Microwave imaging is a science which evolves from detecting techniques to evaluate hidden or embedded objects in a structure or media using electromagnetic (EM) waves in the microwave range (i.e., from 300 MHz to 300 GHz). Microwave imaging is often associated with radar detection such as target detection and tracking, weather pattern recognition, and underground surveillance which are far-field applications. In recent years, due to microwave's characteristic that allows penetration into optically opaque media, short-range applications including medical imaging, nondestructive testing and quality evaluation, through-the-wall imaging, and security screening have been utilized. Microwave near-field imaging occurs when detecting the profile of an object within the short-range: when the distance apart from the sensor to the object is from less than one wavelength to several wavelengths --This is coarse definition because the size of the antenna and the object is often comparable to the range between them.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. Typically many sensors are placed near the object and either a quantitative or a qualitative algorithm is applied to the collected data. 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. Examples can be found are the canine kidney imaging experiment carried by Jacobi and Larsen in the 1970s [1]-[3], and active microwave imaging for horse kidney by Jofre and Bolomey [4]. In 1990s, researchers started being able to use microwave signal higher than 1 GHz in real imaging systems. One example is by Bolomey and Pichot, who developed a practical system [5] and designed a planar microwave camera, both operating at This paper has been accepted by IEEE Microwave Magazine and published on a future issue.2.45 GHz [6]. 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 overv...

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A novel compact unidirectional UWB antenna is presented in this paper. First a novel planar omnidirectional UWB antenna with CPW-feed is designed. The antenna is composed of a half-elliptical disc with a small ground plane. A slot is inserted on the patch as a novel technique to improve the gain bandwidth of the antenna at higher frequencies. The omnidirectional antenna shows UWB matching and gain bandwidth of 2 GHz to 6.5 GHz. Furthermore, to make the radiation pattern of the omnidirectional antenna unidirectional, a rectangular shape metallic reflector without bottom wall is used on the backside of the antenna. The unidirectional antenna with a total dimension of 0.52λ m × 0.33λ m × 0.18λ m (λ m wavelength of the minimum operating frequency) has a matching bandwidth of 1.5 GHz to 7.7 GHz with a gain of 5 dBi to 10.2 dBi over 1.7 GHz to 6.5 GHz, and flat group delays of less than 1 nsec. To validate the proposed design, the antenna is fabricated, and measured results are compared with simulations.

A phase shift and sum (PSAS) algorithm to image objects in dispersive media is presented. The algorithm compensates the phase shift of the scattered field from the receiver to the source for each frequency component in an ultrawideband (UWB) and then integrates all the frequency responses. This method resolves the multispeed and multipath issue when UWB signals propagate in dispersive media. In addition, a multipath effect due to refraction on a curved boundary is also explored. By collecting data using a customized microwave measurement system of two different objects placed in a plastic graduated cylinder filled with glycerin, along the measured dielectric parameters of glycerin (a dispersive medium), highquality reconstructed images are formed using PSAS. Quantitative and qualitative comparisons with two other traditional time-shift radar-based microwave imaging algorithms for the same objects under test demonstrate the advantages of PSAS.Index Terms-Dispersive media, microwave imaging, microwave measurement, radar-based method, ultra-wideband (UWB).