Find the loaded Q and fractional bandwidth of a resonant antenna or circuit from its centre frequency and bandwidth.
Q is the ratio of centre frequency to bandwidth: Q = f0 / bandwidth, where bandwidth is usually measured between the 2:1 SWR points. A high Q means a narrow, sharply tuned antenna; a low Q means a broad one that covers more of the band without retuning.
Q (quality factor) is the centre frequency divided by the bandwidth: Q = f0 / BW. It describes how sharply tuned the antenna is. A short, loaded antenna has a high Q and narrow bandwidth; a full-size or fat-element antenna has a lower Q and wider bandwidth.
For antennas it is usually the width between the points where the SWR rises to 2:1. So a 20m antenna centred on 14.2 MHz with 2:1 points at 14.0 and 14.4 MHz has a 0.4 MHz bandwidth and a Q of about 35.
A high-Q antenna needs retuning as you move across a band and is more easily detuned by its surroundings, but it rejects out-of-band signals better. Lower Q gives easy broadband operation at the cost of some selectivity.