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The presence nearby of a parallel metallic plane usually impairs the radio-electric performance of planar ultrawideband (UWB) elements such as monopole and slot antennas. Two comparative approaches to this problem are studied, in the frequency and time domains. Conservation of the UWB performance of a slot antenna is investigated in order to obtain a mono-directional quasi-planar element suitable for an UWB array. First, the addition of a planar reflector is studied: up to a certain distance, wide impedance bandwidth is maintained, but the antenna offers a non-uniform gain. Secondly, the addition of a cavity overcomes this disadvantage and makes the cavity-backed slot antenna one of the best candidates for an UWB quasi-planar array.