Volume 78, Issue 1-2

The distorted Born iterative method (DBIM) is a popular quantitative reconstruction algorithm for solving electromagnetic inverse scattering problems. These problems are non-linear and ill-posed. As a result, the efficiency of the method is limited by local minima. To overcome this, a correct initial guess solution is needed to obtain a satisfactory result. The U-Net based Convolutional Neural Network (CNN) is used in this study to make a good initial guess for the DBIM technique. The permittivity estimate produced at the output of U-Net is then refined using an existing iterative optimization process. This method’s findings are compared with the conventional DBIM approach. Strong scattering profiles of synthetic and experimental datasets with homogeneous and heterogeneous scatterers are investigated to validate the efficiency of the proposed technique. The results suggest that the use of the deep learning technique for an initial guess of DBIM improves accuracy and convergence rate significantly.

In this work, the design of broadband, wide-angle, direction-independent metamaterial (MM) electromagnetic wave (EM) absorber for K-band frequency application is investigated and validated experimentally. The unit cell of the metamaterial absorber consists of four 90° rotated L-shaped metallic patches imprinted on a dielectric substrate backed by a metallic sheet. The structure yield absorption in the broad frequency ranges from 22.5 to 29.3 GHz for both TE and TM polarized waves with more than 90 % absorptivity having a wide fractional bandwidth of (6.8 GHz) 25.8 %. The structure is four-fold symmetric and hence yields polarization insensitivity for different angles of polarization under both TE and TM polarized waves. The structure is also investigated under oblique incidence where the 80 % absorptivity holds up to 45° incident angles for both TE and TM waves. The absorption mechanism is explained with the help of top and bottom surface current distribution, induced electric field, and parametric analysis. To verify the resonance in the structure, characteristic mode, and equivalent circuit analysis have been carried out and presented. A prototype of the absorber has been fabricated and simulated results are validated with measured results. Measured results are showing good agreement with the simulated responses. The novelty of the proposed absorber lies in its unique metallic pattern on a λ 0 /8 (concerning the canter frequency of absorption bandwidth) thin FR-4 substrate while showing the wide absorption bandwidth and direction independence to normal and oblique incidence. The compact nature of the absorber and broadband response with good polarization insensitivity at normal and oblique incidence makes it commercially suitable for the reduction of radar cross section (RCS) in stealth applications at the K-band.

In this article, an angle independent Metamaterial Microwave Absorbers (MMA) is designed and demonstrated. The design consists of two concentric hexagon connected through consolidated resistors. The absorber is imprinted on a metal-backed low cost FR4 dielectric substrate with a thickness of 3.2 mm ( λ /0.07) and a dielectric constant of 4.3. The proposed structure exhibits a wide absorption bandwidth ranging from 2.8 to 10.42 GHz with absorptivity above 90 % covering S, C and X Band. In the area of interest, the current and electric field distribution has been examined, for two maximum peaks at a frequency of 3.66 and 9.54 GHz, with maximum absorptivity of 99.99 % and 99.44 % respectively. The presented absorber is examined under different polarization angles for phi and theta variation. The fabricated sheet consists of an array of a unit cell, which is examined inside the anechoic chamber with the help of two horn antennas and VNA. The tested and simulated results are compared and it was found that they are almost similar to each other with little variation due to fabrication tolerance. The presented absorber can be practically used for defence applications for Radar Cross Sections (RCS) reduction.

A Wilkinson power divider (PD), which can suppress the second, third, and fourth harmonics is proposed. The PD is implemented based on an asymmetric meta-structure. The meta-structure is constructed be combing defect ground structure (DGS) and spoof surface plasmon polariton (SSPP). It can not only realize the impedance conversion function in the operating frequency band, but also realize the suppression of the second, third, and fourth harmonics. The simulation and measurement results show that this structure can achieve −32 dB suppression for the second harmonic, −31 dB suppression for the third harmonic, and −28 dB suppression for the fourth harmonic respectively.

In the present work, a study of a metamaterial complementary split ring resonator (CSRR) microwave planar sensor for dielectric liquid characterization is carried out using HFSS software. The design consists in a CSRR-loaded two ports rectangular patch microstrip-fed grounded planar structure. The investigated liquid sample is put in a capillary glass tube lying parallel to the surface of the sensor. The liquid test tube is deposited horizontally parallel to the surface of the planar sensor. The advantage of the design lies on the fact that it allows different orientations and multiple size possibilities of the test tube. This makes it possible to explore different resonant frequencies in the 2.1 GHz frequency band. Moreover, an optimization study is carried out to achieve a high sensitivity and a high-quality factor of the proposed sensor. To better understand the operation and to further verify the feasibility of the equivalent circuit, a parallel RLC resonant circuit is used to obtain the desired Z parameter responses Z 11 , Z 22 , Z 21 . A T-shaped electrical model of the proposed sensing structure is established using Advanced Design System (ADS) software. This latter constitutes one of the principal novelties of this work, which has never been addressed so far.

This manuscript presents a novel aperture-coupled fed slotted dumbbell-shaped (SDS) dielectric resonator antenna (DRA) with enhanced bandwidth for 5G sub-6 GHz, short-range wireless communications, and C-band applications. The DRA is excited by HEM 11 δ mode. The proposed SDS-DRA is designed by assembling the four frustums of conical DRAs with the base and apex of one on top of another. The measured operating frequency of SDS-DRA is from 3.5 GHz to 9.6 GHz (impedance bandwidth of 93.1 %). The SDS-DRA shows a peak gain of 9.7 dBi in the operational frequency range, with an average peak gain of 6.02 dBi. The antenna’s average radiation efficiency is found to be 84.6 %. The proposed SDS-DRA produces omnidirectional-like radiation patterns with significantly low cross-polarization in the broadside direction. The size ( l  ×  w  ×  h ) of the proposed SDS-DRA is 1.55 λ g  × 1.55 λ g  × 0.67 λ g where, λ g is the guided wavelength in mm at 3.5 GHz for effective permittivity ( ϵ eff ), 7.1. To validate the simulation results, the proposed antenna structure is built, and its performance is measured. The findings of simulation and measurement results are found to be quite congruent.

In this paper, we introduce a novel design for a high-gain, low-profile quad-feed folded transmitarray antenna (FTA) to enable a more compact system. The proposed antenna system consists of a transmitarray, a reflectarray, and four identical planar microstrip U-slot patch antennas placed on the same surface of the reflectarray with an adjacent distance greater than 1 λ . To compare the effectiveness of our design, we developed three different antenna array systems with the same aperture size: a single-feed transmitarray antenna (TA) system, the proposed quad-feed FTA system, and a single-feed FTA system. Our experimental results demonstrate that employing four symmetrical feeders with an adjacent distance of 3.2 λ effectively reduces the height by about 76.7 % in comparison to the height of a single-feed TA, and by 30 % when compared to the height of a single-feed FTA. We also present the design, fabrication, and testing of a prototype of the proposed quad-feed FTA operating in the Ku-band. The measured results of the prototype confirm the effectiveness of our design.

The terahertz (THz) technology has fascinated lot of attention due to its enormous potential for a wide range of applications in the public, private and enterprise domains. This research work demonstrates the design structure and analysis of graphene based waveguide fed hybrid plasmonic THz patch antenna (HPTPA) constructed at around 3 THz. Hybrid plasmonic THz waveguide (HPTW) as a feeding line for the proposed THz patch antenna can increases antenna efficiency. The graphene is sandwiched between gallium arsenide (GaAs) and silver (Ag) to confine THz waves efficiently. Using mode analysis in finite element method for the proposed HPTW the propagation length, effective refractive index has been thoroughly examined. Based on the finite element method (FEM) approach, the results of the designed graphene-based waveguide fed HPTPA shows a high effective refractive index of 2.9, large propagation length of 230 µm, gain of 2.29 dBi, bandwidth of 200 GHz and efficiency of 85 % has obtained. The proposed graphene-based waveguide fed HPTPA could be beneficial to enable several photonic integrated circuit applications in next-generation wireless communication.