Maximum Gain, Effective Area, and Directivity

Fundamental bounds on antenna gain are found via convex optimization of the current density in a prescribed region. Various constraints are considered, including self-resonance and only partial control of the current distribution. Derived formulas are valid for arbitrarily shaped radiators of a given conductivity. All the optimization tasks are reduced to eigenvalue problems, which are solved efficiently. The second part of the paper deals with superdirectivity and its associated minimal costs in efficiency and Q-factor. The paper is accompanied with a series of examples practically demonstrating the relevance of the theoretical framework and entirely spanning wide range of material parameters and electrical sizes used in antenna technology. Presented results are analyzed from a perspective of effectively radiating modes. In contrast to a common approach utilizing spherical modes, the radiating modes of a given body are directly evaluated and analyzed here. All crucial mathematical steps are reviewed in the appendices, including a series of important subroutines to be considered making it possible to reduce the computational burden associated with the evaluation of electrically large structures and structures of high conductivity.Comment: 12 pages, 15 figures, submitted to TA

The spiral-pole antenna: An electrically small, resonant hybrid dipole with structural modification for inherent reactance cancellation

A small “spiralpole” antenna – the hybrid structure where one dipole wing is kept, but another wing is replaced by a coaxial single-arm spiral, is studied both theoretically and experimentally. Such a structure implies the implementation of an impedance-matching network (an inductor in series with a small dipole) directly as a part of the antenna body. The antenna impedance behavior thus resembles the impedance behavior of a small dipole in series with an extra inductance, which is that of the spiral. However, there are two improvements compared to the case when an equivalent small dipole is matched with an extra lumped inductor. First, the spiralpole antenna has a significantly larger radiation resistance – the radiation resistance increases by a factor of two or more. This is because the volume of the enclosing sphere is used more efficiently. Second, a potentially lower loss is expected since we only need a few turns of a greater radius. The radiation pattern of a small spiralpole antenna is that of a small dipole, so is the first (series) resonance. The Q-factor of the antenna has been verified against the standard curves. The antenna is convenient in construction and is appealing when used in conjunction with passive RFID tags such as SAW temperature sensors

Metamaterial-Inspired Efficient Electrically Small Antenna

Abstract—Planar two-dimensional (2D) and volumetric threedimensional (3D) metamaterial-inspired efficient electrically-small antennas that are easy to design; are easy and inexpensive to build; and are easy to test; are reported, i.e., the EZ antenna systems. The proposed 2D and 3D electrical- and magnetic-based EZ antennas are shown to be naturally matched to a 50 source, i.e., without the introduction of a matching network. It is demonstrated numerically that these EZ antennas have high radiation efficiencies with very good impedance matching between the source and the antenna and, hence, that they have high overall efficiencies. The reported 2D and 3D EZ antenna designs are linearly scalable to a wide range of frequencies and yet maintain their easy-to-build characteristics. Several versions of the 2D EZ antennas were fabricated and tested. The measurement results confirm the performance predictions. The EZ antennas systems may provide attractive alternatives to existing electrically-small antennas. Index Terms—Antenna efficiency, antennas, electrically small antenna (ESA), metamaterials. I

Doctor of Philosophy

dissertationThree-dimensional (3D) rapid prototyping holds significant promise for future antenna designs. Many complex designs that would be unmanufacturable or costly are realizable on a 3D printing machine. The ability to create 3D designs of virtually any configuration makes it possible to build compact antennas that can form fit to any space. These antennas build on the concept that small antennas can best reach the ideal operating limit when utilizing the entire 3D space in a sphere surrounding the antenna. Antennas require a combination of dielectric and conductive materials. 3D rapid prototyping is already well advanced for plastics and dielectric materials (with more options coming online). Prototyping with conductive materials has lagged behind; due mainly to their higher melting points, but this is advancing as well. This dissertation focuses on 3D rapid prototyping for antenna design. A 3D antenna made from small cubical cells is optimized for 2.4-3GHz using a genetic algorithm (GA). The antennas are built using 3D printing of plastic covered by conductive paint. The effects of the conductivity of the paint and number of layers on the resonance and gain of the antenna are evaluated. These results demonstrate the feasibility of using 3D rapid prototyping for antenna design