Looking Back in Time…

In 1865, the propagation

of radio waves were
In 1887, Heinrich Hertz
predicated
confirmed the predictions
mathematically by James
of Maxwell when he
Clerk Maxwell.
discovered and generated
electromagnetic waves
and demonstrated its
existence.
Electromagnetic Waves

An electromagnetic wave consists of

time-varying electric and magnetic fields
that are perpendicular to each other. It
travels in a direction perpendicular to
both the E and M fields, hence the term
transverse electromagnetic (TEM) waves.

1 E
c c
E = electric field (V/m)
B = magnetic field (T)
 0 0 B
The Electromagnetic
Spectrum
How are EM Waves Generated?

All EM waves are generated by moving

electric charges. Whenever a charged
particle accelerates, it radiates energy.

Antennas are used to generate radio

waves. They are basically transducers
because they convert the electrical
energy into EM radiation.
Propagation of Radio Waves

Once the radio wave is generated, it will

then propagate through a given medium.

Radio waves can propagate through any

dielectric material.

Practical communication systems use

free-space or the Earth’s atmosphere as
the medium of transmission.
Velocity of Propagation

All EM waves including radio waves

travel at the speed of light in vacuum.

The velocity of propagation of radio

waves in a given medium is determined
by its dielectric constant.

c εr → dielectric constant
v
r
Velocity of Propagation

The velocity of an EM wave travelling in

any medium is related to its frequency
and wavelength.

v = λf
The Characteristic Impedance of
Free Space

The ratio of the electric field to the

magnetic field intensity in a given
medium is constant, and is equal to its
impedance.
E 
Z Z (lossless medium)
H 

For non
free magnetic
space, 377
Z Z  0  377
Ω Ω
medium, 0 r
Example:

Determine the characteristic impedance

of polyethylene whose dielectric constant
is 2.3.

Ans: Z = 249 Ω
Power Density

The rate at which the energy of a wave

passes through a given surface area is
called its power density, which is equal to
the product of the electric and magnetic
field intensities at any instant.

P = EH (W/m 2 )
E2
P
E = electric field intensity (V/m) Z
H = magnetic field intensity (A-t/m)
Example:

What is the maximum power density of

an electromagnetic wave in air?

Ans: P = 23.9 GW/m2

The Isotropic Radiator

Conceptually, the simplest source of

electromagnetic waves is a single point in
space that radiates EM waves equally in
all directions. Such a source is called an
isotropic radiator.
Rays and Wavefronts

A ray is a line drawn from the

source directed outward
along the direction of
propagation of the wave.

An imaginary surface formed

by joining the points in which
all the waves have the same
phase is called a wavefront.
Polarization

The polarization of an EM wave refers to

the orientation of the electric field vector
with respect to the surface of the Earth.

Linear: horizontal and vertical

Circular & elliptical polarization

The Power Density and the Inverse
Square Law

When a wave propagates in free space, no

losses are incurred, but the wave is
attenuated as it spreads out as described
by the inverse square law.
The power density of a spherical wave at a
distance r from an isotropic source is

Pt
P
4 r 2
Example:

A 100-W power is transmitted by an

isotropic radiator. What is the power
density 10 km away from the source?

Ans: P = 79.6 nW/m2

The Electric Field Strength of a
Radio Wave

The electric field strength or intensity of a

radio wave is much easier to measure
than its power density.

When a radio wave is transmitted at a power

Pt, its electric field strength at a distance r
from the source is

30Pt
E
r
Example:

What is the field strength of a 100-W

signal 10 km from its source?

Ans: E = 5.48 mV/m

Power Density and ERP for Practical
Antennas

Practical antennas do not radiate the

energy equally in all directions.
If the transmitting antenna radiates more
power in a certain direction, then it has a
directive gain or simply gain, G. The power
density at a distance r from the antenna is
The effective radiated
GPt power is
P
4 r 2 ERP  GPt
Free-Space Path Loss

The free-space path loss or wave

attenuation is due to the spreading of
the wave as it propagates. It is only a
function of the distance and frequency.
The path loss is given by
2
 4 d 
Lp   
This one reduces to   
Lp  32.44  20log d(km)  20log f(MHz )
Example:

A transmitter has a power output of 150

W at a carrier frequency of 325 MHz. It is
connected to an antenna having a gain of
12 dBi. The receiving antenna is 10 km
away and has a gain of 5 dBi. Calculate
the power delivered to the receiver
assuming free-space propagation and
matched conditions.

Ans: Pr = 405 nW
Optical Properties of Radio Waves

Just like light, radio waves and other

electromagnetic waves exhibit the
following properties.

reflection diffraction
refraction interference
Reflection

When a wave
Reflection of is reflected,
radio wavesitoccurs
is normal
when that
it a
certain
bounces amount of phase
off a surface (a shift is introduced.
conductor or the
The wave
earth’s is shorted
surface). Thewhen
anglethe
of Eincidence
field is
parallel
always to the conducting
equals the angle ofsurface.
reflection.
Refraction

As a wave travels from one medium to

another having different densities, its
velocity changes. This results to a change
in the direction of the wave, as if the
wave bends. This is called refraction.
θ1 n1 < n 2
n1 medium 1
v1 > v2
n2 medium 2
θ2 θ1 > θ2
Snell’s Law of Refraction

The angle at which the wave is refracted

can be determined using Snell’s Law:

n 1 sinθ 1 = n 2 sinθ 2
θ1 n  r c
n1 n
v
sin1 r sin1 v 1
n2  2
θ2 
sin 2 r sin 2 v 2
1
Reflection and Refraction

In many practical
cases, some of the
energy is transmitted
or absorbed by the
reflecting surface and
sometimes, a portion
of it is refracted and
the rest is reflected.
Total Internal Reflection

When a wave is travelling into a medium

having a considerably lower refractive index,
and the angle of incidence is large, the angle of
refraction becomes greater than 90° and the
wave is refracted back to the medium where it
came from, as if it is reflected. This is called
total internal reflection.

The critical angle is given by

1 n2
 c  sin n1 > n 2 TIR occurs when θi > θc
n1
Diffraction

Diffraction is the scattering or divergence

of a wave from its path as it encounters
obstructions and sharp edges.
Huygen’s Principle
Diffraction is based on Huygen’s principle:

“All points in a given wavefront act as

isotropic point sources of secondary waves
called wavelets.”
Interference

The radio waves that propagate in space

interfere with other EM waves.

If the source of interference is another

radio transmitter, it is called RFI.

If the interfering signal comes from

elsewhere, usually of natural origin, then
it is called EMI.
 Radio Frequency Spectrum

DESIGNATION ABBR. FREQUENCY APPLICATIONS

RANGE
Extremely low Frequency ELF 30 Hz – 300 Hz Power transmission
Voice Frequency VF 300 Hz – 3 kHz Audio
Very Low Frequency VLF 3 kHz – 30 kHz Navigation
Low Frequency LF 30 kHz – 300 kHz Navigation
Medium Frequency MF 300 kHz – 3 MHz AM broadcast, navigation
High Frequency HF 3 MHz – 30 MHz Shortwave broadcasting
Very High Frequency VHF 30 MHz – 300 MHz TV and FM broadcasting
Ultra High Frequency UHF 300 MHz – 3 GHz Cellular, sat comm, PCS
Super High Frequency SHF 3 GHz – 30 GHz Radar, radioastronomy
Extremely High Frequency EHF 30 GHz – 300 GHz
Mode of Propagation

Radio waves may propagate in different ways

depending on the signal frequency. The
following are the various modes of radio wave
propagation:

Ground wave
Sky wave
Space wave
Ground Wave Propagation

Ground waves are radio waves that follow the

earth’s curvature and travels along the surface of
the earth (surface waves).
 The main mode of propagation for frequencies up to 2 MHz
 Ground waves are vertically polarized
 Suffers from tilting as the wave propagates
 Affected by the terrain in which it propagates and is highly attenuated by
conductive surfaces
 Travels better over water
 Attenuation increases with frequency, not effective above 2 MHz
 Reception is not affected by weather changes
 Provides reliable long distance communications
 Requires high power and physically large antennas
 Used for AM broadcasting, navigation systems and navigational aids
Pros and Cons of Ground Wave
Propagation

PROS
• Given enough power, it can be used to
communicate between any two points on earth
• Relatively unaffected by changing atmospheric
conditions
CONS
• Require relatively high transmission power
• Requires larger antennas
• Losses vary considerably with surface material
Sky wave Propagation

Sky waves are radio waves that directed

above the horizon and are refracted by a
region of the atmosphere known as
ionosphere. Propagation of sky waves is
also called ionospheric propagation.

Sky wave propagation is the choice in

propagating HF signals. An example is the
long-distance shortwave communications.
The Ionosphere

The ionosphere is a region in the upper

atmosphere where the air molecules are
ionized by solar radiation. It has an
approximate altitude of about 50 to 400 km.
 The Ionosphere
 Altitude varies from 60 to 400 km above the Earth’s surface
 The ionosphere is divided into 3 regions: D, E and F, with the F
divided into F1 and F2 layers.
 Ionization increases with altitude. At night, D and E layers disappear;
F1 and F2 combines into a single F layer.
 The transmitted signal returns to the Earth by refraction. Higher
frequency requires greater ionization.
 At daytime, D and E layers absorb frequencies below 10 MHz.
Frequencies up to 30 MHz are refracted by F layers.
 At night, lower frequencies reach the F layer. This makes long
distance communications better at night.
 At night, higher frequencies pass through all the layers. Propagation
at frequencies above 10 MHz is better during day time. Long
distance propagation is best during sunspot cycles.
 Multi-hop propagation is possible but reduces signal strength.
Important Terms

• Ionospheric sounding
• Critical frequency
• Maximum usable frequency
• Optimum working frequency
• Skip zone
Example:

Determine the maximum usable

frequency for a transmitting station if
the required angle of incidence for
propagation to a desired destination is
70°, if the critical frequency is 11.6 MHz.

Ans: fm = 33.9 MHz

Pros and Cons of Sky Wave
Propagation
• Allows long distance communication
• Relatively simple equipments are
necessary
• Does not require very high transmit
power
• The propagation is noisy and uncertain
• Prone to phase shifting and fading
• Limited only to HF applications
Line-of-sight Propagation

The transmission of radio waves directly

from transmitter to receiver in direct
path or through ground reflections is
called line-of-sight propagation. It is also
called direct wave, space wave and
tropospheric propagation.
 LOS Propagation
 Applicable to VHF and UHF signals.
 The communication distance for terrestrial propagation is
limited by the Earth’s curvature.
 The radio horizon is approximately one-third greater than the
visual horizon.
 The antenna height is important; the higher the farther the
radio horizon.
Radio Horizon calculation

• Radio
The horizon is value
approximate aboutfor
80%thegreater
maximumthan
line of sight
distance because
between of diffraction
transmitter effectsfor
and receiver
LOS transmission is given by the formula

d  17ht  17hr

d = maximum distance in km
ht = transmit antenna height in meters
hr = receive antenna height in meters
Example:

A taxi company uses a central dispatcher

with an antenna at the top of a 15 m
tower, to communicate with taxi cabs.
The taxi antennas are on the roofs
approximately 1.5 m from the ground.
Calculate the maximum communication
distance (a) between the dispatcher and
the taxi (b) between two taxis.

Ans: (a) 21 km (b) 10.1 km

 LOS Propagation
 LOS range can be extended by diffraction. However, the signal
strength is reduced requiring more powerful transmitters and
more sensitive receivers.
 Ground reflected waves interfere with direct waves causing
fading.
 Diffracted signals may cause signal cancellation
 The signal is prone to multipath distortion
Example:

Determine the radio horizon for a

transmit antenna that is 100 ft high and a
receiving antenna that is 50 ft high.

Ans: d = 24.14 mi
Example:

Determine the maximum usable

frequency for a critical frequency of 10
MHz and an angle of incidence 45°.

Ans: MUF = 14.14 MHz

Example:

For a radiated power of 10 kW,

determine the voltage intensity at a
distance 20 km from the source

Ans: E = 27.39 mV/m

Question:

 Radio waves were first predicted

mathematically by:

A. Armstrong
B. Hertz
C. Maxwell
D. Marconi
Question:

 Radio waves were first demonstrated

experimentally by:

A. Armstrong
B. Hertz
C. Maxwell
D. Marconi
Question:

 Which of the following are

electromagnetic:

A. Radio waves
B. Light
C. Gamma rays
D. All of the above
Question:

 The electric and magnetic fields of a

radio wave are:

A. perpendicular to each other

B. perpendicular to the direction of
travel
C. both a and b
D. none of the above
Question:

 Which polarization can be reasonably

well received by a circularly polarized
antenna:

A. vertical
B. horizontal
C. Circular
D. All of the above
Question:

 An antenna has "gain" as compared to:

A. an isotropic radiator
B. a vertically polarized radiator
C. a ground-wave antenna
D. none of the above
Question:

 The "attenuation of free space" is due to

A. losses in the characteristic impedance of

free space
B. losses due to absorption in the upper
atmosphere
C. the decrease in energy per square meter
due to expansion of the wavefront
D. the decrease in energy per square meter
due to absorption of the wavefront
Question:

 Ground waves are most effective

A. below about 2 MHz

B. above about 20 MHz
C. at microwave frequencies
D. when using horizontally polarized
waves
Question:

 Radio waves would most strongly

reflect off

A. a flat insulating surface of the right

size
B. a flat dielectric surface of the right size
C. a flat metallic surface of the right size
D. a flat body of water
Question:

 Radio waves sometimes "bend" around

a corner because of

A. reflection
B. diffraction
C. refraction
D. dispersion
Question:

 Space waves are

A. line-of-sight
B. reflected off the ionosphere
C. same as sky waves
D. radio waves used for satellite
communications
Question:

 Sky waves

A. are line-of-sight
B. "bounce" off the ionosphere
C. are same as space waves
D. are radio waves used for satellite
communications
Question:

A 20-dB reduction in the strength of a

radio wave due to reflection is called

A. fading
B. diffraction
C. Frequency diversity
D. Space diversity
Question:

 "Ghosts" on a TV screen are an example

of

A. fading
B. diffraction
C. Multipath distortion
D. Cancellation due to reflection
Source: Radiation and Presentation
by:

Engr. Jefril M. Amboy