Glenn D. Hopkins

Inflatably Deployed Membrane Waveguide Array Antenna for Space

As an alternative to parabolic antennas and Synthetic Aperture Radar (SAR) systems, waveguide arrays offer another method of providing RF transmit/receive communication apertures for spacecraft. The advantage of the membrane waveguide array concept, in addition to its lightweight and low packaged volume, is its inherent shape. Relative to parabolic antennas, the requirement to make an accurate doubly curved surface is removed. L’Garde and Langley Research Center (LaRC), are currently working in this area to develop lightweight waveguide array technologies utilizing thin film membrane structures. Coupled with an ultra-lightweight inflatably deployed rigidizable planar support structure, the system offers a very compelling technology in the fields of space-based radar, communications, and earth resource mapping.

Two-dimensional, nonlinear oscillator array antenna

This paper details the design and performance of a 25-element, two-dimensional, nonlinear oscillator array antenna operating near the L1 GPS (1.57542 GHz) frequency. A simple theoretical framework for understanding its principle of operation (a reliance on the synchronization properties of coupled, nonlinear oscillators for phase coherence and inertia-less scanning) is provided. A unique feature of this design, the integration of diagnostic circuitry for the real-time simultaneous measurement of element-level amplitude and phase for calibration and control, is described.

Mechanism of apparent gain observed in focused beam measurements of a planar FSS

Apparent gain in focused beam measurements (tenths of a dB above 0 dB for calibrated VNA transmission) has been observed on many occasions for low loss resonant structures that include photonic/electromagnetic band-gap (PBG/EBG) structures [1] or frequency selective surface (FSS) stacks [2]. The authors are unaware of any previously published explanations perhaps due to the small magnitude of deviation. However, for a device with tight specifications, tenths of a dB deviation without explanation may be unacceptable, and transmission coefficient data greater than 0 dB can create controversy. This paper will demonstrate that the apparent gain seen in focused beam measurements occur due to radiation via mutual coupling from FSS elements not directly illuminated. Radiation from these elements increases the directivity of the stack resulting in an effective gain as measured by a VNA.

Performance evaluation of a membrane waveguide array antenna

NASA Langley Research Center (LaRC) has pursued the development of tensioned membrane antenna technology for several years. For many applications, it is desirable to have space-based antennas which are very large. As the physical size increases, antennas constructed of conventional materials quickly become too bulky and heavy to be practical for space applications. For some of these applications, such as earth remote sensing, reducing electromagnetic losses in the antenna is also critical. For this reason, there is interest in finding new methods of fabricating large antennas which not only exhibit lighter weight than conventional antennas, but also meet stringent electromagnetic performance criteria. One configuration which has been investigated by NASA LaRC is a slotted waveguide array antenna constructed of a metallized thin membrane material. Several test articles have been built and tested to validate this configuration. This concept will lend itself to compact packaging for launch, and can be tensioned on an inflatable, rigidizable support structure, which would deploy once on orbit. One major advantage of this planar configuration is that there is no need for fabricating a doubly curved surface (such as for a reflector antenna) from membrane material. One of the most challenging technical issues to be faced in constructing antennas such as the ones described above is that of designing the antenna and feed network to minimize electromagnetic losses while maintaining compatibility with a light, thin, membrane structure. This paper describes the electromagnetic evaluation of an eight element metallized membrane L-band waveguide array antenna with an integrated membrane feed network. Though there are still some technical hurdles to overcome before this technology is applicable for earth remote sensing from space, the result of the performance evaluation shows that further investigation is warranted.

Development of a novel faceted, conformal, slotted-waveguide subarray for sensor applications with full 360° azimuth tracking capabilities

This paper describes the design and implementation of a conformal subarray antenna that was developed as part of a larger circular array which is intended to provide full 360deg azimuth coverage for a sensing system operating from 16 - 16.6 GHz. Despite the fact that several technologies from prior art were incorporated into the design of the presented center-fed, narrow-wall slotted waveguide array, the innovative character of this approach lies in the development of a faceted conformal waveguide subarray and its use with neighboring similar arrays to provide azimuth angular tracking information. The measured reflection coefficient and radiation patterns of two fabricated prototypes demonstrate good agreement with those predicted by FEM simulations.

Aperture Efficiency of Amplitude Weighting Distributions for Array Antennas

Array antennas offer a wide range of opportunities in the variation of their directivity patterns through amplitude and phase control. Peak sidelobe levels may be reduced via amplitude control or weighting across the array aperture. Several authors have made significant contributions in detailing processes for synthesizing these aperture amplitude distributions for the purpose of sidelobe level control. One of the basic trade-offs when implementing amplitude weighting functions is that a trade between low sidelobe levels and a loss in main beam directivity always results. Some of the commercially available pattern calculation programs that can implement sidelobe level control do not provide calculations of the aperture efficiencies given different amplitude weightings. Calculation of the aperture efficiency can be somewhat confusing, particularly with regards to the difference between tapering via attenuation versus redistribution. The purpose of this paper is to define these terms, to provide a review of the proper normalization technique that is important in obtaining accurate aperture efficiency estimation. Descriptions of the amplitude tapers and their utility will be presented. A design example will be presented which will compare theoretical efficiencies with those obtained via finite element method simulation.

A K-band microstrip phased array radiator for airborne antennas

A K-band microstrip radiator has been developed and prototyped for application in phased arrays for airborne antenna systems. The electrical requirements for the radiator included broad impedance and polarization bandwidths (19.2 - 21.2 GHz) over a full field-of-view scan volume ( /spl plusmn/60/spl deg/ in azimuth and elevation). Mechanical requirements included a thin form factor, fabrication using conventional printed circuit techniques, and direct integration with multilayered feed and control circuitry. This paper presents the design details of the radiator and measured results from prototyped single radiators and a seven-element test array. Computer predicted performance is presented for larger arrays. Presented data includes reflection coefficient and polarization performance versus frequency. GTRI researchers were awarded U.S. Patent 6,466,171 for the design innovations.

A 2–22 GHz wideband active bi-directional power divider/combiner in 130 nm SiGe BiCMOS technology

An active bi-directional power dividier/combiner circuit based on a distributed topology is proposed. The use of bi-directional amplifiers (BDAs) provides both dividing and combining functions within the same circuit. By utilizing a distributed topology composed of BDAs and artificial transmission lines, a wide operational bandwidth (2-22 GHz) and a large, flat power gain (9 dB) were achieved under DC power consumption of 100 mW. The maximum amplitude and phase imbalances were 0.7 dB and 3°, respectively.

Beam former development for the NASA Hurricane Imaging Radiometer

The Hurricane Imaging Radiometer (HIRAD) is an airborne passive microwave synthetic aperture radiometer designed to provide high resolution, wide swath imagery of surface wind speed in tropical cyclones from a low profile planar array antenna. This paper will present the array radiometer system concept and summarize its development, including multiple flight tests on NASAs Genesis and Rapid Intensification Processes (GRIP, 2010) and Hurricane and Severe Storm Sentinel (HS3, 2012) campaigns. The paper will focus on the design goals, trades, and approach for the array antenna along-track beam former. The paper presents details of the beam former design, implementation, integration approach, and measured performance. The paper concludes with a description of planned improvements for the next generation dual-polarized HIRAD antenna and the resulting impacts on the beam former design and integration.

Simultaneous, multi-frequency synchronous oscillator antenna

As signal bandwidth increases, the inherent limitations of analog-to-digital converter technology become significant. Compounding the difficulty of direct-to-digital operation, at such high frequencies the sheer volume of data generated (on a per-element basis) could result in unattainably high throughput rates and processing power requirements. This paper reviews the design, construction and testing of coupled nonlinear oscillator arrays capable of phase-shifterless beam-steering and null generation at multiple simultaneous frequencies. This ongoing effort by Georgia Tech Research Institute (GTRI) and SPAWAR-SSC is intended to yield significant insight into this novel approach which exploits coupling and nonlinearities in antenna design in order to provide improved performance (increased null depth, simplified parameter controls, BER reduction) on SIGINT platforms. If successful, this design could represent a possible enabling technology for the realization of low-cost, phased arrays at micro- and millimeter wave frequencies, impacting a diverse range of potential commercial, military and aerospace applications.