Alignment fixture for millimeter waveguide

An alignment fixture for handling the precise waveguide connection procedures required in performing gain measurements of millimeter-wave antennas is described. Two versions of the fixture were developed: one for small devices, such as standard gain horns, which can be supported by the fixture, and another for larger devices. These systems, along with a properly-installed flange, can reduce the measurement uncertainty associated with the connection from greater than 1 dB to a few hundredths of a decibel. For measurements in the WR-22 and WR-15 waveguide bands, the overall gain error for millimeter-frequency antenna measurements has decreased from 0.31 dB to 0.10 dB     

Accurate measurement of antenna gain and polarization at reduced distances by an extrapolation technique

A new technique is described for determining power gain and polarization of antennas at reduced range distances. It is based on a generalized three-antenna approach which, for the first time, permits absolute gain and polarization measurements to be performed without quantitative a priori knowledge of the antennas. The required data are obtained by an extrapolation technique which includes provisions for rigorously evaluating and correcting for errors due to proximity and multipath interference effects. The theoretical basis provides a convenient and powerful approach for describing and solving antenna measurement problems, and the experimental method employed illustrates the utility of this approach. Examples of measurements are included which exhibit errors in gain as small aspm0.11dB (3sigma).     

Water regulation in aestivating snails. Ultrastructural and analytical evidence for an unusual cellular phenomenon

Urinary excretion of immunoglobulins of the A, G and M classes was studied in 14 children suffering from viral hepatitis without HBAg antigenemia, and in 12 children with viral hepatitis accompanied by HBAg antigenemia. The control group consisted of 11 healthy children. Over fourfold increase in excretion of IgA, and only twofold increase in IgG were observed. The difference in excretion of A and G immunoglobulins, despite their similar molecular weight, was attributed to increased active excretion of IgA by the kidneys, probably due to stimulation by the virusemia. In no case were immunoglobulins of the M class encountered in the urine.     

Towards real-time GOMS: a model of expert behaviour in a highly interactive task

We present an analysis of an expert performing a highly interactive computer task. The analysis uses GOMS models, specifying the Goals, Operators, Methods, and Selection rules used by the expert. Two models are presented, one with function-level operators which perform high-level functions in the domain, and one with keystroke-level operators which describe hand movements. For a segment of behaviour in which the expert accomplished about 30 functions in about 30 s, the function-level model predicted the observed behaviour well, while the keystroke-level model predicted only about half of the observed hand movements. These results, including the discrepancy between the models, are discussed.     

Broadband CPN demonstrator using wavelength and time divisionmultiplexing

RACE Project 1036 is developing the technology for a broadband customer premises network (CPN) using wavelength and time division multiplexing (WTDM). Features include close wavelength spacing (4 nm in the 1500 nm band), high-speed time multiplexing (2.5 Gbit/s, STM-16), and a layered control protocol. An initial demonstrator with two wavelengths carrying TV signals at 155 Mbit/s was exhibited in the autumn of 1990 and is now evolving into the final demonstrator due at the end of 1992 which will show 16 wavelengths and a 2.5 Gbit/s multiplex of HDTV, TV, and sound. The authors describe the devices made in the Project, the demonstrators, and the performance results achieved so far     

Interfaces for the ordinary and beyond

Several ways in which current computer interfaces discriminate against those who do not fit the accepted profile of the average user are described. A needs continuum for describing users' abilities and characteristics is presented. The parameters affecting a person's position within the continuum are discussed. The ways in which interface designers take into account a range of needs is also discussed     

A real-time electrical impedance tomograph

Electrical properties of tissues in the human body can be imaged using a technology known as Electrical Impedance Tomography. In this modality, sinusoidal electrical currents are applied to the body using electrodes attached to the skin, and voltages that are developed on the electrodes are measured. Using these data, a reconstruction algorithm computes the conductivity and permittivity distributions within the body. This paper describes the reconstruction algorithm, image display algorithm, and hardware of a real-time Electrical Impedance Tomograph known as the Real-Time Imaging System. The reconstruction algorithm, executed by a commercially available coprocessor board that resides in a 386-based personal computer, is a modification of the Newton's One Step Error Reconstructor (NOSER) that minimizes algorithm execution time by precomputing many quantities. The image display algorithm, also executed by the coprocessor board, maps the output of the reconstruction algorithm into an image which is displayed using a video graphics board. The architecture of the system and execution times of algorithms implemented by the system are discussed. Using the continuous data acquisition mode of the Real-Time Imaging System, data from the thorax of a normal human subject were collected. Admittivity changes in the chest, as a result of respiration and the cardiac cycle, are presented. Data that were collected from the leg of a normal subject are shown which demonstrate capabilities of the triggered data acquisition mode of the system, allowing data acquisition synchronization with an electrocardiogram     

Cytotoxic T cell responses to multiple conserved HIV epitopes in HIV-resistant prostitutes in Nairobi

Many people who remain persistently seronegative despite frequent HIV exposure have HIV-specific immune responses. The study of these may provide information about mechanisms of natural protective immunity to HIV-1. We describe the specificity of cytotoxic T lymphocyte responses to HIV in seronegative prostitutes in Nairobi who are apparently resistant to HIV infection. These women have had frequent exposure to a range of African HIV-1 variants, primarily clades A, C, and D, for up to 12 yr without becoming infected. Nearly half of them have CTL directed towards epitopes previously defined for B clade virus, which are largely conserved in the A and D clade sequences. Stronger responses are frequently elicited using the A or D clade version of an epitope to stimulate CTL, suggesting that they were originally primed by exposure to these virus strains. CTL responses have been defined to novel epitopes presented by HLA class I molecules associated with resistance to infection in the cohort, HLA-A*6802 and HLA-B18. Estimates using a modified interferon-gamma Elispot assay indicate a circulating frequency of CTL to individual epitopes of between 1:3,200 and 1:50,000. Thus, HIV-specific immune responses-particularly cross-clade CTL activity- may be responsible for protection against persistent HIV infection in these African women.     

A VLSI for real-time linear operations and transforms

A VLSI system, utilizing 16 systolic array multipliers, designed to compute vector-matrix products at a rate of 640×10⁶ MACs is presented. The 448,000-transistor, 1.6-μm CMOS device incorporates a dual timing scheme which allows multiplexing of hardware units over identical operations. This hardware balances maximum internal operating frequency with external data bandwidth and results in an improved ration of the signal throughput to silicon area. This system has wide application because of its ability to compute correlation, convolution, linear transforms, and connections in multilayer perceptrons