Near-Field Focused Array Antenna With Frequency-Tunable Focal Distance

An
$8\times8$
microstrip planar array antenna with near-field focused beams is presented in this paper, whose elements are series fed in both E- and H-planes. The focus is designed at a distance of 200 mm above the array plane at 10 GHz, and by carefully designing the feed network, the focal distance can be frequency tunable. Eventually, a steerable focal distance from 78 to 249 mm is achieved as frequency varies from 9.25 to 10.5 GHz, with near-field sidelobes below −8 dB. Theoretical analysis is performed first, which is further proven by simulations and experiments. Finally, the method of tuning the sensitivity and range of frequency scan is discussed, which is proven by numerical study. To the best of our knowledge, the proposed design is the first array approach to realize tunable focal distance without any beam-forming components.