Band-Gap (EBG) materials exploiting proper
Bloch waves supported by the lattice rather than its bandgaps.
So far such a method has only relied on the dispersion
diagram of the periodic structure, i.e., on the eigenvalues of the
lattice, neglecting the particular configuration of the associated
eigenfunction. This paper explores the radiation mechanism
under a novel viewpoint, which is mostly focused on the electric
field pattern of lattice modes, allowing a deeper understanding
of the underlying physics. Such an approach is profitably used
here to improve the performance of antennas based on different
Bloch waves by reducing spurious lattice modes. Geometrical
configurations coupled to a line source are provided with little
adjustments, getting EBGs to work at a frequency where they
had no band-gap originally. As a proof-of-concept of proposed
perspectives, a compact antenna using a cheap, low-permittivity
dielectric is conceived, fabricated and successfully tested
On 2D photonic crystals and the shaping of radiation diagrams
No full textPhotonic band-gap materials have been widely used to mold the flow of light. The scaling property of crystals has allowed studying their behaviour at microwaves, where periodic structures have been profitably adopted to shape the radiation pattern of low-directivity antennas. In particular spatial filters can be achieved using two-dimensional periodicity in two different manners: the one relies on the excitation of a proper Bloch wave at the edge of the band-gap, whereas the other exploits the interference between standing waves in a crystal defect. Strengths and weaknesses of the two mechanisms are discussed comparing the radiative and electrical parameters of several optimized geometries. These are made by circular dielectric rods, arranged in either square or triangular lattices, and fed by a current wire radiating a TM-polarized cylindrical wave. The method using the forbidden band is shown to achieve the highest directivities for low profile structures, whereas the other mechanism, besides being more efficient, is more suitable when antennas consist of few cylinders per row. Suggestions for an experimental validation of drawn conclusions are given
Low-permittivity EBG materials for antenna superstrates
No full textSuitability of low-permittivity EBGs to implement
superstrates of high-gain resonator antennas is explored. Unit cell
of the EBG is a square lattice with circular cross-section rods,
with band-gap in the X-band. Permittivity of the rods is typical of
PLA plastic filament used in 3D printing, with r = 2.76 and tan
= 0.014. EBG cavity made by two identical mirrors separated by
a resonating spacing is used to derive the antenna layout. The
primary radiator is a rectangular patch, covered by the EBG
mirror at a resonating distance, displaying a good gain
enhancement. A further resonator antenna is devised,
introducing a non-uniform spacing in the EBG rods along each
layer of the superstrate. In this new layout, antenna gain and Side
Lobe Level are considerably improved
3D Additive Manufacturing of Tapered EBG Layers for a Resonant-Cavity Antenna
No full text3D Fusion Deposition Modelling (FDM) technique is applied, in combination with numerical machining technique, to the fabrication of an Electromagnetic Band-Gap (EBG) multilayer used as superstrate of a Resonant Cavity Antenna (RCA). The primary source of the RCA is a rectangular waveguide WR90 in the X-band, and it is backed by a metallic plate, to form a cavity with the EBG placed above, which behaves as a partially reflecting surface. The EBG superstate is designed alternating three dielectric and planar layers, through materials with a permittivity contrast, i.e., by alternating layers having a high permittivity to a layer with low permittivity. This scheme gives to the EBG material a broadband response. In the in-plane design, each layer is shaped as a tapered grid of rectangular holes, to reduce the field scattered at the edge of the cavities, and thus lowering the Side Lobe Level in the antenna radiation patterns. The fabrication of the low-permittivity layer has been performed with additive manufacturing, through a PLA filament which has a relative permittivity of 2.76. The choice of PLA over ABS filament as printing material has been dictated by the different handling of the inner parts of the fabricated samples by the 3D printer. The high-permittivity layers of the superstate, instead, are fabricated from pre-formed vetronite layers, shaping the grid layout through numerically controlled machining technique. The antenna prototype has been measured in anechoic chamber, and experimental results are in very good agreement with the numerical simulations
On the use of corrugations in mode filters for oversized rectangular waveguides
No full textThe filtering of unwanted modes that can propagate in oversized rectangular waveguides is addressed, focusing on longitudinal corrugations partially filled with an absorber. Lengthwise slots, in the middle of top and bottom waveguide walls, can indeed extract the power of some modes from the main waveguide with negligible insertion losses for the working dominant mode. Once some power is coupled through the slot into the junction, it travels toward the absorbing material, where it is damped. This behavior is studied comparing the performance of a single continuous slot, where various modes can propagate, with the one achieved by several shorter single-mode apertures. The excitation and absorption of modes in the corrugations is estimated by means of analytical expressions under a small coupling approximation. Mono-modal corrugations achieve better performances with respect to the overmoded junction
Directive scattering by a line source coupled to infinite or finite Electromagnetic Band-Gap media
No full textRadiation from a line source coupled to an Electromagnetic Band-Gap material is analyzed with two different methods. Using lattice sums technique combined with generalized reflection matrix method, the scattering from a crystal of infinite extension is rigorously investigated. The radiation from an electromagnetic crystal of finite extension is also solved with Cylindrical Wave Approach. The two methods show perfect agreement when the source is localized within the crystal, achieving a directive radiation. As a spacing between the two Electromagnetic Band-Gap layers is introduced, the finite structure analyzed with the Cylindrical Wave Approach shows significant edge diffraction at the boundaries. Agreement between results from the methods is recovered introducing a suitable layout of absorbers in the cavit
Directive Propagation in Two EBG Structures: a Comparison
No full text"\"Two methods for the enhancement of directive propagation in two-dimensional EBG structures, consisting of either square or triangular lattices of dielectric rods, are compared. The benchmark is carried out in terms of main radiation and geometrical parameters for a set of configurations, chosen among several arrangements having different number of layers and cylinders per layer. In one mechanism a resonator antenna is attained by inserting a line source in between an EBG cover and a ground plane, while, in the other, the same source is embedded within an EBG material, working near its band-gap edge. The favourable configurations of the two methods are identified together with their strengths and weaknesses. \"
Physical mechanisms and design principles in mode filters for oversized rectangular waveguides
No full textMode filters, based on oversized rectangular waveguides with periodic symmetrical junctions, are addressed through analytical models under a small coupling approximation to explain the physical mechanisms of their behavior, and provide guidelines for their design. In the junctions, which are partially filled with a lossy dielectric, both propagating and evanescent modes play a role. Filter parameters are studied with respect to their impact on the excitation, propagation, and absorption of such modes in the junctions. Particular attention is paid to practical choices a designer has to face in real applications. The performance of single versus multiple lengthwise slots with the same total length is compared, and high-power matters such as voltage and thermal breakdown are discussed. A novel junction with two absorbing layers is proposed to help designers in maximizing absorption, while fulfilling thermal constraints
Transmitter and HVPS Architectures in the Ion-Cyclotron Radio Frequency System of DTT
No full textThe design choices for the conversion of electrical energy into radio frequency (RF) waves is critically reviewed in this paper to conceive the most suitable architectures for RF transmitters in nuclear fusion. Ion Cyclotron Resonance Heating (ICRH) systems, using RF waves to heat fusion plasmas, present specific and demanding requirements in terms of power supplies that are tackled here with reference to the DTT, a new advanced fusion machine currently under construction. After the selection of the technology for the amplification stages of ICRH transmitters, the electrical specifications of the power supplies are determined on the basis of several parameters such as amplification, efficiency and loading conditions. The study presented in this paper concerns the choice of the most suitable architecture of the high voltage power supplies. It is based on available literature and technical documentation as well as on the calculation of operational points for the high-power amplification stages in mismatching conditions. Several possible solutions have been analyzed but the peculiar needs of ICRH system, such as the high performance in terms of accuracy, stability and fast response, lead to the adoption of unconventional solutions, resulting in a custom design. Finally, some alternatives and improvements are proposed
Gain enhancement of dipolar antennas employing EBG media
No full textDifferent methods based on Electromagnetic Band-Gap materials are used to enhance the directivity of dipoles with a back reflector and compared. The EBG media consist of a periodic arrangement of dielectric cylinders that are excited either inside or at the edge of the band-gap by a half-wavelength dipole, while a ground plane with finite size acts as reflector against backscattering. The strengths and weaknesses of the two methods are assessed considering both square and triangular lattices
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