Although a resonant antenna has a purely resistive feed-point impedance at a particular frequency, many (if not most) applications require using an antenna over a range of frequencies. An antenna's <answer>bandwidth</answer> specifies the range of frequencies over which its performance does not suffer due to a poor impedance match. Also in the case of a Yagi-Uda array, the use of the antenna very far away from its design frequency reduces the antenna's directivity, thus reducing the usable <answer>bandwidth</answer> regardless of impedance matching.
What term can be used to refer to the usable spectrum of an antennas frequency?
bandwidth