As an integral part of the electromagnetic system, antennas are becoming more advanced and versatile than ever before, thus making it necessary to adopt new techniques to enhance their performance. Machine Learning (ML), a branch of artificial intelligence, is a method of data analysis that automates analytical model building with minimal human intervention. The potential for ML to solve unpredictable and non-linear complex challenges is attracting researchers in the field of electromagnetics (EM), especially in antenna and antenna-based systems. Numerous antenna simulations, synthesis, and pattern recognition of radiations as well as non-linear inverse scattering-based object identifications are now leveraging ML techniques. Although the accuracy of ML algorithms depends on the availability of sufficient data and expert handling of the model and hyperparameters, it is gradually becoming the desired solution when researchers are aiming for a cost-effective solution without excessive time consumption. In this context, this paper aims to present an overview of machine learning, and its applications in Electromagnetics, including communication, radar, and sensing. It extensively discusses recent research progress in the development and use of intelligent algorithms for antenna design, synthesis and analysis, electromagnetic inverse scattering, synthetic aperture radar target recognition, and fault detection systems. It also provides limitations of this emerging field of study. The unique aspect of this work is that it surveys the state-of the art and recent advances in ML techniques as applied to EM.

A new approach to distributed synchronization (frequency alignment) for the coordination of nodes in open loop coherent distributed antenna arrays to enable distributed beamforming is presented. This approach makes use of the concept of consensus optimization among nodes without requiring centralized control. Decentralized frequency consensus can be achieved through iterative frequency exchange among nodes. We derive a model of the signal received from a coherent distributed array and analyze the effects on beamforming of phase errors induced by oscillator frequency drift. We introduce and discuss the average consensus protocol for frequency transfer in undirected networks where each node transmits and receives frequency information from other nodes. We analyze the following cases: 1) undirected networks with a static topology; 2) undirected networks with a dynamic topology, where connections between nodes are made and lost dynamically; and 3) undirected networks with oscillator frequency drift. We show that all the nodes in a given network achieve average consensus and the number of iterations needed to achieve consensus can be minimized for a given cluster of nodes. We derive the theoretical phase error resulting from both processing noise and measurement noise, and derive an optimal update interval that minimizes the error. Numerical simulations demonstrate that the consensus algorithm enables tolerable errors to obtain high coherent gain of greater than 90% of the ideal gain in an error-free distributed phased array.

A novel guiding channel that is electromagnetic bandgap (EBG) based is presented. The structure is formed in a two‐dimensional slab of a square lattice of multi‐layer metallic open square rings. Dispersion diagrams and eigenfield distributions of the in‐gap‐guided mode are shown. The channel guide offers many advantages such as easy and flexible design for band‐select applications while retaining its electromagnetic properties, simple fabrication (no drilling through the substrate) and compatibility with integrated circuit technology. Two EBG waveguide configurations with a feeding structure on a circuit board (PCB) were designed, fabricated and tested. It is shown experimentally that the length of the EBG guide does not impact the insertion loss significantly, which establishes low insertion loss of the guide.

This paper presents a novel electromagnetic bandgap (EBG) structure composed of stacked U-shaped 3-D lattice of metallic open square rings and its potential application to selected waveguide configurations. Five key geometrical parameters are shown to impact the bandgap and modal structure of the EBG. Using this insight, we demonstrate two different configurations for line defect that realize guiding through the structure at Ku-band. The proposed EBG structure overcomes the fabrication complexity of conventional 2-D EBGs that are composed of dielectric rods or holes in a dielectric material. Selected EBG waveguide configurations with transitions to conventional microwave guiding structures are fabricated and tested.

A dielectric-based multilayer structure composed of U-shaped rings (ML-UR) is used to develop a class of novel electromagnetic band gap (EBG) slab waveguide. The structure has two band gaps that narrow down as dielectric constant is increased. The EBG slab waveguide is created by embedding a single-layer line defect inside the 3D crystal of the EBG slab guide. Unlike our previously published foam-based EBG structure, the use of dielectric spacer in the EBG waveguides offers significant advantages in terms of overall size, structure reliability, and design flexibility. The waveguide structures reported in this paper are designed to operate at X-band (8–12 GHz) while being fed by coplanar-slotline transitions. Prototypes were fabricated and characterized experimentally. The insertion loss decreases by decreasing the number of full lattices of ML-UR surrounding the channels. The proposed waveguide has potential in microwave components such as directional couplers, phase shifters, and antenna array feeding networks.

This article examines the effect of multipath channels on the performance of distributed arrays that employ time reversal. A model of the signal received from a distributed array is formulated, and a statistical analysis of the variation in signal power in the presence of phase noise and multipath is given. We present the impact these nonidealities have on received signal power, and we analyze the received power for three specific cases: continuous waveform, impulse waveform, and modulated rectangular pulse waveform in the presence of standard channel models. It is shown that for larger arrays in multipath channels, the change in power between coherent and incoherent states converges to the line-of-sight channel. It is further shown that in a line-of-sight channel time-reversal completely cancels unknown channel delays resulting in coherent signals from all nodes in a distributed array, while in a multipath channel only the main diagonal round-trip paths are coherent. Nevertheless, this additional benefit improves signal coherence in complex channels and can aide in distributed array synchronization using two-way time transfer.