Wireless Communications

1
Wireless Communications

• Also Known as Radio
• Part I Radio waves

2
Imagine standing at the side of a pond. In the
shallow water near the edge of the pond is a
small twig floating on the surface. The water is
absolutely calm and still. Pick up a pebble, and
throw it toward the center of the pond. When it
hits the water, a series of waves radiate outward
from the point where the pebble hit the water,
forming concentric rings growing ever larger as
they move outward.
Pebble
Twig
3
The wave crests are perhaps a few inches apart,
and the distance from one crest to the rest is
very nearly constant. That is, the distance from
the first crest to the second is almost exactly
the same as the distance from the second to the
third, from the third to the fourth, and so on.
Before long the first wave reaches the twig,
which begins to bob up and down as the succeeding
waves pass it. The twig serves as a device for
detecting disturbances on the surface of the pond.
Pebble
Twig
4
Viewed from a point right at the surface of the
water the passing waves might look something like
this. The water doesnt actually move from left
to right, but the waves travel from left to right
at the velocity of propagation.
twig
Direction of travel
Wavelength
height
distance from pebble
If we take a snapshot of the waves passing by at
a particular instant, we can measure the distance
between one crest and the next. The distance
between successive crests is called the
wavelength.
5
As the wave crests pass the twig the twig rises,
and it falls as each trough passes it. The
number of times it rises and falls in one second
is called the frequency.
twig
Direction of travel
Wavelength
The wavelength is the distance between two
successive wave crests. If the velocity of the
wave motion is represented by c, the frequency by
f, and the wavelength by l (the Greek letter
lambda), the following relationship is true
6
In this example, the pebble is like a radio
transmitter. It creates a disturbance in the
water, causing waves to radiate outward from it
in all directions. The twig is like a radio
receiver, it detects passing waves. The water is
like empty space, because electromagnetic waves
(radio waves) apparently move through nothing
like waves moving across the surface of the water.
Pebble
Twig
7
Radio waves dont actually move up and down like
waves on the surface of a pond, so heres another
example, using sound waves. Imagine an opera
singer (this may be a painful image for some)
striking a particular note in the same room as a
wine glass. The singers pitch happens to match
the natural frequency (when discussing sound
waves, pitch and frequency mean the same
thing) of the glass, so the glass begins to
vibrate at the same pitch.
If you dont know what is meant by the glasss
natural frequency, set a wine glass on a table,
moisten the tip of your finger, and lightly
stroke the rim of the glass round and round. The
glass will start to sing at a particular pitch,
which is its natural frequency.
Glass
Singer
8
In this example, the singer represents the radio
transmitter, transmitting on a particular
frequency (pitch). The wine glass is like a
radio receiver, tuned to the frequency of the
transmitter. If a second transmitter appears, at
a slightly different frequency, (higher or lower
pitch), the receiver does not respond to it.
In this example, the sound waves dont move up
and down. Sound waves are waves of pressure
Higher pressure followed by lower pressure. They
dont move across the surface of the air, they
move through the volume of air. The wave crests
are concentric spheres, centered on the singer,
and constantly moving outward.
Glass
Singer
9
We could plot the sound waves by plotting air
pressure as a function of distance from the
singer. The velocity of sound waves in air is
the speed of sound, about 331 meters per second.
A pitch of 1000 Hertz (Hz), 1000 wave crests
passing a given point per second, has a
wavelength of
Air pressure
Direction of travel
Wavelength
distance from singer
10
We could also plot the air pressure at one
particular distance from the transmitter as a
function of time, as shown below. The time which
elapses between successive crests is called the
period of the waveform, and is usually
represented by T. One complete wave, from crest
to crest, is called a cycle. The frequency of
the waveform is the number of cycles per second
(one cycle per second is the same as one Hz.),
and the period is the number of seconds (or
milliseconds) per cycle. Thus,
Period (T)
Air pressure
time (t)
11
Keep in mind that the sound waves dont actually
move up and down, like the surface of the pond.
The plot shown below goes up and down, but this
represents air pressure which alternately rises
above and below the normal atmospheric pressure
(represented by the dotted line).
Atmospheric pressure (14.7 psi)
Period (T)
Air pressure
time (t)
12
Radio waves are similar to this in some ways, but
are waves of electromagnetic force instead of air
pressure. The direction of the electromagnetic
field measured at any stationary point reverses
twice per cycle. Radio waves have the
properties of frequency and period, which are
defined exactly like the frequency and period of
sound waves.
Electromagnetic Field Direction
Period (T)
positive
time (t)
negative
13
Radio waves radiate outward from the radio
transmitter in all directions. This is very
similar to the way sound waves radiated outward
from the singer in the previous example. If a
radio receiver is located at some distance from
the transmitter, it can detect these waves.
In this example, wave crests are represented by
solid circles and troughs are represented by
dashed circles. Each circle actually represents
a sphere, with all spheres centered at the
transmitters antenna.
Receiver
Transmitter
14
Radio waves, and all electromagnetic waves
(including light waves) travel (or propagate) at
a much higher velocity than sound waves, and can
propagate through a vacuum (unlike sound waves).
The velocity of propagation of electromagnetic
waves is the same as the speed of light,
300,000,000 meters per second (3 x 108 meters per
second in scientific notation) or 186,000 miles
per second.
Electromagnetic Field
Direction of travel
Wavelength (l)
distance from transmitter
15
The wavelength of a radio signal can be
calculated using the same formula as was used for
sound waves. We saw earlier that the wavelength
of a 1000 Hz. Sound wave in air was 0.331 meters.
Because radio signals propagate at a much
greater velocity, the wavelength would be much
longer
Electromagnetic Field
Direction of travel
Wavelength (l)
distance from transmitter
16
In the early days of radio (which was called
wireless in those days, so we have come full
circle in a sense) the tuning of a transmitter or
receiver was designated in terms of wavelength.
More recently, frequency has been used. However,
in amateur radio the various bands of frequencies
which are allocated (by international agreement)
for amateur radio operators are often referred to
by their approximate wavelength. Here is a table
of some of the amateur radio bands
Our project for this semester is a 20 - meter
receiver.
17
The lowest frequency used for radio transmission
is 76 Hz, with a wavelength of 3947 kilometers.
That system operated in the frequency range
designated SLF (Super Low Frequency), and was
used until 2004 to communicate with submerged
submarines. Here are other frequency-range
designations
 
 
18
In contrast, the wavelength of visible light
ranges from 700 nanometers down to 400 nm.
Wavelengths longer than 700 nm are considered
infrared, and those from 400 nm down to 10 nm
(the longest x-rays) are considered ultraviolet.
Radio waves behave somewhat like light waves.
This similarity grows closer as the frequency of
a radio wave increases (and its wavelength
decreases). Like light waves, radio waves tend
to travel in a straight line. Also like light
waves, the straight-line path of radio waves can
be altered by phenomena such as reflection,
refraction, diffraction and scattering. The
manner and degree to which these phenomena affect
a radio signal depend on its wavelength. Some
effects also exhibit daily and yearly variation,
and are dependent sunspot number and the
geomagnetic field.
 
 
19
Consider a radio transmitter and receiver
separated by a distance which is sufficient that
the transmitter is over the horizon from the
viewpoint of the receiver. The straight-line (or
line-of-sight) path is obscured by the earth, so
no signal is received. This is normally the case
for signals in the VHF range and above As shown
here, over-the-horizon communication via the
line-of-sight path is impossible. The signal
either is blocked by the ground, or goes off into
space.
Transmitter
Receiver
 
 
20
Relatively low frequencies, including the AM
broadcast band, tend to follow the curvature of
the earth. This is called groundwave
propagation. You may be able to receive WLS
(the big 89, 890 KHz) in Chicago on your car
radio. This would not be possible without
groundwave propagation. Unfortunately, signals
following the groundwave path are quickly
absorbed by the earth. This is why you probably
wouldnt receive WLS more than 200 to 300 miles
from Chicago during daylight hours.
Transmitter
Receiver
 
 
21
At still higher frequencies, including 20 meter
Amateur band, radio waves are refracted or bent
by certain layers within the ionosphere (a region
of charged particles, ionized by the suns
radiation, from 50 km to 400 km above the surface
of the earth). Under good conditions, the
waves may be bent sharply enough that they return
to the earth instead of going off into space.
The effect is almost as if they were reflected by
a mirror many kilometers above the earth. This
is called skywave propagation.
Ionosphere
Transmitter
Receiver
 
 
22
This effect depends primarily on the amount of
ultraviolet light from the sun which reaches the
ionosphere, which varies daily and seasonally.
The amount of UV radiated by the sun also depends
on the amount of sunspot activity, which exhibits
an eleven-year cycle. Increasing UV radiation
increases the thickness of the ionospheric layers
which refract radio waves, which increases the
maximum frequency at which signals return to the
earth.
Ionosphere
Transmitter
Receiver
 
 
23
This effect also depends on the angle at which a
signal strikes the ionosphere (the angle of
incidence), which is greater for short paths. At
an angle which is too great, the signal does not
return to earth. This angle depends on the
thickness of the ionosphere (thicker is better)
and the signal frequency. Lower frequencies
refract at greater angles than higher
frequencies.
Ionosphere
Transmitter
Receiver
 
 
24
For a given signal frequency, there may be a zone
beyond the maximum line-of-sight range and the
minimum skywave range. Within this skip zone,
reception is not possible. For any given path,
there is a Maximum Usable Frequency, or MUF,
above which skywave propagation does not occur.
The MUF is generally higher at night than during
daylight, and increases with increasing sunspot
activity.
Ionosphere
Skip Zone
Transmitter
Receiver
 
 
25
For a given signal frequency, there may be a zone
beyond the maximum line-of-sight range and the
minimum skywave range. Within this skip zone,
reception is not possible. For any given path,
there is a Maximum Usable Frequency, or MUF,
above which skywave propagation does not occur.
The MUF is generally higher at night than during
daylight, and increases with increasing sunspot
activity.
Ionosphere
Skip Zone
Transmitter
Receiver
 
 
26
AM broadcast signals do exhibit skywave
propagation, but during daylight the D layer of
the ionosphere absorbs most of the energy at
these relatively low frequencies, so little
energy reaches the F layer which is responsible
for skywave propagation. At night, the D layer
disappears, and AM band skywave propagation takes
place. At night, you may be able to pick up AM
stations many hundreds or even thousands of miles
away. Try listening for WCBS-AM at 880 KHz some
night.
Ionosphere
Skip Zone
Transmitter
Receiver
 
 
27
D-layer absorption decreases as frequency
increases, so if the MUF is great enough skywave
propagation during daylight is often possible.
The best frequency for a given path would be
close to (but not greater than) the MUF for that
path because this minimizes D-layer absorption.
Ionosphere
Skip Zone
Transmitter
Receiver
 
 
28
The 20-meter amateur band, where our project will
operate, is a particularly good compromise. It
often supports worldwide skywave propagation
during daylight hours, even during low points of
the sunspot cycle. During sunspot maxima, it
supports skywave propagation as much as 24 hours
per day.
Ionosphere
Skip Zone
Transmitter
Receiver