Radiation of exponentially tapered coplanar stripline antennas used in coherent microwave transient spectroscopy technique

With the advent of reliable sources of ultrashort optical pulses, photoconductive switches, photoconductive sampling have been turned into methods that allow one to generate and to detect ultra-fast electric pulses. Experiments have shown that commuting devices of optoelectronic pulses associated with appropriate antennas can be used to generate, control and detect picosecond bursts of electromagnetic radiation. It leads to various applications such as using the transient radiation properties of antennas for the characterization of materials by coherent microwave transient spectroscopy. This paper deals with an electromagnetic analysis of these exponentially tapered coplanar stripline antennas. In particular, the purpose of this electromagnetic study is to understand the radiation mechanism of this antenna type and to optimize it. For this analysis and for the considered frequency bands, the finite difference time domain (FDTD) method is a suitable method because it takes all the propagation and radiation modes into account. The FDTD method based on a discrete form of Maxwell´s equations leads directly to the time evolution of the radiated field.