Thesis title: Wideband and Ultra-Wideband Antennas for Wireless Communications
The recent progress of wireless communication technologies is driving the development and design of increasingly innovative antennas. In fact, modern communication systems have to satisfy increasingly stringent requirements such as higher data rates, reduced system complexity, compact size, low latency and path loss, and stable radiation pattern. To meet these requirements, the development of multiband, broadband and ultra-wideband (UWB) antennas, characterized by high gain, small size and excellent radiative properties both in frequency and time domain, may be required. The main applications of modern wireless technology concern the mobile communications, the implementation of local and personal area networks, the monitoring of people’s vital biological parameters, the realization of sensor networks for the safety of the environment, things and people, the realization of radars for special applications. To meet the requirements of each application it is essential to identify the most appropriate design strategy. To this end, the research activity has been finalized at identifying new antenna geometries and design strategies suitable for improving the performance of broadband and ultra-wideband antennas useful to operate both in frequency and time domain. An in-depth investigation concerning the state of the art of high-gain wideband antennas for wireless communications, radar applications, etc., has allowed to identify in the class of Vivaldi and dielectric resonator antennas (DRAs) two research areas suitable to satisfy the aforementioned requirements. In the first phase of the research activity, a new Vivaldi geometry operating in linear (horizontal/vertical) and circular polarization in the 650 MHz and 6 GHz frequency band, whose characteristics has been identified with the aim of satisfying the requirements of the Wireless Communications and of the Through-the-Wall Imaging systems, was developed. To this end, new techniques for widening the operating band and increasing and equalizing the antenna gain, as well as a new dielectric lens named spherical–axicon lens which has been integrated into the radiant structure to further increase the gain and compact the antenna size, have been introduced. Further studies concerned the development of a new high-gain dielectric resonator antenna excited by a suitable wideband slot coupled to a microstrip line. This radiating system works from 3 to 12.4 GHz and it is useful for wireless, UWB short-range communications, meteorological and satellite applications. Both designs feature a dielectric lens to improve gain in the band of interest and other performance-enhancing elements useful to reduce side lobe level, and increase the front-to-back ratio. Furthermore, since in air and space applications as well as in through imaging applications, the weight of the radiating system could represent a limit in practical applications, a research activity aimed at identifying lightweight dielectric lenses suitable for integration in high gain broadband antennas was carried out. This new class of lenses is characterized by low manufacturing costs, and excellent radiative performances both in the frequency and time domain. Finally, using an accurate approach based on the incomplete Hankel functions electromagnetic structures consisting of thin truncated cylinders used in radiating systems, like planar composite lenses useful to improve antenna directivity, have been analyzed.