Back-to-Back Reflector Antennas With Reduced Moment of Inertia for Spacecraft Spinning Platforms

A back-to-back reflector antenna system with reduced moment of inertia is proposed in order to address the demanding problem of supporting large reflector antennas on spinning platforms. The configuration provides additional potential advantages, such as reducing the spinning speed by half for a given sampling rate when both back-to-back reflectors are utilized. Geometrical parameters of the reflector are determined such that the moment of inertia of the rotating system is reduced. It is shown that these back-to-back reflectors suffer from a high cross-pol level in the asymmetrical plane due to the large feed offset angle. Two different methods are explored to alleviate the high cross-pol level problem. In the first method, a sub reflector is utilized to minimize the cross-pol level by satisfying the Mizugutchi condition. In the second method, a tri-mode matched feed horn is suggested to achieve a similar result. The suppressed cross-pol level puts forward the gravitationally balanced back-to-back reflector antenna systems as a potential candidate for future spacecraft antennas on spinning platforms.     

Stacked Microstrip-Patch Arrays as Alternative Feeds for Spaceborne Reflector Antennas

The feasibility of using stacked microstrip-patch arrays as feeds for offset reflector antennas is investigated in this paper. It is shown that patch arrays can be used as alternatives to the conventionally used horn feeds, which tend to be bulky. In particular, patch arrays can be of interest for spacecraft applications where reduced size and light-weight feeds are highly desirable. In this paper, patch arrays were tailored to provide radiation characteristics similar to those of horn feeds by varying the element spacing and excitation. A reduction in weight was mainly realized by the planar construction of the patch arrays. A full-wave analysis of the feed array, using finite-difference time-domain (FDTD) and PO-based UCLA reflector-analysis codes, was used to test the results of the proposed feed, operating at 1.413 GHz for radiometer applications, and 1.26 GHz for radar applications. A dual-polarized and dual-frequency stacked microstrip-patch element was fabricated and tested. It was then demonstrated that a seven-element hexagonal array design seemed to be the best match to the horn feeds for a 12 m offset-reflector antenna.     

Microwave antenna imaging,diagnostics, and phaseless reconstructions

Microwave antenna imaging techniques are a practical and popular method for antenna diagnostic analysis. Phase retrieval methods, however, are just beginning to emerge as an alternative microwave antenna measurements technique when phase cannot be directly measured. This article focuses on recent advances in microwave antenna imaging, diagnostic techniques, and phase retrieval methods for bi-polar planar near-field antenna measurements. An overview of the bi-polar planar near-field technique is included. The application of optimal sampling interpolation, holographic imaging and diagnostics, and iterative Fourier phase retrieval for the bi-polar planar near-field modality is explored in detail. Experimental results for a waveguide-fed slot array antenna are presented to illustrate these methods. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 396–406, 1997     

Nonuniform Luneburg and two-shell lens antennas: radiationcharacteristics and design optimization

Design optimization of radially nonuniform spherical lens antennas is the focus of this paper. In particular, special attention is given to the optimal design of nonuniform Luneburg (1964) lens antennas. One of the important engineering objectives of designing an optimal Luneburg lens antenna is to use as small number of shells as possible while maintaining an acceptable gain and sidelobe performance. In a typical radially uniform design, by reducing the number of shells, the gain is decreased and the grating lobes are increased. This deficiency in the radiation performance of the uniform lens antenna can be overcome by designing the nonuniform lens antenna. This necessitates the optimum selection of each layer thickness and permittivity. A genetic algorithm (GA) optimizer with adaptive cost function is implemented to obtain the optimal design. In this manner, the GA optimizer simultaneously determines the optimal material and its thickness for each shell by controlling the gain and sidelobes envelope of the radiation pattern. Various lens geometries, including air gaps and feed offset from the lens surface, are analyzed by using the dyadic Green's functions of the multilayered dielectric sphere. Many useful engineering design guidelines have been suggested for the optimum construction of the lens. The results have been satisfactory and demonstrate the utility of the GA/adaptive cost-function algorithm. Additionally, the radiation characteristics of a novel two-shell lens antenna have been studied, and its performance is compared to the Luneburg lens     

Periodic bandgap and effective dielectric materials in electromagnetics: characterization and applications in nanocavities and waveguides

The main objectives of this paper are to characterize and develop insight into the performance of photonic bandgap (PBG) periodic dielectric materials and to integrate the results into some novel applications. A powerful computational engine utilizing the finite-difference time-domain technique with periodic boundary conditions/perfectly matched layers integrated with Prony's method is applied to provide an in-depth look at the physics of PBG/periodic bandgap structures. Next, the results are incorporated into two classes of applications in the areas of nanocavity lasers and guidance of electromagnetic (EM) waves in sharp bends. A two-dimensional PBG structure with finite thickness is presented to strongly localize the EM waves in three directions and design a high-Q nanocavity laser. It is shown that the periodic PBG/total internal reflections remarkably trap the EM waves inside the defect region. The effect of the number of periodic cells and defect's dielectric constant on the Q of structure is investigated. It has been found that a seven-layer PBG with a dielectric impurity defect can be used in the design of a laser with a Q as high as 1050. Additionally, potential applications of the PBG structures for guiding the EM waves in sharp bends, namely, 90/spl deg/ and 60/spl deg/ channels are demonstrated. It is shown that shaping the bend by introducing small holes can noticeably improve the guidance of the waves at the bends and channel the EM waves with great efficiency. A comparative study between PBG and effective dielectric materials in controlling the EM waves is also provided and it is observed that the novel characteristics of the PBG cannot be modeled using the effective material for the frequencies within the bandgap.     

Dual band FSS with fractal elements

Experimental and computed results of a frequency selective surface (FSS) based on a certain type of fractal element are presented. The fractal element is a two iteration Sierpinski gasket dipole. Owing to the dual band behaviour of the two iteration Sierpinski gasket dipole, two stopbands are exhibited within the operating frequency band. This behaviour is obtained by arraying one simple element in a single layer frequency selective surface (FSS)     

Linear spiral sampling for the bipolar planar near-field antennameasurement technique

This paper investigates linear spiral sampling for bipolar planar near-field antenna measurements. This sampling scheme is, depending on range implementation, the most rapid polar near-filed data acquisition mode. The near-field to far-field transformation is performed using a modified optimal sampling interpolation (OSI)/fast Fourier transform (FFT) approach. Measured far-field pattern results for a waveguide-fed slot array antenna are presented and are shown to have excellent agreement with results obtained from a conventional bipolar measurement     

EM interaction of handset antennas and a human in personalcommunications

In personal communications, the electromagnetic interaction between handset-mounted antennas and the nearby biological tissue is a key consideration. This paper presents a thorough investigation of this antenna-tissue interaction using the finite-difference time-domain (FDTD) electromagnetic simulation approach with detailed models of real-life antennas on a transceiver handset. The monopole, side-mounted planar inverted F, top-mounted bent inverted F, and back-mounted planar inverted F antennas are selected as representative examples of external and internal configurations. Detailed models of the human head and hand are implemented to investigate the effects of the tissue location and physical model on the antenna performance. Experimental results are provided which support the computationally obtained conclusions. The specific absorption rate (SAR) in the tissue is examined for several different antenna/handset configurations. It is found that for a head-handset separation of 2 cm, the SAR in the head has a peak value between 0.9 and 3.8 mW/g and an average value between 0.06 and 0.10 mW/g for 1 W of power delivered to the antenna. Additionally, the head and hand absorb between 48 and 68% of the power delivered to the antenna     

Analysis of near-field Cassegrain reflector: plane wave versuselement-by-element approach

A near-field Cassegrain reflector (NFCR) is an effective way to magnify a small phased array into a much larger aperture antenna for limited scan applications. Traditionally the pattern wave approach, i.e. the field from the feed array incident on the subreflector is approximated by a truncated collimated beam with planar phase and tapered amplitude distribution. This approach simplifies the computation tremendously, but fails to provide design information about the most critical component of the whole antenna system, namely, the feed array. With the help of today's computers, it is now feasible to calculate the pattern of a NFCR by a more exact element-by-element approach. Each element in the feed array is considered individually and the diffraction pattern from the subreflector is calculated by the geometrical theory of diffraction (GTD). The field contributions from all elements are superimposed at the curved main reflector surface, and a physical optics integration is performed to obtain the secondary pattern