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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