Analytical calculation of focal spot size for lens antenna

An analytical method was presented to calculate the focal spot size of the focal lens antenna. Firstly, the formulation for the 3dB focal spot size on the focal plane of the focal lens antenna was derived by Huygens-Fresnel principle. Then this formulation was simplified for convenience and the approximate expression was obtained. Secondly, the field distribution on the plane that parallels the focal plane was discussed. In order to demonstrate the accuracy and efficiency of this new method, a focal lens antenna at Ka band was designed and simulated as an example. Moreover, two focal lens antennas were fabricated for comparison. And these two lens antennas were measured by the planar-near-fields (PNF) method. Finally, the results obtained by the proposed method were compared with those of numerical method and measured data. It is found that the results of 3dB focal spot size computed by the approximate expression are in good agreement with the experimental data.