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In traditional approaches to antenna characterisation, spectral and angular dispersion are modelled separately. In the paper, the dependence of the power radiated from ultra wideband (UWB) antennas jointly on frequency and direction is established experimentally. It is demonstrated that both omni-directional and directional antennas typically exhibit higher directivity with frequency, with the former experiencing a seven-fold increase with respect to the lowest frequency in the FCC UWB frequency range (3.1-10.6 GHz). The consequences of this behaviour are highlighted and the effect on the communications link is quantified. The effective available bandwidth of a system is found to be highly sensitive to the angular disposition of the antenna at each end of the link. It is shown that the bandwidth is severely limited in some directions, and careful orientation of the antennas is required to achieve full UWB operation for a range of antenna designs. The 10 dB bandwidth of a vertically polarised UWB antenna can be as little as 2 GHz in the equatorial plane when the joint dispersion is considered. The distortion caused to a UWB signal is investigated and significant variation in the radiated signal waveform with angle is demonstrated.