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A new approach is proposed to develop aperture-shared dual-band antennas using partially reflecting surfaces (PRSs). A fully aperture-shared dual-band antenna consisting of a low-band antenna (LBA) and a high-band antenna (HBA) right below the LBA is first investigated. The aperture sharing has little influence on the LBA but causes a severe shielding effect on the HBA. To alleviate the shielding effect, a low-pass PRS is inserted between the LBA and HBA, forming an air-filled quasi-Fabry–Perot cavity, which can make compensation for the radiation from the HBA but with little influence on the LBA. The second PRS is introduced above the LBA to enhance the bandwidth (BW) for HBA. It is shown that the fully aperture-shared dual-band antenna achieves an impedance BW ($\vert \text{S}11\vert < -15$ dB) of ~43.7% (0.68–1.07 GHz) for the LBA and ~45% (1.7–2.7 GHz) for the HBA with an average gain of ~8 dBi and stable radiation patterns. Based on the PRSs, an aperture-shared dual-band antenna subarray is finally developed for base-station applications, which covers the frequency bands of 0.68–0.96 and 1.7–2.7 GHz for 2G/3G/4G mobile communications. [ABSTRACT FROM AUTHOR]

In this paper, after a brief review on the working principles of synthetic aperture radiometers exploiting interferometry and mechanical rotation of the antenna, analytical expressions are provided which allow to easily compute the receiving performance of the interferometer starting from given locations of its sensors. Then, an innovative and general approach is developed and discussed for the optimal synthesis of the locations of the array sensors in order to fulfill requirements of crucial interest. Moreover, an innovative approach is also developed for the synthesis of the optimal weighting of the different collected signals. Finally, the performances of the developed synthesis approaches are discussed with reference to the GEO Atmospheric Sounder (GAS) array instrument. [ABSTRACT FROM AUTHOR]

A method of estimating spatial degrees of freedom (DoF) from measured multipath propagation channels in the multiple-input single-output regime is presented. The DoF of the multipath channels on the transmit (Tx) side is derived by means of the spherical-wave expansion of electromagnetic fields radiated from a Tx antenna array having a certain aperture size. The DoF estimates are independent of particular realization of antenna elements on the Tx aperture. For a given aperture size, the DoF provides the number of the Tx antenna elements for efficient improvement of the system performance by utilizing the spatial diversity. Having confirmed the soundness of our DoF estimation method by channel measurements in an anechoic chamber, the DoF of indoor multipath channels is analyzed. The DoF on the Tx side of the considered multipath channels reveals larger values when the antenna aperture size is increased at a fixed frequency, and when the frequency increases for a fixed antenna aperture size. For a fixed frequency, increasing the antenna aperture size is more effective in observing extra DoF in the obstructed and non-line-of-sight channels than in the line-of-sight channels. Furthermore, for a fixed antenna aperture size, the use of the higher frequency brings larger DoFs in many propagation scenarios. The results also show that electrically smaller antennas are more efficient in observing the DoF. Finally, the solid angle of multipath is derived as a Tx antenna-independent metric of the multipath richness. It is defined as an angular range of dominant multipath clusters subtended on a unit sphere. Our analysis method is extendable to the multiple-input multiple-output regime in a straightforward manner. [ABSTRACT FROM PUBLISHER]

A new directive ultra-wideband antenna based on rectangular waveguide aperture antennas is presented. Conventional rectangular aperture antennas are limited by the single mode operation of the waveguide as well as relatively long and narrowband resonant feeds. To overcome these limitations, we developed a new rectangular waveguide antenna feeding structure that can establish an aperture field distribution very similar to that of the fundamental waveguide mode with a relatively short waveguide section and over two octaves of bandwidth. The overall antenna size is 0.55 \lambdam \times 0.27\lambda m \times 0.18 \lambdam where \lambdam is wavelength at the minimum operating frequency. Due to the planar aperture of the antenna, a polarizer could be added to the antenna to ensure good cross-polarization performance with fabrication tolerances. The proposed feed requires a balun structure, and thus an integrated balun design is presented. It is found that a standard balun would interact with the antenna structure and thus a modified design was implemented. The antenna was fabricated and measured. The antenna had VSWR < 2.5 from 1.08 to 4.9 GHz and the measured gain showed almost constant aperture gain which increases from 5.3 dBi at the low frequency end to 11 dBi at the high frequency edge. [ABSTRACT FROM PUBLISHER]

New configurations of small-size shielded metallic Fabry–Perot (FP) antennas with improved performance over a large frequency band are presented in S-band for space missions. The bandwidth enlargement is obtained by stacking two FP cavities of different size, each of them presenting a low quality factor. Their radiating apertures measure around \lambda0 and 2\,\times\lambda0, respectively. Concentric corrugations are also introduced between both cavities to control the higher-order modes that are excited systematically in shielded small-size FP antennas due to lateral resonances. The obtained results are compared to those of a single-stage FP cavity antenna with the same aperture size. Several prototypes have been fabricated and measured. An aperture efficiency higher than 70%, a reflection coefficient smaller than - 15 dB, and sidelobe levels lower than - 20 dB have been obtained experimentally, over a wide frequency band (2.4–2.66 GHz). These characteristics make stacked FP cavity antennas very attractive to replace global coverage horn antennas, or to be used in feed clusters of multiple-beam antennas, especially in C- and S-bands, where they lead to more compact and less bulky solutions compared to classical feed horns. [ABSTRACT FROM PUBLISHER]

Antenna array configurations have significant influence on the radiometric sensitivity of aperture synthesis microwave radiometers. In this paper, we propose a minimum degradation array (MDA) for optimum sensitivity. First, the degradation factor (DF) is defined to characterize the effect of redundant spatial frequency samples formed by an array on the sensitivity. Aiming at minimizing DF, a simulated annealing (SA) based method is proposed to search for an MDA, which, combining with a concept of augmented maximum baseline (CAMB), can effectively locate the true global minimum of DF. Numerical results validate the effectiveness of the proposed method as well as CAMB in optimizing DF. Further, the lower bound of DF is discussed. Finally, simulation and experiment results demonstrate that the proposed method as well as CAMB is of significance in achieving optimum sensitivity. [ABSTRACT FROM AUTHOR]

The near-field radiation of several electrically large apertures has been computed based on the aperture field convolution method, including a square aperture, a circular aperture, and a serrated edge aperture. The frequency range is 8.0-12.0 GHz. The aperture diameters are limited to 5.00 m and the corresponding electrical size is 133-200 wavelengths. The time domain spectrum has been obtained by chirp z-transform with a Hamming window and the mechanism for the generation of the aperture near-field radiation in time domain is analyzed. The calculation results demonstrate that the near-field radiation of the aperture can be approximately seen as the synthesis of a plane wave from the aperture, cylindrical waves from the edges, and spherical waves from the corners. By this method, the direction, position, and magnitude of incoming waves in the near-field region can be estimated, and the aperture design can be modified to meet the requirements of the near-field radiation. The near field of the three apertures mentioned above has been compared, and it is shown that the serrated edge aperture has more uniform direct wave and lower diffraction waves, making it a candidate for compact range (CR) aperture design. [ABSTRACT FROM AUTHOR]