Unit -V

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UNIT IV

RADIO WAVE PROPAGATION

Dr.T.V.Padmavathy Professor/ECE

RMKCET

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Presentation Outline

 Introduction

 Electromagnetic Spectrum

 Types of Waves

 Radio Frequency Bands

 Propagation Mechanisms

 Radio Propagation Effects

 Free-space Propagation

 Ground-Wave Propagation

 Slow Fading

 Fast Fading

 Propagation Modes

 Ionospheric Propagation

 Other Propagation Modes

 Usable Frequency and Angles

 Ducting

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 Radio waves are one form of electromagnetic radiation

 Electromagnetic radiation has a dual nature:

 In some cases, it behaves as waves

 In other cases, it behaves as particles (photons)

 For radio frequencies the wave model is generally more appropriate

 Electromagnetic waves can be generated by many means, but all

them involve the movement of electrical charges

Unit - V Radio Wave Propagatin 3

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Introduction

System

Frequency

Wavelength

AC current

60 Hz

5,000 km

FM radio

100 MHz

3 m

Cellular

800 MHz

37.5 cm

Ka band satellite

20 GHz

15 mm

Ultraviolet light

10

15

_Hz

₁₀

-7

_m

Light speed = Wavelength x Frequency

= 3 x 108_{m/s = 300,000 km/s} Speed, Wavelength, Frequency

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Electromagnetic Spectrum

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Types of Waves

Earth Sky wave

Space wave

Ground wave

Troposphere (0 - 12 km) Stratosphere

(12 - 50 km) Mesosphere

(50 - 80 km) Ionosphere (80 - 720 km)

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Radio Frequency Bands

Classification

Band

Initials Frequency Range Characteristics

Extremely low ELF < 300 Hz

Ground wave Infra low ILF 300 Hz - 3 kHz

Very low VLF 3 kHz - 30 kHz

Low LF 30 kHz - 300 kHz

Medium MF 300 kHz - 3 MHz Ground/Sky wave

High HF 3 MHz - 30 MHz Sky wave

Very high VHF 30 MHz - 300 MHz

Space wave Ultra high UHF 300 MHz - 3 GHz

Super high SHF 3 GHz - 30 GHz

Extremely high EHF 30 GHz - 300 GHz

Tremendously high THF 300 GHz - 3000 GHz

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Unit - V Radio Wave Propagatin 8  Three properties are shared by light and radio waves are

 Reflection

 Refraction

 Diffraction

 For both reflection and refraction, it is assumed that the surfaces

involved are much larger than the wavelength; if not, diffraction will

occur

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Propagation Mechanisms

Reflection

 Propagation wave impinges on an object which is large as

compared to wavelength

- e.g., the surface of the Earth, buildings, walls, etc.

Diffraction

 Radio path between transmitter and receiver obstructed by

surface with sharp irregular edges

 Waves bend around the obstacle, even when LOS (line of

sight) does not exist

Scattering

 Objects smaller than the wavelength of the propagation wave

- e.g. foliage, street signs, lamp posts

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Reflection

Unit - V Radio Wave Propagatin 10 Reflection of waves from a smooth surface (specular reflection)

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Other Types of Reflection

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Refraction

A transition from one medium to another results in the bending of

radio waves, just as it does with

light

Snell’s Law governs the behavior

of electromagnetic waves being

refracted:



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 As a result of diffraction, electromagnetic waves can appear to “go around corners”

 Diffraction is more apparent when the object has sharp edges, that is when the dimensions are small in comparison to the wavelength

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Radio Propagation Effects

Transmitter d Receiver

h_b

h_m

Diffracted Signal

Reflected Signal Direct Signal

Building

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Free-space Propagation

• The received signal power at distance d:

where P_t is transmitting power A_e is effective area

G_t is the transmitting antenna gain

Distance d h_b h_m 2 r

4 P

d

P

G

A

e t t





Transmitter Receiver

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Ground-Wave Propagation

 Most of the time, radio waves are not quite in free space

 Terrestrial propagation modes include:

 Line-of-sight propagation

 Space-wave propagation

 Ground waves

 Sky waves

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Land Propagation

 The received signal power:

where G_r is the receiver antenna gain,

L is the propagation loss in the channel, i.e.,

L = L_P L_S L_F

L

P

G

P

_r



t r t

Fast fading

Slow fading

Path loss

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Slow Fading

 The long-term variation in the mean level is known as slow fading

(shadowing or log-normal fading).

 This fading caused by shadowing.

 Log-normal distribution:

 The pdf of the received signal level is given in decibels by

where M is the true received signal level m in decibels, i.e., 10log₁₀m,

is the area average signal level, i.e., the mean of M,

 is the standard deviation in decibels

Unit - V Radio Wave Propagatin

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Fast Fading

 The signal from the transmitter may be reflected from objects such as hills, buildings, or vehicles.

 When MS far from BS, the envelope distribution of received signal is Rayleigh distribution.

The pdf is

 

2

,

0

2





r

e



r

p

r



where  is the standard deviation

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Propagation Modes

Ground or Surface Wave

Follow earths contour

 Affected by natural and man-made terrain

 Salt water forms low loss path

 Several hundred mile range

 2-3 MHz signal

Space Wave

 Line of Sight (LOS) wave

 Ground Diffraction allows for greater distance

 Approximate Maximum Distance, D in miles is

_D



₂

_h

_tx



₂

_h

_rx

h_rx h_tx

 No Strict Signal Frequency Limitations

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Sky Waves

 Reflected off ionosphere (20-250 miles high)

 Large ranges possible with single hop or multi-hop

 Transmit angle affects distance, coverage, refracted energy

ionosphere transmitted

wave

reflected wave refracted

wave

skip distance

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Ionospheric Propagation

 Long-range communication in the high-frequency band is possible

because of refraction in a region of the upper atmosphere called the

ionosphere

 The ionosphere is divided into three regions known as the D, E, and

F layers

 Ionization is different at different heights above the earth and is

affected by time of day and solar activity

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Ionosphere

 It is a layer of partially ionized gasses below troposphere

 ionization caused by ultra-violet radiation from the sun

 affected by: available sunlight, season, weather, terrain

 free ions & electrons reflect radiated energy

 It consists of several ionized layers with varying ion density each layer has a central region of dense ionization

Layer Altitude (miles)

Frequency Range

Availability

D 20-25 several MHz day only

E 55-90 20MHz day, partially at night F₁ 90-140 30MHz 24 hours

F₂ 200-250 30MHz 24 hours

F₁ & F₂ separate during daylight, merge at night

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Other Propagation Modes

 Tropospheric Scatter - makes use of the scattering of radio waves in the troposphere to propagate signals in the 250 MHz –5 GHz range

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Meteor-Trail Propagation

 Meteors are constantly entering the earth’s atmosphere and being

destroyed

 The meteors that enter the atmosphere leave behind an ionized trail

that can be used for communication. It is not suitable for voice

communication

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Usable Frequency and Angles

Critical Frequency

 Frequency that won’t reflect vertical transmission

 Critical frequency is relative to each layer of ionosphere

 as frequency increases  eventually signal will not reflect

m

C

N

f



9

Critical frequency

Maximum density

C

f

m N

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Usable Frequency and Angles

Frequency Trade-Off

 High frequency signals eventually will not reflect back to ground

 Lower frequency signals are attenuated more in the ionosphere

Critical Angle

Angle of radiation: transmitted energy relative to surface tangent

 Smaller angle requires less ionospheric refraction to return to earth

 Too large an angle results in no reflection

 3o_-60o_{are common angles}

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Critical angle

 Maximum angle of radiation that will reflect energy to earth

Usable Frequency and Angles

angle of radiation ionosphere

Maximum Usable Frequency (MUF)

Highest frequency useful for reflected transmissions

 absorption by ionosphere decreases at higher frequencies

 absorption of signal energy = signal loss

 best results when MUF is used

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Virtual height

Height to which a short pulse of energy sent vertically upward and traveling with the speed of light would reach taking the same two ways travel time as does the actual pulse reflected from the layer.

2 cT

h

height

Virtual

_v



T-Time required for the signal to travel from point A to C after reflection from point B.

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Ducting

 Under certain conditions, especially over water, a super refractive

layer can form in the troposphere and return signals to earth

 The signals can then propagate over long distances by alternately

reflecting from the earth and refracting from the super refractive

layer

 A related condition involves a thin tropospheric layer with a high

refractive index, so that a duct forms

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Examples of Ducting

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