Design of a Self-Phased Quadrifilar Helix Antenna for Satellite Communication

The objective of this study is the design and implementation of a Quadrifilar Helix Antenna (QHA) for telemetry, tracking and control of a Low Earth Orbit (LEO) satellite. Because of its cardioid-shaped circularly polarized beam, QHA can satisfy requirements of satellite communications completely. In this paper, a variation of QHA referred to as “self-phased QHA” with a single feeding circuit, has been proposed to reduce implementation complexity. The designed antenna has been analyzed and the experimental results show that the radiation pattern provides good wide-beam in the desired frequency while the antenna’s bandwidth is around 200MHz when VSWR is less than 2

About Gravitomagnetism

The gravitomagnetic field is the force exerted by a moving body on the basis of the intriguing interplay between geometry and dynamics which is the analog to the magnetic field of a moving charged body in electromagnetism. The existence of such a field has been demonstrated based on special relativity approach and also by special relativity plus the gravitational time dilation for two different cases, a moving infinite line and a uniformly moving point mass, respectively. We treat these two approaches when the applied cases are switched while appropriate key points are employed. Thus, we demonstrate that the strength of the resulted gravitomagnetic field in the latter approach is twice the former. Then, we also discuss the full linearized general relativity and show that it should give the same strength for gravitomagnetic field as the latter approach. Hence, through an exact analogy with the electrodynamic equations, we present an argument in order to indicate the best definition amongst those considered in this issue in the literature. Finally, we investigate the gravitomagnetic effects and consequences of different definitions on the geodesic equation including the second order approximation terms.Comment: 16 pages, a few amendments have been performed and a new section has been adde

Comparison between the Plasma Levels of Long Noncoding RNA BDNF-AS in Patients with Alzheimerś disease and Healthy Subjects

BACKGROUND AND OBJECTIVE: Diagnosis of Alzheimer's disease usually occurs when serious damages have occurred in the brain and common treatments are ineffective in preventing it. One of the RNAs involved in Alzheimer's disease is a long noncoding RNA, called BDNF antisense (BDNF-AS). The aim of this study is to determine the presence and compare the BDNF-AS levels in plasma of Alzheimer's patients and healthy subjects, and to evaluate its potential as a plasma marker for Alzheimer's disease. METHODS: In this case-control study, 30 patients with late-stage Alzheimer's disease and 30 healthy subjects without neurological disease who matched the patients in terms of age were selected by a specialist according to the criteria for clinical diagnosis of Alzheimer's disease and their intravenous blood samples were collected. The plasma of the blood samples was isolated and total plasma RNA was extracted. After cDNA synthesis, the presence of BDNF-AS in plasma was examined by PCR. Finally, the relative level of BDNF-AS transcripts in plasma samples of patients with Alzheimer's disease and healthy subjects was evaluated using Real Time PCR. FINDINGS: The results of this study showed that long noncoding RNA BDNF-AS was present in the plasma of patients and controls. Comparison of Real Time PCR data showed that BDNF-AS levels in the plasma of patients (0.107±0.021) showed significant increase compared to healthy subjects (0.039 ± 0.006). CONCLUSION: The results of this preliminary study indicate that the levels of long noncoding RNA BDNF-AS in plasma can be used as a blood/plasma marker for the diagnosis of Alzheimer's disease

Boundary Term in Metric f(R) Gravity: Field Equations in the Metric Formalism

The main goal of this paper is to get in a straightforward form the field equations in metric f(R) gravity, using elementary variational principles and adding a boundary term in the action, instead of the usual treatment in an equivalent scalar-tensor approach. We start with a brief review of the Einstein-Hilbert action, together with the Gibbons-York-Hawking boundary term, which is mentioned in some literature, but is generally missing. Next we present in detail the field equations in metric f(R) gravity, including the discussion about boundaries, and we compare with the Gibbons-York-Hawking term in General Relativity. We notice that this boundary term is necessary in order to have a well defined extremal action principle under metric variation.Comment: 12 pages, title changes by referee recommendation. Accepted for publication in General Relativity and Gravitation. Matches with the accepted versio

On Higher Order Gravities, Their Analogy to GR, and Dimensional Dependent Version of Duff's Trace Anomaly Relation

An almost brief, though lengthy, review introduction about the long history of higher order gravities and their applications, as employed in the literature, is provided. We review the analogous procedure between higher order gravities and GR, as described in our previous works, in order to highlight its important achievements. Amongst which are presentation of an easy classification of higher order Lagrangians and its employment as a \emph{criteria} in order to distinguish correct metric theories of gravity. For example, it does not permit the inclusion of only one of the second order Lagrangians in \emph{isolation}. But, it does allow the inclusion of the cosmological term. We also discuss on the compatibility of our procedure and the Mach idea. We derive a dimensional dependent version of Duff's trace anomaly relation, which in \emph{four}-dimension is the same as the usual Duff relation. The Lanczos Lagrangian satisfies this new constraint in \emph{any} dimension. The square of the Weyl tensor identically satisfies it independent of dimension, however, this Lagrangian satisfies the previous relation only in three and four dimensions.Comment: 30 pages, added reference

Naked Singularity Formation In f(R) Gravity

We study the gravitational collapse of a star with barotropic equation of state $p=w\rho$ in the context of $f({\mathcal R})$ theories of gravity. Utilizing the metric formalism, we rewrite the field equations as those of Brans-Dicke theory with vanishing coupling parameter. By choosing the functionality of Ricci scalar as $f({\mathcal R})=\alpha{\mathcal R}^{m}$, we show that for an appropriate initial value of the energy density, if $\alpha$ and $m$ satisfy certain conditions, the resulting singularity would be naked, violating the cosmic censorship conjecture. These conditions are the ratio of the mass function to the area radius of the collapsing ball, negativity of the effective pressure, and the time behavior of the Kretschmann scalar. Also, as long as parameter $\alpha$ obeys certain conditions, the satisfaction of the weak energy condition is guaranteed by the collapsing configuration.Comment: 15 pages, 4 figures, to appear in GR

Horizon Problem Remediation via Deformed Phase Space

We investigate the effects of a special kind of dynamical deformation between the momenta of the scalar field of the Brans-Dicke theory and the scale factor of the FRW metric. This special choice of deformation includes linearly a deformation parameter. We trace the deformation footprints in the cosmological equations of motion when the BD coupling parameter goes to infinity. One class of the solutions gives a constant scale factor in the late time that confirms the previous result obtained via another approach in the literature. This effect can be interpreted as a quantum gravity footprint in the coarse grained explanation. The another class of the solutions removes the big bang singularity, and the accelerating expansion region has an infinite temporal range which overcomes the horizon problem. After this epoch, there is a graceful exiting by which the universe enters in the radiation dominated era.Comment: 13 pages, 2 figures, to appear in GER

Chameleonic Generalized Brans--Dicke model and late-time acceleration

In this paper we consider Chameleonic Generalized Brans--Dicke Cosmology in the framework of FRW universes. The bouncing solution and phantom crossing is investigated for the model. Two independent cosmological tests: Cosmological Redshift Drift (CRD) and distance modulus are applied to test the model with the observation.Comment: 20 pages, 15 figures, to be published in Astrophys. Space Sci. (2011