Below is an image of a Vivaldi antenna that we grabbed from U. S. patent 6,043,785, Broadband fixed-radius slot antenna arrangement invented by Ronald A. Marino. This figure was provided as "prior art", and provides a good picture of how the antenna works. A microstrip line (dotted) on the opposite site crosses a slot line. The microstrip line is terminated in an open circuit a quarterwave away, while the slot-line is terminated in a short circuit, one quarter-wave away. The effect of this interface is that the energy in the microstrip is fed to the slot line, and propagates to the right. The slot line gap opens up according to an equation which we will ignore for now.