Chapt-10.pdf

CHAPTER 10

RADIO WAVES

ELECTROMAGNETIC WAVE PROPAGATION

1000. Source of Radio Waves

at zero, increases to a maximum as the rotor completes one

quarter of its revolution, and falls to zero when the rotor

completes one half of its revolution. The current then

approaches a negative maximum; then it once again returns

to zero. This cycle can be represented by a sine function.

The relationship between the current and the magnetic

field strength induced in the conductor through which the

current is flowing is shown in Figure 1001 . Recall from the

discussion above that this field strength is proportional to the

magnitude of the current; that is, if the current is represented

by a sine wave function, then so too will be the magnetic field

strength resulting from that current. This characteristic shape

of the field strength curve has led to the use of the term

“wave” when referring to electromagnetic propagation. The

maximum displacement of a peak from zero is called the

amplitude . The forward side of any wave is called the wave

front . For a non-directional antenna, each wave proceeds

outward as an expanding sphere (or hemisphere).

One cycle is a complete sequence of values, as from crest

to crest. The distance traveled by the energy during one cycle

is the wavelength , usually expressed in metric units (meters,

centimeters, etc.). The number of cycles repeated during unit

time (usually 1 second) is the frequency . This is given in hertz

(cycles per second). A kilohertz (kHz) is 1,000 cycles per

second. A megahertz (MHz) is 1,000,000 cycles per second.

Wavelength and frequency are inversely proportional.

The phase of a wave is the amount by which the cycle

Consider electric current as a flow of electrons along a

conductor between points of differing potential. A direct

current flows continuously in the same direction. This would

occur if the polarity of the electromotive force causing the

electron flow were constant, such as is the case with a battery.

If, however, the current is induced by the relative motion

between a conductor and a magnetic field, such as is the case

in a rotating machine called a generator , then the resulting

current changes direction in the conductor as the polarity of the

electromotive force changes with the rotation of the

generator’s rotor. This is known as alternating current .

The energy of the current flowing through the

conductor is either dissipated as heat (an energy loss

proportional to both the current flowing through the

conductor and the conductor’s resistance) or stored in an

electromagnetic field oriented symmetrically about the

conductor. The orientation of this field is a function of the

polarity of the source producing the current. When the

current is removed from the wire, this electromagnetic field

will, after a finite time, collapse back into the wire.

What would occur should the polarity of the current

source supplying the wire be reversed at a rate which

exceeds the finite amount of time required for the electro-

magnetic field to collapse back upon the wire? In this case,

another magnetic field, proportional in strength but exactly

opposite in magnetic orientation to the initial field, will be

formed upon the wire. The initial magnetic field, its current

source gone, cannot collapse back upon the wire because of

the existence of this second electromagnetic field. Instead,

it propagates out into space. This is the basic principle of a

radio antenna, which transmits a wave at a frequency

proportional to the rate of pole reversal and at a speed equal

to the speed of light.

1001. Radio Wave Terminology

The magnetic field strength in the vicinity of a

conductor is directly proportional to the magnitude of the

current flowing through the conductor. Recall the

discussion of alternating current above. A rotating

generator produces current in the form of a sine wave. That

is, the magnitude of the current varies as a function of the

relative position of the rotating conductor and the stationary

magnetic field used to induce the current. The current starts

Figure 1001. Radio wave terminology.

151

152

RADIO WAVES

. Generally, the origin is not important,

principal interest being the phase relative to that of some

other wave. Thus, two waves having crests 1/4 cycle apart

are said to be 90

gamma rays, and cosmic rays. These are included in Table

1002. Waves shorter than 30 centimeters are usually called

microwaves .

Within the frequencies from 1-40 gHz (1,000-40,000

MHz), additional bands are defined as follows:

“out of phase.” If the crest of one wave

occurs at the trough of another, the two are 180

out of

L-band: 1-2 gHz (1,000-2,000 MHz)

S-band: 2-4 gHz (2,000-4,000 MHz

C-band: 4-8 gHz (4,000-8,000 MHz)

X-band: 8-12.5 gHz (8,000-12,500 MHz)

Lower K-band: 12.5-18 gHz (12,500-18,000 MHz)

Upper K-band: 26.5-40 gHz (26,500-40,000 MHz)

phase.

1002. The Electromagnetic Spectrum

The entire range of electromagnetic radiation frequen-

cies is called the electromagnetic spectrum . The

frequency range suitable for radio transmission, the radio

spectrum , extends from 10 kilohertz to 300,000 mega-

hertz. It is divided into a number of bands, as shown in

Table 1002.

Below the radio spectrum, but overlapping it, is the au-

dio frequency band, extending from 20 to 20,000 hertz.

Above the radio spectrum are heat and infrared, the visible

spectrum (light in its various colors), ultraviolet, X-rays,

Marine radar systems commonly operate in the S and

X bands, while satellite navigation system signals are found

in the L-band.

The break of the K-band into lower and upper ranges is

necessary because the resonant frequency of water vapor

occurs in the middle region of this band, and severe absorp-

tion of radio waves occurs in this part of the spectrum.

Band

Abbreviation

Range of frequency

Range of wavelength

Audio frequency

20 to 20,000 Hz

15,000,000 to 15,000 m

Radio frequency

10 kHz to 300,000 MHz

30,000 m to 0.1 cm

Very low frequency

VLF

10 to 30 kHz

30,000 to 10,000 m

Low frequency

30 to 300 kHz

10,000 to 1,000 m

Medium frequency

300 to 3,000 kHz

1,000 to 100 m

High frequency

3 to 30 MHz

100 to 10 m

Very high frequency

VHF

30 to 300 MHz

10 to 1 m

Ultra high frequency

UHF

300 to 3,000 MHz

100 to 10 cm

Super high frequency

SHF

3,000 to 30,000 MHz

10 to 1 cm

Extremely high

frequency

EHF

30,000 to 300,000 MHz

1 to 0.1 cm

Heat and infrared*

10 6 to 3.9

10 8 MHz

0.03 to 7.6

10 -5 cm

Visible spectrum*

3.9

10 8 to 7.9

10 8 MHz 7.6

10 -5 to 3.8

10 -5 cm

Ultraviolet*

7.9

10 8 to 2.3

10 10 MHz 3.8

10 -5 to 1.3

10 -6 cm

X-rays*

2.0

10 9 to 3.0

10 13 MHz 1.5

10 -5 to 1.0

10 -9 cm

Gamma rays*

2.3

10 12 to 3.0

10 14 MHz 1.3

10 -8 to 1.0

10 -10 cm

Cosmic rays*

>4.8

10 15 MHz

<6.2

10 -12 cm

* Values approximate.

Table 1002. Electromagnetic spectrum.

has progressed from a specified origin. For most purposes it

is stated in circular measure, a complete cycle being

considered 360

RADIO WAVES

153

1003. Polarization

frequencies, but becomes more prevalent as frequency

increases. In radio communication, it can be reduced by

using directional antennas, but this solution is not always

available for navigational systems.

Various reflecting surfaces occur in the atmosphere. At

high frequencies, reflections take place from rain. At still

higher frequencies, reflections are possible from clouds,

particularly rain clouds. Reflections may even occur at a

sharply defined boundary surface between air masses, as

when warm, moist air flows over cold, dry air. When such a

surface is roughly parallel to the surface of the Earth, radio

waves may travel for greater distances than normal The

principal source of reflection in the atmosphere is the

ionosphere.

Radio waves produce both electric and magnetic fields.

The direction of the electric component of the field is called

the polarization of the electromagnetic field. Thus, if the

electric component is vertical, the wave is said to be

“vertically polarized,” and if horizontal, “horizontally

polarized.”

A wave traveling through space may be polarized in

any direction. One traveling along the surface of the Earth is

always vertically polarized because the Earth, a conductor,

short-circuits any horizontal component. The magnetic field

and the electric field are always mutually perpendicular.

1004. Reflection

1005. Refraction

When radio waves strike a surface, the surface reflects

them in the same manner as light waves. Radio waves of all

frequencies are reflected by the surface of the Earth. The

strength of the reflected wave depends upon angle of

incidence (the angle between the incident ray and the

horizontal), type of polarization, frequency, reflecting

properties of the surface, and divergence of the reflected

ray. Lower frequencies penetrate the earth’s surface more

than higher ones. At very low frequencies, usable radio

signals can be received some distance below the surface of

the sea.

A phase change occurs when a wave is reflected from

the surface of the Earth. The amount of the change varies

with the conductivity of the Earth and the polarization of

the wave, reaching a maximum of 180

Refraction of radio waves is similar to that of light

waves. Thus, as a signal passes from air of one density to

that of a different density, the direction of travel is altered.

The principal cause of refraction in the atmosphere is the

difference in temperature and pressure occurring at various

heights and in different air masses.

Refraction occurs at all frequencies, but below 30 MHz

the effect is small as compared with ionospheric effects,

diffraction, and absorption. At higher frequencies,

refraction in the lower layer of the atmosphere extends the

radio horizon to a distance about 15 percent greater than the

visible horizon. The effect is the same as if the radius of the

Earth were about one-third greater than it is and there were

no refraction.

Sometimes the lower portion of the atmosphere

becomes stratified. This stratification results in nonstandard

temperature and moisture changes with height. If there is a

marked temperature inversion or a sharp decrease in water

vapor content with increased height, a horizontal radio duct

may be formed. High frequency radio waves traveling

horizontally within the duct are refracted to such an extent

that they remain within the duct, following the curvature of

the Earth for phenomenal distances. This is called super-

refraction . Maximum results are obtained when both

transmitting and receiving antennas are within the duct.

There is a lower limit to the frequency affected by ducts. It

varies from about 200 MHz to more than 1,000 MHz.

At night, surface ducts may occur over land due to

cooling of the surface. At sea, surface ducts about 50 feet

thick may occur at any time in the trade wind belt. Surface

ducts 100 feet or more in thickness may extend from land

out to sea when warm air from the land flows over the

cooler ocean surface. Elevated ducts from a few feet to

more than 1,000 feet in thickness may occur at elevations of

1,000 to 5,000 feet, due to the settling of a large air mass.

This is a frequent occurrence in Southern California and

certain areas of the Pacific Ocean.

A bending in the horizontal plane occurs when a

groundwave crosses a coast at an oblique angle. This is due

and the two signals have the same amplitude. This interac-

tion of waves is called wave interference .

A phase difference may occur because of the change of

phase of a reflected wave, or because of the longer path it

follows. The second effect decreases with greater distance

between transmitter and receiver, for under these condi-

tions the difference in path lengths is smaller.

At lower frequencies there is no practical solution to

interference caused in this way. For VHF and higher fre-

quencies, the condition can be improved by elevating the

antenna, if the wave is vertically polarized. Additionally,

interference at higher frequencies can be more nearly elim-

inated because of the greater ease of beaming the signal to

avoid reflection.

Reflections may also occur from mountains, trees, and

other obstacles. Such reflection is negligible for lower

for a horizontally

polarized wave reflected from sea water (considered to

have infinite conductivity).

When direct waves (those traveling from transmitter to

receiver in a relatively straight line, without reflection) and

reflected waves arrive at a receiver, the total signal is the

vector sum of the two. If the signals are in phase, they rein-

force each other, producing a stronger signal. If there is a

phase difference, the signals tend to cancel each other, the

cancellation being complete if the phase difference is 180

154

RADIO WAVES

to a marked difference in the conducting and reflecting

properties of the land and water over which the wave travels.

The effect is known as coastal refraction or land effect .

reflection of LF and VLF waves during daylight.

1007. The Ionosphere and Radio Waves

1006. The Ionosphere

When a radio wave encounters a particle having an

electric charge, it causes that particle to vibrate. The

vibrating particle absorbs electromagnetic energy from the

radio wave and radiates it. The net effect is a change of

polarization and an alteration of the path of the wave. That

portion of the wave in a more highly ionized region travels

faster, causing the wave front to tilt and the wave to be

directed toward a region of less intense ionization.

Refer to Figure 1007a, in which a single layer of the

ionosphere is considered. Ray 1 enters the ionosphere at

such an angle that its path is altered, but it passes through

and proceeds outward into space. As the angle with the

horizontal decreases, a critical value is reached where ray 2

is bent or reflected back toward the Earth. As the angle is

still further decreased, such as at 3, the return to Earth

occurs at a greater distance from the transmitter.

A wave reaching a receiver by way of the ionosphere

is called a skywave . This expression is also appropriately

applied to a wave reflected from an air mass boundary. In

common usage, however, it is generally associated with the

ionosphere. The wave which travels along the surface of the

Earth is called a groundwave . At angles greater than the

critical angle, no skywave signal is received. Therefore,

there is a minimum distance from the transmitter at which

skywaves can be received. This is called the skip distance ,

shown in Figure 1007a . If the groundwave extends out for

less distance than the skip distance, a skip zone occurs, in

which no signal is received.

The critical radiation angle depends upon the intensity

of ionization, and the frequency of the radio wave. As the

frequency increases, the angle becomes smaller. At fre-

quencies greater than about 30 MHz virtually all of the

energy penetrates through or is absorbed by the ionosphere.

Therefore, at any given receiver there is a maximum usable

frequency if skywaves are to be utilized. The strongest sig-

nals are received at or slightly below this frequency. There

is also a lower practical frequency beyond which signals are

too weak to be of value. Within this band the optimum fre-

quency can be selected to give best results. It cannot be too

near the maximum usable frequency because this frequency

fluctuates with changes of intensity within the ionosphere.

During magnetic storms the ionosphere density decreases.

The maximum usable frequency decreases, and the lower

usable frequency increases. The band of usable frequencies

is thus narrowed. Under extreme conditions it may be com-

pletely eliminated, isolating the receiver and causing a

radio blackout.

Skywave signals reaching a given receiver may arrive

by any of several paths, as shown in Figure 1007b . A signal

which undergoes a single reflection is called a “one-hop”

signal, one which undergoes two reflections with a ground

reflection between is called a “two-hop” signal, etc. A

Since an atom normally has an equal number of

negatively charged electrons and positively charged

protons, it is electrically neutral. An ion is an atom or group

of atoms which has become electrically charged, either

positively or negatively, by the loss or gain of one or more

electrons.

Loss of electrons may occur in a variety of ways. In the

atmosphere, ions are usually formed by collision of atoms

with rapidly moving particles, or by the action of cosmic

rays or ultraviolet light. In the lower portion of the

atmosphere, recombination soon occurs, leaving a small

percentage of ions. In thin atmosphere far above the surface

of the Earth, however, atoms are widely separated and a

large number of ions may be present. The region of

numerous positive and negative ions and unattached

electrons is called the ionosphere . The extent of ionization

depends upon the kinds of atoms present in the atmosphere,

the density of the atmosphere, and the position relative to

the Sun (time of day and season). After sunset, ions and

electrons recombine faster than they are separated,

decreasing the ionization of the atmosphere.

An electron can be separated from its atom only by the

application of greater energy than that holding the electron.

Since the energy of the electron depends primarily upon the

kind of an atom of which it is a part, and its position relative

to the nucleus of that atom, different kinds of radiation may

cause ionization of different substances.

In the outermost regions of the atmosphere, the density

is so low that oxygen exists largely as separate atoms, rather

than combining as molecules as it does nearer the surface of

the Earth. At great heights the energy level is low and

ionization from solar radiation is intense. This is known as

the F layer . Above this level the ionization decreases

because of the lack of atoms to be ionized. Below this level

it decreases because the ionizing agent of appropriate

energy has already been absorbed. During daylight, two

levels of maximum F ionization can be detected, the F 2

layer at about 125 statute miles above the surface of the

Earth, and the F 1 layer at about 90 statute miles. At night,

these combine to form a single F layer.

At a height of about 60 statute miles, the solar radiation

not absorbed by the F layer encounters, for the first time, large

numbers of oxygen molecules. A new maximum ionization

occurs, known as the E layer . The height of this layer is quite

constant, in contrast with the fluctuating F layer. At night the

E layer becomes weaker by two orders of magnitude.

Below the E layer, a weak D layer forms at a height of

about 45 statute miles, where the incoming radiation

encounters ozone for the first time. The D layer is the

principal source of absorption of HF waves, and of

RADIO WAVES

155

Figure 1007a. The effect of the ionosphere on radio waves.

Figure 1007b. Various paths by which a skywave signal might be received.

“multihop” signal undergoes several reflections. The layer

at which the reflection occurs is usually indicated, also, as

“one-hop E,” “two-hop F,” etc.

Because of the different paths and phase changes oc-

curring at each reflection, the various signals arriving at a

receiver have different phase relationships. Since the densi-

ty of the ionosphere is continually fluctuating, the strength

and phase relationships of the various signals may undergo

an almost continuous change. Thus, the various signals may

reinforce each other at one moment and cancel each other

at the next, resulting in fluctuations of the strength of the to-

tal signal received. This is called fading . This phenomenon

may also be caused by interaction of components within a

single reflected wave, or changes in its strength due to

changes in the reflecting surface. Ionospheric changes are

associated with fluctuations in the radiation received from

the Sun, since this is the principal cause of ionization. Sig-

nals from the F layer are particularly erratic because of the

rapidly fluctuating conditions within the layer itself.

The maximum distance at which a one-hop E signal can be

received is about 1,400 miles. At this distance the signal leaves

the transmitter in approximately a horizontal direction. A one-

hop F signal can be received out to about 2,500 miles. At low

frequencies groundwaves extend out for great distances.

A skywave may undergo a change of polarization

during reflection from the ionosphere, accompanied by an

alteration in the direction of travel of the wave. This is

called polarization error . Near sunrise and sunset, when

rapid changes are occurring in the ionosphere, reception

may become erratic and polarization error a maximum.

This is called night effect .

1008. Diffraction

When a radio wave encounters an obstacle, its energy is re-

flected or absorbed, causing a shadow beyond the obstacle.

However, some energy does enter the shadow area because of

diffraction. This is explained by Huygens’ principle, which

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: