Electromagnetic Wave Theory II

Lecture 8

Ground Wave Propagation Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example

AM radio

Disadvantages .Requires relatively high transmission power

.They are limited to very low, low and medium frequencies which require large antennas

.Losses on the ground vary considerably with surface material

Ground Wave Propagation

Advantages

Given enough power they can be used to communicate between any two points in the world

They are relatively unaffected by changing atmospheric conditions

Space wave propagation This includes radiated energy that travels in

the lower few miles of the earth’s atmosphere. They include both direct and ground reflected waves.

Direct waves travel in essentially a straight line between the transmitting and receiving antennas. The most common name is line of sight propagation.

The field intensity at the receiving antenna depends on the distance between the two antennas and whether the direct and ground reflected waves are in phase.

Line-of-Sight Propagation

Line-of-Sight Propagation Transmitting and receiving antennas must be within

line of sight Satellite communication – signal above 30 MHz not reflected

by ionosphere Ground communication – antennas within effective line of

site due to refraction Refraction – bending of microwaves by the atmosphere

Velocity of electromagnetic wave is a function of the density of the medium

When wave changes medium, speed changes Wave bends at the boundary between mediums

Line-of-Sight Equations Optical line of sight

Effective, or radio, line of sight

d = distance between antenna and horizon (km) h = antenna height (m) K = adjustment factor to account for refraction,

rule of thumb K = 4/3

hd 57.3

hd 57.3

Line-of-Sight Equations Maximum distance between two antennas

for LOS propagation:

h1 = height of antenna one

h2 = height of antenna two

2157.3 hh

LOS Wireless Transmission Impairments Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise

Sky Wave Propagation

Signal reflected from ionized layer of atmosphere back down to earth

Signal can travel a number of hops, back and forth between ionosphere and earth’s surface

Reflection effect caused by refraction Examples

Amateur radio CB radio

Sky Wave Propagation

For many years, numerous organisations have been employing

the High Frequency (HF) spectrum to communicate over long

distances. It was recognised in the late 30's that these

communication systems were subject to marked variations in

performance, and it was hypothesised that most of these

variations were directly related to changes in the ionosphere.

Sky Wave Propagation

Considerable effort was made to investigate ionospheric

parameters and determine their effect on radio waves and

the associated reliability of HF circuits. World-wide noise

measurement records were started and steps were taken to

record observed variations in signal amplitudes over various

HF paths.

Sky Wave Propagation

The results of this research established that ionised regions

ranging from approximately 70 to 1000 km above the earth's

surface provide the medium of transmission for electromagnetic

energy in the HF spectrum (2 to 30 MHz) and that most

variations in HF system performance are directly related to

changes in these ionised regions. The ionisation is produced in

a complex manner by the photoionization of the earth's high

altitude atmosphere by solar radiation.

Within the ionosphere, the recombination of the ions and

electrons proceeds slowly enough (due to low gas densities) so

that some free electrons persist even throughout the night. In

practice, the ionosphere has a lower limit of 50 to 70 km and

no distinct upper limit, although 1000 km is somewhat

arbitrarily set as the upper limit for most application purposes.

Sky Wave Propagation

The vertical structure of the ionosphere is changing

continuously. It varies from day to night, with the seasons of

the year, and with latitude. Furthermore, it is sensitive to

enhanced periods of short-wavelength solar radiation

accompanying solar activity. In spite of all this, the essential

features of the ionosphere are usually identifiable, except

during periods of unusually intense geomagnetic disturbances.

Sky Wave Propagation

The presence of free electrons in the ionosphere produces the

reflecting regions important to High Frequency (HF) radio-wave

propagation. In the principal regions, between the approximate

heights of 75 km and 500 km, the electrons are produced by the

ionising effect of ultraviolet light and soft x-rays from the sun. for

convenience in studies of radio-wave propagation, the ionosphere is

divided into three regions defined according to height and ion

distribution: the D,E, and F regions.

PREDICTABLE IONOSPHERIC

PARAMETERS

Each region is subdivided into layers called the D,E, Es, F1, and F2

layers, also according to height and ion distribution. These are not

distinctly separated layers, but rather overlapping regions of

ionisation that vary in thickness from a few kilometres to hundreds of

kilometres. The number of layers, their heights, and their ionisation

(electron) density vary both geographically and with time. At HF, all

the regions are important and must be considered in predicting the

operational parameters of radio communication circuits.

Sky Wave Propagation

The D region lies between the approximate limits of 75 and 90 km

above the earth's surface.

The electron density is relatively small compared with that of the other regions, but, because of collisions

between the molecules of the atmosphere and free electrons excited by the presence of an electromagnetic

wave, pronounced energy loss occurs. This energy loss, dissipated in the form of thermal energy of the

electrons or thermal (electromagnetic) noise, is termed absorption. Higher in the E and F regions, electron

collisions with atmosphere molecules can also affect the condition for reflection that occurs wherever there is a

marked bending of the wave. This is explained by the fact that as the wave nears its reflecting level, there is a

slowing down or retardation effect, which allows additional time for collisions to occur and thus for absorption

to take place. Absorption of this type is called deviative absorption.

Because of the low electron density, the D region does not reflect useful transmissions in the frequency range

above 1 MHz. However, D-region absorption is important at all frequencies and, because its ionization is

produced by ultraviolet solar radiation, it is primarily a daytime phenomenon

The degree of absorption is expressed by the absorption factor. After sunset in the D region, ionization

decreases rapidly and non-deviative absorption becomes negligible 2 to 3 hours later.

Non-deviative D-region absorption is the principal cause of the attenuation of HF sky waves, particularly at the

lower frequencies during daylight hours.

The D region

The approximate true height range of the regular E layer is

well established at 90 to 130 km and it is assumed that the

maximum electron density occurs at 110 km and the semi-

thickness is 20 km.

For communication, the most important characteristic feature of the E region is the temporal

and geographic variation of its critical frequency. In almost all other respects, the features of

the E layer are very predictable compared with those of the F2 layer.

A large volume of vertical-incidence ionosonde data has been collected over about three solar

cycles, and many features of the E region are therefore well known. The minimum virtual

height of the E region and the variation of maximum electron density within this region as a

function of time and geographic location are readily obtained from the ionograms.

THE E REGION

THE F REGION

For HF radio communications, the F region is the most important part

of the ionosphere. It is not regular and because of its variability, short time scale

estimates of the important F-region characteristics are required if predictions of the

operational parameters of HF radio systems are to be meaningful

There are many characteristic features of the F region important to HF

radio communications. This layer is actually divided into two separate

layers, F1 and F2 layers.

The F1 layer is of importance to communication only during daylight

hours or during ionospheric storms; it lies in the height range of about

200 to 250 km and undergoes both seasonal and solar cycle variations,

which are more pronounced during the summer and in high sunspot

periods.

The F2 layer is located between 250 to 350 km above the earth’s

surface. During the night the F1 and F2 layers combine into a single

layer

If we consider a wave of frequency , f incident on an ionospheric layer

whose maximum density is N then the refractive index of the layer is

given by

Effects of the Ionosphere on the Sky wave

2

811

f

Nn

If the frequency of a wave transmitted vertically is

increased, a point will be reached where the wave will not

be refracted sufficiently to curve back to earth and if this

frequency is high enough then the wave will penetrate the

ionosphere and continue on to outer space. The highest

frequency that will be returned to earth when transmitted

vertically under given atmospheric conditions is called the

critical frequency.

Critical Frequency

Nfc 9

There is a best frequency for communication between any two

points under specific ionospheric conditions. The highest frequency

that is returned to earth at a given distance is called the Maximum

Usable Frequency (MUF).

Maximum Usable Frequency

sec9 Nfmuf

This is the frequency which provides the most consistent

communication and is therefore the best to use. For transmission

using the F2 layer it is defined as

Optimum Working Frequency

sec985.0 Nfowf

This is set by the attenuation in the ionosphere. A practical

value of this is usually taken as 3 MHz.

Lowest Usable Frequency

Satellite Communication

In these systems a communication satellite is placed into

synchronous orbit about 22 000 mi above the earth’s surface. The

transmitter sends a signal using a highly directional antenna to

the satellite. This signal is reamplified within the satellite and

transmitted back to earth. This allows transoceanic links,

frequencies range from 1 GHz to 40 GHz. The received signals

and the retransmitted signals are usually at different carrier

frequencies.

Satellite-Related Terms Earth Stations – antenna systems on or near earth Uplink – transmission from an earth station to a

satellite Downlink – transmission from a satellite to an

earth station Transponder – electronics in the satellite that

convert uplink signals to downlink signals

Ways to CategorizeCommunications Satellites Coverage area

Global, regional, national Service type

Fixed service satellite (FSS) Broadcast service satellite (BSS)

General usage Commercial, military, amateur, experimental

Classification of Satellite Orbits Circular or elliptical orbit

Circular with center at earth’s center Elliptical with one foci at earth’s center

Orbit around earth in different planes Equatorial orbit above earth’s equator Polar orbit passes over both poles Other orbits referred to as inclined orbits

Altitude of satellites Geostationary orbit (GEO) Medium earth orbit (MEO) Low earth orbit (LEO)

Geometry Terms Elevation angle - the angle from the

horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite

Minimum elevation angle Coverage angle - the measure of the portion

of the earth's surface visible to the satellite

Minimum Elevation Angle Reasons affecting minimum elevation angle

of earth station’s antenna (>0o) Buildings, trees, and other terrestrial objects

block the line of sight Atmospheric attenuation is greater at low

elevation angles Electrical noise generated by the earth's heat

near its surface adversely affects reception

GEO Orbit Advantages of the the GEO orbit

No problem with frequency changes Tracking of the satellite is simplified High coverage area

Disadvantages of the GEO orbit Weak signal after traveling over 35,000 km Polar regions are poorly served Signal sending delay is substantial

LEO Satellite Characteristics Circular/slightly elliptical orbit under 2000 km Orbit period ranges from 1.5 to 2 hours Diameter of coverage is about 8000 km Round-trip signal propagation delay less than 20

ms Maximum satellite visible time up to 20 min

LEO Categories Little LEOs

Frequencies below 1 GHz 5MHz of bandwidth Data rates up to 10 kbps Aimed at paging, tracking, and low-rate messaging

Big LEOs Frequencies above 1 GHz Support data rates up to a few megabits per sec Offer same services as little LEOs in addition to voice

and positioning services

MEO Satellite Characteristics Circular orbit at an altitude in the range of 5000 to

12,000 km Orbit period of 6 hours Diameter of coverage is 10,000 to 15,000 km Round trip signal propagation delay less than 50

ms Maximum satellite visible time is a few hours

Frequency Bands Available for Satellite Communications

Satellite Link Performance Factors Distance between earth station antenna and

satellite antenna For downlink, terrestrial distance between earth

station antenna and “aim point” of satellite Displayed as a satellite footprint (Figure 9.6)

Atmospheric attenuation Affected by oxygen, water, angle of elevation, and

higher frequencies

Satellite Footprint

Sat

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Con

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ratio

ns

The power relation between a transmitted and received power of

any space wave is given as follows

Power Budget for SATCOM

dBfdGGP

PdBrdBt

dBt

r1010 log20log205.32

where Pr is the received power

Pt is the transmitted power

Gt is the gain of the transmitting antenna

Gr is the gain of the receiving antenna

d is the distance (km) between the antennas

f is the frequency in MHz

1. What is the horizon for a transmitting antenna height 225 feet above ground level? What is the total horizon if the receiver is of height 25 feet above ground level?

2. If the transmitting antenna is 1000ft above ground level and the receiving antenna is 20 ft high what is the radio horizon?

Examples

3. Determine the distance to the radio horizon for an antenna

40 ft above sea level

4. Calculate the radio horizon for a 500 ft transmitting antenna

and receiving antenna of 20 ft. calculate the required increase in

height for the receiving antenna if a 10% increase in radio horizon

were required.

5. Calculate the power received at a satellite given the following

conditions

Power gain of the transmitting antenna is 30 000

The transmitter drives 2 kW of power into the antenna at a

carrier frequency of 6.21 MHz

The satellite receiving antenna has a power gain of 30

The transmission path is 45 000 km

6. Determine the maximum distance between identical

antennas equally distant above sea level