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The theory of characteristic modes is proposed for the analysis of simple band-notched UWB planar monopole antennas, since the information provided by these real modes (i.e. resonant frequency and radiating bandwidth) provides a very interesting physical insight into the radiation phenomena taking place in this type of antennas. Such an in-depth understanding paves the way for the proposal of novel designs of UWB antennas with increased functionality, through the control of the excitation and resonance of specific modes. Modal analysis of band-notched UWB antennas illustrates the existence of resonant modes linked to the embedded narrowband slot structure (slot modes), whose location within the planar geometry of the monopole determines the effect over the rest of the radiating modes of the wideband structure, and consequently, over the behavior of the antenna. By electronically controlling the excitation of the first slot mode, an UWB antenna with switchable band-notched behavior is proposed. Similarly, a tunable band-notched UWB antenna can be proposed by controlling the resonance of this slot mode. Prototypes for both the switchable and tunable band-notched antenna have been fabricated and measured, demonstrating the expected behavior. The understanding of the radiating behavior of this type of antennas can be easily extrapolated to other configurations, such as coplanar band-notched UWB antennas, which are more compact than the designs presented here, and may help to propose other antennas of this type.