Introduction

At microwaves and RFs, antennas are fundamental devices for wireless communication systems, and they are found around us in everyday use probably more often than we even realize. In our homes and offices we can probably count tens of antennas operating in the same environment, each capable of transmitting and receiving wireless radio signals at different frequencies for a variety of purposes. The Latin word antenna was commonly used well before the discovery of electromagnetic radiation to describe the long stylus on a ship connected to the sail, the sensing appendage of several arthropods in the animal world, as well as the central pole of a tent. For electromagnetic radiation, the term was introduced by the Italian radio-wave pioneer Guglielmo Marconi to describe the vertical pole he was using as the apparatus capable of transmitting and receiving wireless electromagnetic signals at a distance. In general, an antenna is designed as an efficient transducer to convert electromagnetic waves freely propagating in free-space into confined electric signals, and vice versa. From the first attempts to create such a bridge to modern antenna technology, more than a century has passed and antenna technology has evolved tremendously. Nowadays, microwave antenna designers have a variety of powerful tools in order to match the requirements of the specific application of interest.

Recent progress in nanofabrication technology allows the possibility of realizing metallic NPs of arbitrary shape that may provide strong scattering resonances associated with the plasmonic features of metal at optical frequencies.