Small Antennas: Downsizing Techniques and Its Index Factor

In this paper, we present the classification of small antennas based on statistical data. The three categories of downsizing methods are loading a matching circuit, changing the current path, and using dielectric/magnetic materials. These categories are explained using several examples. In this paper, we show that the miminum Q value as a fundamental limit defined by an infinitesimal dipole is effective for determining the index factor of small antennas. Radiation efficiency measurements for small antennas are also discussed.