Passive millimeter-wave imaging has emerged as a useful tool in many remote sensing applications, including resource re-mote sensing, material classification and target detection. This paper presents a method to classify specular objects based on their material composition from passive millimeter-wave polarimetric imagery. Passive Degree of Polarization (PDoP) is proposed and cal-culated by using the results of passive millimeter-wave polarization measurement. The PDoP values of typical ground targets are further analyzed. Outdoor experiments are conducted, and the PDoP image is generated from the brightness temperature images. The PDoP values of pixels in the image are statistically analyzed, and the threshold values are set based on the statistical analysis results, then every pixel is recognized. Experimental results indicate that this method is highly effective for distinguishing among various materials of interest.

Surface normal vector is essential for object recognition and three-dimensional reconstruction. This article establishes a passive millimeter-wave polarization observation model of object thermal radiation. Then, a polarization-based method is proposed to acquire the surface normal vector from multipolarization measurements. The simulation results show that our method has high accuracy and robustness. The outdoor imaging experiment was conducted, and the polarization characteristics of several typical objects in the imaging were analyzed. Experimental results show that our method can successfully estimate the surface normal vector. The possible applications of our method include object recognition and pavement inspection.

The alias-free field of view (AF-FOV) is inversely proportional to spatial frequency sampling spacing in synthetic aperture interferometric radiometers. When the physical size of antenna elements are large relative to the wavelength, the AF-FOV is limited. This paper focuses on extending the AF-FOV from the perspective of array design. By introducing the sampling spacing compression factor, we define a type of large AF-FOV linear array (LFOVLA) that can generate consecutive spatial frequency samples with sampling spacing less than the minimum antenna spacing. An SA-based algorithm is presented to generate double AF-FOV linear array (DFOVLA), which has the advantage of generating lower redundancy DFOVLA and reducing time cost compared with the previous method. A general pattern method is also proposed to seek large DFOVLA directly. DFOVLA configurations below 26 elements are given. And simulation results show that DFOVLA has a larger AF-FOV than the traditional array in synthetic aperture interferometric radiometers.

A millimeter wave (MMW) one-dimensional leaky-wave antenna (1-D LWA) with high gain is proposed. By using a dielectric resonator (DR) operating on its higher-order transverse electric mode (TE113) as a radiating element of a substrate integrated waveguide (SIW), the gain and radiation efficiency of the 1-D LWA can be improved. Meanwhile, the open stopband (OSB) is suppressed through a tilted slot coupling method, achieving broadside direction at 35 GHz. Finally, a prototype was produced and measured. It has the measured maximum gain of 17.03 dBi and radiation efficiency of 90% at 39 GHz. The measured results demonstrate that the 1-D LWA can continuously scan from -61° to 20° within a 38.8% impedance bandwidth.

In this letter, the substrate integrated waveguide (SIW) TE10/TE30 mode broadband circularly polarization (CP) leaky-wave antennas (LWAs) are proposed. Initially, the surface current distribution characteristics of the SIW fundamental mode (TE10) and the higher-order TE30 mode are analyzed, and the CP mechanism of the transverse asymmetric cross-shaped slot is explored. Then, a pair of orthogonal modes of the dielectric resonator (DR) is excited through slot magnetic coupling to enhance CP radiation. Meanwhile, the unit cell loaded with DR increases the attenuation constant, thereby enhancing the gain. Additionally, the transverse asymmetric cross-shaped slot effectively suppresses the open-stopband (OSB) for both unit cells. The SIW TE10 and TE30 mode LWAs have achieved CP radiation with beam scanning of 53° and 43° in 27-40 GHz (38.8%) and 30-40 GHz (28.6%), respectively. Notably, the SIW TE30 mode LWA has a more compact size. Finally, the correctness of the proposed approach is verified through the fabrication of prototypes and measurement.

This paper discusses the near-field imaging issue of synthetic aperture interferometric radiometers (SAIR). It is challenging for near-field SAIR to realize accurate and fast imaging. To handle this issue, we use convolution to describe the relationship between the near-field visibility function and the brightness temperature and demonstrate that near-field imaging is a deconvolution operation. Then, a band-limited filtering function is constructed to compensate for the synthetic phase factor introduced by the angular spectrum propagation. Finally, the band-limited filtering near-field imaging (BFNI) algorithm is proposed to generate the spectrum of the near-field brightness temperature and realize accurate near-field imaging. Moreover, we present the fast BFNI (FBFNI) algorithm to achieve fast and precise imaging based on the sub-arrays. Unlike the time-domain-based (TDB) imaging algorithm, the proposed BFNI and FBFNI algorithms adopt the frequency-domain-based methods to recover the 2-D brightness temperature image from the 4-D visibility function. Simulation and experiment results show that the proposed frequency-domain algorithms can achieve the same imaging quality as the time-domain algorithm.