3. INTRODUCTION
The term Radio waves arbitrarily applied to
electromagnetic waves in the frequency range of 0.001 hertz to hertz
and the wavelength ranging from 3× m to 3× m.
Radio waves comprises of both Electric and Magnetic fields.
The two fields are at right-angles to each other and the direction of
propagation is at right-angles to both fields.
X
Y
Electric
field , E
Direction of
propagation
4. RADIO WAVE PROPAGATION
Radio Waves propagate outward from an antenna , at the speed
of light. The exact nature of these waves is determined by the
transmission medium. i.e., In free space they travel in straight
lines , whereas in the atmosphere they generally travel in curved
path.
In a unguided medium, radio waves propagate in TEM mode
while in confined or guided medium radio waves do not
propagate in the TEM mode but in TM or TE mode.
Radio waves can be reflected and refracted in a manner similar
to light . They are effected by ground terrain, atmosphere and
other objects.
6. DIFFERENT MODES OF PROPAGATION
Radio waves
Ground waves
Space waves or Line
of Sight waves
Direct waves
Ground reflected waves
Sky waves
MODES OF PROPAGATION FREQUENCY APPLICATION
1. Ground waves VLF
LF
MF
1.Submarine Communication
2.AM ,FM and television
broadcasting
2. Space or line of sight waves HF
VHF
UHF
1.Satellite communication
2.Mobile communication
3. Sky waves 2Mhz-30Mhz 1.Microwave link
2.Radar communication
7. GROUND WAVE PROPAGATION
It propagates from transmitter to receiver by gliding over the surface of the
earth in which both antennas are close to the surface of the earth. It follows
the curvature of the earth.
Earth is assumed to be an ideal conductor ,EM waves cannot penetrate ideal
conductor only exist in dielectric medium above conducting surface that’s
why ground wave propagation is possible.
The Ground waves are traverse in nature. Horizontal polarised antennas are
not preferred as the horizontal component of the electric field in contact with
the earth is short circuited by the earth.
8. WAVE TILT
The earth is not an ideal conductor , so there will remain a tangential component of electric
field resulting wave tilt.
The earth is characterised by σ and εr then Surface impedanc for earth:
Intrinsic impedance for dielectric medium:
Since there is surface impedance which is non-zero there will be tangential component of
electric field i.e., EH and Ev be vertical component of electric field and if H is the
magnetic field
then, and
9. Then,
From the equation we see that the two component are not in phase and as
in general
The resultant electric field is elliptically polarised with plane of
polarisation in longitudinal direction .as conductivity is infinite for
ideal conductor the wave is vertically polarised as the conductivity
decreases the ellipse become more tilted. This phenomena is known as
WAVE TILT.
wave tilt for decreasing value of conductivity
10. SKY WAVE PROPAGATION
• The sky waves are reflected from ionized layer of atmosphere back down to
the earth surface and is useful for very long range distance communication
• They can travel a number of layers, back and forth between ionosphere and
earth’s surface.
• The sky wave, often called the ionospheric wave.
11. 11
D layer – 50 to 90 km.
Disappears at
night.
N=400e/cc
fc=180KHz
Mainly reflects
ULF and LF
E layer – 90 to 140 km
Disappears at
night.
N=2* 10^5 e/cc
fc=4mhz
IONOSPHERIC LAYERS
Sporadic E layer:
Temporary layer
Summer time
thin layer of intense
ionisation clouds.
reflect frequency up to
224 MHz
90-130 km
F1 layer- 140 to
250km
fc=5Mhz
N=2*10^5 to
4*10^5 e/cc
F2 layer- 250 km and
beyond
fc=8Mhz at
day and
6Mhz at
night
N=2*10^6
e/cc
12. CRITICAL FREQUENCY
Critical frequency (fc) for given layer is defined as the highest frequency
of wave that will be returned to the earth by that layer having been
beamed normal ly at it.
Nmax = maximum ionisation density i.e., no. of electrons per unit
volume .
MUF is defined as the highest frequency at which it is reflected by the ionospheric
layer at the angle of incidence other than normal incidence.
MUF depends on time ,day ,distance
direction and solar activity.
MUF is the highest frequency that can be
used by sky waves.
MAXIMUM USABLE FREQUENCY(MUF)
13. SKIP DISTANCE
The minimum distance from the transmitter to the point on ground at
which of a given frequency will return to the earth by the ionosphere is
called skip distance.
For flat earth: 1.Dskip=skip distance
2.h=height at which the
reflection occurs.
3.fMUF=maximum
usable frequency.
4.fc= Critical frequency
Dskip
h
i
T R
ionosphere
14. VIRTUAL HEIGHT
The virtual height of an ionospheric layer is the equivalent altitude
of a reflection that would produce the same effect as the actual
refraction.
c=speed of light=3×10⁸
T=round trip time
APPLICATION OF SKY WAVES:
1.Satellite Communication
2. Mobile communication
15. FADING:
Fading is defined as the fluctuation in the received signal strength at the
receiver or a random variation in the received signal.
Fading may be classified in terms of duration of variation in signal
strength as :
1) Rapid Fluctuations
2)Short Term Fluctuations
3)Long term Fluctuations
The various types of fading are as follows:
Selective Fading: At high frequency.
Interference Fading
Absorption Fading
Polarization Fading
Skip Fading
16. SPACE WAVE PROPAGATION(line of sight)
These waves occur within the lower 20 km of the atmosphere i.e.
troposphere .
In this mode of wave propagation electromagnetic waves after
transmitted from transmitting antenna reaches the receiving antenna
either directly or after reflection from earth’s surface and tropospheric
region. i.e., direct wave and ground reflected wave or indirect wave.
17. SHADOW ZONE
Ideally field strength of receiving antenna should be
But sometimes when receiving antenna and transmitting antenna is not
in l.o.s a zone is created where the field radiations are not utilised. This
is called Shadow zone.
18. RADIO HORIZON
The radio horizon of an antenna is defined as the distant points locus at which
antenna’s direct ray become tangential to planetary surface.
a
Let h1=transmitting antenna’s
height
h2= receiving antenna’s height
a= radius of the earth
19. Duct propagation
Duct is a leaky waveguide through which E.M waves move in the air by
successive reflection and refraction. When the signal move through
different layers, signal may suffer from some loss.
APPLIATION OF SPACE WAVES:
1.Radar communication
2.Microwave linking
3.Mobile systems and satellite systems.
20. CONCLUSION
Radio wave signals are mainly used for communication purpose. Starting
from transmission of television broadcast signals to the communication
with aircrafts, every mode of communication and transportation is
dependent on signals transmitted in radio wave frequency .The main
objective of radio wave propagation is to transmit signals securely
without any error. Mobile phones use radio signals for communication
purpose. It can be concluded that Radio wave Propagation is an
Important branch of Communication studies.
