Antennas for ultra-wideband systems

Wide-band wireless systems are used in a wide range of applications including ground-penetrating radars, biomedical imaging systems, high-data-rate short range wireless local networks, communication systems and medium- and long-range military radars due to their high spatial and temporal resolution. As is the case in conventional wireless communication systems, an antenna plays a crucial role in wide-band systems. However, there are additional challenges when designing an antenna to operate over a wideband or an ultra-wide-band. In the recent past the interest of academia and industry developed in the Federal Communication Commission (FCC)-sanctioned UWB systems standard, which makes use of the spectrum from 3.1 GHz to 10.6 GHz.To utilise this spectrum fully, the challenges in designing a viable antenna are great. Antenna design involves achieving a 110% bandwidth, stable radiation patterns over the bandwidth, compact size and low manufacturing cost particularly for consumer electronics applications. In some important applications such as point-to-point high-speed data-communication systems, in addition to the above characteristics a small gain variation over the bandwidth may required. Several compact planar monopole antenna geometries have been investigated for such UWB applications. Most of these planar antennas have a significant gain variation (1 dBi to 5 dBi) at lower frequencies (3 GHz to 6 GHz) and then the gain becomes nearly constant. On the other hand some slot antennas have a relatively constant gain between 2 dBi and 3 dBi and only 1 dB variation in gain over the complete bandwidth. High-gain slot antennas have exhibited a stronger gain variation, between 2 dBi to 7 dBi. Indeed, gain enhancement is a challenging task for UWB systems where gain flatness across 3 GHz to 10 GHz is preferred. In this work, efforts have been made for enhancing the gain, and the gain variation with frequency, of printed monopole antennas. Extensive investigations were also carried out on different types of concepts in enhancing the gain of UWB antennas. The types of antenna studied in this thesis are based on monopole and slot antennas. A concept of proximity coupling has been proposed for the printed monopoles and it has been demonstrated with practical implementation and testing. This concept helps to enhance the angular stability of the printed monopoles. Based on the understanding of UWB antennas, two more compact versions of TEM horns are proposed. Their integration with printed-circuit boards has been studied. Both of them are fed using printed monopoles; one fed by a circular monopole and the the other by a co-planar waveguide (CPW)-based semicircular monopole. Short horn antennas designed in this thesis are comparable to conformal or integrated antennas printed on the same substrate. Apart from enhancing the gain performance of antennas, significant work has been carried out in the field of Frequency Selective Surfaces (FSS). The behaviour of FSS-based reflectors in enhancing the gain while maintaining the operating bandwidth has been demonstrated for UWB applications. A concept of phase coherence over the UWB bandwidth has been proposed in this work and it is demonstrated with some practical examples of antenna design and implementation. These multi-layer FSS reflector surfaces help to achieve a good gain enhancement in terms of flatness and directional radiation properties over the UWB bandwidth. A gain flatness of ±0.5 dB has been demonstrated over a 110 % impedance bandwidth.