Impact on focal parameters for near‐field‐focused aperture antennas

Near‐field‐focused (NFF) antennas have been studied to focus the electromagnetic power in the near‐field (NF) region of the antenna aperture over a long time. Some focal parameters have been adopted to evaluate the focused performance of NFF antennas. Research work on smaller NFF aperture antennas with electrically large size is limited for the similarities between array antennas and aperture ones increasing with their size increasing. It is meaningful to study smaller aperture (several wavelengths) antennas because the physical size of antennas is limited for low‐frequency applications. This paper investigates the impacts of four factors on six focal parameters for NFF aperture antennas when the electrical size is not large enough. The results of six focal parameters are given as functions of the focal distance for different aperture shapes, aperture sizes, and aperture amplitude distributions, respectively. Finally, the differences of two focal parameters between the focal plane and the focal sphere are discussed as well. Unlike the previous work on the scalar diffraction field of NFF aperture antennas using the Fresnel region field approximation, the integral expression of scalar diffraction field is calculated directly and exactly in this paper to produce the focused field at an arbitrarily selected point.