Fundamental Bounds on Performance of Periodic Electromagnetic Radiators and Scatterers
Time: Fri 2020-02-07 13.00
Subject area: Electrical Engineering
Doctoral student: Andrei Ludvig-Osipov , Elektroteknisk teori och konstruktion
Opponent: Professor Christophe Caloz, KU Leuven, Department of Electrical Engineering (ESAT)
Supervisor: Professor B. Lars G. Jonsson, Teoretisk elektroteknik, Elektrotekniska system, Elektroteknisk teori och konstruktion
In this thesis, the optimal bandwidth performance of periodic electromagnetic radiators and scatterers is studied. The main focus is on the development and application of methods to obtain fundamental physical bounds, relating geometrical parameters, frequency bandwidth, efficiency and radiation characteristics of periodic electromagnetic structures.
Increasing demand on the performance of wireless electromagnetic systems in the modern world requires miniaturization, high data rates, high efficiency, and reliability in harsh electromagnetic environments. Attempts to improve all these design metrics at once confront the inevitable physical limitations. For example, an antenna’s size is fundamentally bounded with bandwidth performance, and attempts to decrease size result in reduced performance capabilities. Knowledge of such physical bounds is vital to achieve high performance: to gain an understanding of the trade-off between parameters and requirements, or to evaluate how optimal the realized design is.
Periodic structures are indispensable components in many wireless systems. As antenna arrays, they are in base stations of mobile phone networks, in radio astronomy, in navigation systems. As functional structures, they are used as frequency-selective filters, polarizers and metamaterials.
In this thesis, methods to construct fundamental bounds on Q-factor – a quantity inversely proportional to bandwidth – are presented for periodic structures. First, the Q-factor representation is derived in terms of the electric current density in a unit cell. Then, the bounds are obtained by minimizing the Q-factor over all current densities, that are supported in a specified spatial subset of a unit cell, with possibly additional constraints (e.g. on conductive losses, or on polarization) imposed.
Moreover, an alternative approach for obtaining fundamental bandwidth bounds is investigated – the sum rules, that are based on representing a physical phenomenon as a passive input-output system. Transmission of a plane wave through a periodically perforated metal screen is described by a passive system, and the sum rule bounds the transmission bandwidth with the static polarizability of the unit cell. Such a bound is shown to be tight for simulated and measured perforated screens.