DEVICES AND CIRCUITS FOR MILLIMETER WAVE AND THz APPLICATIONS Impact on focal parameters for near‐field‐focused aperture antennas Weimin Wang | Huaqiang Gao | Yongle Wu | Yuanan Liu Beijing Key Laboratory of Work Safety Intelligent Monitoring, School of Electronic Engineering, Beijing University of Posts and Telecommunications, Beijing 100876, China Correspondence Yongle Wu, Beijing Key Laboratory of Work Safety Intelligent Monitoring, School of Electronic Engineering, Beijing University of Posts and Telecommunications, Beijing 100876, China. Email: wuyongle138@gmail.com Funding information National Natural Science Foundation of China, Grant/Award Numbers: 61701041 and 61327806 Abstract Near‐field‐focused (NFF) antennas have been studied to focus the electromag- netic 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 perfor- mance 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 investi- gates 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. KEYWORDS aperture antennas, focal distance, focal parameters, near‐field‐focused (NFF), scalar diffraction field 1 | INTRODUCTION Near‐field‐focused (NFF) antennas have been becoming an interesting research field in some applications such as NF radiometry, 1 medical imaging, 2 and tissue hyperthermia. 3 The basic idea is to control the phase of the radiation sources on the antenna's aperture so as to enhance the electromagnetic power density in a specific region near the antenna aper- ture. 4-12 In addition, compared with the NFF antennas with tapered phase distributions of the radiation sources, far‐ field‐focused antennas are with uniform phase distributions of the radiation sources. There are some focal figures of merit adopted to characterize NFF antennas such as the half power beam width (BW), the -3‐dB spot diameter (Δ s ), the sidelobe level (SLL), the focal shift, the depth of focus, and the focusing gain. 13-17 In particular, the focal shift (FS) is the location deviation between the focal point and the field amplitude peak. The depth of focus (DoF) is defined as the range between the -3‐dB axial points around the point of power density peak, in the direction from antennas center to focal point (ordinarily in the z‐axis direction when the focal point lies on the z‐axis). The focusing gain Received: 15 May 2018 Revised: 21 July 2018 Accepted: 3 August 2018 DOI: 10.1002/jnm.2510 Int J Numer Model. 2018;e2510. https://doi.org/10.1002/jnm.2510 © 2018 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/jnm 1 of 13