Atmospheric Effects in RF Propagation

The atmospheric effects of interest for RF propagation are refraction/reflection, scattering, and absorption/attenuation.With the exception of refraction, these effects are all minimal below 30MHz. Between 30MHz and 1GHz, refraction/reflection is the primary concern. Above 1GHz or so, attenuation starts to be a significant factor and refraction/reflection becomes less of an issue except for nearly horizontal paths. Atmospheric multipath also starts to be observed above 1GHz and can cause extreme fading on terrestrial microwave links. The effects of interest for propagation analysis are in the troposphere and, to a limited extent, the tropopause.The stratosphere is well approximated as free-space.

Refractive and scattering effects of the atmosphere include:

• Refraction on horizontal paths resulting in alteration of the radio horizon due to ray curvature.

• Troposcatter, from localized fluctuations in the atmospheric refractive index, which can scatter electromagnetic waves.

• Temperature inversion, abrupt changes in the refractive index with height causing reflection.

• Ducting, where the refractive index is such that electromagnetic waves tend to follow the curvature of the earth.

These effects vary widely with altitude, geographic location, and weather conditions. The effects can permit beyond-the-horizon communication (or interference), or produce blockage and diffraction from terrain that appears to be below the line of sight and multipath fading.

Reference:

An excerpt from John S. Seybold, "Introduction to RF Propagation", John Wiley and Sons Inc., 2005.

Authors in LaTeX!

To produce

write this code just below the title

\author{ \textbf{Syed Ahsaan Rizvi \& Muhammad Ali Khalid} \\ \small Electrical Engineering Department, Military College of Signals\\ \small National University of Sciences and Technology (NUST), Pakistan}

For authors with different affiliations, check this link
http://fourlovesfour.blogspot.com/2009/10/latex-tip-author-affiliations.html

Large Scale Data Analysis

The past decade has seen the increasing availability of very large scale data sets, arising from the rapid growth of transformative technologies such as the Internet and cellular telephones, along with the development of new and powerful computational methods to analyze such datasets. Such methods, developed in the closely related fields of machine learning, data mining, and artificial intelligence, provide a powerful set of tools for intelligent problem-solving and data-driven policy analysis. These methods have the potential to dramatically improve the public welfare by guiding policy decisions and interventions, and their incorporation into intelligent information systems will improve public services in domains ranging from medicine and public health to law enforcement and security.

In Pakistan, there are so many organizations that are always in need of better mechanism to handle such a large data and use to take decisions effectively and quickly. Take the telecom sector, for instance, almost all the decisions are made by taking out KPIs from a large set of data. A lot of research work is going on worldwide in this arena and there still exists a lot of potential!

Equations in LaTeX!

1) Latex Code for

\begin{equation}
E(\theta,\phi) = -j\frac{Z_{0}}{2\lambda}\frac{e^{-jkr}}{r}\sum\limits_{m=0}^{M} le_{m}(\theta)i_{m}a_{m}(\theta,\phi)
\end{equation}


2) Latex Code for 

\begin{equation}
\min_{x}{\frac{1}{2}x^{T}Hx+c^{T}x+b}
\end{equation}

NUST graduate’s book published in Germany

NUST graduate’s book published in Germany

Reconfigurable Antennas - Beam Steering

Recently, reconfigurable antennas have attracted significant interest to implement several operating characteristics by using a single antenna. Various methods are reported in the literature to achieve reconfigurability in frequency, polarization, and radiation pattern. Printed microstrip antennas are widely preferred in reconfigurable antenna applications because of their low profile and easy integration with active devices.

Beam steering refers to the changing of the direction of the wave lobe that contains the maximum power of a radiation blueprint. This is the lobe that displays the greatest field strength, which basically means the one with the best reach and the strongest of signals. Typically, beam steering with reflective apertures is performed using mechanical rotation of the reflecting surface. Limited electronic steering can be accomplished using a focusing reflective surface by changing the feed location or direction of incidence. To achieve beam steering more than 10 degrees, different phase shifting mechanisms using varactors can be used such as 90 hybrid loaded by the varactor diodes. Furthermore, the use of MEMS capacitors will also increase the overall efficiency of the antenna because the insertion loss of MEMS capacitances is small compared with varactor diodes.

Microstrip Line

Printed transmission lines are widely used in circuits and antennas as they are compact, light weight and broadband in frequency. The microstrip line is a transmission-line geometry with a single conductor trace on one side of a dielectric substrate and a single ground plane on the opposite side. Since it is an open structure, microstrip line has a major fabrication advantage over stripline.

Another advantage of microstrip over stripline is that all active components can be mounted on top of the board. The disadvantages are that when high isolation is required such as in a filter or switch, some external shielding may have to be considered. Given the chance, microstrip circuits can radiate, causing unintended circuit response. A minor issue with microstrip is that it is dispersive, meaning that signals of different frequencies travel at slightly different speeds. Microstrip does not support a TEM mode, because of its filling factor.

All microstrip equations are approximate. The effective dielectric constant is a seen to be a function of the ratio of the width to the height of the microstrip line (W/H), as well as the dielectric constant of the substrate material.

[1] http://mcalc.sourceforge.net/

[2] http://paginas.fe.up.pt/~hmiranda/etele/microstrip_basics.pdf