introduction to radio wave propagation

2. Transfer of electromagnetic waves or radio waves from one point
to another
• Electrical energy that has escaped into free space.
• It travels in a straight line at approximately the speed of light and are made of magnetic and electric
fields that are right angles to each other and right angles to the direction of propagation.
• Other forms include infrared, ultraviolet, X rays, and gamma rays.
TWO COMPONENTS:
 Electric Field= E
 Magnetic Field= H
• Form of electromagnetic radiation similar to light and heat.
• They differ from these other radiations in the manner in which they are generated and detected and in
their frequency range.
• It consists of traveling electric and magnetic fields, with the energy evenly divided between two types of
fields

3. The essential properties of radio waves are frequency, intensity, direction travel, and
plane of polarization.
Frequency (f) - is the number of cycles per second of a wave
Wavelength (λ) - is the distance that a wave travels in the time of one
cycle
Wave Velocity (c) - is the speed of the wave depending on the type and
nature of the propagation of the medium.
It travels fastest on FREE SPACE
λ= c/f
c = 3x108 m/s

4. POLARIZATION
The polarization of a wave is the direction of the
electric field vector.
• Refers to the physical orientation of the radiated
waves in space dictated by the direction of the
electric field
• Polarization may be horizontal or vertical and can
also be circular or elliptical if the electric field vector
rotates as it moves through space.

5. Linear Polarization
- Electric vector has a particular direction in free
space
- Waves have the same alignment in space
VERTICAL– Electrical vector is vertical, or it lies
in a vertical plane.
HORIZONTAL– Electrical vector is horizontal, or
it lies in a horizontal plane
- Electric vector rotates about the axis of the
direction of propagation
Elliptical Polarization
- Electric vector rotates about the axis of the
direction of propagation but the amplitudes of
its two linearly polarized components are
unequal
Circular Polarization
Random Polarization
- There is no fixed pattern of polarization va
Isotropic Source
- Radiates uniformly in all possible
direction in space

6.  RAYS AND WAVEFRONTS are aids to illustrating the effects of
electromagnetic wave propagation through free space.
 RAY – is a line drawn along the direction of propagation of an
electromagnetic wave
– is used to show the relative direction of electromagnetic wave
propagation; however it does not necessarily represent the propagation
of a single electromagnetic wave.
 WAVEFRONT – shows a surface of constant phase of electromagnetic
wave
– is formed when points of equal phase on rays propagated
from the same source are joined together.

7. o The figure shows a wavefront with a surface that is perpendicular to the
direction of propagation (rectangle ABCD).
o When a surface is plane, its wavefront is perpendicular to the direction
of propagation. The closer to the source, the more complicated the
wavefront becomes.
A plane wave

8.  POINT SOURCE – is a single location from which rays propagate equally in all
directions (an isotropic source).
• The figure shows a point source, several rays propagating from it, and the
corresponding wavefront.
A wavefront from a point source.

9. o The wavefront generated from a point source is simply a sphere
with radius R and its center located at the point of origin of the
waves.
o In free space and a sufficient distance from the source, the rays
within a small area of a spherical wavefront are nearly parallel.
o The farther from the source, the more wave propagation appears
as a plane wavefront.

10. Radio waves are electromagnetic waves simply because they are
made up of an electric and a magnetic field.
Magnetic Field
- Invisible force field produced by a magnet, such as a conductor when
current is flowing through it.
The strength of a magnetic field (H) produced around a conductor is expressed
mathematically as:
Where
H = magnetic field (ampere turns per meter)
d = distance from wire (meters)

11. Electric fields
Invisible force fields produced by a difference in voltage potential between two conductors.
Electric filed strength (E) is expressed mathematically as:
Where:
E = electric field strength (volts per meter)
Q = charge between conductors (coulombs)
∈ = permittivity (farads per meter)
d = distance between conductors (meters)
*Permittivity
- Is the electric constant of the material separating the two conductors.
- The permittivityof air or space is approximately 8.85 x 10^-12 F/m.k

13. Introduction
 Waves propagate through space as
transverse electromagnetic (TEM)
waves.
 This means that the electric field,
the magnetic field, and the direction of
travel of the wave are all mutually
perpendicular.

14. Radio-Wave Propagation
• Radio waves are generated by electrons moving in a
conductor, or set of conductors, called an antenna.
• Once a radio signal has been radiated by an antenna, it travels
or propagates through space and ultimately reaches the
receiving antenna.
• The energy level of the signal decreases rapidly with distance
from the transmitting antenna.
• The electromagnetic wave is affected by objects that it
encounters along the way such as trees, buildings, and other
large structures.
• The path that an electromagnetic signal takes to a receiving
antenna depends upon many factors, including the frequency

15. *FREE-SPACE
– Space that not interfere with the normal radiation or propagation of radio waves; no magnetic or
gravitational fields, no solid bodies, no ionized particles present
- Propagation of electromagnetic waves often calledradio-frequency (RF) propagation or simply radio
propagation.
Once launched, electromagnetic waves can travel through free space and through
many materials.
Any good dielectric will pass radio waves; the material does not have to be
transparent to light.
The waves do not travel well through lossy conductors, such as seawater, because
the electric fields cause currents to flow that dissipate the energy of the wave very
quickly.
Radio waves reflect from good conductors, such as copper or aluminum.

16. The speed of propagation of radio waves in free space is the same as that of light,
approximately 300 × 106 m/s. In other media, the velocity is lower.
The propagation velocity is given by
where

17.  POWER DENSITY – the rate at which energy passes through a given surface
area in free space. It is energy per unit time per unit of area and is usually
given in watts per square meter.
where: P = power density (watts per meter squared)
E = rms electric field intensity (volts per meter)
H = rms magnetic field intensity (ampere turns per meter)
 FIELD INTENSITY – is the intensity of the electric and magnetic fields of an
electromagnetic wave propagating in free space.

18.  The electric and magnetic field intensities of an electromagnetic wave in
free space are related through the characteristic impedance (resistance) of
free space.
 The characteristics impedance of a lossless transmission medium is equal
to the square root of the ratio of its magnetic permeability to its electric
permittivity.
where: Zs = characteristic impedance of free space (ohms)
µo = magnetic permeability of free space (1.26 x 10-6 H/m)
o = electric permittivity of free space (8.85 x 10-12 F/m)

19. substituting into the equation;
therefore, using Ohm’s law:

20.  ISOTROPIC RADIATOR – a point source that radiates power at
a constant rate uniformly in all directions and is
closely approximated by an omnidirectional antenna.
 An isotropic radiator produces a spherical wavefront
with radius R. All points distance R from the source lie on
the surface of the sphere and have equal power densities.
spherical wavefront
from an isotropic
source
 The power density at any point on the sphere is the total radiated power
divided by the total area of the sphere.

21.  power density at any point on the surface of a spherical
wavefront:

22.  INVERSE SQUARE LAW – the power density is inversely proportional to the square of the
distance from the source.
 The power density at any point on the surface of the outer sphere is:
 The power density at any point on the surface of the inner sphere is:
Therefore,

23. o As the distance from the source doubles, the power
density decreases by a factor of 22 or 4.
o When deriving the inverse square law of radiation, it
was assumed that the source radiates isotropically,
although it is not necessary; however, it is necessary
that the velocity of propagation in all directions be
uniform. Such a propagation medium is called an
isotropic medium.

24.  WAVE ATTENUATION – the reduction in power density due to the inverse
square law presumes free-space propagation (a vacuum or nearly a vacuum)
– sometimes called the space attenuation of the wave
because the attenuation is due to the spherical spreading of the wave.
 Wave attenuation is generally expressed in terms of the common logarithm
of the power density ratio (dB loss) . Mathematically;

25.  ABSORPTION – the reduction in power density due to nonfree-
space propagation.
 Since absorption of energy is dependent on the collision of
particles, the greater the particle density, the greater the
probability of collisions and the greater the absorption.
 For a homogeneous medium (one with uniform properties
throughout), the absorption experienced during the first mile of
propagation is the same as for the last mile.
 Atmospheric absorption ɳ for a wave propagating from R1 to
R2 is: ϒ (R2 – R1), where ϒ is the absorption coefficient.
 Wave attenuation depends on the ratio R2/R1, and wave
absorption depends on the distance between R1 and R2.

26. • Radio waves act much like light waves.
• Light waves can be reflected, refracted, diffracted,
and focused by other objects.
• The focusing of waves by antennas to make them
more concentrated in a desired direction is
comparable to a lens focusing light waves into a
narrower beam.

27. • Any conducting surface looks like a mirror to a radio
wave, and so radio waves are reflected by any
conducting surface they encounter.
• Radio-wave reflection follows the principles of light-
wave reflection.
• The angle of reflection is equal to the angle of
incidence.
• The direction of the electric field approaching the
reflecting surface is reversed from that leaving the
surface. This is equivalent to a 180° phase shift.

28. How a conductive surface reflects a radio wave.

29. ◦ Refraction is the bending of a wave due to the physical makeup
of the medium through which the wave passes.
◦ Index of refraction is obtained by dividing the speed of a light
(or radio) wave in a vacuum and the speed of a light (or radio)
wave in the medium that causes the wave to be bent.
◦ The relationship between the angles and the indices of refraction
is given by a formula known as Snell’s law:
n1 sin Θ1 = n2 sin Θ2
where
n1 = index of refraction of initial medium
n2 = index of refraction of medium into which wave passes
Θ1 = angle of incidence
Θ2 = angle of refraction

30. How a change in the index of refraction causes bending of a radio wave.

31. Diffraction is the bending of waves around an
object.
Diffraction is explained by Huygen’s Principle:
◦ Assuming that all electromagnetic waves radiate as spherical waveforms from
a source, each point on a wave front can be considered as a point source for
additional spherical waves.
◦ When the waves encounter an obstacle, they pass around it, above it, and on
either side.
◦ As the wave front passes the object, the point sources of waves at the edge of
the obstacle create additional spherical waves that penetrate and fill in the
shadow zone.

32. Diffraction causes waves to bend around obstacles.