01- BASIC RADIO THEORY

8/7/2019 01- BASIC RADIO THEORY

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BASIC RADIO THEORY

1. Introduction

2. Basic principles

Phase difference

Frequencies

Radio signal classification

3. Modulation

AM

FM

Pulse

4. Antennas

Electromagnetic wave Characteristics

Types of antennas

Polarisation

5. Wave propagation

Factors

Surface propagation

Sky propagation

Space propagation

Doppler effect


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1- INTRODUCTION

The wave travels at a constant speed.

The form of the wave moves outward.

The wave becomes smaller as it moves awayfrom the source.

The wave is sinusoidal.


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2- BASIC PRINCIPLES

Cycle: a complete sequence of positive and

negative values (passing twice through one point).

Wavelength (): the physical distance traveled by a

radio wave during one cycle of transmission.

Amplitude (A): the maximum deflection/height of the

wave. It can be positive or negative.


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Velocity (c): the speed and direction of movement

through a given medium. Is the speed of the light(300 000 000 m/sec. = 162 000 NM/sec.)

c = 300 106 m/s

c = f = cte

Period (T): the duration of one cycle. T = 1 / f

Phase (): the fraction of one wavelength expressed

in degrees.


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Frequency (f): the number of cycles

occurring in one second in a radio wave.Expressed in Hertz (Hz).

1hz = 1 cycle/sec.

The higher the frequency, the shorter the

wavelength, and vice versa.

Kilo: Khz = 103 hzMega: Mhz = 103 Khz = 106 hz

Giga: Ghz = 103 Mhz = 106 Khz = 109 hz


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2.2- FREQUENCIES

Very Low Frequency (VLF) 3 30 Khz

Low Frequency (LF) 30 300 Khz Medium Frequency (MF) 300 3000 Khz

High Frequency (HF) 3 30 Mhz

Very High Frequency (VHF) 30 300 Mhz

Ultra High Frequency (UHF) 300 3000 Mhz Super High Frequency (SHF) 3 30 Ghz

Extremely High Frequency (EHF) 30 300 Ghz


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2.3- RADIO SIGNAL CLASSIFICATION

A radio signal may be classified by three

symbols in accordance with the ITU radioregulation.

First symbol indicates the type of modulation of

the main carrier.

Second symbol indicates the nature of the signalmodulating the main carrier.

Third symbol indicates the nature of the

information to be transmitted.


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3- MODULATION

Modulation: is the technical term for the

process of impressing and transportinginformation by radio waves.

Carrier wave: is the radio wave acting as the

carrier or transporter.

Keying: Interrupting the carrier wave to breakit into dots and dashes. Morse Code.


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3.1- AMPLITUDE MODULATION

AM: The information is impressed onto the

carrier wave by altering the amplitude of thecarrier.

Used in ADF


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3.2- FREQUENCY MODULATION

FM: The information is impressed onto the

carrier wave by altering the frequency of thecarrier.

Used in VOR, Radio Altimeters, Doppler.


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3.3- PULSE MODULATION

PULSE MODULATION: A modulation form used in

radar, by transmitting short pulses followed by largerinterruptions.

Phase Modulation: used in GPS where the phase of

the carrier wave is reversed.


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4.- ANTENNAS

A wave type transducerfor the process of

converting a line AC into afree

electromagnetic wave.


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4.2- ANTENNAS CHARACTERISTICS

The simplest type of antenna is a dipole which is a

wire of length equal to one half of the wavelength.**

In a wire which is fed with an AC (alternating

current), some of the power will radiate into space.

In a wire parallel to the wire fed with an AC butremote from it, an AC will be induced.

Aerial feeders are the connection between the

transmitter/receiver and the aerial. The type of

feeder depends upon the frequency to be used.


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4.3- TYPES OF ANTENNAS


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The common different kinds of directional

antennas are:

Loop antenna used in old ADF receivers.

Parabolic antenna used in weather radars.

Slotted planar array used in more modernweather radars.

Helical antenna used in GPS transmitters.


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4.4- POLARISATION

The polarisation of an electromagnetic wavedescribes the orientation of the plane of

oscillation of the electrical component of the

wave with regard to its direction of

propagation.


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A vertically polarised antenna transmits an

electromagnetic wave with theE field perpendicular

to the Earth's surface. Simple vertical antennas can

be used to provide Omnidirectional / all directions

communication.

A Horizontally polarised antenna transmits a radio

wave with the E field parallel to the Earth's surface.

A simple horizontal antenna is bi-directional. This

characteristic is useful when you desire to minimize

interference from certain directions.


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5- WAVE PROPAGATION


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5.1- FACTORS ON PROPAGATION

Reflection: Some surfaces exhibit reflective

properties to radio energy, accepting theincoming signal and then re-radiating it.


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Refraction: The speed of the radio wave is

affected differently on either side of thecentreline of the ray. Bends the wave.


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Diffraction: A radio wave travels close to the

Earths surface. The signal is attenuated andslowed. Bends and follows the Earths

curvature.

Attenuation of an RF signal is the reductionin signal strength due to absorption,

scattering or dispersion and diffraction.


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5.2- SURFACE PROPAGATION

The surface waves travel along the surface

of the Earth, attenuates quite quickly owingto absorption.


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5.3- SKY PROPAGATION

The sky waves are refracted by the ionosphere and

returned to Earth. It allows the signal field to carry

over much larger distances than the surface wave.


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Ionosphere is the ionized component of the

Earths upper atmosphere. It is from 60 to 400 km above the surface.

It is vertically structured in three regions or

layers (layers D, E and F) and their depth

varies with time.

The electromagnetic waves refracted from

the E and F layers of the ionosphere are the

sky waves.


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5.4- SPACE PROPAGATION

The space waves are line of sight, they

travel through the air directly from thetransmitter to the receiver.


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5.5- DOPPLER EFFECT

The Doppler effect is the phenomena that the

frequency of an electromagnetic wave willincrease or decrease if there is relativemotion between the transmitter and thereceiver.

The frequency will increase if the transmitterand receiver are converging.

The frequency will decrease if they arediverging.

01- BASIC RADIO THEORY

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