Realizable feed-element patterns and optimum aperture efficiency inmultibeam antenna systems

The results of an earlier paper by Y. Rahmat-Samii et al. (ibid., vol.AP-29, pp.961-3, 1981), regarding realizable patterns from feed elements that are part of an array that feeds a reflector antenna, are extended. The earlier paper used a cos^q &thetas; model for the element radiation pattern, whereas here a parametric study is performed, using a model that assumes a central beam of cos^q &thetas; shape, with a constant sidelobe level outside the central beam. Realizable q-values are constrained by the maximum directivity based on feed element area. The optimum aperture efficiency (excluding array feed network losses) in an array-reflector system is evaluated as a function of element spacing using this model as well as the model of the earlier paper. Experimental data for tapered slot antenna arrays are in agreement with the conclusions based on the model     

RF characterization of an inflatable parabolic torus reflectorantenna for space-borne applications

Space-borne satellite applications provide a vast array of services extending from global connectivity to Earth observation systems. The soil moisture radiation mission is a proposed space-borne passive microwave system complementary to the existing Earth observing system operating at low microwave frequencies and requiring an antenna with multibeam, high-beam efficiency, and dual polarization capabilities. To achieve both the large reflector size and the multibeam pattern at the operational frequencies an innovative multibeam reflector antenna design was needed. The advances in inflatable antenna technology has been proposed to overcome the launch vehicle size and weight restrictions. This paper describes a novel offset parabolic torus reflector antenna design that produces the desired multibeam pattern and is compatible with the inflatable antenna technology. Using the system requirements of this mission as an example, the design process for an inflatable parabolic torus reflector antenna is outlined, the development of suitable distortion models is given, and representative RF characteristics are presented. These RF characteristics include far-field patterns, beam contour patterns, beam efficiency, and other key performance parameters. The development of an advanced analytical modeling/numerical tool in support of the design effort is also detailed     

An iterative current-based hybrid method for complex structures

This paper presents a general unified hybrid method for radiation and scattering problems such as antennas mounted on a large platform. The method uses a coupled electric-field integral equation (EFIE) and magnetic-field integral equation (MFIE) formulation, referred to as the hybrid EFIE-MFIE (HEM), in which the EFIE and MFIE are applied to geometrically distinct regions of an object. The HEM is capable of modeling arbitrary three-dimensional (3-D) metallic structures, including wires and both open and closed surfaces. We show that current-based hybrid techniques that utilize physical optics (PO) are an approximation of the HEM formulation. A numerical solution procedure is given that combines the moment method (EFIE) with an iterative Neumann series technique (MFIE). This permits one to effectively utilize the PO approximation when appropriate, and provides a general and systematic mechanism to correct the errors introduced by PO. Consequently, the HEM overcomes the inherent limitations of hybrid techniques which rely upon ansatz-based improvements of PO. The method is applied to the problem of radiation from objects that can be modeled using wires and metallic surfaces as fundamental elements. A representative example is given to demonstrate that the method can handle the difficult problem of a parasitic monopole located in the deep shadow region     

A novel array antenna for MSAT applications

The issue of reducing the cost of phased array vehicle antennas through the use of a lens feeding arrangement instead of phase shifters at each element is addressed. In particular, the economic viability of a mobile satellite system (MSAT) is largely dependent on the efficient use of the allocated scarce spectrum and orbit as well as the satellite power. The type of vehicle antenna used will play a critical role in achieving this efficiency. A standard design approach for an electronically steered array uses phase shifters at each element to provide beam steering. A method for reducing the required number of phase shifters by using an R-KR lens feed network is outlined. The authors briefly discuss the phase shifter approach to beam steering, examine various lens feed techniques, and describe the R-KR lens approach. The lens feed network architecture is examined, a computer model for simulation of the array is presented, and the results of analysis of a suggested design for the MSAT application are given. In addition, satellite acquisition and tracking considerations are investigated     

The field equivalence principle: illustration of the establishmentof the non-intuitive null fields

The field equivalence principle, one of the fundamental concepts in electromagnetics, has numerous applications. However, for a beginning student, it is not easy to understand this concept thoroughly and to appreciate it. The dilemma faced by beginning students is illustrated. We have sources in a finite Region I, and an arbitrary mathematical surface separating Regions I and II. The equivalent problems for the exterior and interior regions are specified with the use of electric and magnetic equivalent currents impressed on the boundary surface. The acceptance of the establishment by the equivalent sources of the non-intuitive null field for the exterior problem (by the equivalent sources and the original source for the interior problem) is commonly bothersome and not comfortably realized. In order to clarify this, we revisit Love's and Schelkunoff s forms of the equivalence principle. Subsequently, we discuss two simple, analytically tractable illustrative examples, consisting of plane-wave fields in two half-space regions, separated by an infinite planar surface. In particular, the emphasis is on the establishment of the non-intuitive null fields developed by these equivalent sources. Various forms of equivalence are illustrated by simple analytical field expressions     

Switchable dual-band circularly polarised patch antenna with singlefeed

A novel patch antenna with switchable slots is proposed for dual-band circularly polarised (CP) operation. Two diode controlled slots are incorporated into the patch for dual-band operation and a pair of tuning stubs are utilised to tune the CP performance. The antenna design and experimental results are presented     

Analysis and characterization of multilayered reflector antennas:rain/snow accumulation and deployable membrane

There are important engineering issues in designing reflector antennas that cannot be addressed by simply assuming a perfect electric conductor (PEC) reflector surface. For example, coatings may exist on antenna surfaces for protection, rain or snow can accumulate on outdoor reflectors, and the deployable mesh or inflatable membrane antennas usually do not have solid PEC reflector surfaces. Physical optics (PO) analysis remains the most popular method of reflector analysis owing to its inherent simplicity, accuracy, and efficiency. The conventional PO analysis is performed under the assumption of perfectly conducting reflector surface. To generalize the PO analysis to arbitrary reflector surfaces, a modified PO analysis is presented. Under the assumptions of locally planar reflector surface and locally planewave characteristic of the waves incident upon the reflector surface, the reflection and transmission coefficients at every point of the reflector surface are determined by the transmission-line analogy to the multilayered surface structure. The modified PO currents, taking into account by the finite transmissions of the incident waves, are derived from the reflection and transmission coefficients. Applications on the analyses of the rain and snow accumulation effects on the direct-broadcast TV antennas and the effects of finite thickness and finite conductivity of the metal coating on a 15-m inflatable antenna are described and results are presented     

Reconfigurable patch antennas for steerable reflectarray applications

Reconfigurable reflectarrays have been designed with patch elements which can vary the reflected phase by varying the height of the patches. These patches have been designed using a periodic method of moments simulation. Reflectarrays incorporating elements of varying heights have been built and tested. The first design is a 33 element array comprised of stacked patches which operates at 7.31 GHz. The second design is a 120-element dipole array over a ground plane which operates at 5.2 GHz. Microelectrical, mechanical systems actuation technology could be used to implement these designs and a potential concept is suggested.     

An array feed approach to compact range reflector design

A novel methodology is presented for the design of array feeds for compact range reflectors. The principal design strategy is to illuminate the rim of a standard, circular-aperture reflector with an array pattern null. This results in a reduction in the level of edge-diffracted fields present in the quiet zone. Since low-frequency performance is improved without dedicating reflector surface area to edge treatment, the potential quiet zone size is maximized. Implementation of the technique requires only a few real-valued array excitation coefficients. It is demonstrated that various quiet zone field performance trade-offs can be made by varying either the excitation coefficients or the array size. The quality of the quiet zone field is compared with that of a uniformly illuminated serrated reflector. The operational bandwidth and the effects of both random and systematic array excitation coefficient errors are evaluated