chapter 1-radio wave july 08

8/4/2019 Chapter 1-Radio Wave July 08

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CHAPTER 1

Radio-Wave Propagation

Chapter 1

Prepared by: Pn Siti Zura A. Jalil 1


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Introduction

Chapter 1Prepared by: Pn Siti Zura A. Jalil

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Radio waves are one form of electromagnetic radiation.

Other forms include infrared, visible light, ultraviolet, X-rays and gamma rays.

Radio frequencies occupy the range of 15 kHz to 300GHz.

Electromagnetic Spectrum


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Introduction


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Once launched, radio waves can travel throughfree space and through any dielectric materials,including air.

Any good dielectric will pass the radio waves.


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Electromagnetic (EM) Wave


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Another name for radio wave.

An Electromagnetic waveinvolves the creation of electricfield (E) and magnetic field (H) in free space or in somephysical medium.

Thus, the waves that propagate are known as Transverseelectromagnetic (TEM) waves.

Both field are perpendicular to

each other and also perpendicularto the propagation direction.

This means that the electric field,the magnetic field and the

direction of travel of the wave areall mutually perpendicular

(H)


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Polarization


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The polarization of an EM wave is determinedby the direction of electric field (E) vector.

The E field ishorizontal (x direction),

and the wave istherefore said to behorizontally polarized.


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Wavefront


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The simplest source of electromagneticwaves would be a point in space, with wavesradiating equally in all directions.

Such as source is called an isotropicradiator.

A surface joining all points of equal phase is

called wavefront.


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Free-Space Propagation


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Radio waves propagate through free spacein a straight line at speed of light(300,000,000 m/s = 300 x 106 m/s).

There is no loss of energy in free space, butthere is attenuation due to the spreading ofthe waves.


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Attenuation of Free Space


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An isotropic radiator would produce spherical wavessince no energy would be absorbed by free space, sothat its radiates equally in all directions.

The power density of an isotropic radiator is simply be

the total power divided by the surface area of thesphere, according to the square-law:

PD

Pt

4r

2

where PD = power density in watts persquare meter

Pt = total power in watts

r = distance from the antenna in meters


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Characteristic Impedance of freespace


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An EM wave propagating through space consists ofelectric and magnetic fields, perpendicular both to eachother and to the direction of travel of the wave.

The relationship between electric and magnetic fieldintensities is analogous to the relation between voltageand current in circuits.

This relationship is expressed by:

Z of free space is 377 ohm.

H

EZ )377(


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Power Density


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Similar to power and voltage relate in electriccircuit, the power density, PD and the electricfield E are related to impedance as:

Z

EPD

2

where PD

= power density in watts per squaremeter

E = electric field strength in volts per meter

Z = impedance of the medium in ohms


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Chapter 1Prepared by: Pn Siti Zura A. Jalil11

PD = Pt / 4r2

PD = E2 /Z

E = 30 Pt ; Z = 377 r

where E = electric field strength in volts permeter

Pt = total power in watts

r = distance from the source in meters

The strength of a signal is often given in terms of its electricfield intensity rather than power density. So that,


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Questions:

1. What power density is required to produce anelectric field strength of 100 v/m in air?

2. A signal has a power density of 50 mW/m2 in

free space. Calculate its electric and magneticfield strengths.

3. A power of 100 W is supplied to an isotropicradiator. What is the power density at a point 10km away?

4. Find the electric field strength for the signal inthe question 3.


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Effect of environment

Once radio signal has been radiated, it travels orpropagates through space and ultimately reaches thereceiver.

The energy level of the signal decreases rapidly with thedistance from the transmitter.

The EM wave also affected by objects that it encountersalong the way such as trees, buildings, and other largeobjects.

The path of EM signal takes to receiver depends uponmany factors, including the frequency of the signal,atmospheric conditions and time of day.

All these factors can be taken into account to predict thepropagation of radio waves from transmitter to receiver.Chapter 1Prepared by: Pn Siti Zura A. Jalil

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Optical properties of Radio waves


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Reflection, Refraction, and Diffraction.

These three properties are shared by lightand radio waves.

Radio waves are identical to light wavesexcept for the frequency.


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Reflection

Light waves are reflected by a mirror.

Any conducting surface looks like mirror to radio wave,and so radio waves are reflected by any conductingsurface they encounter along a propagation path.

Example of conducting surface: Any metallic objectssuch as building parts, water towers, automobiles,airplanes and power line .

Example of partially conducting surface: earth andbodies of water.

Reflection of waves from a conducting surface results inthe angle of reflection being equal to the angle ofincidence.


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Other Types of Reflection

Chapter 1 Prepared by: Pn Siti Zura A. Jalil 16

Corner reflector Parabolic reflector Diffuse Reflection

same angle ofincidence andreflection but differentorientation.

Have focus point to pass energy


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Refraction


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Refraction is referred as the bending of the radiowaves path.

Refraction occurs when waves pass from a

medium of one density to another medium withdifferent density.

Snells Law governs the behavior of

electromagnetic waves being refracted:n1sin1 n2 sin2


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Diffraction

Light and radio waves travel in a straight line.

If an obstacle appears between transmitter and receiver,some of the signal is blocked, creating a shadow zone.

A receiver located in the shadow zone cannot receive acomplete signal.

However, some signal usually gets through due tophenomenon of diffraction, the bending of waves aroundan object.

Diffraction is the phenomenon whereby waves travelingin straight paths bend around an obstacles.

The EM waves can appear to go around corners.



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Diffraction

Sharp edge acts as a new isotropic source ofradio waves, radiates as spherical wave frontsfrom the source.

This effect is the result of Huygens principle,advances by Dutch astronomer ChristianHuygens in 1690.

The principle states that each point on aspherical wavefront may be considered as thesource of a secondary spherical wavefront.


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Diffraction around an object

Chapter 1

20

Prepared by: Pn Siti Zura A. Jalil


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Radio-Wave Propagation ThroughSpace


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Most of the time, radio waves are not quite infree space.

Terrestrial propagation modes include:

Ground wavesSky waves

Space-wave (Line-of-sight )propagation

Tropospheric Scatter


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Ground-Wave Propagation


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Propagates follow the surface of the earth.

The wave follow the ground and the curvature of theearth, and can propagate far beyond the horizon.

It has vertical polarization to propagated from an

antenna.

to minimize currents induced in the ground itself,which results in losses.

Horizontally polarized waves are absorbed or shorted by

the earth.

Operate at frequencies3 kHz to 2 MHz (VLF, LF, MF).

Above 2 MHz the ground waves attenuated veryquickly.


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As the distance of waves propagation increases fromthe transmitter, there is a tendency for the waves to tilt

toward the horizontal, increasing losses andeventually, the ground waves will dies down.

The waves tilt toward the horizontal depend to thefrequency value;

higher frequency the waves tilt quickly -transmission at closer distance.

Short transmission distance: At High frequency with Low power.

Long transmission distance:

At Low frequency with High power.


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At LOW frequency with the sufficient power, the ground waves ableto propagates around the world more than one times before its

energy loss.

The conductivity of the earth also determines how well ground

waves are propagated. The better the conductivity, the less theattenuation and the greater the distance the waves can travel.

It much better to propagate over water (especially salt waterexcellent conductor) than very dry (poor conductivity) desertterrain.

Applications Example:- Millitary TX = 76 Hz,

- International navigation (LORAN-C)= 100 kHz.

- Standard AM broadcast

- submarine communications- ELF, 30 to 300Hz


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Space-Wave Propagation


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Used for signals transmission in the VHF andhigher range.

Two types of space waves:

Direct waveReflected wave

The direct wave is most widely used mode.

uses direct radiation from the transmitter tothe receiver

But this direct space wave does have severelimitation limited to line-of-sighttransmissiondistances.


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Line of sight Propagation

Direct or space waves are not refracted, not dothey follow the curvature(go through in straightline) of the earth.

Because of their straight-line nature, direct wavesignal travel horizontally from the transmittingantenna until they reach the horizon, at whichpoint they are blocked.

If a direct wave signal is to be received beyond thehorizon, the receiver must be high enough tointercept it.


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The Maximum Distance of SpaceWave


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To calculate the maximum distance depends tothe height of both transmitter and receiverantenna.

The higher height above the ground the better.

The maximum distance between transmitter andreceiver:

d = 17hT + 17hRwhere

d = maximum distance in kilometers (km)

hT = height of the transmitting antenna inmeters

hR = height of the receiving antenna in


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Questions:

1. A taxi company uses a central dispatcher, with anantenna at the top of a 20 m tower, to communicate withtaxi cabs. The taxi antennas are on the roofs of the cars,approximately 1.8 m above the ground. Calculate the

maximum communication distances:I. Between the dispatcher and taxi

II. Between two cabs

2. A transmitter tower located at Block S UTM KL has aheight of 350 m above the sea level. It can transmit asignal for a distance of 160 km. A receiver tower islocated on a 200 m hill above sea level. Calculate theheight of receiver tower.


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Line of sight Propagation

Line of sight communication is characteristic of mostradio signals with frequency above 30 MHz, particularlyVHF, UHF and microwave signals.

Transmission distances at those frequencies are

extremely limited, and it is obvious why very hightransmitting antennas must be used for FM and TVbroadcast.

The antennas for transmitters an receivers operating at

VHF are typically located on top of the tall buildings oron mountains, to increases the range of transmissionand reception.

To extend the communication distance, special

technique named repeater stations have beenChapter 1Prepared by: Pn Siti Zura A. Jalil

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Li f i ht P ti


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Line of sight Propagation -Repeater

A repeater is combination of a receiver and transmitteroperating on separate frequencies.

The receiver picks up a signal from a remote transmitter,amplifies it, and retransmits it (on another frequency) toa remote receiver.

Usually, the repeater is located between the transmittingand receiving stations, therefore extends thecommunication distance.

Repeaters have extremely sensitive receivers and highpower transmitters, their antennas are located at highpoints.


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Ghosting in TV reception


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This occurs when the signal reflect from the largeobjects like hills or buildings.

There not only phase cancellation but also timedifferences between the direct and reflected waves.

This condition results a ghosts that appear in Televisionreception.

Ghosts: When the same signal arrives at the TVreceiver at two different times; the reflected signal hasfarther to travel and is weaker than the direct signal,resulting in a double image.

For fixed receivers this problem is reduce by usingdirectional receiving antenna.


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Sky-Wave (Ionospheric) Propagation


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Most frequent used method for long distancetransmission in the high-frequency (HF) band.

The concept of radiating is the radiating signal towardthe ionosphere and refracted back by the ionosphere to

the ground, reflected to the ionosphere and so on.

The refracting and reflecting action is called skipping.

(lantunan)


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Atmosphere Layers


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Free

spaceearth

troposphere stratosphere ionosphere

Sky-wavepropagation


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Ionosphere layers


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vary in height from 40 to 400 km above theearths surface.

can be divided into 3 regions known as the D, E,

and F (F1 & F2) layers.

Earth D E F1 F2

Ionosphere layers

40 km

360 km


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Ionosphere layers


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The presence or absence of these layers in the ionosphere andtheir height above the earth vary with the position of the sun.

The sun (solar radiation) is an agent of ionization.

Ionization is different at different heights above the earth and is

affected by time of day and solar activity. At high noon, radiation from the sun in the ionosphere is greatest,

while at night it is minimal.

The level of ionization is increases with height above the earth andgreater in the daytime.

At night, when the solar radiation is not received, the D and Eregions disappear and the F1 & F2 layers combine into a single Flayers.

The F layers remains during the night.


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Chapter 1Prepared by: Pn Siti Zura A. Jalil36

Layers of the ionosphere


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Layers of the ionosphere


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D layer Ranges from 40 km to 88 km

Has low ionization level (farthest from the sun).

Exists only during daylight hours.

E layer

Approximately 80 km to 145 km.

Known as Kennely-Heaviside layer.

Exists only during daylight hours.

F layer

Exists from about 45 km to 400 km. Closest to the sun, are the most highly ionized.

During the daylight hours, it consist of two layers, F1& F2.

Responsible for HF signal transmission.


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Ionosphere Propagation


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The signal returned from the ionosphere by aform of refraction.

If the signal is not refracted enough to reach the

earth, it may absorbed or may pass rightthrough the atmosphere into space.

In the daytime, the D&E layers absorbfrequencies below 8 or 10 MHz.

Frequencies above this (up to 30 MHz) arerefracted by F1 & F2 layers and may return toearth.


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Ionosphere Propagation


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At night, the D&E layers virtually disappear,allowing lower frequencies to reach the Flayers without being absorbed.

These frequencies are refracted by F layers.

Thus, propagation at lower frequencies isbetter at night than during the day.

The higher frequencies pass right through all the

layers at night. So, propagation at frequencies above 10

MHz tends to be better during the daylighthours.


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Multi-Hop or Multi-skip


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The signal reflect from the ground and make two ormorehops before reaching its final destination.

Each reflection from the ground or refraction in theionosphere greatly reduces its strength.

For strong signals and ideal ionospheric conditions, asmany 20 hops are possible.

Multi-hop transmission can extend the communicationrange by thousand of km.

The maximum distance of single hops is about 3220km, but with multi-hops, transmission all the wayaround the world are possible.


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Frequency Diversity


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Used by large users of high-frequencycommunications (shortwave broadcastingand the military).

They transmit on a number of differentfrequencies that span the HF spectrum.

Hope that at least one of these will work at

a given time.

Ionospheric so nding & Critical


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Ionospheric sounding & Critical

Frequency


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Ionospheric sounding is used to make somemeasurements on which to base propagationpredictions.

A signal is sent straight up, then the signal willbe returned to earth and can be picked up bythe receiver.

At low frequencies, the signal will be absorbed.

The highest frequency that is returned toearth in the vertical direction is called thecritical frequency.


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Critical Angle


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Above the certain frequency, when wavestransmitted vertically, it will continue to freespace.

However, if the angle of propagation is lowered(from the vertical), a portion of high frequencywaves below the critical frequency is returned toearth.

The highest angle at which a wave of a specificfrequency can be propagated and still returnedto the earth is called critical angle.

Maximum Usable Frequency


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Maximum Usable Frequency(MUF)


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The highest frequency that is returned to earth over agiven distance is called the maximum usable frequency(MUF).

Since absorption decreases with increasing frequency, it

would be expected that a frequency at or just below theMUF would give best results.

85% of MUF is called optimum working frequency(OWF).

To predict the MUF : fm = fc / cos 1Where fm = MUF

fc = critical frequency

1

= angle of incidence


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Skip Zone & Skip Distance

Skip zone is determine as theregion on which the ground-wave completely dissipated tothe first refraction of sky-wave.

No signal will be heard.

This region called quiet or skipzone.

Skip distance is the minimum

distance from the TX to wherethe sky wave returned to earthand also occurs for energypropagated at the critical

angle. Chapter 1

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Special case of sky-wave propagation.

It makes use of the scattering of radio waves in thetroposphere to propagate signals in the 350 MHz to 10GHz range.

But, the best and most widely used frequencies are 0.9,2 and 5 GHz.

Troposcatter can give reliable communicationdistances up to 644 km.


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The TX antenna is aimed inthe direction of the RX, butthe RX is over the horizon.

Most of the transmitted

energy continues on intospace, but a small portionof it is scattered, and asmall fraction of the

scattered energy reachesthe receiver.

Thus, it requiring morepowerful transmitter and

more sensitive receiver. Chapter 1

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It is an inefficient system,requiring larger transmitter

power, antenna with highergain and more sensitivereceivers than line-of-sightcommunication.

It is also subject to fading

can be reduced by usingspatial diversity at bothends, each station has atleast two antennasseparated by 100

wavelength or more.

Troposcatter can operate at amuch greater range than line-of-

sight communication. thus, reducing the

requirement for repeaterstations.

Great benefit when thecommunications path is over

water or over difficult terrain suchas mountains, between islandsor when a foreign, possiblyunfriendly government controlsthe territory between ends of link.

Chapter 1

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Disadvantages Advantages

Common propagation problem


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Common propagation problem -

Fading

Fading is one of the primary effect of radio wavespropagation.

Fading is the variation in signal amplitude at the receivercaused by characteristics of signal path and changes in it.

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

Fading is caused by four factors:

Variation in distance between transmitter and receiver, Changes in the environmental characteristics of the

signal path,

The present of the multiple signal paths,

Relative motion between transmitter and receiver.Chapter 1Prepared by: Pn Siti Zura A. Jalil

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Common propagation problem


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Fading can occur on signals of anyfrequency, but it is most pronounced inUHF and microwave communication,

where the signal wavelengths are veryshort compared to path distances and sizeof reflecting surfaces.

Fading is a problem of long distanceshortwave communication.


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Common propagation problem -

Fading


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Solutions to Fading

To accommodate severe fading problems, diversityreception is used

Frequency diversity - transmission of the sameinformation on slightly different frequencies.

Spatial diversity - comprises two or more receivingantennas separated by 50 wavelengths or more. Bestreceived signal will be selected.

Angle diversity - transmission of the same

information at two or more slightly different angles.

Polarized diversity - the capability of receivinghorizontally or vertically polarized signals.

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chapter 1-radio wave july 08

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