What happens as the Q of an antenna increases?

The Q, or Quality Factor, of an antenna represents the ratio of energy stored in its reactive fields to the energy dissipated (radiated or lost as heat) per cycle. It is a direct measure of how sharply resonant the antenna system is. In any resonant circuit, including antennas, Q and bandwidth are inversely related. Therefore, as the Q of an antenna increases, its resonance curve becomes sharper. This means the frequency range over which the SWR remains acceptably low (the SWR bandwidth) decreases. Higher Q does not intrinsically reduce gain or increase common-mode current. It simply narrows the usable frequency window before the SWR rises to an unacceptable level, which is why high-Q antennas often require frequent retuning when changing frequencies.