Near-field Measurement of Gaussian Beam behind Dielectric Lens

A horn antenna with the gain equal to 20 dB at the frequency of 40 GHz has far-field in the distance of 0.4 m for the plane wave radiation and has a very wide beam-width. In case of adding and using of dielectric lens, the plane wave is created at the distance of 92 mm with the Gaussian amplitude distribution and focused into the very narrow beam. This energy focusing is used in the microwave, millimeter-wave and terahertz spectroscopies in order to reach the optimal coupling of energy into the measuring cuvette. This paper discusses the differences between focusing by a dielectric lens in the optical meaning with the ideal point source or Gaussian beam with respect to the real source represented by the horn antenna. The results of this paper consist of the location, where the focused beam is the narrowest, and the power distribution in this distance.