In this paper we derive that a Vivaldi antenna behaves much the same as a TEM horn. However a Vivaldi antenna is built on a print circuit board, while the TEM horn is basically a 3D antenna. Hence a Vivaldi antenna can be easily manufactured and has more solid mechanical structure than a TEM horn antenna. We can say that we have found a new way to build a simple structure TEM horn antenna. We have used the method of designing a TEM horn antenna to design a Vivaldi antenna. The test results show that the designed antenna, which has the smallest opening at the antenna end, has a better performance than the other Vivaldi antennas tested in respect of the received signal magnitude.