A HYBRID RF PHOTONICS system, which utilizes optical links and an RF transducer at the antenna, may reportedly provide data transmission with a bandwidth beyond 100 GHz. Many are therefore eying an ultra-wideband (UWB) antenna array as a target application. Such an array requires an RF aperture or antenna to be integrated with an opto-electronic circuit in both a simple and compact manner. At the University of Delaware, Jian Bai, Shouyuan Shi, and Dennis W. Prather have suggested a Vivaldi antenna for such a design.
Generally, the bandwidth of Vivaldi antennas is in proportion to their length and aperture. To keep their antenna compact, the engineers altered traditional Vivaldi antennas by introducing a loading structureeither a circular shape or slot loadto match the termination of a traditional Vivaldi antenna. The physical mechanisms of the two loads, which allow the antenna to radiate below the cutoff frequency, are different. Yet both antennas can compensate for the restriction on antenna length and aperture.
Both antennas offered UWB performance beyond 46 GHz. The slot-loaded antenna actually is capable of wider bandwidth. The slot length and width simply have to be manipulated to allow a lower-frequency resonance to be merged. While the circular-shape-loaded Vivaldi antenna is characterized by lower return loss, the slot-loaded version exhibited almost 2 GHz wider impedance bandwidth with typical return loss below -10 dB (except at a few frequencies). A demonstration antenna had an impedance bandwidth beyond 25:1. From 4 to 50 GHz, it offered gain of 3 to 12 dBi. It also exhibited symmetric radiation patterns in the E- and H-planes. See "Modified Compact Antipodal Vivaldi Antenna for 4-50-GHz UWB Application," IEEE Transactions On Microwave Theory And Techniques, April 2011, p. 1051.