1,968 research outputs found
Effect of curvature on the backscattering from leaves
Using a model previously developed for the backscattering cross section of a planar leaf at X-band frequencies and above, the effect of leaf curvature is examined. For normal incidence on a rectangular section of a leaf curved in one and two dimensions, an integral expression for the backscattered field is evaluated numerically and by a stationary phase approximation, leading to a simple analytical expression for the cross section reduction produced by the curvature. Numerical results based on the two methods are virtually identical, and in excellent agreement with measured data for rectangular sections of coleus leaves applied to the surfaces of styrofoam cylinders and spheres of different radii
On Impedance Bandwidth of Resonant Patch Antennas Implemented Using Structures with Engineered Dispersion
We consider resonant patch antennas, implemented using loaded
transmission-line networks and other exotic structures having engineered
dispersion. An analytical expression is derived for the ratio of radiation
quality factors of such antennas and conventional patch antennas loaded with
(reference) dielectrics. In the ideal case this ratio depends only on the
propagation constant and wave impedance of the structure under test, and it can
be conveniently used to study what kind of dispersion leads to improved
impedance bandwidth. We illustrate the effect of dispersion by implementing a
resonant patch antenna using a periodic network of LC elements. The analytical
results predicting enhanced impedance bandwidth compared to the reference
results are validated using a commercial circuit simulator. Discussion is
conducted on the practical limitations for the use of the proposed expression.Comment: 4 pages, 7 figure
Plane wave solutions for right‐angled interior impedance wedges
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95650/1/rds5420.pd
Diffraction of a Dipole Field by a Perfectly Conducting Half Plane
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/116080/1/rds19672111339.pd
A Theory of Radar Scattering by the Moon
A theory is described in which the moon is regarded as a "quasi-smooth" scatterer at radar frequencies. A scattered pulse is then composed of a number of individual returns each of which is provided by a single scattering area. In this manner it is possible to account for all the major features of the pulse, and the evidence in favor of the theory is presented. From a study of the measured power received at different frequencies, it is shown that the scattering area nearest to the earth is the source of a specular return, and it is then possible to obtain information about the material of which the area is composed. The electromagnetic constants are derived and their significance discussed
High-latitude artificial aurora using the EISCAT high-gain HF facility
The EISCAT high-frequency (HF) transmitter facility at Ramfjord, Norway, has been used to accelerate F-region electrons sufficiently to excite the oxygen atoms and nitrogen molecules, resulting in optical emissions at 630, 557.7 and 427.8 nm. During O-mode transmissions at 5.423 MHz, using 630 MW effective radiated power, in the hours after sunset on 12 November 2001 several new observations were made, including: (1) The first high-latitude observation of an HF induced optical emission at 427.8 nm and (2) Optical rings being formed at HF on followed by their collapse into a central blob. Both discoveries remain unexplained with current theories
Self-Assembly in the Growth of Precious Opal
It is proposed that primary nucleation of amorphous microspherulites of
hydrated silica in natural proto-precious-opal can be followed by a long range
superlattice ordering process by means of electrostatic self-assembly.
Necessary conditions in the thermodynamics are a high surface charge density on
microspherulite surfaces, a long Debye length and an appropriate number density
of nucleation centres. A further chemical requirement is a high alkaline
environmental pH from 9 to 10. It is also proposed that the characteristic
concentric spherical shell-like structure of spherulites, centred on primary
nuclei, are due to sequential deposition of intrinsic salts which precipitate
out when the corresponding solubility limits in the liquid are successively
exceeded. It can be that the better-known sedimentation of microspherulites
under gravity only plays part in the final stabilization period of overall
growth.Comment: 12 pages pdf http://dx.doi.org/10.1016/j.jcrysgro.2009.09.04
Derivation and application of a class of generalized impedance boundary conditions, part 2
Boundary conditions involving higher order derivatives are presented by simulating surfaces whose reflection coefficients are known analytically, numerically, or experimentally. Procedures for determining the coefficients of the derivatives are discussed, along with the effect of displacing the surface where the boundary conditions are applied. Provided the coefficients satisfy a duality relation, equivalent forms of the boundary conditions involving tangential field components are deduced, and these provide the natural extension to non-planar surfaces. As an illustration, the simulation of metal-backed uniform and three-layer dielectric coatings is given. It is shown that fourth order conditions are capable of providing an accurate simulation for the uniform coating at least a quarter of a wavelength in thickness. Provided, though, some compromise in accuracy is acceptable, it is also shown that a third order condition may be sufficient for practical purposes when simulating uniform coatings
The domain of organizational cognitive neuroscience:theoretical and empirical challenges
In this editorial, the authors respond to the 2011 article in the Journal of Management by Becker, Cropanzano, and Sanfey, titled “Organizational Neuroscience: Taking Organizational Theory Inside the Neural Black Box.” More specifically, the authors build on the ideas of Becker et al. first to clarify and extend their work and then to explore the critical philosophical issues involved in drawing inferences from neuroscientific research. They argue that these problems are yet to be solved and that organizational researchers who wish to incorporate neuroscientific advances into their work need to engage with them
A comparison of overshoot modelling with observations of polar mesospheric summer echoes at radar frequencies of 56 and 224 MHz
We have compared radar observations of polar mesospheric summer echoes (PMSEs) modulated by artificial electron heating, at frequencies of 224 MHz (EISCAT VHF) and 56 MHz (MORRO). We have concentrated on 1 day of observation, lasting ∼ 3.8 h. The MORRO radar, with its much wider beam, observes one or more PMSE layers all the time while the VHF radar observes PMSEs in 69% of the time. Statistically there is a clear difference between how the MORRO and the VHF radar backscatter reacts to the heater cycling (48 s heater on and 168 s heater off). While MORRO often reacts by having its backscatter level increased when the heater is switched on, as predicted by Scales and Chen (2008), the VHF radar nearly always sees the "normal" VHF overshoot behaviour with an initial rapid reduction of backscatter. However, in some heater cycles we do see a substantial recovery of the VHF backscatter after its initial reduction to levels several times above that just before the heater was switched on. For the MORRO radar a recovery during the heater-on phase is much more common. The reaction when the heater was switched off was a clear overshoot for nearly all VHF cases but less so for MORRO. A comparison of individual curves for the backscatter values as a function of time shows, at least for this particular day, that in high layers above ∼ 85 km height, both radars see a reduction of the backscatter as the heater is switched on, with little recovery during the heater-on time. These variations are well described by present models. On the other hand, the backscatter in low layers at 81-82 km can be quite different, with modest or no reduction in backscatter as the heater is switched on, followed by a strong recovery for both radars to levels several times above that of the undisturbed PMSEs. This simultaneous, nearly identical behaviour at the two very different radar frequencies is not well described by present modelling
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