In the context of narrowband, concepts of antennas are usually only defined in the frequency domain because their parameters present little variation through the operational band of interest. As an example, let us recall the discussion of the central frequency in Section 4.2, where it was found that both the upper and the lower cut-off frequencies are very close for a rectangular patch antenna. Nevertheless, when the antenna bandwidth is broadened, some antenna responses could significantly vary (see Figure 4.1 of Chapter 4, for instance). Thus, any variation in the frequency of the signal has an impact on it in time. Hence it is important to analyze the antenna response not only in the frequency domain, but also in the time domain.