2. Electromagnetic Waves
 Electromagnetic wave consists of oscillating electric
and magnetic fields in certain directions with
propagate .
 Propagate through free space at the velocity of light

3. Electromagnetic Radiation
 Includes radio waves, light, X-rays, gamma rays
VLF 3 – 30 kHz
LF 30 – 300 kHz
MF 300 – 3000 kHz
HF 3 – 30 MHz
VHF 30 – 300 MHz
UHF 300 – 3000 MHz
Radio waves of our interest

5. TEM Propagation
 Radio waves in space are transverse
electromagnetic waves (TEM)
 Electric field, magnetic field and direction
of travel of the wave are mutually
perpendicular
 Waves will propagate through free space
and dielectrics
 Conductors have high losses due to induced
current

7. Propagation Velocity
 Speed of light in free space: 3 108 m/s
 In dielectric and plasma the velocity of propagation is
lower:
r
c
v

8. Electromagnetic Waves
 Wavelength is :
fV p
/
Where,
Vp is the phase velocity
is the wavelength
f is the frequency

9. Ohm’s Law in Space
HEZ /

10. Electric and Magnetic Fields
 For waves we use the following units:
 Electric field strength E (V/m) Magnetic field
strength H (A/m) Power density PD (W/m2)
 Ohm’s law holds if characteristic impedance Z of
medium is used
 For free space, Z = 377 Ohm

11. Power Density
EH
H
E
PD
Z
Z
2
2

12. Plane and Spherical Waves
 Waves from a point in space are spherical
 Plane waves are easier to analyze
 At a reasonable distance from the source,
spherical waves look like plane waves, as long
as only a small area is observed

13. Isotropic
antenna
radiating
equally in
every
direction

14. Free-space Propagation
 Assume an isotropic radiator at the center of a
sphere
 Let receiving antenna be on surface of sphere
 As we move farther from transmitter the amount
of power going through the surface remains the
same but surface area increases

15. Power flux density
Power flux density= E X H

16. Geometrical loss
2
4πr
PPD
Because of the power P on the spherical surface is constant
for every spherical surface (4π r2 ) we consider, the power
flux density at the distance r from the isotropic antenna must
decrease as 1/4πr2.
If an isotropic antenna radiates 10 W of power at the
distance of 1 km the power flux density (PD)is about 0.796
microW/m2

17. Attenuation of Free Space
 Power density is reduced with increasing
distance r
 Power density is total power divided by
surface area of sphere
 Unit: watts/meter
2
4 r
P
P t
D

18. Free Space Electric Field
 Electric field strength is relatively easy to
measure
 Often used to specify signal strength
 Unit: volts/meter
r
P
E t
30

19. Absorption
 No absorption in free space
 EM wave are absorbed in atmosphere as energy is
transferred to atoms and molecules
 Electromagnetic waves are absorbed in the
atmosphere according to wavelength. Two
compounds are responsible for the majority of
signal absorption: oxygen (O2) and water vapor
(H2O).
 Absorption below 10 Ghz is quit insignificant

21. Reflection
 Specular reflection: smooth surface
 Angle of incidence = angle of reflection
 Diffuse reflection: rough surface
 Reflection in all directions because angle of
incidence varies over the surface due to its
roughness

22. Specular Reflection

23. Diffuse reflection

24. Polarization
 Polarization of a wave is the direction of the
electric field vector
 Linearly polarized waves have the vector in
the same direction at all times
 Horizontal and vertical polarization are common
 Circular and elliptical polarization are also
possible
 It is a physical orientation of radiated waves
in space

25. Circular polarization

26. linear polarization

28. Cross Polarization
 If transmitting and receiving antennas have
different polarization, some signal is lost
 Theoretically, if the transmitting and
receiving polarization angles differ by 90
degrees, no signal will be received
 A circularly polarized signal can be received,
though with some loss, by any linearly
polarized antenna

29. Refraction
 Refraction takes place when EM wave pass from
one medium to another medium with diff density.

30.  Atmospheric density changes with height
 Slight refraction of wave
 Increases Radio horizon

31. Refraction
 Occurs when waves move from one medium to
another with a different propagation velocity
 Index of refraction n is used in refraction
calculations
r
n

32. Snell’s Law
 Angles are measured with respect to the
normal to the interface
2211
sinsin nn

33. refraction

34. Angle of Refraction
 If n1<n2 then ray bends toward the normal (away
from the interface)
 If n1>n2 then ray bends away from the normal
(toward the interface)

35. Diffraction
 Occurs when radiation passes an object with
dimensions small compared with wavelength
 The object appears to act as a source of radiation
 Allows radio stations to be received on the shadow
side of obstacles

36. EM WAVE PROPAGATION

38. Layer of atmosphere

39. Terrestrial Propagation
 Propagation over earth’s surface
 Different from free-space propagation
 Curvature of the earth
 Effects of the ground
 Obstacles in the path from transmitter to receiver
 Effects of the atmosphere, especially the
ionosphere

40. Ground-Wave Propagation
 Happens at relatively low frequencies
 up to about 2 MHz
 Only works with vertically polarized waves
 Waves follow the curvature of earth
 range varies from worldwide at 100 kHz and less to
about 100 km at AM broadcast band frequencies
(approx. 1 MHz)

42. Ionospheric Propagation
 Useful mainly in HF range (3-30 MHz)
 Signals are refracted in ionosphere and returned
to earth
 Worldwide communication is possible using
multiple “hops”

43. Ionospheric Layers
 D layer: height approx. 60-90 km
 E layer: height approx. 90-150 km
 F1 layer: height approx. 150-250 km
 F2 layer: height approx. 250-400 km
 D, E layers disappear at night
 F layers combine into one at night

44. Ionospheric Activity
 More ionization causes signals to bend more
 Ionization caused by solar radiation
 greater during daytime
 greater during sunspot cycle peaks (we are about at
a decreasing value now-2004)
 D,E layers are less highly ionized than F layer and
usually just absorb signals

45. Refraction of Signals
 Bending of signals by atmosphere decreases with
increasing frequency
 Bending of signals by atmosphere increases with
increasing ionization

46. Daytime Propagation
 D and E layers absorb lower frequencies, below
about 8-10 MHz
 F layers return signals from about 10-30 MHz

47. Nighttime Propagation
 D, E layers disappear
 F layer returns signals from about 2-10 MHz
 Higher frequencies pass through ionosphere into
space

48. Ionospheric Sounding
 Transmit signal straight up
 Note the maximum frequency that is returned
 This is the critical frequency

49. Important Frequencies in HF
Propagation
 Critical frequency
 Highest frequency that is returned to earth
 Maximum Usable Frequency (MUF)
 Highest frequency that is returned at a given point
 MUF= fcsecθ
 Optimum Working Frequency (OWF)
 85% of MUF for more reliable communication

50. Skip Zone
 Region between maximum ground-wave distance
and closest point where sky waves are returned
from the ionosphere,