This thesis investigates omnidirectional and directional ultra wideband (UWB) antennas for communication and microwave imaging applications. To reduce interference with existing technologies, monopole antennas with efficient band-stop functions are introduced. Single and double slots acting as series resonators are used. Reduction in the antenna gain in the stop-band regions of about 19.5 dB is achieved. Central metal removal and ground plane size effects on the antenna performance are investigated.

To eliminate signal distortion caused by such monopole antennas, phase centre behaviour over the entire frequency band of operation is investigated at different principle planes, which have not been done before. This study will also show how these antennas act in different communication scenarios and where the radiation will be coming from at different frequencies. The effect of including different slots with different shapes on the performance of phase centre of these antennas is also investigated. Different methods to minimize the antenna phase centre movement are studied.

Novel microstrip antennas with UWB impedance and radiation pattern bandwidth and low cross polarization components are introduced to work over the frequency band from 3 to 20 GHz. The antennas introduced are double-layer structures in which the radiator is sandwiched between two identical partial ground planes or a partial ground plane is sandwiched between two radiators. Results show a significant reduction in the cross polarization components at all frequencies. A novel high gain UWB Vee dipole antenna with a UWB coaxial balun feed is introduced to cover the existing and future UWB communication applications. Different type of loadings such as a reflecting ground below the antenna, a dielectric sleeve over the UWB balun and conical dielectrics between the Vee plates are also used and studied that show enhanced gains and lower sidelobes. A miniaturized-type UWB Vee dipole antenna is also investigated for microwave imaging applications. The antenna has a small radiation aperture which makes it a good candidate for array type applications. Full wave analysis of studied antennas are done using Ansoft HFSS, finite-element-methods based software. Experimental investigations are done to confirm the accuracy of simulated results.