Roberto J. Acosta

A method for producing a shaped contour radiation pattern using a single shaped reflector and a single feed

Eliminating the corporate feed network in shaped contour beam antennas will reduce the expense, weight, and RF loss of the antenna system. One way of producing a shaped contour beam without using a feed network is to use a single shaped reflector with a single feed element. For a prescribed contour beam and feed, an optimization method for designing the reflector shape is given. As a design example, a shaped reflector is designed to produce a continental-United-States (CONUS) coverage beam. The RF performance of the shaped reflector is then verified by physical optics. >

Compensation of reflector antenna surface distortion using an array feed

Various aspects of reflector surface distortion compensation are explored by first assuming the reflector distortion is given and then designing the compensating feed array. The sensitivity of boresight directivity to changing surface distortion parameters for fixed feed-array geometries is examined. It is found that feed array compensation is feasible only for distortions with low spatial frequency content, such as those distortions induced by thermal and gravitational effects. The optimum directivity methods for determining element excitation is found to yield slightly better values of directivity than those for the conjugate field matching (CMF) technique. However, the CFM technique has, in general, much lower sidelobe levels and lends itself to simple realization in hardware. In view of these results, distortion compensation using an array feed is concluded to be a reasonable approach to improving antenna performance for large, space-based reflector antennas that are not easily accessible for tuning and have time-dependent surface distortions. >

Compensation of reflector surface distortions using conjugate field matching

The feasibility of compensating for reflector surface distortions has been investigated. The performance characteristics (gain, sidelobe level, null location, beamwidth, etc.) of space communication reflector antenna systems degrade as the reflector surface distorts due to thermal effects from a varying solar flux. The technique reported here will maintain the design radiation performance independently of thermal effects on the reflector surface. With the advent of monolithic microwave integrated circuits (MMIC), a greater flexibility in array-fed reflector system design can be achieved. MMIC arrays provide independent control of amplitude and phase for each of many radiating elements of the feed array. The conjugate field matching technique provides a basis for obtaining the required element excitations under surface distortion for maintaining the design radiation performance. It is assumed that the surface characteristics (x, y, z, 1st derivaties, and 2nd derivatives) under distortion are known.

Microstrip antenna array with parasitic elements

Discussed is the design of a large microstrip antenna array in terms of subarrays consisting of one fed patch and several parasitic patches. The potential advantages of this design are discussed. Theoretical radiation patterns of a subarray in the configuration of a cross are presented.

Predicting Sparse Array Performance From Two-Element Interferometer Data

Widely distributed (sparse) ground-based antenna arrays are being considered for deep space communications applications with the development of the proposed Next Generation Deep Space Network. However, atmospheric-induced phase fluctuations can impose daunting restrictions on the performance of such an array, particularly during transmit and particularly at Ka-band frequencies, which have yet to be successfully resolved. In this paper, an analysis of the uncompensated performance of a sparse antenna array, in terms of its directivity and pattern degradation, is performed utilizing real data. The theoretical derivation for array directivity degradation is validated with interferometric measurements (for a 2-element array) recorded at Goldstone, CA, from May 2007-May 2008. With the validity of the model established, an arbitrary 27-element array geometry is defined at Goldstone, CA, to ascertain its theoretical performance in the presence of phase fluctuations based on the measured data. Therein, a procedure in which array directivity performance can be determined based on site-specific interferometric measurements is established. It is concluded that a combination of compact array geometry and atmospheric compensation is necessary to minimize array loss impact for deep space communications.

Prototype antenna elements for the next-generation TDRS enhanced multiple-access array

This paper summarizes a study performed to produce prototype antenna elements for the next-generation enhanced Tracking and Data Relay Satellite Continuation (TDRS-C) multiple-access (MA) S-band phased-array antenna. Compared to the multiple-access antenna on the current class of TORS, the enhanced multiple-access antenna requires elements that achieve greater on-axis gain, simultaneous circular polarization capability, and increased beamwidth. To demonstrate that array elements could be realized meeting these requirements, designs that were successful in simulation were fabricated and tested. These included a helical antenna; a novel short backfire antenna, excited with a circular waveguide (cup waveguide) with integrated polarizer and orthomode transducer (OMT); and a corrugated-horn antenna with integrated polarizer and OMVT. The paper describes the design process for the novel elements, and compares measured and simulated results. It also compares the elements in terms of performance, size, and mass.

A microstrip patch-fed short backfire antenna for the tracking and data relay satellite system-continuation (TDRSS-C) multiple access (MA) array

A novel microstrip patch-fed short backfire antenna has been presented here for the first time as a candidate antenna element for the next generation TDRSS-C MA array. This design meets nearly all the specifications required for the enhanced MA array antenna element and promises to provide a lightweight low cost alternative over other designs presently being considered (i.e., corrugated horn, helix, waveguide-fed SBA). Simulations performed using IE3D and MWS agree with measured data, thus far, and indicate an 11% 15-dB bandwidth and a maximum directivity of 15.2 dBi. Future work involves further optimization of the radiation characteristics and the construction of the complete short backfire antenna structure to verify simulated results

Advanced Communication Technology Satellite (ACTS) multibeam antenna analysis and experiment

The in-house developed NASA Lewis Research Center thermal/structural/RF analysis program was designed to accurately simulate the ACTS in-orbit thermal environment and predict the RF antenna performance. The program combines well-established computer programs (TRASYS, SINDA, and NASTRAN) with a dual-reflector-physical-optics RF analysis program. The ACTS multibeam antenna configuration is analyzed, and several thermal cases are presented and compared with measurements (preflight). The overall simulation results were in agreement with the measurements. The thermal/structure/RF analysis program could provide an indirect means of verifying in-orbit antenna performance.<<ETX>>

Detection of reflector surface error from near-field data: Effect of edge diffracted field

The surface accuracy of large reflector antennas must be maintained within certain tolerances if high gain/low sidelobe performance is to be achieved. Thus the measurement of the surface profile is an important part of the quality control procedure when constructing antennas of this type. An efficient method for surface profile measurement has been proposed, i.e., the reflector surface is calculated from the measured near-field phase data using the theory of geometric optics. For a surface profile calculation of this kind, it is necessary to know the margin of error built into the method of calculation. This will enable a specification of the tolerance from which the surface profile can be determined. When calculating the surface profile from near-field phase data, there are two main sources of error. The first is the measurement error in near-field phase data. The second arises from the edge diffracted fields that are superimposed on the reflected fields in the measured near-field data. The error in the calculated surface profile produced by the edge diffracted fields is examined.

Adaptive feed array compensation system for reflector antenna surface distortion

The feasibility of a closed-loop adaptive feed array system for compensating reflector surface deformations is investigated. The performance characteristics (gain, sidelobe, level, pointing, etc.) of large communication antenna systems degrade as the reflector surface distorts mainly due to thermal effects from a varying solar flux. The compensating systems described here can be used to maintain the design performance characteristics independent of thermal effects on the reflector surface. The proposed compensating system uses the concept of conjugate field matching to adjust the feed array complex excitation coefficients. To demonstrate the concept a simulated distorted reflector case is presented with several element patterns.<<ETX>>