TO FIND GAPS in the finite frequency spectrum and then alter its characteristics to operate within those bands, a cognitive-radio system requires a transmit/receive antenna that can monitor broad bandwidths and be reconfigured as necessary. An ultra-wideband (UWB) sensing antenna continuously monitors the wireless channel in its search for unused carrier frequencies. In contrast, a reconfigurable transmit/receive antenna provides traditional transmission and reception functions. Both of these antennas have been incorporated onto the same substrate by Y. Tawk and C.G. Christodoulou from the University of New Mexico, J. Constantine from California State University Fullerton, and K. Avery from the US Air Force Research Laboratory.
While the UWB antenna "scans" the spectrum to discover white-space frequency bands from 2 to 10 GHz, the reconfigurable section is tuned to communicate within those bands. Reconfigurability is attained by switching among different antenna patches. Such frequency agility is achieved with the aid of the rotational motion of the antenna patch, which is controlled by a stepper motor mounted on the back of the antenna structure. Each patch operates at a different frequency band when fed by a stripline transmission line, which excites each particular shape upon rotation. See "Implementation of a Cognitive Radio Front-End Using Rotatable Controlled Reconfigurable Antennas," IEEE Transactions On Antennas And Propagation, May 2011, p. 1773.
