Variable permittivity dielectric material loaded stepped-horn antenna

Stepped-horn antenna loaded with dielectric material of variable permittivity is proposed to improve radiation characteristics and/or to increase the electrical dimensions of the radiating structure compared to unloaded empty one. A hybrid numerical technique is used to analyze such an antenna. The tapered section of the horn antenna is modeled by multi-stepped waveguide structures filled with variable dielectric constant material. Generalized scattering matrix representation of the tapered section of the horn antenna is obtained using mode matching technique. The radiating aperture problem is solved by the method of moments, under the assumption that the horn is terminated by an infinite metallic flange. Input return loss, gain, aperture efficiency and cross-polarization characteristics are studied. Comparisons indicate that any kind of loading with dielectric material along the taper tend to improve the gain level for the entire bandwidth, aperture efficiency may increase and cross-polarization level may decrease for relatively narrow bandwidths. It is observed that this is partly due to increase in the electrical size of the aperture and partly due to excitation of higher order modes. The goal of the optimization in a stepped-horn antenna is to adjust the magnitude and phase of the excited TE30 mode to achieve more uniform aperture field distribution. This has been accomplished by optimizing step length and permittivity of the enclosed dielectric material. As a result of optimization, improved gain, reduced cross-polarization and enhanced aperture efficiency characteristics have been achhieved in a stepped-horn antenna. Instead of loading only one material, using materials with different permittivities gave extra parameters to control characteristics such as the input return loss. These study can be extended for further multi-stepped structures with variable dielectric materials

Anisotropic massive Brans-Dicke gravity extension of the standard Λ\LambdaCDM model

We present an explicit detailed theoretical and observational investigation of an anisotropic massive Brans-Dicke (BD) gravity extension of the standard $\Lambda$CDM model, wherein the extension is characterized by two additional degrees of freedom; the BD parameter, $\omega$, and the present day density parameter corresponding to the shear scalar, $\Omega_{\sigma^2,0}$. The BD parameter, determining the deviation from general relativity (GR), by alone characterizes both the dynamics of the effective dark energy (DE) and the redshift dependence of the shear scalar. These two affect each other depending on $\omega$, namely, the shear scalar contributes to the dynamics of the effective DE, and its anisotropic stress --which does not exist in scalar field models of DE within GR-- controls the dynamics of the shear scalar deviating from the usual $\propto(1+z)^6$ form in GR. We mainly confine the current work to non-negative $\omega$ values as it is the right sign --theoretically and observationally-- for investigating the model as a correction to the $\Lambda$CDM. By considering the current cosmological observations, we find that $\omega\gtrsim 250$, $\Omega_{\sigma^2,0}\lesssim 10^{-23}$ and the contribution of the anisotropy of the effective DE to this value is insignificant. We conclude that the simplest anisotropic massive BD gravity extension of the standard $\Lambda$CDM model exhibits no significant deviations from it all the way to the Big Bang Nucleosynthesis. We also point out the interesting features of the model in the case of negative $\omega$ values; for instance, the constraints on $\Omega_{\sigma^2,0}$ could be relaxed considerably, the values of $\omega\sim-1$ (relevant to string theories) predict dramatically different dynamics for the expansion anisotropy.Comment: 27 pages, 6 figures, 1 tabl

CORPORATE GOVERNANCE, OWNERSHIP STRUCTURE, AND CREDIT RATINGS OF HOSPITALITY FIRMS

This study examines interrelated connections of corporate governance, ownership structure, and credit ratings. From the agency relationship perspective, the study analyzes this multiple association by accounting for firm-specific and ownership characteristics for the period between 1990 and 2007. In this context, logistic functions are used in regression models to predict the probable outcomes of these multiple relationships. Primary findings of this study revealed that hospitality firms with higher anti-takeover provisions (less shareholder power) enjoy higher credit ratings. Findings also revealed high coefficients of Gompers, Ishii, and Metrick (2003) index (the GIM index), suggesting that hospitality firms have strong governance provisions, reduced agency conflicts, and higher chances of getting better credit ratings