Radiation & Propagation of Waves

8/11/2019 Radiation & Propagation of Waves

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RADJATION PROPAGATIO OJ: W VES

EFFECfS OF THE ENVIRONMENT

NORMAL

1.

REFLECfION

OF W VES

the incident ray, reflected ray, and the normal to

til('

point of incidence

Mt

in om'

plane,

m eCI$:l

7~~lf~ V V o . { S

Mf (C -

1 ; d :

/

~

REI'LE(. TING SURFACE

both incident roflectcd waves travel at the same wlodty, but there is .1

r ed uc ti on

in lilt

s

igncll

strength

tho reflected WcWE'S strlkes the ground and bounces hack up to the f(..'(:eiving

antenna. is seriously attenuated

as

tl

result

of >trikin~~the ground, but this

is

.1

bonus condition, because the wave also changes the phase

y

I~O (h~gn,; t. s.The

reflected

wave

cancels some of the dlrl~t wave ('ncrgy.

2.

REFRACTION

takes place whcn clectrornegnetic waves PM, from om' prop,\gClting medium to ,I

m edium having a dif ferent density

till

refraction

process bends the wave d ue to 111('d

i t f lIn~s

in

th,'

density of the

,IiI .The rt'fl'Mt('(i

wave does

not change phase

th ere fo re w ill

add

to the wave

that clrriv(.'s by the direct path.

NORMAL

.. _ _ _


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3. DIFFR TION

the behavior of the electromagnetic waves is affected

by

the presence of small

slits in a conducting plane or sharp edges of obstacle.

HUYGEN S PRINCIPLE - states that every point on a given spherical)

wavefront may be regarded as a source of waves from which further waves are

radiated outward.

4. INTERFEREN E

occurs when two waves that left one source traveled ydifferent paths arrive

at a point.

In the difference between paths 1

l

is

n

there is complete cancellation

if

the

ground is a perfect reflector

f

the difference between path 2

2 is 1Athere is reinforcement

at VHF and below - interference is not significant

at UHF and above - interference must be definitely taken into account

1 1 .

~~

) \ 1 c

, O , I G

( Y t l l l t vt

; / j t


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D LAYER: \ (NJ~

~\O

* t:.t ~09,I )~)

exists during daytime only, disappears at night

least important layer from the point of view of HF propagation L~~

reflects some VLF and LF waves

absorbs MF

HF waves to a certain extent

prevents low-freq daytime skip-wave propagation, bu at night thesae low-freq

waves may propagate great distance

used for signals up to several megahertz

E LAYER (.

t.{){) ell - W( 'Ji')I(I~

LV\'f~'()

most useful at the sun s noon peak, but disappears at night due to the

recombmation of the ions into molecules

aids MF surface-wave propagation a little

reflects some HF waves in daytime.

Used at freqs up to about 20 MHz

Es LAYER (Sporadic E Layer)

a thin layer of very high ionization density

when it does occur, it persists during the night also

does not have an important part in long-distance propagation, but it does

sometimes permit unexpectedly good reception

1~

2.4CI01 \ fS FI LAYER ..- _ H Fe~

( - some HF waves are reflected from it, but most pass through to be reflected from

\ the F21ayer

\, main effect to provide more absorption for HF waves

F2 LAYER

/most important reflecting medium for HF radio waves

-combines with the

Fl

layer at night

available around the full

24

hours

The basic idea of a

s y

wave is to radiate the signal toward the

ionospheric layers have it refract and return to earth a

substantial distance away

Some of the signal passes through then layers out into space,

but enough returns to earth to be picked up by a sensitive ,-

receiver.


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Additional distance is possible when the signal reflects from the

earth

goes back up to the ionosphere layers for another HOP.

These multiple hops are what provide the capability for globe

spanning comm unications.

MAXIMUM USABLE FREQUENCY (MUF) - the highest frequency that can be used for sky

wave commumunications between two given points on earth.

normal values: 8 - 35 MHz

~e- ....

t>

ec,1 1 ( c;n+>

~

c cs

FADING - the fluctuation in signal strength at a receiver and may be rapid or slow, general or

frequency selective

rl

k (\0< ,,,~IW { - . it is due to interference between the two waves which left the same source but

\J IJCr,\,. J _ fU~I~

arrived at the destination by different paths.

S

Q V ~ c

d.lv~~'1

f

c . . __ , ,- ,, ,

Most likely to occur at the higher freqs (i.e wave with smaller wavelengths)

i../f. o. \\ ~

- 0

~because the signal received at any instant is the vector sum of all the waves

r 'eC~ived;alternate cancellation & reinforcement will result if there is a length

(

._ _ variation as large as a half-wavelength between any two points.

~~ Normal variations - seasonal height

thickness changes

Abnormal variations - due mainly to the fact that the sun is a variable star

*

SID's (Sudden Ionospheric Disturbances) / Dellinger Dropouts

- caused by solar flares

- only the sunlit side of the earth is affected

- VLF propagation is actually improved

Ionospheric Storms - caused by particle emissions from the sun, generally

and 1 3 rays.

- - highest freqs are most affected

' Sporadic E Layer - when present, this layer has the twin effects of preventing

long distance HF communications

permitting over-the-horizon VHF communication

waves that travel in straight Jines

depends on line-of-sight conditions, thus space waves are limited in their

propagation by the curvature of the earth


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- RADIO HORIZON - about 4/3 as far as the optical horizon due to the varying density of the

atmosphere because of diffraction around the curvature of the earth

or

where: dt

distance from transmitting antenna

ht = height of transmitting antenna above the ground

Visual iiorizon

Curl uture

the eartli

Rculit hnriron

Approrimcrtdy 5~~;

beyond the true horiron

[ A person 6

ft

tall standing on a shoreline would see the horizon at a distance of 3 mi. off shore

hence the 3-m ile limit that borders every country].

Ex. A horizontally polarized antenna is placed on top of a 6\t_S ft tower. The distance to the radio

horizon is 35 mi. . A receiving antenna is 53 mi. from the transmitter would need to be raised to

an elevation of 162 to see the direct path from the xmtng antenna.

The radio horizon ciTffersslightly in that radio waves have a slight bending fill-in effect behind

tall obstructing objects. A receiving antenna immediately behind a tall hill may receive no signal

from a station but if it is moved farther from the station the signal strength increases this void

condition is called the SHADOW EFFECT.

On the other hand any object large enough to cast a radio shadow will if it is a good conductor

cause back reflections also thus in areas in front of

it

a form of interference known as

GHOSTING may be observed on the screen on a TV receiver.

SUPERREFRALTION / DUCTING :

- under certain atmospheric conditions a layer of warm air may be trapped above cooler

ground often over the surface of water the result is that the refractive index

will

decreases far

more rapidly with higher than is usual and this causes complete bending down of microwave

freqs to take place. Microwaves are thus continuously refracted in the duct

reflected by the

ground. Main requirement for formation of atmospheric ducts is the so called temperature

inversion.


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lV ll fll

l\ir lHfLSs

S. TROPHOSHERIC SCAITER PROPAGATION

also known as TROPOSCA ITER or FORWARD SCATIER PROPAGATION

a means of beyond-the-horizon propagation for UHF signals

if

two directional antennas are pointed so that their beams intersect midway

between them, above the horizon, at about km. Or 6.5 mi. from the ground,

they Interact in a manner similar to ducting

high transmitting power are needed since the actual proportion of forward

scatter to signals incident on the scatter volume is very tiny, between -60 dB

dB

Tropospheric scattering is a sytem of xmsn that falls in the same category as

magnetism, gravity,

light energy. We can explain what happens in its

presence, we can predict control its behavior to makeit work for us, but no one

really knows what it is

Tropospheric scatter propagation is subject to two forms of fading

a Rayleigh fading - caused by multi path propagation

- fast occurs several times per minute, with max signal strength variations

in excess of 2O-dB

b fading caused by variation in atmospheric conditions along the path

To obtain best results antennas are elevated and then directed down toward the horizon. Also

because of fading problems, diversity systems space diversity, frequency diversity, quadruple

diversity are employed


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6. EXTRATERRESTRIAL COMM/ TRANSIONOSPHERIC SPACE-WAVE PROPAGTION

/ SATELLITE WAVES

involves the use of various satellite relays

frequencies used are well above normal critical frequencies to minimize their

refraction be able to propagate through the ionosphere

refractions becomes insignificant at freqs above 100 MHz and atmospheric

absorption is negligible up to about 14 GHz

FARADAY EFFECT - problems encountered in transionospheric propagation

- causes the polarization of the radio waves to rotate as

passes through

the ionosphere

is a complex process involving the presence of ionized

particles the earth s magnetic field

- solution: use an antenna with circular polarization

satellite wave systems use freqs which are much higher than the critical freq.

High enough to penetrate the ionosphere without refracting back to the

transmitter.

Major problem: high path loss caused by the large distances. The

electromagnetic energy spreads With distance

relatively little reaches the

receiver.

PREPARED BY:ENGR. TUARIZO

ECE FACULTY

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Radiation & Propagation of Waves