A frequency agile Vivaldi antenna whose operating frequency band can be switched between two selected bands is
proposed in this paper for spectrum monitoring and cognitive radio applications. A RF switch is introduced into the back-slot of a
Vivaldi antenna to allow switching of the operational band. The realised gains of the antenna are 10.5 dBi in the low band around
3.1 GHz, and 12 dBi in high band around 4.1 GHz. The radiation pattern is stable and its direction is consistent across the two
bands. This design can be applied to multiple reconfigurable bands by using more RF switches to tune the desired operating
frequency. A set of reliable design equations has been provided as well. This reconfigurable antenna offers improved gain and
isolation over multiple, wideband and multiband antennas without increasing the cost and size when compared to those designs
reported