**2. Vivaldi antenna**

A Vivaldi antenna gives significant advantages of efficiency, high gain, wide bandwidth and simple geometry. The Vivaldi antenna, having an exponentially tapered slot profile, is a type of tapered slot antenna (TSA). Lewis et al. [1] introduced tapered slot antenna as a broadband strip line array element capable of multi octave bandwidths in his study in 1974. Vivaldi antenna, is an exponentially tapered slot antenna, was originated by Gibson [2]. These antennas operate in the frequency range from below 2 to above 40 GHz and offer significant gain and linear polarization.

Yngvesson et al. [3] compared three different TSAs, linearly tapered slot antenna (LTSA), constant width slot antenna (CWSA) and Gibson's exponentially tapered slot antenna, Vivaldi antenna.

Gazit [4] proposed two important changes to the traditional Vivaldi design. The use of a low dielectric substrate (cu clad, ε = 2.45) instead of alumina and an antipodal slot line transition. This type of transition offers relatively wider bandwidth but, antipodal slot line transition has high cross polarization problem.
