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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Radiation by cavity-backed antennas on a circular cylinder

Conformal antenna arrays are popular antennas for aircraft, spacecraft and land vehicle platforms due to their inherent low weight, cost and drag properties. However, to date there has been a dearth of rigorous analytical or numerical solutions to aid the designer. In fact, it has been common practice to use limited measurements and planar approximations in designing such non-planar antennas. The finite element-boundary integral method is extended to radiation by cavity-backed structures in an infinite, metallic cylinder. The formulation is used to investigate the effect of cavity size on the radiation pattern for typical circumferentially and axially polarized patch antennas. Curvature effect on the gain, pattern shape, and input impedance is also studied. Finally, the accuracy of the FE-BI approach for a microstrip patch array is demonstrated.

Kempel, Leo C.↗
NASA NTRS1993

Dynamic Termination On Radiating Coaxial Cable

Radiation pattern dithered to reduce adverse effect of nulls. In improved system for radio communication between base station and portable units within building, tunnel, ship, or other large structure, radiating or "leaky" coaxial cable serves as base-station antenna, and radiation pattern of cable dithered by dithering impedance of termination at end of cable remote from base station.

Lombardi, Robert↗
NASA NTRS1993

System overview on electromagnetic compensation for reflector antenna surface distortion

The system requirements and hardware implementation for electromagnetic compensation of antenna performance degradations due to thermal effects was investigated. Future commercial space communication antenna systems will utilize the 20/30 GHz frequency spectrum and support very narrow multiple beams (0.3 deg) over wide angle field of view (15-20 beamwidth). On the ground, portable and inexpensive very small aperture terminals (VSAT) for transmitting and receiving video, facsimile and data will be employed. These types of communication system puts a very stringent requirement on spacecraft antenna beam pointing stability (less than .01 deg), high gain (greater than 50 dB) and very lowside lobes (less than -25 dB). Thermal analysis performed on the advanced communication technology satellite (ACTS) has shown that the reflector surfaces, the mechanical supporting structures and metallic surfaces on the spacecraft body will distort due thermal effects from a varying solar flux. The antenna performance characteristics (e.g., pointing stability, gain, side lobe, etc.) will degrade due to thermal distortion in the reflector surface and supporting structures. Specifically, antenna RF radiation analysis has shown that pointing error is the most sensitive antenna performance parameter to thermal distortions. Other antenna parameters like peak gain, cross polarization level (beam isolation), and side lobe level will also degrade with thermal distortions. In order to restore pointing stability and in general antenna performance several compensation methods were proposed. In general these compensation methods can be classified as being either of mechanical or electromagnetic type. This paper will address only the later one. In this approach an adaptive phased array antenna feed is used to compensate for the antenna performance degradation. Extensive work has been devoted to demonstrate the feasibility of adaptive feed compensation on space communication antenna systems. This paper addresses the system requirements for such a system and identify candidate technologies (analog and digital) for possible hardware implementation.

Acosta, R. J.↗
NASA NTRS1976

Analysis of aircraft wing-mounted antenna patterns

High frequency radiation patterns of aircraft wing mounted antennas are analyzed. Basic antenna types using ray optical techniques are studied. The aircraft is modelled in its most basic form so that this study is applicable to general type aircraft. The fuselage is modelled as a perfectly conducting finite elliptic cylinder. The wings and horizontal and vertical stabilizers are modelled as perfectly conducting "n" sided flat plates that can be arbitrarily attached to the fuselage or to themselves. The antenna locations are assumed to be on the surfaces of the wings at locations removed from engines and stores such that these effects are negligible. Volumetric patterns are calculated for several aircraft. The validity of the solution is shown by comparing the results against scale model measurements. The application of this solution to practical airborne antenna problems has shown its versatility in designing antennas and predicting their radiation patterns in an accurate and efficient manner.

Marhefka, R. J.↗
NASA NTRS1994

Dual-Antenna Microwave Reception Without Switching

Receiver remains connected to both antennas, transmitter switched to connect it to one or other. Combination of hybrid junction, circulators, and filter provides simultaneous reception paths from both antennas without significantly altering radiation patterns of antennas. Communication system considered for use in spacecraft and in which mechanical switch permitted on downlink but not on uplink. Applicable to terrestrial microwave communication stations subject to dual-antenna requirements.

Hartop, Robert W.↗
NASA NTRS2005

Further Developments in Microwave Ablation of Prostate Cells

A report presents additional information about the subject matter of Microwave Treatment of Prostate Cancer and Hyperplasia (MSC-23049), NASA Tech Briefs, Vol. 29, No. 6 (June 2005), page 62. To recapitulate: the basic idea is to use microwaves to heat and thereby kill small volumes of unhealthy prostate tissue. The prostate is irradiated with microwaves from one or more antennas positioned near the prostate by means of catheters inserted in the urethra and/or colon. The microwave frequency, power, and exposure time, phasing, positions, and orientations of the antennas may be chosen to obtain the desired temperature rise in the heated region and to ensure that the location and extent of the heated region coincides with the region to be treated to within a few millimeters. Going beyond the description in the cited previous article, the report includes a diagram that illustrates typical placement of urethra and colon antenna catheters and presents results of computationally simulated prostate-heating profiles for several different combinations of antenna arrangements, frequencies, and delivered- energy levels as well as experimental results within phantom materials. The advantage of the two-antenna technology is that the heat generated at each antenna is significantly reduced from that associated with only one antenna. The microwave energy radiated from each antenna is focused at the tumor center by adjusting the phasing of the irradiated microwave signal from the antennas.

Arndt, G. Dickey↗
NASA NTRS2013

Large/Complex Antenna Performance Validation for Spaceborne Radar/Radiometeric Instruments

Over the past decade, Earth observing missions which employ spaceborne combined radar & radiometric instruments have been developed and implemented. These instruments include the use of large and complex deployable antennas whose radiation characteristics need to be accurately determined over 4 pisteradians. Given the size and complexity of these antennas, the performance of the flight units cannot be readily measured. In addition, the radiation performance is impacted by the presence of the instrument's service platform which cannot easily be included in any measurement campaign. In order to meet the system performance knowledge requirements, a two pronged approach has been employed. The first is to use modeling tools to characterize the system and the second is to build a scale model of the system and use RF measurements to validate the results of the modeling tools. This paper demonstrates the resulting level of agreement between scale model and numerical modeling for two recent missions: (1) the earlier Aquarius instrument currently in Earth orbit and (2) the upcoming Soil Moisture Active Passive (SMAP) mission. The results from two modeling approaches, Ansoft's High Frequency Structure Simulator (HFSS) and TICRA's General RF Applications Software Package (GRASP), were compared with measurements of approximately 1/10th scale models of the Aquarius and SMAP systems. Generally good agreement was found between the three methods but each approach had its shortcomings as will be detailed in this paper.

Antenna Modeling↗
NASA NTRS1975

MLS airborne antenna research

The geometrical theory of diffraction was used to analyze the elevation plane pattern of on-aircraft antennas. The radiation patterns for basic elements (infinitesimal dipole, circumferential and axial slot) mounted on fuselage of various aircrafts with or without radome included were calculated and compared well with experimental results. Error phase plots were also presented. The effects of radiation patterns and error phase plots on the polarization selection for the MLS airborne antenna are discussed.

Yu, C. L.↗
NASA NTRS1993

Recent activities in printed Antennas at LeRC

This paper will report two recent R&D efforts in printed antennas at NASA Lewis Research Center. These efforts are: (1) to enhance the current antenna performance in gain, bandwidth and pattern characteristics, and (2) to develop coplanar waveguide/aperture coupled feeding technique for dual excitation of a patch antenna. Research in area (1) has led to the development of a nonplanar linearly tapered slot antenna (LTSA) which has exhibited over 10 dB gain with broad bandwidth and excellent radiation patterns. This endfire antenna element is most suitable for use in MMIC arrays of 'brick' construction. A space power amplifier composed of active LTSA has been demonstrated and shown to have a gain of 30 dB at 20 GHz. In each of the antenna elements, a GaAs monolithic microwave integrated circuit (MMIC) three-stage power amplifier is integrated with two LTSA's. A single active LTSA has also been demonstrated and exhibited a power gain of 6.7 dB with the MMIC amplifier turned on. The aperture coupled feeding technique with coplanar waveguide feeds has demonstrated high coupling efficiency on both LTSA and patch antennas. Recent efforts have been focused on applying this technique for dual excitation (dual frequency and/or dual polarization) of a patch antenna. Preliminary results confirm the feasibility of this approach. Further development is required to improve the coupling efficiency and antenna radiation characteristics.

Lee, Richard Q.↗
NASA NTRS1996

Microstrip patch antenna receiving array operating in the Ku band

Microstrip patch antennas were first investigated from the idea that it would be highly advantageous to fabricate radiating elements (antennas) on the same dielectric substrate as RF circuitry and transmission lines. Other advantages were soon discovered to be its lightweight, low profile, conformability to shaped surfaces, and low manufacturing costs. Unfortunately, these same patches continually exhibit narrow bandwidths, wide beamwidths, and low antenna gain. This thesis will present the design and experimental results of a microstrip patch antenna receiving array operating in the Ku band. An antenna array will be designed in an attempt to improve its performance over a single patch. Most Ku band information signals are either wide band television images or narrow band data and voice channels. An attempt to improve the gain of the array by introducing parasitic patches on top of the array will also be presented in this thesis.

Walcher, Douglas A.↗
NASA NTRS2007

Near Field Radiation Characteristics of Implantable Square Spiral Chip Inductor Antennas for Bio-Sensors

The near field radiation characteristics of implantable Square Spiral Chip Inductor Antennas (SSCIA) for Bio-Sensors have been measured. Our results indicate that the measured near field relative signal strength of these antennas agrees with simulated results and confirm that in the near field region the radiation field is fairly uniform in all directions. The effects of parameters such as ground-plane, number of turns and microstrip-gap width on the performance of the SSCIA are presented. Furthermore, the SSCIA antenna with serrated ground plane produce a broad radiation pattern, with a relative signal strength detectable at distances within the range of operation of hand-held devices for self-diagnosis.

Nessel, James A.↗
NASA NTRS1975

Radiation pattern synthesis of planar antennas using the iterative sampling method

A synthesis method is presented for determining an excitation of an arbitrary (but fixed) planar source configuration. The desired radiation pattern is specified over all or part of the visible region. It may have multiple and/or shaped main beams with low sidelobes. The iterative sampling method is used to find an excitation of the source which yields a radiation pattern that approximates the desired pattern to within a specified tolerance. In this paper the method is used to calculate excitations for line sources, linear arrays (equally and unequally spaced), rectangular apertures, rectangular arrays (arbitrary spacing grid), and circular apertures. Examples using these sources to form patterns with shaped main beams, multiple main beams, shaped sidelobe levels, and combinations thereof are given.

Stutzman, W. L.↗
NASA NTRS2006

Analysis of the Optical Design for the SAFIR Telescope

SAFIR, the Single Aperture Far Infra Red Observatory, is a very powerful space mission that will achieve background-limited sensitivity in the far infrared-submillimeter spectral region. Many processes of enormous interest to astronomers can best be studied in this wavelength range, but require the demanding combination of high sensitivity, good angular resolution, and spectroscopic capability. SAFIR is a 10m class telescope to offering good angular resolution, cooled to below 5 K in order to achieve background-limited sensitivity, and equipped with a complement of large-format cameras and broadband spectrometers. Successful operation of such a facility is critically dependent on achieving the level of sensitivity expected, but this is rendered difficult by potential pickup from unwanted sources of radiation. This problem is exacerbated by the fact that the emission from the optical system itself is minimal due to its low temperature, thus emphasizing the importance of minimizing pickup from unwanted astronomical sources of radiation, including the emission from dust in our solar system (analogous to the zodiacal light, hence 'zodi'), and the emission from warm dust in the Milky Way (Galactic 'cirrus').

antenna radiation patterns↗