  A near-field microwave imaging system attempts to reveal the presence of an
object and/or an electrical property distribution by measuring the scattered
field from many positions surrounding the object. Over the past few decades,
both the hardware and software components of a near-field microwave imaging
system technology have attracted interest throughout the world. Due to
limitations of hardware technology (unavailability of data acquisition
apparatus), experimental microwave imaging is very challenging for the
pioneers. However, Probably due to the hardware cost, most of the studies
(operating at a few GHz) were still focused on software only. The feasibility
of using microwave approaches to image different types of objects have been
tested and verified by simulations in a variety of applications. Further, work
has been conducted on improving both quantitative and qualitative algorithms to
improve simulated reconstruction results. Nowadays, benefitting from the
hardware progress and reduction of their cost, researchers are eager to pursue
real experimental validations instead of simulations, and, more unique
prototypes and commercial systems have been built for various applications.
These prototypes and systems are a result of years of dedicated work and it is
important to review the advancements in developed prototype systems. The
article will provide an overview of the many of the systems designed from
different research groups throughout the world, for applications of near-field
microwave imaging. The article further outlines challenges faced in current
microwave near-field imaging, developmental tendencies of engineers and
scientists, and the future outlook.
