Among their diverse uses, Vivaldi antennas have been utilized in Ultra Wideband (UWB) designs and in less traditional fields, such as microwave imaging. Yet little work has delved into the theoretical aspects of Vivaldi antenna design and synthesis. To fill this gap, Marco Chiappe and Gian Luigi Gragnani of Italy's University of Genoa studied the geometrical and electrical features of the frequency-independent antenna theory.
The researchers derived a scaling principle for the exponential tapering of the Vivaldi antenna. They also produced a closed-form model for the current distribution. To obtain the correct phase variation, the current propagation was modeled along the exponential curvilinear coordinate. The closed-form model that was proposed may have use in further works aimed at attaining an easier and faster analytic formulation of an antenna's radiation parameters.
To make the design of the antennas more systematic, a practical feeding system—based on a double-Y balun—was developed and tested. The second part of the paper is devoted to this feeding system. Because the Vivaldi antenna requires a balanced feeding, a balun is required. This balun has to be frequency independent for at least two octaves. It is quite difficult to design wideband baluns, however. It also was assumed that the antenna had to be as simple as possible and very inexpensive. As a result, a single-sided printed antenna was used. When testing a possible solution to improve bandwidth, the double-Y balun was chosen to provide the transition between the antenna slotline and a coplanar waveguide. Being an all-pass network, this type of balun should provide an infinite band of operation. See "Vivaldi Antennas for Microwave Imaging: Theoretical Analysis and Design Considerations," IEEE Transactions on Instrumentation and Measurement, December 2006, Vol. 55, No. 6, p. 1885.
