Transient Characteristics of Small Antennas

As the size of an antenna decreases, its Q factor increases. In this paper, we focus on the transient aspects of high-Q radiators and present a comprehensive investigation on the time-domain behavior of resonant-type antennas. The concept of energy storage and its connection with size and Q factor is described by means of an equivalent circuit of the radial-wave impedance for spherical modes. The impact of antenna structure on the field response is studied by analyzing the transient radiation of the planar inverted-F antenna as a typical example of small antennas. Transient mismatch characteristics due to the high amount of stored energy are observed by evaluating the time-domain reflection at the input terminal. Without losing generality, the transient behavior of high-Q antennas can be mimicked by a simple RLC resonator. We show that time-domain characteristics of the fundamental radiated mode are very well matched to that of a high-Q resonator with the same Q and resonant frequency. Early-time response to the Gaussian pulse in the near and far field is closely observed, and its correlation with the exciting pulse is studied. To investigate the effect of switching on the time-domain fields, antennas are fed with a sinusoidal-modulated pulse signal and it is shown that the Q factor and time constant of the fields can be controlled by switching to different source impedances during the off state of the input pulse.