This work aims to provide guidelines on the design of wideband flat lenses based on artificial dielectric layers (ADLs). Planar lenses based on metasurfaces are typically narrowband, due to the phase wrapping over the period of 2\pi that is strongly frequency-dependent. On the contrary, true-time-delay (TTD) planar lenses, which do not resort to phase discontinuities, can achieve large bandwidths. One convenient way to design wideband TTD lenses is by means of ADLs, which are stacks of subwavelength-period patch arrays embedded in a host medium to increase its effective permittivity. Tradeoffs including bandwidth, focal ratio, lens diameter, and thickness are discussed and related to the manufacturing constraints of artificial dielectrics, such as the smallest features realizable in printed circuit board (PCB) technology, which define the maximum achievable effective permittivity. An example of design is also presented, operating from 30 to 60 GHz and experimentally validated.

We present an analytical model to describe arrays of connected slots fed by parallel plate waveguides (PPWs). Connected slot arrays are planar ultra-wideband arrays with wide scanning capability. PPW feeds can be used to reduce the complexity of the unit cell design. However, existing analytical expressions of the active input impedance of the array cannot account for the presence of PPWs. Here, we develop a new model that includes PPW structures in the stratification, enabling the optimization of the design together with the feed. An equivalent circuit of the unit cell is derived, where the PPW sections are represented in terms of equivalent transmission lines for each Floquet mode. Closed-form expressions are also derived for the capacitance associated with step discontinuities of the PPW and the inductance associated with the feed. Full-wave simulations are used to validate the model.

A semi-analytical method is presented for the design of gradient index (GRIN) flat lenses. Closed-form expressions are derived to define the refractive index distribution of the lens, for several cases: collimating lenses with on-axis feed, collimating lenses with off-axis feed, lenses converting spherical wavefronts with different wavenumbers, lenses changing the focal number of a quasi-optical system, and Fresnel zone lenses. The design equations are validated by ray-tracing simulations in inhomogeneous media, implemented by numerical solution of the Eikonal equation.

We present a dual-polarized connected array of slots with an artificial dielectric layer (ADL) radome for mobile communication applications operating in the sub-6 GHz and the upper 6 GHz bands of 5G. The radiating slots are combined with two interchangeable ADL radomes with different thicknesses, targeting the bands 6-8 and 2-8 GHz, respectively. This highlights the main property of the ADL radome, which realizes an impedance transformer whose bandwidth is proportional to the height of the structure. Moreover, the ADL anisotropy allows for wide scanning, up to 60° in the main planes for both radomes, without scan blindness. An $8\,\, \times \,\, 8$ prototype array has been manufactured and tested with the two ADL radomes. The measured results of the active voltage standing wave ratio (VSWR) and the radiation patterns are reported to validate the design.

The characteristic cross-polarization (X-pol) of wide angle impedance matching (WAIM) structures is investigated. The study considers an ideal linearly polarized current sheet in the presence of various dielectric and artificial dielectric superstrates, analyzed using transmission line models representing the stratified media. The main mechanism that causes increased X-pol is highlighted and linked to the anisotropy of the superstrate. We then propose an approach to reduce the X-pol by including vertical vias within the WAIM dielectrics, to control the vertical component of the permittivity tensor. The intrinsic X-pol performance of a set of artificial dielectric layers (ADLs) with and without vias is experimentally verified by placing the WAIM above an open-ended waveguide that acts as a linearly polarized source. The proposed WAIM with vias can be used in wideband wide-scanning array designs to improve polarization purity.

In this work, we investigate antenna architectures to implement dual-mode operation in phased array designs. Planar slot antenna elements are used in array configuration, in combination with artificial dielectrics layers (ADLs) located in the close proximity of the array, to achieve pattern shaping. The artificial dielectric superstrate supports the propagation of leaky waves that can be optimized to enhance the gain in a specific angular region or to enlarge the array field of view. By controlling the amplitude and phase of the antenna elements, the radiation patterns can be combined to realize either wide or narrow beams. This concept present advantages for both millimeter-wave (mm-wave) communication and radar applications. A design of a four-element array fabricated in standard printed circuit board (PCB) technology validates the feasibility of the dual-mode operation. The measured results also show good agreement with simulations.

Artificial Dielectric Layers (ADLs) have recently been exploited to improve the radiation and impedance performance of integrated antennas at millimeter wave (mmWave) and terahertz (THz) frequencies. The ADLs are composed by layers of sub-wavelength periodic metal patches that can be arranged within a host medium to synthesize an equivalent anisotropic material. Thanks to the availability of closed-form expressions for the modeling, ADLs can be conveniently designed to realize matching layers and impedance transformers when used in the closed proximity of antennas, to improve their bandwidth and the front-to-back ratio. An overview of different applications that benefit from this concept is given. Moreover, recent developments on the use of ADLs for wideband flat lenses are described.

Low-Earth orbit (LEO) constellations are revolutionizing the world of satellite communication (Satcom), providing new opportunities to manufacturers and operators and enabling innovative and attractive services to users.

This article presents the development of a focal plane array (FPA) for terahertz imaging applications with a near diffraction-limited resolution achieved through a very tight sampling of the focal plane. The antenna array is integrated with direct detectors in a 22-nm CMOS technology and operates from 200 to 600 GHz. The tight sampling of the focal plane is realized by using a combination of leaky-wave radiation and a dual-polarized connected array configuration that closely resembles a chessboard. By utilizing both the polarizations in the chessboard design, the number of array elements per unit area is effectively doubled. The geometry of the chessboard array was co-optimized together with that of a silicon elliptical lens to achieve both high aperture efficiency and beam overlap. Measurements in the WR2.2 band of a fabricated demonstrator showed that an aperture efficiency of −4.1 dB was realized at 400 GHz. The average gain roll-off between two diagonally adjacent array elements was measured to be −1.5 dB at 400 GHz. Compared to the reference configuration of an idealized, equivalently sampled hexagonal FPA, the improvement in gain at the edge of coverage yields 1.2 dB, which includes 1.9 dB of ohmic losses in the chessboard array. The agreement between measurements and simulations proved to be within 1 dB from 325 to 475 GHz.

In this study, an efficient power combiner for mm-wave frequency transmitters is investigated. The combiner is based on a parallel plate waveguide (PPW) excited with multiple parallel feeds. The Doherty power combiner scheme is also integrated in the proposed concept, to increase the efficiency of the amplifiers when implementing amplitude modulation. The advantage of the proposed PPW combiner with respect to other concepts, for example, the ones based on substrate-integrated waveguide, is the wider bandwidth and the scalability to an arbitrary number of inputs. Measured results from a demonstrator realised in standard printed circuit board technology are presented. Two variations of the combiner are implemented, one terminated with a 50 Ω coaxial output, and another integrated with an antenna. In the latter case, the waveguide is folded so that both the power combiner and the antenna fit within a half wavelength size, and thus would be compatible with a dense antenna array implementation.