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NASA NTRS1991

Sliding backshorts for planar circuits

An adjustable planar imbedding circuit based on coplanar transmission lines is presented to deal with the impedance mismatch associated with the use of the Superconductor-Insulator-Superconductor tunnel junction. The planar embedding circuit is developed with movable noncontacting shorting elements that consist of thin metallic plates with optimized arrangements of rectangular holes. The rectangular holes are placed along the insulated metallic transmission line to provide a periodic variation of the line impedance. A large reflection coefficient is demonstrated with a scale model of the sliding backshort, and values of more than -0.5 dB are reported. The shorting elements are incorporated into a low-frequency tuning circuit to test the practical tuning ranges. The backshort can be used to fashion tuning stubs with variable post-fabrication electrical lengths which relaxes the design constraints of planar integrated circuits.

Lubecke, Victor M.↗

NASA NTRS1992

Applications of submillimeter wave technology for SDI

This paper examines the potential use of using submillimeter wave technology (actually 3 mm to 100 microns, or frequencies between 100 GHz and 3 THz) for SDI applications, especially for endo-KEW systems. The requirements and functions for a multipurpose weapon sensor are considered. The emphasis is on active sensors, but certain elements of passive sensors are mentioned that could play an important role. The rest of the paper provides information on the state-of-the-art in submillimeter wave technology, especially current component activity, and future plans.

Kile, J. N.↗

NASA NTRS1992

H I Lyman alpha emission from Saturn (1980-1990)

The characteristics of the H I Ly alpha dayglow and auroral emissions from Saturn are investigated by analyzing observations made aboard the IUE satellite on Saturn over the 1980-1990 decade (covering a complete solar activity cycle). It was found that, in this period, the dayglow emission from Saturn had significant long-term variability, with decreasing disk brightness between 1980 and 1984 and increasing brightness, as well as increased short-term (days to months) variability, between 1987 and 1990. Both the long-term and short-term variability in the Saturn Ly alpha dayglow were strongly correlated with several different solar indices. Neither the dayglow nor the auroral emission exhibited any dependence on magnetic longitude. Attempts to observe auroral activity associated with a major solar flare which produced unprecedented geomagnetic activity in March 1989 were not successful.

Mcgrath, Melissa A.↗

NASA NTRS1993

Tuners For Coplanar-Strip Transmission Lines

Sliding radio-frequency short circuits easily adjusted for optimum impedance. Tuning elements developed for use in planar integrated circuits operating at frequencies from about 10 to more than 1,000 GHz. Purpose of such adjustment to adjust impedance presented to circuit connected to nonshorted end of transmission line and thereby optimizes performance of circuit. Mixers, oscillators, amplifiers, couplers, and phase shifters among circuits improved by incorporating tuners of this type.

Mcgrath, William R.↗

NASA NTRS1992

Analysis of a novel non-contacting waveguide backshort

A new non-contacting waveguide backshort has been developed for millimeter and submillimeter wave frequencies. The design consists of a metal bar with rectangular or circular holes cut into it, which is covered with a dielectric (mylar) layer to form a snug fit with the walls of a waveguide. Hole geometries are adjusted to obtain a periodic variation of the guide impedance on the correct length scale, in order to produce efficient reflection of RF power. It is a mechanically rugged design which can be easily fabricated for frequencies from 1 to 1000 GHz and is thus a sound alternative to the miniaturization of conventional non-contacting shorts. To aid in high-frequency design, a rigorous full-wave analysis has been completed, which will allow variations of the size, number and spacing of the holes to be easily analyzed. This paper will review the backshort design and the method developed for theoretical characterization, followed by a comparison of the experimental and numerical results. Low frequency models operating from 4-6 GHz are shown to demonstrate return loss of greater than -0.2 dB over a 33 percent bandwidth. The theory is in good agreement with measured data.

Weller, T. M.↗

NASA NTRS1992

Silicon micromachined waveguides for millimeter and submillimeter wavelengths

The majority of radio receivers, transmitters, and components operating at millimeter and submillimeter wavelengths utilize rectangular waveguides in some form. However, conventional machining techniques for waveguides operating above a few hundred GHz are complicated and costly. This paper reports on the development of silicon micromachining techniques to create silicon-based waveguide circuits which can operate at millimeter and submillimeter wavelengths. As a first step, rectangular WR-10 waveguide structures have been fabricated from (110) silicon wafers using micromachining techniques. The waveguide is split along the broad wall. Each half is formed by first etching a channel completely through a wafer. Potassium hydroxide is used to etch smooth mirror-like vertical walls and LPCVD silicon nitride is used as a masking layer. This wafer is then bonded to another flat wafer using a polyimide bonding technique and diced into the U-shaped half wavelengths. Finally, a gold layer is applied to the waveguide walls. Insertion loss measurements show losses comparable to those of standard metal waveguides. It is suggested that active devices and planar circuits can be integrated with the waveguides, solving the traditional mounting problems. Potential applications in terahertz instrumentation technology are further discussed.

Yap, Markus↗

NASA NTRS1992

Onset of dispersion in Nb microstrip transmission lines at submillimeter wave frequencies

We have measured the dispersion in phase velocity of a Nb-SiO(x)-Nb microstrip transmission line resonator over a frequency range from 50 GHz to 800 GHz. A submicron Nb/Al-AlO(x)/Nb Josephson junction was used as a voltage-controlled oscillator to excite the high order modes in the resonator. The same junction is used as a direct detector resulting in a series of step-like structures in the DC current-voltage characteristic at the position of each mode frequency. The transmission line is dispersionless up to about 500 GHz where the phase velocity begins to decrease. This is well below the gap frequency f(sub g) approx. equals 700 GHz. Results agree qualitatively with the expected theoretical behavior near f(sub g). This onset of dispersion and loss in Nb transmission lines will have a significant impact on the design of submillimeter wave RF circuits.

Javadi, H. H. S.↗

NASA NTRS1992

Dispersion in Nb microstrip transmission lines at submillimeter wave frequencies

We have measured the effects of dispersion on the resonant mode frequencies of open-ended Nb-SiO(x)-Nb microstrip transmission lines over a frequency range from 50 to 800 GHz. Submicron Nb/Al-AlOx/Nb Josephson junctions were used as both voltage-controlled oscillators and detectors to sample the high order modes of the resonators. The resonator modes are equally spaced up to about 550 GHz where the mode spacing start to decrease gradually to a minimum above the gap frequency of about 700 GHz and then increases. Results are in good agreement with the expected theoretical behavior based on the Mattis-Bardeen conductivity of the superconductor line.

Javadi, H. H. S.↗

NASA NTRS1993

The spectrum of the Jovian dayglow observed at 3 A resolution with the Hopkins ultraviolet telescope

Ultraviolet spectra of the Jovian equatorial dayglow in the spectral range 830-1850 A were obtained at about 3 A resolution. The observed spectrum is dominated by electron impact excitation of the H2 Lyman and Werner band systems. Solar Lyman-beta induced fluorescence in the (6, nu-double prime) Lyman band progression is clearly identified in five distinct P(1) lines, and the contribution of solar fluorescence to the total 2.3 kR slit-averaged H2 emission rate is estimated to be 17-22 percent. The electron excitation spectrum is characterized by a relative weakness of the Werner band system and the absence of cascade contributions to the Lyman system and is very similar to that of the south polar aurora. The integrated H2 emission rate in the 900-1100 A band is a factor of two lower than that measured by the Voyager UVS. Based on model calculations, photoelectron excitation does not appear able to account for the amount of observed electron-excited H2 emission.

Feldman, P. D.↗

NASA NTRS1993

Computational study of a conical wing having unit aspect ratio at supersonic speeds

A study was conducted to identify and assess a computational method as a preliminary analysis and design tool for advanced military aircraft designs. The method of choice for this study was the Euler Marching Technique for Accurate Computation (EMTAC). Computational and experimental results were compared for a thick unit aspect ratio delta wing at Mach 2.8 and 4.0. This geometry along with the associated flow physics is representative of advanced aircraft designs. The comparisons of the lift and drag coefficients show that the computations agree with experimentally obtained data at Mach 2.8 and 4.0. Further, comparison between EMTAC and experiment shows that the computations accurately predict the overall shape and levels of the surface pressure distributions at Mach 2.8 and 4.0. Qualitative assessment of the computed flow-field properties shows that EMTAC captures the basic flow-field characteristics representative of advanced aircraft designs. The study further suggests that EMTAC can be successfully used in the preliminary analysis and design of advanced military aircraft.

Mcgrath, Brian E.↗

NASA NTRS1993

Silicon micromachined waveguides for millimeter-wave and submillimeter-wave frequencies

Rectangular waveguide is commonly used up to high millimeter-wave frequencies. However, conventional machining techniques for waveguides operating above a few hundred GHz are complicated and costly. The development of silicon micro-machining techniques to create silicon-based waveguide circuits, which can operate up to high submillimeter-wave frequencies, is reported. As a first step, WR-10 waveguide has been fabricated from (110) silicon wafers. Insertion loss measurements of gold plated silicon waveguide show performance comparable to standard metal waveguides. It is suggested that active devices and planar circuits can be integrated with the waveguides, solving the traditional mounting problems.

Mcgrath, William R.↗

NASA NTRS1993

Stability of the Io plasma torus/atmosphere interaction

The stability of the Io plasma torus-atmosphere interaction is examined. A simple plasma deflection model describes how transients in the plasma flux and the content of the atmosphere affect the ionospheric conductance, limiting the plasma bombardment and, hence, the supply of atmospheric species to the torus. The supply of the torus is seen to be determined by the thermal structure of the plasma, namely, the amount of low energy plasma producing atmospheric erosion vs. that which produces ionization, so that the torus supply rate is not simply proportional to the torus ion density.

Johnson, R. E.↗

NASA NTRS1994

Josephson Junctions Help Measure Resonance And Dispersion

Electrical characteristics of superconducting microstrip transmission lines measured at millimeter and submillimeter wavelengths. Submicron Josephson (super-conductor/insulator/superconductor) junctions used as both voltage-controlled oscillators and detectors to measure frequencies (in range of hundreds of gigahertz) of high-order resonant electromagnetic modes of superconducting microstrip transmission-line resonators. This oscillator/detector approach similar to vacuum-tube grid dip meters and transistor dip meters used to probe resonances at much lower frequencies.

Javadi, Hamid H. S.↗

NASA NTRS1994

Micromachined Silicon Waveguides

Components that handle millimeter and submillimeter wavelengths fabricated conveniently. Micromachining rectangular waveguide involves standard steps of masking, etching, and deposition of metal on silicon. Parts made assembled into half-waveguide and finally into full waveguide. Silicon-micromachining approach enables simultaneous fabrication of several versions of waveguide, with variations in critical parameter, on single wafer of silicon. Performances of versions compared and optimized more quickly and at lower cost than is possible if different versions are fabricated sequentially, by conventional machining techniques.

Mcgrath, William R.↗

NASA NTRS1994

Low-speed longitudinal aerodynamic characteristics of a flat-plate planform model of an advanced fighter configuration

A flat-plate wind tunnel model of an advanced fighter configuration was tested in the NASA LaRC Subsonic Basic Research Tunnel and the 16- by 24-inch Water Tunnel. The test objectives were to obtain and evaluate the low-speed longitudinal aerodynamic characteristics of a candidate configuration for the integration of several new innovative wing designs. The flat plate test allowed for the initial evaluation of the candidate planform and was designated as the baseline planform for the innovative wing design study. Low-speed longitudinal aerodynamic data were obtained over a range of freestream dynamic pressures from 7.5 psf to 30 psf (M = 0.07 to M = 0.14) and angles-of-attack from 0 to 40 deg. The aerodynamic data are presented in coefficient form for the lift, induced drag, and pitching moment. Flow-visualization results obtained were photographs of the flow pattern over the flat plate model in the water tunnel for angles-of-attack from 10 to 40 deg. The force and moment coefficients and the flow-visualization photographs showed the linear and nonlinear aerodynamic characteristics due to attached flow and vortical flow over the flat plate model. Comparison between experiment and linear theory showed good agreement for the lift and induced drag; however, the agreement was poor for the pitching moment.

Mcgrath, Brian E.↗

NASA NTRS1994

Fuzzy-neural control of an aircraft tracking camera platform

A fuzzy-neural control system simulation was developed for the control of a camera platform used to observe aircraft on final approach to an aircraft carrier. The fuzzy-neural approach to control combines the structure of a fuzzy knowledge base with a supervised neural network's ability to adapt and improve. The performance characteristics of this hybrid system were compared to those of a fuzzy system and a neural network system developed independently to determine if the fusion of these two technologies offers any advantage over the use of one or the other. The results of this study indicate that the fuzzy-neural approach to control offers some advantages over either fuzzy or neural control alone.

Mcgrath, Dennis↗

NASA NTRS1994

A Combined Experimental/Computational Study of Flow in Turbine Blade Cooling Passage

Laser velocimetry was utilized to map the velocity field in a serpentine turbine blade cooling passage at Reynolds and Rotation numbers of up to 25.000 and 0.48. These results were used to assess the combined influence of passage curvature and Coriolis force on the secondary velocity field generated. A Navier-Stokes code (NASTAR) was validated against incompressible test data and then used to simulate the effect of buoyancy. The measurements show a net convection from the low pressure surface to high pressure surface. The interaction of the secondary flows induced by the turns and rotation produces swirl at the turns, which persisted beyond 2 hydraulic diameters downstream of the turns. The incompressible flow field predictions agree well with the measured velocities. With radially outward flow, the buoyancy force causes a further increase in velocity on the high pressure surface and a reduction on the low pressure surface. The results were analyzed in relation to the heat transfer measurements of Wagner et al. (1991). Predicted heat transfer is enhanced on the high pressure surfaces and in turns. The incompressible flow simulation underpredicts heat transfer in these locations. Improvements observed in compressible flow simulation indicate that the buoyancy force may be important.

Tse, D. G. N.↗

NASA NTRS1993

The X-ray eclipse of the LMC binary CAL 87

ROSAT-PSPC observations of the LMC eclipsing binary CAL 87 show a short-duration, shallow X-ray eclipse which coincides in phase with the primary optical minimum. Characteristics of the eclipse suggest the X-ray emitting region is only partially occulted. Similarities with the eclipse of the accretion-disk corona in X 1822-37 are discussed. However, no temperature variation through eclipse is found for CAL 87. A revised orbital period, combining published data and recent optical photometry, is given.

Schmidtke, P. C.↗