Propagation of radio waves Possible propagation path Space ...jalilaziz.yolasite.com/resources/Chapter five radio wave propagation.… · Sky wave Propagation of radio waves Possible

Sky wave

Propagation of radio waves

Possible propagation path

Ground

wave

Surface wave Space wave

Reflected

wave Direct wave

Ground wave

A wave is said to be ground wave or surface wave when it propagates from transmitter

to receiver. This wave exists when both transmitting and receiving antennas are close to

the earth and the antennas are vertically polarized. Ground wave is useful at low

frequency broadcast application. The wave is attenuated as it propagates due to

imperfect nature of the earth, the attenuation is mainly as a result of the absorption and

reflection of EM energy by the earth.

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Figure (1) Ground wave between transmitting and receiving antenna

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Ground wave field strength

The ground wave field strength for flat earth is:

Where, E=field strength at a point, V/m.

Eo=field strength of the wave at a unit distance from the transmitting antenna.

A=factor of the ground losses.

d= distance of the point from transmitting antenna.

Eo depends on:

1.Power radiated by the transmitting antenna.

2.Directivity of the antenna.

Factor A depends on:

1.Conductivity, σ mho/m

2.Permittivity of the earth, Єr

3.Frequency of the wave,f

4.Distance from the transmitter,d

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P (numerical distance) =

b(phase constant) =

Df =1.8x1012(σ/f)

Df : is known as dissipation factor of the dielectric

σ: is the conductivity

Єr : relative permittivity of the earth

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Surface wave or tropospheric wave propagation

Space wave also called tropospheric wave, the wave propagates directly from

the transmitter to the receiver in

the tropospheric region. The portion of the atmosphere above the earth and

within 16km is called

troposphere. This is useful for frequencies above 30MHz. FM reception is

normally by space wave propagation.

It is also used for FM,TV,VHF and UHF bands and radar applications.

In space wave propagation, the field strength at the receiver is contributed by:

1.Direct wave from transmitter.

2.Ground reflected wave.

3.Reflected and refracted rays from the troposphere.

4.Diffracted rays around the curvature of the earth, hills and so on.

Field strength due to space wave

Where:

Eo = field strength due to direct ray at unit distance.

mV/m

ht = height of transmitting antenna.

hr = height of receiving antenna

d = distance between transmitter and receiver.

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Figure ( 2)

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Diffraction is when a wave goes through a small hole and has a flared out

geometric shadow of the slit. Diffraction is a characteristic of waves of all

types. We can hear around a corner because of the diffraction of sound

waves. For instance, if a wall is next to you when you yell, the sound will

parallel the wall. The wall may stop, but the voice doesn't; sound will

almost turn the corner of the wall. This is diffraction.

Reflection is when waves, whether physical or electromagnetic, bounce from a

surface back toward the source. A mirror reflects the image of the observer.

Refraction is when waves, whether physical or electromagnetic, are deflected

when the waves go through a substance. The wave generally changes the

angle of its general direction.

Refraction of waves involves a change in the direction of waves as they pass

from one medium to another.

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Considerations in space wave propagation

The space wave field strength is affected by the following:

1.Curvature of earth.

2. Earth’s imperfections and roughness.

3. Hills, tall buildings and other obstacles.

4. Height above the earth.

5. Transition between ground and space wave.

6.Polarization of the wave.

Figure ( 3 )

Effect of hills, buildings and other obstacles

Hills, buildings and other obstacles create shadow zone. As a result, the possible

distance of transmission is reduced.

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Atmospheric effects in space wave propagation

The atmosphere consists of gas molecules and water vapour. The density of

water vapour vary with height. As a result, the dielectric constant and hence

refractive index of air depends on the height. Dielectric decreases with

height. The variation of refractive index with height gives rise to different

phenomena like refraction,

reflection, scattering, duct propagation and fading.By definition, the refractive

index,

n is the square root of the dielectric constant.

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Duct propagation

Duct propagation is a phenomenon of propagation making use of the atmospheric

duct region. The duct region exists between two levels where the variation of

refractive index with height is minimum.

In duct propagation, the ray which is parallel to the earth’s surface round the earth in

a series of hops with successive reflections from the earth. This is shown in

figure below:

Figure ( 4 )

Duct propagation occurs at VHF, UHF and microwave range and in areas of

oceans. Long distance communication is possible when duct phenomenon

takes place.

Maximum distance between antennas

Because of the curvature of the earth there is a maximum distance from the transmitting

antenna at which the receive antenna can be sited and still be able to receive the direct

wave. This distance is known as the (radio horizon).

The radio horizon distance between transmitting and receiving antenna is :

Radio horizon distance,

The effective earth’s radius factor,

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Line of Sight (LOS)

It is defined as the distance that is covered by a direct space wave from the

transmitting antenna to the receiving antenna.

Line of sight depends on :

1.Height of receiving antenna

2.Height of transmitting antenna

3.Effective earth’s surface.

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Ionospheric wave propagation

Ionospheric wave propagation also called sky wave propagation. Electromagnetic

(EM) waves directed upward at some angle from the earth’s surface are called sky

waves. Sky wave propagation is useful in the frequency range of 2 to 30MHz and

for long distance communication.

Ionosphere is the upper region of the atmosphere between approximately 60km

and 400km above the earth which is ionized by absorbing large quantities of

radiation energy from the sun.

Ionization is the process by which a neutral atom or molecule gains or

loses electrons and is left with a net charge

Figure ( 4 ) ionospheric propagation

Characteristics of ionosphere

The physical properties of the ionosphere vary from time to time as the

temperature. Ionosphere is divided into different regions or layers and each

layer exhibits different characteristics. The layers are:

D-layer

E-Layer

F1-Layer

F2-Layer

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Characteristics of D-layer

It is the lowest layer of the ionosphere, it exists at an average height of 70km and

its thickness is 10km. It exists only during day time, its ionization properties

depend on the altitude of the sun. It reflects some VLF and LF waves but absorbs

MF and HF waves. Critical frequency of this layer is (fc = 180 kHz.)

Characteristics of E-layer

It exists next to D-layer with an average height of 100km and a thickness of about

25km. It exists only in day time and disappear during the night, it reflect some HF

waves. Its critical frequency is (fc = 4MHz) with maximum single hop range of

(2350 km).

Characteristics of F1-layer

It exists at height of about 180 km in day time with thickness of about 20 km. It

combines with F2-layer during nights, it reflects HF waves. Its critical frequency is

(fc = 5MHz) with maximum single hop range of (3000 km).

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Characteristics of F2-layer

It is the most important layer for HF communications having an average height of

about 325 km and its thickness is about 200 km. It falls to a height of 300 km at

nights as it combines with F1-layer. It exists at nights also and it highly ionized, It

offers better reflection of HF waves. Its critical frequency is (fc = 8MHz), it has

maximum single hop range of 3800 km.

Refractive index of ionosphere

Refractive index of ionosphere is defined as the ratio of phase velocity of a wave

vacuum to the velocity in ionosphere. Refractive index of ionosphere,

Figure (5) Figure ( 6)

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The relative permittivity of the ionosphere is given by:

Where,

Plasma is a completely ionized gas at a very high temperature

Mechanism of ionospheric propagation-reflection and refraction

Ionospheric propagation involves the reflection of the wave by the ionosphere as shown in figure below:

Figure (7) 19

The bending of a wave produced by the ionosphere follows optical laws. The direction of

propagating wave at a point in the ionosphere is given by Snall’s Law,

: Angle of incident

: Angle of refraction at point P in the figure above

Characteristic parameters of ionospheric propagation

Generally, propagation characteristics of the layers are described in terms of the

following parameters:

1. Virtual height, hv

2.Critical frequency, fc

3. Skip distance, ds

4. Lowest usable frequency, (LUF)

5. Critical angle, θc

6. Optimum working frequency, (OWF)

6. Maximum usable frequency, (MUF)

Virtual height:

Is defined as the height that is reached by a short pulse of energy which has the same

time delay as the original wave. 20

Ionospheric layer

hv

Figure ( 8)

Critical frequency, fc :

Is defined as the highest frequency that the wave will be reflected to earth by that layer,

below which a wave is reflected and above which it penetrates the ionosphere.

N = number of ions/m3 of the layer

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Maximum Usable Frequency, MUF

It is the highest frequency of wave that is reflected by the layer, depends on

time of day, distance, direction, season and solar activity. The common values

of MUF range between 8 to 30MHz.

Figure ( 9 )

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Figure ( 10 )

MUF

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h: height of the layer

d: distance between transmitting and receiving antennas.

Skip distance (ds)

Is defined as the shortest distance from the transmitter, measured along surface of

the earth, at which a sky wave of fixed frequency will return back to earth.

Figure ( 11) 25

Figure (12) Skip distance and skip zone

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Figure (13)

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Figure (14)

Lowest usable frequency (LUF)

The lowest frequency that can be used for communication is called LUF

Critical angle

The critical angle is the angle above which the wave will not return to earth and below which

it bends back to earth Penetrate the ionosphere

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Optimum working frequency

The frequency of wave which is normally used ionospheric communication is

known as optimum working frequency. It is generally chosen to be about 15% less

than the MUF. It is always desirable to use as high a frequency as possible since the

attenuation is inversely proportional to the frequency.

Fading

Fading is basically the undesirable variation in the intensity of the signal received at

the receiver. Hence, the fading is defined as the fluctuations in the received signal

strength caused due to variations in height and density of the ionization in different

layers. Most of the receivers are designed with an automatic volume control (AVC)

circuit which reduces the effect of fading if the change in signal strength is small.

The main causes of fading are:

1. Variation in ionospheric conditions

2. Multipath reception

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Types of fading are:

1.Selective fading: It produces serious distortion of modulated signal, AM are more

distorted by selective fading. Selective fading can be reduced by the use of SSB

system.

2.Interference fading: It produced by the interference of the rays.

3.Absorption fading: This takes place due to absorption of waves by the ionosphere.

4.Polarization fading: This takes place due to change of polarization of EM waves.

5.Skip fading: This occurs near the skip distance.

To minimize the fading, the most common method is to use automatic volume

control (AVC or AGC), in the receiver. But this control con not serve as perfect

solution for reducing fading because the signal generally drops below the noise level.

Hence the best option to minimize the fading is to use a diversity reception system.

1. Space diversity

In space diversity system, two or more receiving antennas spaced at one-half

wavelength apart are used. The receivers are arranged in such a way that the AGC

from the receiver for the strongest signal cuts off other receivers instantaneously.

Thus only the signal with more strength is passed out to common output stages.

2. Frequency diversity

In this, the transmitter will send two or more frequencies simultaneously with the

same modulating information. As the different frequencies will fade differently, one

will always be strong.

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Figure (15) Space diversity to reduce fading

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Figure (16) Frequency diversity to reduce fading

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