Daniele Cavallo

Closed-Form Analysis of Artificial Dielectric Layers—Part II: Extension to Multiple Layers and Arbitrary Illumination

In this second part of a two-paper sequence, we present an analytical formulation to model artificial dielectric layers (ADLs) of finite height. We extend the method developed in Part I (IEEE Trans. Antennas Propag., vol. 62, no. 12, pp. 6256-6264, 2014) describing the single layer to the multilayer case, by including the higher-order interaction between parallel sheets in analytical form. From the equivalent impedance of the layers, we can construct a transmission line model that provides the spectral Greens function of ADLs in closed form. This allows to characterize the propagation through finite ADLs and to study the dispersion characteristics from the investigation of the Greens function singularities. Finally, by integrating the responses of a continuous set of plane waves, sources at finite distances from the ADL slab are treated. The method is validated by comparison with commercial electromagnetic solvers and experimental results obtained from a X-band prototype demonstrator.

Closed-Form Analysis of Artificial Dielectric Layers—Part I: Properties of a Single Layer Under Plane-Wave Incidence

This two-part sequence of papers deals with the analysis of artificial dielectric layers (ADLs). ADLs consist of a cascade of planar periodic surfaces designed to realize dielectric slabs with a desired equivalent permittivity. A single layer is typically composed of an array of electrically small patches periodically arranged in a rectangular lattice. In this paper we present an analytical formulation to model a single layer for arbitrary plane-wave incidence, highlighting the characteristic properties and their frequency range of validity. We derive a closed-form solution for the magnetic current distribution, including the reactive near field on the crossing slots, by expanding the total current with ad-hoc entire-domain basis functions. Simple analytical expressions are also derived for the equivalent sheet impedance of the layer. The second part of the paper extends the analysis to finite ADL slabs and nonplane wave sources.

Design, Fabrication, and Measurements of a 0.3 THz On-Chip Double Slot Antenna Enhanced by Artificial Dielectrics

In this paper, we demonstrate, at 300 GHz and with integrated technology, the effectiveness of artificial dielectric layers to enhance the front-to-back ratio of printed antennas. This concept was previously proposed at microwave frequencies and using printed circuit board technology. The artificial material is now realized by introducing non-resonant metallic inclusions in a silicon dioxide host material. This allows to enhance the permittivity of the host medium and renders it anisotropic. By loading an electrically thin dielectric with these metallic inclusions, an engineered slab with effectively quarter wavelength thickness has been realized. Despite the large effective height and density of the artificial dielectric, the surface wave efficiency of the antenna is 99%. This is entirely due to the anisotropic properties of the material. A prototype antenna was built using an in-house CMOS back-end compatible integrated circuits (IC) process. Measured results from the antenna are presented and show a good agreement with the expected results.

Wideband, Wide-Scan Planar Array of Connected Slots Loaded With Artificial Dielectric Superstrates

Microwave broadband wide-scan antenna arrays are typically implemented resorting to vertical arrangements of printed circuit boards (PCBs). Here, we propose a planar solution realized with a single multi-layer PCB, with consequent reduction in cost and complexity of the array. It consists of an array of connected slots backed by a metallic reflector and loaded with superstrates. Artificial dielectric layers (ADLs) are used in place of real dielectrics to realize the superstrates, as they are characterized by very low surface-wave losses. For the unit-cell design, we developed an analysis tool based on closed-form expressions and thus requiring minimal computational resources. Finite-array simulations are also performed by generalizing the analysis method to account for the truncation effects. The presence of the ADL superstrate allows reducing the distance between the array plane and the backing reflector while maintaining good matching performance. A realistic feed structure is also proposed, which consists of a microstrip line connected to a coaxial feed. Such a solution does not require balanced-to-unbalanced transitions, which often limit the achievable bandwidth. The proposed structure achieves in simulations more than an octave bandwidth (6.5-14.5 GHz), within a scanning range of ±50° in all azimuth planes.

A Rigorous Equivalent Network for Linearly Polarized THz Absorbers

Linearly polarized electromagnetic absorbers are often used in THz space science as means of power detection. In this paper, we present a rigorous and analytical equivalent network suitable for the analysis of these structures. The model includes, on one hand, the vectorial representation of the general propagation and scattering of the dominant electromagnetic waves and, on the other hand, details of the absorber geometry. Unlike existing models for frequency selective surfaces (FSS), the present network highlights the full interaction between TE and TM modes and the current in the strips that couples them. The network is elegant, simple and accurate, providing the explicit description of the dominant physical mechanism involved. The tool derived from the network is used for designing several THz absorber based detectors showing broadband absorption efficiencies. The network is validated with full wave simulations.

A Connected Array of Slots Supporting Broadband Leaky Waves

In this paper, we investigate the properties of a finite array of slots that support broadband leaky waves. The slots are etched on a ground plane located in the close proximity of a dense dielectric half space and each slot is fed at a finite number of locations. A quasi-analytical spectral procedure is presented, which can be used to evaluate the near and far fields that emerge from each separate feed of the array. In fact, the structure can be used to realize hundreds of independent beams in the overlaying dense dielectric over a band in the order of 4:1. The spectral procedure can also be used to evaluate the mutual coupling between the elements. This latter limits the lower frequency at which the different beams are independent and directive. These arrays are mainly proposed to be used as feeders for dielectric lens antennas. The formulation presented here is very general and can be applied to any finite array of long slots, independently from their capability to support leaky waves.

Characterization of a Dual-Polarized Connected-Dipole Array for Ku-Band Mobile Terminals

In this paper, we present the characterization of a Ku-band connected-dipole array for mobile Satcom application. The prototype array consists of 2 × 256 dipole radiators arranged in a square grid, to form 16 × 16 dual-polarized cells. It has been designed to operate over a wideband ranging from 10.7 to 14.5 GHz (30%), accommodating in a single antenna both the transmit (14.0 to 14.5 GHz) and the receive (10.70 to 12.75 GHz) bands for the application. The array has been characterized in terms of active reflection (AR) and radiation patterns. Dedicated test support equipment (TSE) has been developed and used to test the array radiation characteristics. Over the frequency range of operation, the array maintains good matching (ensuring system gain to noise temperature better than 8 dB/K) and good radiation performance while scanning as far as 60° in all the azimuth planes. The cross-polarization suppression is better than 15 dB within most of the scan range and reaches 10 dB for the extreme scan angles in the diagonal plane.

Wideband Dielectric Lens Antenna With Stable Radiation Patterns Fed by Coherent Array of Connected Leaky Slots

A broadband reflector feed is presented, which consists of a dielectric extended hemispherical lens fed by a connected array of leaky-wave slots. The slot elements are coherently combined to generate directive radiation patterns that mostly illuminate the central part of the lens, the most efficient one. The array is capable of producing secondary patterns with almost constant -10 dB beamwitdh over a 3:1 bandwidth. This allows efficient illumination of the reflector over a wide frequency range. Performance is estimated in terms of amplitude taper and phase error losses at the reflector, yielding efficiency of about 80% over the entire 3:1 bandwidth. Although the envisaged applications are in the terahertz (THz) and mm-wave frequency bands, a low-frequency prototype demonstrator has been tested in the 4 to 12 GHz band, for experimental validation of the concept.

The Lateral Wave Antenna

This paper introduces a quasi-planar antenna that provides, over a broad frequency bandwidth, highly directive radiation in one plane, phase center stability as a function of the frequency and high efficiency. The structure is composed by a dielectrically filled parallel plate waveguide that supports the propagation of a transverse electromagnetic (TEM) wave. The waveguide is truncated and shaped as a planar lens to achieve directive beams in the H-plane, while a short flaring is added at the waveguide open end to facilitate radiation into free space. High efficiency is achieved thanks to the peculiar feed structure that supports the propagation of directive lateral waves. An antenna prototype has been manufactured and measured. The results highlight the highest directivity for a quasi-planar broadband antenna (3:1 bandwidth) with very low dispersion and frequency-stable phase center.

Equivalent Transmission Line Models for the Analysis of Edge Effects in Finite Connected and Tightly Coupled Arrays

In this paper, we present an analysis of the edge effects in wideband connected arrays of slots and dipoles. Due to the strong mutual coupling between the elements, these arrays can support the propagation of guided waves along their surface. Such waves arise from the edges of the finite array and can be especially detrimental to the performance, since they can travel within the array without geometrical spreading. In this paper, we introduce Green’s function-based equivalent transmission line models to describe the propagation of the guided waves. The elements’ active impedances are represented as periodic loads on these transmission lines. The equivalent models can be used as a simple and convenient tool to control and minimize the edge effects. Finite array simulations of relevant array structures are discussed. The evidence is that the active impedance and the interelement capacitance can be tuned to attenuate and reflect the edge-born waves.