Backward Lobes Research Articles

Microwave Plasma Pencil for Surface Treatment: Numerical Study of Electromagnetic Radiation and Experimental Verification.

An atmospheric pressure plasma source of the microwave plasma pencil type utilizing a coaxial line is presented. The generated plasma takes the form of a cylinder up to about 30 mm long and up to 5 mm in diameter. It is suitable for surface sterilization, surface treatment, and material processing. This study numerically analyzes the electromagnetic radiation emitted by the plasma pencil, which compromises performance and poses safety risks. Electric field distributions, radiation patterns, the ratio of the power entering the discharge to the incident wave power, and the ratio of radiated power to entering power were numerically investigated for different plasma parameters and pencil lengths. Results indicate that increasing electron density, gas temperature, plasma length, and pencil length increases the radiated power by up to more than 60% of the entering power, and the radiation patterns can be highly non-uniform with strong backward lobe. The numerical finding were qualitatively confirmed experimentally. It was also found that it is possible to reduce radiation from the device by using appropriately designed cones, the presence of which does not impede its performance.

A novel wideband fractal rectangular dielectric resonator antenna with improved radiation performance

A wideband fractal rectangular dielectric resonator antenna (RDRA) based on the hook-shaped dipole is proposed for wireless communication, which exhibits stable, symmetric radiation patterns and low cross-polarization. The antenna consists of a Koch-like fractal rectangular dielectric resonator antenna (FRDRA) and a hook-shaped electric dipole (HSE-Dipole), which are equivalent to the magnetoelectric (ME) dipoles. The symmetrical radiation pattern with gain enhancement and suppression of backward lobe is achieved by the complementary combination of the electric and magnetic dipoles. The Koch-like fractal structure is used to improve the impedance matching of the RDRA with 15% size reduction. The antenna is excited by the substrate integrated waveguide (SIW) feeding network, which takes the advantage of compact size and low insertion loss. Due to the air gap between the FRDRA and HSE-Dipole, the fundamental TE1δ1 and high-order TE1δ2 modes of the RDRA are both generated to obtain the 19.5% impedance bandwidth. The prototype is fabricated and measured to validate the antenna design, which shows remarkable radiation performance including the front to back ratio (FTBR) of >20 dB, the average gain of 5.96 dBi and low cross-polarization.

A phenomenological single scatterer for studies of complex particulate media

A phenomenological single scatterer is presented for multiple-scattering studies involving complex random media. The single-scattering albedo is used to describe the proportion of scattering to absorption. A double Henyey–Greenstein function is introduced for the scattering phase function P11, allowing for realistic forward and backward lobes using the forward, backward, and total asymmetry parameters. The scattering matrix divided by P11 is defined as a weighted sum of modified electric-dipole and magnetic-dipole matrices. Angular stretching is introduced in these matrices by incorporating eccentricities in the scattering-angle arguments, allowing for single scatterers that are negatively polarizing in the backscattering regime and that show their maximum positive polarization at scattering angle differing from 90°. For the two types of scatterers separately, the scattering matrices are pure 4×4 Mueller matrices that derive from 2×2 Jones scattering amplitude matrices. In order to illustrate the application of the phenomenological scatterer, radiative-transfer coherent-backscattering computations are carried out for spherical media of such scatterers, assuming that attenuation due to extinction is exponential and described by the extinction mean-free-path length. The results are seen to be relevant for modeling photometric, polarimetric, and spectroscopic observations of small Solar System objects.

Electron impact double ionization of rare gases: A kinematical analysis of the non-first order effects at intermediate impact energy

The (e,3-1e) four-fold differential cross sections (4DCS) are measured for the electron impact double ionization of rare gas atoms (He, Ne, Ar) in coplanar asymmetric geometry for a wide range of ejected electron energies (5 to 144 eV) and at an incident energy of about 600 – 700 eV. Though we only show here a sample of the results, all experimental angular distributions of the 4DCS are characterized by large angular shifts of the forward and backward lobes with respect to the momentum transfer direction or its opposite, respectively. This validates our previously published results [Lahmam-Bennani et al 2002 J. Phys. B: At. Mol. Opt. Phys. 35 L59] which were questioned by Götz et al [2003 J. Phys. B: At. Mol. Opt. Phys. 36 L77]. A qualitative, kinematical analysis is given which allows relating these shifts and the observed structures (or sub-lobes) in the cross section distributions to the second order, 'two-step 2' double ionization mechanism, which is shown to predominate over the first-order 'shake-off' and 'two-step 1' mechanisms under the present kinematics.

Effect of ground plane size on radiation pattern in IFF/SSR microstrip antenna on thick substrate fed by H type slot

Numerical and experimental study of the effect of the ground plane size on radiation pattern of microstrip antenna on thick substrate is presented in the article. First the results of simulations and measurements of reference microstrip antenna are shown. Next the ground plane sizes were reduced in both x and y directions and the side lobes and backward lobe levels were calculated and measured. It was shown how far we can reduce the size of ground plane without important deterioration of the antenna pattern. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52:2679–2682, 2010; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25568

Predominance of the second-order, two-step mechanism in the electron impact double ionization of helium at intermediate impact energy

The (e,3–1e) four-fold differential cross sections (4DCS) are measured for the double ionization of helium in coplanar asymmetric geometry for a wide range of ejected electron energies and at an incident energy of about 600 eV. The experimental angular distributions of the 4DCS are characterized by large angular shifts of the forward and backward lobes with respect to the momentum transfer direction or its opposite, respectively. This validates our previously published results (Lahmam-Bennani et al 2002 J. Phys. B: At. Mol. Opt. Phys. 35 L59) which were questioned by Götz et al (2003 J. Phys. B: At. Mol. Opt. Phys. 36 L77). A qualitative, kinematical analysis is given which allows relating these shifts and the observed structures in the intensity distributions to the second-order, ‘two-step 2’ double ionization mechanism, which is shown to predominate over the first-order ‘shake-off’ and ‘two-step 1’ mechanisms under the present kinematics.

Measurements of the binaural temporal window using a detection task

Two experiments investigated the shape of the binaural temporal window using a detection task. In experiment 1, a 10-ms tone burst was presented binaurally out-of-phase during a burst of white noise, which changed from being interaurally uncorrelated, to correlated, and back to uncorrelated. The tone occurred during the correlated portion of the noise in one interval of each 2I-FC trial. Detection thresholds were recorded using a 2-down/1-up adaptive procedure. Thresholds were measured for different durations of correlated noise (0–960 ms), frequencies of tone burst (125, 250, 500, and 1000 Hz) and levels of noise [20, 30, 40, and 50 dB(SPL)/Hz]. Window shapes based on nine candidate functions were fitted to the data using the assumption that the binaural masking release was related to the overall interaural correlation of noise admitted by the window. Fitted windows included both a forward and a backward lobe. Gaussian functions tended to give closer fits than exponential and rounded-exponential functions, and simple functions gave more parsimonious fits that those which included dynamic-range-limiting terms. Using simple Gaussian fits, the shape of the window was largely independent of frequency and level, and the windows for individual listeners had equivalent rectangular durations ranging from 55 to 188 ms. The asymmetry was variable, although forward lobes were generally shorter than backward lobes. Experiment 2 ruled out the possibility that the forward lobe might be an artefact caused by distraction of the listener, when the interaural phase change in the noise closely followed the signal. In this experiment, the out-of-phase tone was presented during a burst of partially correlated noise which changed, after a variable interval, to a fully correlated noise. Thresholds for detecting the tone rose (i.e., performance worsened) as the interval was increased. Distraction would have produced the opposite effect.

SuperDARN interferometry: Meteor echoes and electron densities from groundscatter

The SuperDARN radars are now able to measure the angles of arrival of the backscattered radiation. We describe the analysis procedure and present results for meteor scatter from slant ranges below 500 km and ground scatter from either the E region or the F region at greater slant ranges which can be used to determine peak electron densities and their heights. The angle of arrival measurements can also be used to identify “unwanted” backscatter from the backward lobes of the antenna radiation pattern. Electron densities can also be measured in the back lobes.

High-phase-angle observations of Neptune at 2650 and 7500 Å: Haze structure and particle properties

Spatially resolved Voyager 2 PPS photometric and polarimetric observations of Neptune at 2650 and 7500 Å are presented and analyzed. A vertically homogeneous model with Rayleigh scattering and absorption fit the 2650 Å at 137° phase and below, but not the 159° phase data. The 159° phase data imply stratospheric particles of mean radius 0.20 ± 0.05 μm, with a best-fit imaginary refractive index at 2650 Å of 0.03. The optical depth of the stratospheric haze in the 5- to 100-mbar region is 0.19 ± 0.08 at 2650 Å. The 7500 Å data at latitude - 25° constrain the stratospheric haze mean radius to be less than 0.4 μm, with a best-fit value of 0.25 μm. Stratospheric haze optical depth at 7500 Å is 0.05 ± 0.02. A continuum absorber is abundant in the troposphere. If it is confined to the region below the methane haze layer, the optical depth of the methane haze can be no larger than about 0.8 (too much limb brightening is produced for larger optical depths). The methane haze optical depth at 7500 Å must be greater than 0.1. Models with this optical depth as large as 3 produced too much limb darkening if the continuum absorber is mixed in both the methane haze and deeper cloud. Tropospheric aerosols have phase functions with shalloow backward lobes, described by a double Henyey-Greenstein parameterization with g 2 (the asymmetry parameter for the backward lobe) in the range −0.07 to −0.22. The planetary phase integral (the ratio of the spherical albedo to the geometric albedo) at 7500 Å is 1.4 ± 0.1; at 2650 Å it is 1.26 ± 0.05.

Observations of the phase variation of Venus CO 2 equivalent widths

We present equivalent widths of Venus CO 2 scans of the P branch ( P8– P32) of the 5ν 3 band at 8689 Å, the P16 line of the 5ν 3 band, and the P14 line of the ν 1 + 5 ν 3 band at 7820 Å covering phase angles between 5°.1 and 170°. The equivalent widths reach a minimum at 10°, in agreement with a phase function with a backward lobe at 160° which is caused by a single internal reflection within the cloud particles. This is evidence that Venus cloud particles are composed of liquid droplets. Maximum equivalent widths are observed at ∼60°, a value which is closer to the maximum of single-layer Mie scattering models than to that of two-layer models. At high phase angles we observe equivalent widths greater than those computed from homogeneous scattering models, indicating that at high altitudes the mixing ratio of scattering particles to CO 2 increases with depth. At all phase angles, particularly at large phase angles, the temporal and spatial variations in the observed equivalent widths confuse the phase variation.

Ultrasonic Anomalies in Simple Metals due to Backward Scattering of the Conduction Electrons

The acoustic attenuation for a one-band free-electron metal has been derived using a generalized scattering function. If the latter has an appreciable backward lobe, the attenuation exhibits an anomalous peak when the sound wavelength is comparable to the electron mean free path. Similar effects might be observable in the alkalis as a result of umklapp scattering processes. If so, further study could enhance our understanding of the electron-phonon interaction in these metals.

Does spectroscopic evidence require two scattering layers in the Venus atmosphere?

The phase variation of lines in the 7820 and 7883 Å CO 2 bands has been interpreted by Hunt using an inhomogeneous, anisotropic scattering model of the Venus atmosphere. He concluded that the Venus atmosphere contains two scattering layers. We show that the observed phase variation may be due to the strong backward lobe in the Venus cloud phase function and that two cloud layers are not necessarily required.

The first backward lobe in Mie scattering by spheres

An examination is made of the intensity of light scattered nearly backwards by spherical dielectric particles illuminated by natural light. It is shown that, for particles larger than the wavelength, the intensity varies with direction in a characteristic way, agreeing well with an approximate theory at small angles.