Supergain antennas: possibilities and problems

In principle, any desired amount of gain can be developed from an antenna of arbitrary size. The phenomena of high gain from very small antennas is called “supergain”. To see why this statement might be so, the author recalls the construction used in optics, known as Huygens' principle. This states that every point on a wavefront can be regarded as a source of radiation. At the end of a short period of time, the envelope of all of these individual wavelets forms the new wavefront. For example, this construct explains why a shadow is not perfectly sharp, and why interference fringes form. End fire antennas, dipole antennas, Yagi antennas and quad antennas are examined     

Note: The intermodulation lockin analyzer

Nonlinear systems can be probed by driving them with two or more pure tones while measuring the intermodulation products of the drive tones in the response. We describe a digital lockin analyzer which is designed explicitly for this purpose. The analyzer is implemented on a field-programmable gate array, providing speed in analysis, real-time feedback, and stability in operation. The use of the analyzer is demonstrated for intermodulation atomic force microscopy. A generalization of the intermodulation spectral technique to arbitrary drive waveforms is discussed.     

Combining propensity score matching and group-based trajectory analysis in an observational study.

In a nonrandomized or observational study, propensity scores may be used to balance observed covariates and trajectory groups may be used to control baseline or pretreatment measures of outcome. The trajectory groups also aid in characterizing classes of subjects for whom no good matches are available and to define substantively interesting groups between which treatment effects may vary. These and related methods are illustrated using data from a Montreal-based study. The effects on subsequent violence of gang joining at age 14 are studied while controlling for measured characteristics of boys prior to age 14. The boys are divided into trajectory groups based on violence from ages 11 to 13. Within trajectory group, joiners are optimally matched to a variable number of controls using propensity scores, Mahalanobis distances, and a combinatorial optimization algorithm. Use of variable ratio matching results in greater efficiency than pair matching and also greater bias reduction than matching at a fixed ratio. The possible impact of failing to adjust for an important but unmeasured covariate is examined using sensitivity analysis. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Combining group-based trajectory modeling and propensity score matching for causal inferences in nonexperimental longitudinal data.

A central theme of research on human development and psychopathology is whether a therapeutic intervention or a turning-point event, such as a family break-up, alters the trajectory of the behavior under study. This article describes and applies a method for using observational longitudinal data to make more transparent causal inferences about the impact of such events on developmental trajectories. The method combines 2 distinct lines of research: work on the use of finite mixture modeling to analyze developmental trajectories and work on propensity score matching. The propensity scores are used to balance observed covariates and the trajectory groups are used to control pretreatment measures of response. The trajectory groups also aid in characterizing classes of subjects for which no good matches are available. The approach is demonstrated with an analysis of the impact of gang membership on violent delinquency based on data from a large longitudinal study conducted in Montréal, Canada. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Phase imaging with intermodulation atomic force microscopy

Intermodulation atomic force microscopy (IMAFM) is a dynamic mode of atomic force microscopy (AFM) with two-tone excitation. The oscillating AFM cantilever in close proximity to a surface experiences the nonlinear tip-sample force which mixes the drive tones and generates new frequency components in the cantilever response known as intermodulation products (IMPs). We present a procedure for extracting the phase at each IMP and demonstrate phase images made by recording this phase while scanning. Amplitude and phase images at intermodulation frequencies exhibit enhanced topographic and material contrast.     

Direct demonstration of decoupling of spin and charge currents in nanostructures

The notion of decoupling of spin and charge currents is one of the basic principles underlying the rapidly expanding field of spintronics. However, no direct demonstration of the phenomenon exists. We report a novel measurement in which a nonequilibrium spin population is created by a pointlike injection of current from a ferromagnet across a tunnel barrier into a one-dimensional spin channel and detected differentially by a pair of ferromagnetic electrodes placed symmetrically about the injection point. We demonstrate that the spin current is strictly isotropic about the injection point and, therefore, completely decoupled from the unidirectional charge current.