Tlap Report Faisal

7/30/2019 Tlap Report Faisal

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Muhammad Faisal Bashir

2008-EE-47

Transmission Lines and Antenna

Propagation

DEPARTMENT OF ELECTRICAL

ENGINEERING

Bahauddin Zakariya University,

Multan.


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Important Terms:

Tropospheric Scatter:The propagation of electromagnetic waves by scattering as a result of irregularities or discontinuities in the physical

properties of the troposphere. At the frequencies above 150 MHz, the atmosphere has a scattering effect on electromagnetic

fields. The scattering allows over-the-horizon communications at very high, ultrahigh, and microwave frequencies. This

mode of communication is called tropospheric scatter, or troposcatter . Scattering can take place at low altitudes, but mostly

it is at about 10 miles. To realize communication via troposcatter, overlong distances, high-gain antennae, high-power

transmitters, and sensitive receivers are a must as path loss is high.

The magnitude of the received signal depends on the number of turbulences

causing scatter in the desired direction and the gain of the receiving antenna.

The scatter area used for tropospheric scatter is known as the scatter volume.

The angle at which the receiving antenna must be aimed to capture the scattered energy is called the scatter angle.

The scatter volume and scatter angle are shown in figure above:


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Direct Wave:

A radio wave that is propagated directly through space from transmitter to receiver without being refracted by the

ionosphere. An interesting example of direct communications is satellite communications. If a satellite is placed in an orbit

22,000 miles above the equator, it appears to stand still in the sky, as viewed from the ground. A high gain antenna can be

pointed at the satellite to transmit signals to it.

The satellite is used as a relay station, from which approximately of the earths surface is visible. The satellite receives

signals from the ground at one frequency, known as the uplink frequency, translates this frequency to a different frequency

known as the downlink frequency, and retransmits the signal. Because two frequencies are used, the reception and

transmission can happen simultaneously. A satellite operating in this way is known as a transponder. The satellite has a

tremendous line of sight from its vantage point in space and many ground stations can communicate through a single

satellite.

Reflected Waves:

The reflected wave reflects off the earth in going from the transmitting antenna to the receiving antenna.

Together, the reflected wave and the direct wave are called the space wave.


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Ionospheric waves:

It is a sky wave that is reflected by the ionosphere. Also a radio wave that is reflected back to earth by

e ionosphere or a communications satellite; permits transmission around the curve of the earth's surface.

Sky waves:

The sky wave, often called the ionospheric wave, is radiated in an upward direction and returned to Earth at some distant

location because of refraction from the ionosphere. This form of propagation is relatively unaffected by the Earth's surface

and can propagate signals over great distances. Usually the high frequency (hf) band is used for sky wave propagation.

Ground Waves:

It is a wave used for radio communications signal propagation on the long, and medium wave bands for local radio

communications.


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Spherical Waves:

A wave whose equiphase surfaces form a family of concentric spheres; the direction of travel is always perpendicular

to the surfaces of the spheres.

Space Field:

A space field is a physical quantity associated with each point of spacetime. A field can be classified as a scalar field, a

vector field, a spinor field, or a tensor field according to whether the value of the field at each point is a scalar, a vector,

a spinor (e.g., a Dirac electron) or, more generally, a tensor, respectively.


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Ionospheric Propagation:

Propagation of radio waves over long distances by reflection from the ionosphere, useful at frequencies up to about 25megahertz.

The ionosphere is important for radio wave (AM only) propagation.ionosphere is composed of the D, E, and F layersthe D layer is good at absorbing AM radio wavesD layer disappears at night. the E and F layers bounce the waves back to the earththis explains why radio stations adjust their power output at sunset and sunrise

Diffraction:

Diffraction is the slight bending of light as it passes around the edge of an object. The amount of bending depends

on the relative size of the wavelength of light to the size of the opening. If the opening is much larger than the

light's wavelength, the bending will be almost unnoticeable. However, if the two are closer in size or equal, the

amount of bending is considerable, and easily seen with the naked eye.


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Refraction:

Refraction is responsible for image formation by lenses and the eye. Refraction is the bending of a wave when it enters a

medium where it's speed is different. The refraction of light when it passes from a fast medium to a slow medium bends

the light ray toward the normal to the boundary between the two media. The amount of bending depends on

the indices of refraction of the two media and is described quantitatively by Snell's Law.

Reflection:Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront retur

into the medium from which it originated. Common examples include the reflection of light, sound and water waves.

The law of reflection says that for specular reflection the angle at which the wave is incident on the surface equals the ang

at which it is reflected. Mirrors exhibit specular reflection.


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Huygenss Principle:

The principle that any point on a wave front of light may be regarded as the source of secondary waves and that the surface that is

tangent to the secondary waves can be used to determine the future position of the wave front. A surface tangent to the wavelets

constitutes the new wave front and is called the envelope of the wavelets. If a medium is homogeneous and has the same properties

throughout (i.e., is isotropic), permitting light to travel with the same speed regardless of its direction of propagation,

the three-dimensional envelope of a point source will be spherical; otherwise, as is the case with many crystals, the envelope will be

ellipsoidal in shape.

Inverse square Law:

Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law.

This comes from strictly geometrical considerations. The intensity of the influence at any given radius r is the source strength divided

the area of the sphere. Being strictly geometric in its origin, the inverse square law applies to diverse phenomena. Point sources of

gravitational force, electric field, light, sound or radiation obey the inverse square law. It is a subject of continuing debate with a sour

such as a skunk on top of a flag pole; will it's smell drop off according to the inverse square law?


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Polarization:

When light travels through a linear polarizing material, a selected vibration plane is passed by the polarizer, while electric

vectors vibrating in all other orientations are blocked. Linearly polarized light transmitted through a polarizer can be eithe

passed or absorbed by a second polarizer, depending upon the electric vector transmission azimuth orientation of the

second polarizing material. This tutorial explores the effect of rotating two polarizers on an incident beam of white light.

Linear Polarization:

A linear polarized antenna radiates wholly in one plane containing the direction of propagation. Where a circular polarized

antenna, the plane of polarization rotates in a circle making one complete revolution during one period of the wave. If the

rotation is clockwise looking in the direction of propagation, the sense is called right-hand-circular (RHC). If the rotation

counter clockwise, the sense is called left-hand-circular (LHC).


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Circular Polarization:

Circularly polarized light consists of two perpendicular electromagnetic plane waves of equal amplitude and 90 difference

in phase. The light illustrated is right- circularly polarized. Circularly polarized light may be produced by passing

linearly polarized light through a quarter-wave plate at an angle of 45 to the optic axis of the plate.

Elliptical Polarization:

Elliptically polarized light consists of two perpendicular waves of unequal amplitude which differ in phase by 90.

If the thumb of your right hand were pointing in the direction of propagation of the light, the electric vector would be

rotating in the direction of your fingers.

Types of propagation paths:

Line of sight propagation path: The simplest and most easily understood way in which a signal travels from one antenna

to another is by 'line-of-sight ' propagation. Line-of-sight propagation requires a path where both antennas are visible to

one another and there are no obstructions. VHF and UHF communication typically use this path.

Unless you are VERY close to your destination, you need to keep the antenna as high as possible. Because radio waves

follow a straight-line in this mode, they simply go off into space as the curvature of the earth causes the ground to drop

away beneath the radio waves.

As we elevate the antenna, the distance to the horizon gets further and further away. With enough power to reach the

other antenna and a high enough antenna to see it, we can talk without problems. VHF repeaters are usually mounted on

high buildings or mountain tops for this very reason. When you are operating with a small VHF hand held, your signal

must be able to travel in a straight-line to the repeater or your signal will be lost to someone beyond line-of-sight.


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Ground Wave: Ground-wave propagation is NOT a variation of line-of-sight propagation. In fact, ground-waves

will travel further because the curved surface of the earth pulls the signal down and keeps it along the ground. You can

reach an antenna which is below the horizon with ground-wave propagation, one you can't 'see' by line-of-sight

propagation. BUT, since the signal interacts with the ground, it loses a lot of energy as it travels, severely limiting it's

range.

Sky wave: HAVE to use sky-wave propagation. In this mode, the signal you send out radiates UP towards the

ionosphere, 30 to 250 miles above the surface of the earth. Depending on how ionized it is and the frequency you're

using, it will act more or less like a mirror to reflect your signal back to the earth some distance away. Your signal will

literally SKIP over a wide section of country and come back down to earth many miles away. At this point, it can bereflected again to bounce a second time (and some times even more) back to earth again. It's like shining a searchlight up

and having it reflected back to illuminate the ground many miles from the light. You can only talk to those people in the

patch of earth that your antenna is 'illuminating' with it's signal. The place that the signal skipped over is called the skip

zone.

Troposphere:

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and

99% of its water vapor and aerosols. The average depth of the troposphere is approximately 17 km (11 mi) in the middle

latitudes. It is deeper in the tropics, up to 20 km (12 mi), and shallower near the polar regions, at 7 km (4.3 mi) in

summer, and indistinct in winter. The lowest part of the troposphere, where friction with the Earth's surface influences air

flow, is the planetary boundary layer. This layer is typically a few hundred meters to 2 km (1.2 mi) deep depending on the

landform and time of day. The border between the troposphere and stratosphere, called the tropopause, is a temperature

inversion.


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