PrinCom [Encoded Handout VI].pdf

Ateneo de Naga University College of Engineering

ECEM416 Principles of Communications [Encoded Handout Edition VI] [@jjfm11] Page 1

RADIATION AND PROPAGATION OF WAVES

The travel of electromagnetic waves or sound waves through a medium, or the sudden electric

disturbance along a transmission line is called propagation of waves.

Such waves are either:

Light, radio, sound; heat, infrared, x-ray, ultraviolet ray, gamma radiation and

cosmic rays.

Before a wave can propagate it must be first radiated or is allowed to escape from a system.

In communication, radio wave is most

commonly employed.

Electromagnetic radiation comprises both an

Electric and a Magnetic field.

The two fields are at right-angles to each other

and the direction of propagation is right-angles

to both fields.

The Plane of the electric field defines the

Polarization of the wave.

ELECTROMAGNETIC WAVE

According to the theory Maxwell, an electromagnetic wave has the following important

characteristics:

1. It is composed of electric field and magnetic field, which are mutually perpendicular to each

other.

2. It is an oscillatory disturbance in free space that travels with a certain velocity. The light

velocity (

) in free space medium is the reference velocity.

3. It is a transverse wave (with oscillations perpendicular to the direction of propagation or

TEM wave.

4. It is a polarized wave. The basic polarizations are: Polarization refers to the physical

orientation in space of the electric field vector component of the electromagnetic wave.

Horizontal- electric field is parallel to the ground.

Vertical-electric field is perpendicular to the ground.

Elliptical-electric field rotates about the axis of the propagation direction but with

the electric and magnetic fields of unequal magnitudes.

Circular- electric field rotates about the axis of the propagation direction but the

electric and magnetic fields of equal magnitudes.

5. It is affected by the resulting medium to its attenuation or loss.



EFFECTS OF REAL ENVIRONMENT TO PROPAGATION OF RADIO WAVES (Propagation Mechanisms)

The actual propagation of waves is

normally happening closer to the ground

or along earths ground and not in free

space medium where the surrounding

environment has a direct of radio waves.

The following are important effects

attributed by the actual medium, the

earths atmosphere, to the radio wave

propagation.

Reflection of radio wave is the return or

change in direction of radio waves

striking or travelling from one medium to

another. It occurs at any boundary

between materials of differing dielectric

constants.

Refraction radio wave is the bending of

radio wave as it passes from one medium

to another in which the velocity of

propagation is different. It occurs when

the radio wave passes from one medium

to another of different density, the degree

of bending a wave at the boundary

increases with frequency.

Diffraction of radio wave is the bending of radio wave as

it passes the edges of an object or opening. It is caused

by interference between radio wave components

scattered by different parts of the object.

As a result of diffraction, electromagnetic waves can

appear to go around orners.



THE HUYGENS PRINCIPLE

A principle authored by Christian Huygens tried to

explain the existence of diffraction phenomenon of

electromagnetic waves.

A wave front is the line or curve of the crests

and troughs.

The direction of travel is perpendicular to the

wave front.

Each individual point is the center of its own

circular wave front.

A ording to his prin iple ea h point on wavelets.

PROPAGATION METHODS

Propagation methods refer to how a radio wave arrives

from a radio transmitting antenna into the receiving

antenna.

1. Ground wave method

2. Space wave method

3. Sky wave method

GROUND WAVE METHOD

The ground (surface) wave propagation method, the radio wave is radiated directly towards the

ground (earths ground is a good refle tor provided it is a good ondu tor ground).

The ground refle ts the radio wave towards the upper region of the earths atmosphere for it to be

refracted back.

It is used world-wide communication in the VLF and LF bands and for broadcasting in the MF

bands

Generally effective only up to 2MHz.

The ground wave signal should be vertically

polarized (vertical antennas are used) for it to

propagate along earths ground.

SPACE WAVE METHOD

Space wave (Tropospheric) Propagation

Method becomes compulsory when frequency generally exceeds 30MHz and beyond up to

300GHz.

It is used for sound and television broadcasting for radio relay systems, various mobile systems in

the VHF, UHF, and SHF bands.



It is limited by earths urvature (line-of-sight dependent) and heights of transmitting and

receiving antennas.

Follows two distinct paths from the transmitting antenna to the receiving antennaone through

the air directly to the receiving antenna, the other reflected form the ground to the receiving

antenna.

LIGHT-OF-SIGHT PROPAGATION

Signals in the VHF and higher range are not usually returned to earth by the ionosphere.

Most terrestrial communication at these frequencies uses direct radiation from the transmitter to

the receiver.

This type of propagation is referred to space-wave, line-of-sight, or tropospheric propagation.

SKY WAVE

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 unaffe ted by the earths surfa e and an propagate

signals over great distances.

Generally effective above 2MHz up to 30MHz

Used for HF communication systems, including long-distance radio telephony and sound

broadcasting.

EARTHS ATMOSPHERE

With respect to radio wave propagation,

there are only three layers of the

atmosphere:

1. Troposphere

2. Stratosphere

3. Ionosphere

IONOSPHERE

Where is ionosphere?

It is from the upper limit of the stratosphere to a distance approximately 400 kilometers.

Beyond ionosphere is the outer space of free space and is ionized by solar radiation.



Ionization is the conversion of atoms or

molecules into an ion by light (heating or

charging) from the sun on the upper atmosphere.

Ionization also creates a horizontal set of stratum

(layer) where each has a peak density and a

definable width of profile that influences radio

propagation.

LAYERS OF THE IONOSPHERE:

1. D layer

2. E layer

3. F layer

D LAYER

D layer is the lowest layer of the ionosphere

existing at an average daytime height of 60

km and with an average daytime thickness

of 10km.

Degree ionization in the ionosphere

depends on the altitude of the sun

above the horizon.

This layer disappears at night due to recombination process.

The D layer is not an important layer for HF propagation for its main effect is to aid surface

wave propagation.

It can also refract back to earth to VLF and LF waves.

E LAYER

E layer is next to D layer in height. It is existing about 100km during the day and with a thickness

of roughly 25km.

Its main effect is to aid MF surface wave propagation but it can also reflect some HF waves in

daytime up to approximately 20MHz.

It exist at a height of 180km in daytime and combines with layer at night, its daytime thickness

is about 20km.

Although some of HF waves are reflected from it, most pass through to be reflected form the

layer. Thus the main effect of layer is to provide more absorption for HF waves.



LAYER

Layer is the most important layer of the ionosphere for refracting HF radio waves.

Its daytime thickness is approximately 200km with height that ranges from 250-400km during the

day. At night it combines with layer and falls to a height around .

FACTORS AFFECTING THE ABILITY OF INOSPHERE TO REFRACT RADIO WAVES

1. ION DENSITY

It is the refractive ability of the ionosphere increases

with the degree of ionization. The bending of a wave

at any given frequency or wavelength and angle of

radiation will increase with increase in ionization

density. The degree of ionization is greater in

summer than in winter and is also greater during the

day than night.

2. FREQUENCY OF THE RADIO WAVE

It is the bending of the wave at any given density and

angle of radiation will increase with a decrease in

frequency. The lower frequency, the more easily the

signal is refracted. The higher the frequency the more

difficult is the refracting or bending process.

3. ANGLE OF RADIATION (AOR) OR ANGLE OF TRANSMISSION

(AOT)

It is the bending of wave at any given ionization

density and angle of radiation (that is, the wave is

farther from the horizon).

SKY WAVE PROPAGATION PARAMETERS

Maximum Useable Frequency (MUF)

It is the highest frequency wherein the signal is able to return back to earth when beamed

at a certain angle.

f f se

f f

os

where f MU , f riti al fre uen y and angle of in iden e

Ranges from8MHz-35MHz, but may increase to 50MHz.



Lowest Useable Frequency (MUF)

It is the lower limit of the range frequencies that provide useful communications.

Frequency nearest the point where reception becomes unusable would be the LUF.

Skips distance

Minimum distance over which

communication at a given frequency

can be established by means of sky

wave.

Skip zone

Area where no signal can be heard.

Hop

Refers to a single reflection of a radio

wave from the ionosphere back to

earth.

Critical Angle ( )

It is the highest angle of radiation that

will return the wave of earth at a given

density of ionization in the layer for

frequency.

OTHER PROPAGATION MODES: SCATTER PROPAGATION METHODS

Tropospheric Scatter

It make use of the scattering of radio waves in the

troposphere to propagate signals in the 250MHz-

5GHz range

Ionoscatter

It is much similar to tropospheric scatter, except

that scattering medium is the E region of the

ionosphere, with some assistance of D and F layer.

FACTORS AFFECTING OPTIUM OPERATING FREQUENCY

Location and Geography

Ionizing radiation varies with locations and altitudes.

Seasonal Variations

Variations brought by the revolution of the earth around the sun.

Diurnal Variations

Variations brought by the rotation of the earth around its axis.

Cyclical Variation

Variations brought by the solar cycle like the sunspot activities (e.g. 11 years sun spot cycle

and 27 days cycle).



IRREGURALITIES OF SPACE WAVE PROPAGATION

Superrefraction or Ducting occurs when the refractive index of the air decreases with height much

more rapidly than normal.

Duct is a region in which superrefraction occurs.

Subrefraction reduces signal strength by bending the rat away from the receiving point.

FADING

Fading is the variation in signal amplitude at the receiver caused by changes in the characteristics

of the signal path.

Fading causes the received signal to vary in amplitude, typically making it smaller.

Fading is caused by three factors:

1. Variation in distance (due to relative motion) between transmitter and receiver.

2. Changes in the environmental characteristics of the signal path (e.g. an obstruction gets

in the way).

3. The presence of multiple signal paths.

Most significant types of fading:

Shadow fading

It is caused by object coming between the transmitter and receiver.

Multipath fading

The transmitted signal takes multiple paths to the receiver due to reflections (off

buildings, water towers, hills and mountains, moving vehicles); these signals can

add to or subtract from each other at the receiver.

Fading is most pronounced at UHF and microwave frequencieswavelengths are short compared

to path distances and size of reflective surfaces.

Selective fading

It occurs at shortwave frequencies (3-30MHz) caused by relatively rapid variations

in the refraction in the ionosphere.

DIVERSITY

A diversity system uses multiple transmitters, receivers, or antennas to mitigate the problems

cause by multipath signals.

With frequency diversity, two separate sets of transmitters and receivers operating on

different frequencies are used to transmit the same information simultaneously.

Space or spatial diversity uses two antennas spaced as far apart as possible at the receiving

site.

In both types, multiple signals are received and compared or combined to determine the message.

PrinCom [Encoded Handout VI].pdf

Documents

Transcript of PrinCom [Encoded Handout VI].pdf