WAVE PROPAGATION

Mobile Radio Propagation

Antenna and Wave Propagation 1

 Outline
◼ Speed, Wavelength, Frequency
◼ Types of Waves
◼ Radio Frequency Bands
◼ Propagation Mechanisms
◼ Radio Propagation Effects

Antenna and Wave Propagation 2

 Speed, Wavelength, Frequency
Light speed = Wavelength x Frequency
= 3 x 108 m/s = 300,000 km/s

System Frequency Wavelength

AC current 60 Hz 5,000 km
FM radio 100 MHz 3m
Cellular 800 MHz 37.5 cm
Ka band satellite 20 GHz 15 mm
Ultraviolet light 1015 Hz 10-7 m

Antenna and Wave Propagation 3

 Radio Frequency Bands
Classification Band Initials Frequency Range Characteristics
Extremely low ELF < 300 Hz
Infra low ILF 300 Hz - 3 kHz Ground wave
Very low VLF 3 kHz - 30 kHz
Low LF 30 kHz - 300 kHz
Medium MF 300 kHz - 3 MHz Ground/Sky wave
High HF 3 MHz - 30 MHz Sky wave
Very high VHF 30 MHz - 300 MHz
Ultra high UHF 300 MHz - 3 GHz
Super high SHF 3 GHz - 30 GHz Space wave
Extremely high EHF 30 GHz - 300 GHz
Tremendously high THF 300 GHz - 3000 GHz

Antenna and Wave Propagation 4

 Types of Waves

Ionosphere
(80 - 720 km)
Sky wave
Mesosphere
(50 - 80 km)

Space wave Stratosphere

(12 - 50 km)
Ground wave
Troposphere
(0 - 12 km)
Earth
Antenna and Wave Propagation 5
 What is propagation?
How radio waves travel between two
points?
They generally do this in four ways:
•Directly from one point to another
•Follow the curvature of the earth
•Become trapped in the atmosphere and
traveling longer distances
•Refracting off the ionosphere back to earth.
 Basics of Mobile Radio
Propogation
• VLF,LF,MLF bands radio waves follow ground.
AM radio broadcasting uses MF band.

• Surface of earth acts as a guide (curvature taken

into consideration).

• AT HF ground waves tend to be absorbed by

earth.So we go for ionospheric propagation
 Propagation M o d e s

• Ground-wave propagation

• Sky-wave propagation

• Space wave propagation

Ground Wave Propagation
 Ground Wave Propagation
• Follows contour of the earth
• Can Propagate considerable distances
• Frequencies up to 3 MHz
• Example
– AM radio
 Ground-Wave Propagation
• At frequencies up to about 3 MHz, the most
important method of propagation is by ground waves
which are vertically polarized. They follow the
curvature of the earth to propagate far beyond the
horizon. Relatively high power is required.

Direction of wave travel

Increasing Tilt

Earth
 Ground-Wave
• Radio waves follow the Earth’s surface
• AM broadcasts during the day
• Works best at lower frequencies (40, 80, and
160 meters)
• Relatively short-range communications
Space Wave

Antenna and Wave Propagation 13

Line-of-Sight Propagation
 Line-of-Sight
• Signals travel in a straight line from
transmitting to receiving antenna
• Useful in VHF and UHF ranges
• Television, AM/FM broadcast
• Signals are easily reflected, causing problems
with mobile operation
 Line-of-Sight Propagation
• Transmitting and receiving antennas must be within
line of sight

• Refraction – bending of microwaves by the

atmosphere
– Velocity of electromagnetic wave is a function of the
density of the medium
– When wave changes medium, speed changes
– Wave bends at the boundary between mediums
 Propagation Mechanisms
◼ Reflection
◼ Propagation wave impinges on an object which is large as
compared to wavelength
- e.g., the surface of the Earth, buildings, walls, etc.
◼ Diffraction
◼ Radio path between transmitter and receiver
obstructed by surface with sharp irregular edges
◼ Waves bend around the obstacle, even when LOS (line of sight)
does not exist
◼ Scattering
◼ Objects smaller than the wavelength of the
propagation wave
- e.g. foliage, street signs, lamp posts

Antenna and Wave Propagation 17

Ground/ Surface Wave
◼ Vertical polarization is subject to
considerably less attenuation than
horizontally polarized signals

Antenna and Wave Propagation 18

 Sky Wave Propagation
• Signal reflected from ionized layer of
atmosphere back down to earth
• Signal can travel a number of hops, back and
forth between ionosphere and earth’s surface
• Reflection effect caused by refraction
• Examples
– Amateur radio
– CB radio
Ionization

Antenna and Wave Propagation 20

Ionization

Antenna and Wave Propagation 21

Antenna and Wave Propagation 22
 Refraction in Ionosphere

The amount of refraction that occurs

depends on three main factors:
◼ (1) the density of ionization of the layer,

◼ (2) the frequency of the radio wave,

and
◼ (3) the angle at which the wave enters
the layer.

Antenna and Wave Propagation 23

Density

Antenna and Wave Propagation 24

 Frequency
◼ maximum frequency at which radio waves can be transmitted
vertically and refracted back to Earth. This frequency is known as
the CRITICAL FREQUENCY

Antenna and Wave Propagation 25

 Angle of incidence
◼ Any wave that leaves the antenna at an angle greater than the critical
angle will penetrate the ionospheric layer for that frequency and then
be lost in space.

Antenna and Wave Propagation 26

 Angle of incidence
◼ As the frequency of the radio wave is increased, the
critical angle must be reduced for refraction to occur

Antenna and Wave Propagation 27

Skip zone

Antenna and Wave Propagation 28

Propagation Paths

Antenna and Wave Propagation 29

 MOBILE RADIO PROPAGATION

• Mobile radio channel is an

important controlling factor in wireless
communication systems.

• Transmission path between transmitter and

receiver can vary in complexity.

• LOS (Line Of Sight) Simplest

 WHY SHOULD WE HAVE
PROPAGATION MODELS
With propagation models, we can

• Provide installation guidelines

• Mitigate interference
• Design better wireless system
 MOBILE RADIO PROPAGATION

•Wired channels are stationary and predictable; radio

channels are extremely random and have complex
models.

•Modeling of radio channels is done in statistical

fashion based on measurements for each individual
communication system or frequency spectrum.
 Impairments

◼ Absorption
◼ Fading
◼ Multipath Fading
◼ Selective Fading

Antenna and Wave Propagation 33

Antenna and Wave Propagation 34
 Radio Propagation Effects
Building

Direct Signal

hb Reflected Signal
Diffracted
Signal hm

d
Transmitter Receiver

Antenna and Wave Propagation 35

 Free-space Propagation

hb

hm

Transmitter Distance d
Receiver

◼ The received signal power at distance d:

AeGtPt
Pr =
4d 2
where Pt is transmitting power, Ae is effective area, and Gt is the
transmitting antenna gain. Assuming that the radiated power is uniformly
distributed over the surface of the sphere.

Antenna and Wave Propagation 36

 Antenna Gain
◼ For a circular reflector antenna
Gain G =  (  D /  )2
 = net efficiency (depends on the electric field distribution over the
antenna aperture, losses, ohmic heating , typically 0.55)
D = diameter
thus, G =  ( D f /c )2, c =  f (c is speed of light)

Example:
◼ Antenna with diameter = 2 m, frequency = 6 GHz, wavelength = 0.05 m
G = 39.4 dB
◼ Frequency = 14 GHz, same diameter, wavelength = 0.021 m
G = 46.9 dB

* Higher the frequency, higher the gain for the same size antenna

Antenna and Wave Propagation 37

 Land Propagation
◼ The received signal power:
Gt Gr Pt
Pr =
L
where Gr is the receiver antenna gain,
L is the propagation loss in the channel, i.e.,
L = L P LS L F
Fast fading
Slow fading
Path loss

Antenna and Wave Propagation 38

 Path Loss (Free-space)

◼ Definition of path loss LP :

Pt
LP = ,
Pr
Path Loss in Free-space:

LPF (dB) = 32.45 + 20 log10 f c ( MHz) + 20 log10 d (km),

where fc is the carrier frequency.

This shows greater the fc , more is the loss.

Antenna and Wave Propagation 39

 Path Loss (Land Propagation)

◼ Simplest Formula:
Lp = A d-α

where
A and α: propagation constants
d : distance between transmitter and receiver
α : value of 3 ~ 4 in typical urban area

Antenna and Wave Propagation 40

 Path Loss

◼ Path loss in decreasing order:

◼ Urban area (large city)
◼ Urban area (medium and small city)
◼ Suburban area
◼ Open area

Antenna and Wave Propagation 41

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