Radiation Q and Radiation Efficiency of Wideband Small Antennas and their Relation to Bandwidth and Cut-Off of Spherical Modes

Resonators are traditionally characterized by their quality factor Q, which is inversely proportional to the relative bandwidth. Small antennas are often resonant, so they can be characterized by a Q, except for the fact that the correct quality measure of an antenna is rather the inverse Q. i.e. the relative bandwidth. Still, it has become common to study fundamental size limitations of small antennas in terms of a so-called radiation Q (or antenna Q). The paper explains how this intrinsic radiation Q relates to: i) the bandwidth- efficiency product of small single-resonance-type antennas, ii) the gradual cut-off of spherical waves for wideband gradual-transition-type small antennas, and iii) the number of resonances needed to cover a certain frequency band for wideband multiple-resonances-type small antennas. The paper also introduces one intrinsic radiation Q for basic single TE and TM spherical mode sources, and another for combined TE and TM sources.