Download Electromagnetic Waves essay

1st November 2008
Name: Tomasz Waśniewski
Course: Computer Network Administration
Report: Electromagnetic waves
Page 1
Introduction
wall
like
they
make
Electricity can be static, like what holds a balloon to the
or makes your hair stand on end. Magnetism can also be static
a refrigerator magnet. But when
change or move together, they
waves – electromagnetic waves.
Electromagnetic
waves
are
formed
when
an
electric
field
couples with a magnetic field. The
magnetic and electric fields of an
electromagnetic
wave
are
perpendicular to each other and to
the direction of the wave. James
Clerk Maxwell and Heinrich Hertz are
two scientists who studied how electromagnetic waves are formed and
how fast they travel.
About
150
years
ago,
James
Clerk
Maxwell, an English scientist, developed a
scientific theory to explain electromagnetic
waves. He noticed that electrical fields and
magnetic fields can couple together to form
electromagnetic waves. Neither an electrical
field (like the static which forms when you
rub your feet on a carpet), nor a magnetic
field (like the one that holds a magnet onto
your
refrigerator)
will
go
anywhere
by
themselves. But, Maxwell discovered that a
changing magnetic field will induce a changing electric field and
vice-versa.
An electromagnetic wave exists when the changing magnetic field
causes a changing electric field, which then causes another changing
magnetic field, and so on forever. Unlike a static field, a wave
cannot exist unless it is moving. Once created, an electromagnetic
wave will continue on forever unless it is absorbed by matter.
Heinrich Hertz, a German physicist, applied
Maxwell's
theories
to
the
production
and
reception of radio waves. The unit of frequency
of a radio wave - one cycle per second - is named
the hertz, in honour of Heinrich Hertz.
Hertz proved the existence of radio waves
in the late 1880s. He used two rods to serve as a
receiver and a spark gap as the receiving
antennae. Where the waves were picked up, a spark
would jump. Hertz showed in his experiments that
these signals possessed all of the properties of
electromagnetic waves. With this oscillator,
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Hertz solved two problems. First, timing Maxwell's waves. He had
demonstrated, in the concrete, what Maxwell had only theorized that the velocity of radio waves was equal to the velocity of light!
(This proved that radio waves were a form of light!) Second, Hertz
found out how to make the electric and magnetic fields detach
themselves from wires and go free as Maxwell's waves.
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Laboratory equipment
In the laboratory, during our experiments, we used the following
equipment:
-
Horn Transmitter (Tx) and Receiver (Rx) [to originate and
receive the signal]
Two dual-height stands [to support both Tx and Rx]
Micro amp Meter [to get the readings from Rx]
Power Supply Unit [to supply Tx with electric power]
Copper cables [ to connect Rx to Micro amp Meter and Tx to
Power Supply Unit]
The length of the wave that Transmitter produced was 2.8 cm. Wanting
to know its frequency (f) we calculated it using the following
equation:
f = c / λ
, where c= 3*108 m/s [speed of light] and λ
= 2.8 cm = 0.028 m [wave length]
Putting the numbers in the place of letters we came out with the
actual value of frequency that both devices (Tx and Rx) were working
on:
f = (3*108 m/s) / (0.028m) = 10.714 GHz
Voltage that we used for our Transmitter was 12V.
We could sketch a graph showing a sine
calculating earlier the periodic time (T):
T = 1 / f = 1 / (10.714*109) = 0.09 ns
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wave
we
were
using,
Experiment 1 – Signal strength and polarisation
During the first experiment we analysed the basic properties of
electromagnetic waves and measured the strength of the signal
depending on different conditions. We placed the Transmitter and
Receiver opposite to each other as shown on the diagram below:
We noticed that as the distance between the two devices was
getting bigger the less current got to the receiver. Following the
same logic as we brought them closer to each other the signal was
stronger.
We also tried to put different materials between the Tx and Rx
and got different readings written down (the distance between
transmitter and receiver was equal to 1 meter):
-
paper (12 µA)
glass (11 µA)
cardboard (6 µA)
copper plate (0 µA)
My personal notice was that the thicker and denser the material
the poorer the signal was. And so paper almost did not affect the
signal at all. Glass had a minor impact, while thicker materials
like
copper
plate
or
aluminium
nearly
did
not
let
the
electromagnetic waves through.
Later on we tried to put those materials close to, but not on
the direct path of the waves. The results varied, but there was one
interesting thing worth to mention here. When we placed the metal
grid parallel to the wave path, close to the transmitter, the
strength of the signal received by receiver was much stronger. This
is probably more because of reflection though.
Polarisation
Polarisation is a property of waves that describes the
orientation of their oscillations. For transverse waves, it
describes the orientation of the oscillations in the plane
perpendicular to the wave's direction of travel. For electromagnetic
waves such as light, the polarisation is described by specifying the
direction of the wave's electric field.
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Polarisation has many applications among different domains of
science. It is widely used in biology, geology, chemistry, astronomy
and even in art. One of the most common inventions using the laws of
polarisation is probably sunglasses. These work as a polarisation
filter, and enable our eyes to look straight into sun. Polarising
filters remove light polarised at 90o to the filter's polarisation
axis, and so they can be used to observe the effect of palarisation
by looking through them at the reflected light at different angles.
Another interesting use of polarisation has been observed in
nature. Many animals are capable of perceiving the polarisation of
light, which is generally used for navigational purposes, since the
linear polarisation of sky light is always perpendicular to the
direction of the sun. This ability is very common among the insects
(like bees) or other species like octopus, squid, cuttlefish or
mantis shrimp.
The naked human eye is weakly sensitive to polarisation,
without the need for intervening filters. Polarised light creates a
very faint pattern near the center of the visual field, called
Haidinger's brush. This pattern is very difficult to see, but with
practice one can learn to detect polarised light with the naked eye.
All radio transmitting and receiving antennas are intrinsically
polarised, special use of which is made in radar. Most antennas
radiate either horizontal, vertical, or circular polarisation
although elliptical polarisation also exists. Vertical polarisation
is most often used when it is desired to radiate a radio signal in
all directions such as widely distributed mobile units. AM and FM
radio uses vertical polarisation. Television uses horizontal
polarisation. Alternating vertical and horizontal polarisation is
used on satellite communications (including television satellites),
to allow the satellite to carry two separate transmissions on a
given frequency, thus doubling the number of customers a single
satellite can serve.
Polarising filters are also used in photography. They can
deepen the color of a blue sky and eliminate reflections from
windows and standing water.
Polarisation is used also in art.
Some artists apply special techniques
using
polarisation
rules
to
create
colorful and often changing images. 3-D
movies are based on polarisation too, but
the 3-D effect works only on a silver
screen since it maintains polarisation,
whereas
the
scattering
in
a
normal
projection screen would void the effect.
The last example of a device using
polarisation laws I will mention about is
Liquid
Crystal
Display
(LCD)
screen.
Nowadays it is replacing old Cathode Ray
Tube (CRT) monitors and television sets,
as the LCD monitor provides better quality
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of image and takes much less space. It also uses small amount of
power and so it is very commonly used in small electronic devices
such as calculators or alarm clocks.
To prove that the devices we used during these labs were
vertically poralised we used metal grille rods and put them between
transmitter and receiver. First we put them vertically. No signal
was received by the receiver. Then we changed the position of the
rods and put them horizontally. This time signal was not affected by
them and we got the same reading as if there was nothing between the
Tx and Rx. That experiment proved that our waves were vertically
polarised, because when we put metal grille rods vertically on the
wave path they were picked up by the rods which acted in this case
as the antenna.
Finally we did one last experiment. We turned the receiver for
and measured the signal. The reading was 0, what was a proof that
the waves we were using were vertically polarised.
90o
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Experiment 2 – Reflection
Our second experiment was about the basic reflection of
electromagnetic waves. Again we measured the strength of the signal
– this time depending on the position of transmitter and receiver
and the material reflecting the signal and the amount of
reflections. General setting looked very much like this below:
During this experiment we tried to keep both the transmitter
and receiver at the same angle of 45o to the reflecting material as
we know from the lectures that the angle of incidence is equal to
angle of reflection, and so to get best readings it is best to keep
the wave path as close to the Receiver as possible. The initial
distance between two devices pointed at the reflecting material at
an angle of 45o was 1 m. As both angles of incidence and reflection
were making 90o angle, and we knew that the distance between
transmitter and reflecting material was approximately equal to the
distance between receiver and reflecting material, we could work out
those approximate distances from the Pythagoras’ theorem:
(1 m)2 = a2 + a2 = 2a2
or Rx and reflecting material
, where a is a distance between Tx
And so we came out with the value of a:
a =
12 / 2 = 0.707 m
, which is approximately 70 cm.
Basic reflection
First we tried different reflecting materials. We used very
similar materials to those we used during first experiment.
Similarly we got different reading, only this time the results were
opposite:
-
paper (0 µA)
glass (0,5 µA)
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-
cardboard (2 µA)
copper plate (8 µA)
aluminium plate (15-20 µA)
From this instance I came to conclusion that the more dense and
firm the material was the better it reflected the wave. The
interesting notice is that the aluminum plate, though lighter than
copper, reflected the signal very effectively. This is probably
because aluminum is very similar to silver when it comes to
reflecting transverse waves, as the silver is a very good reflecting
material. The other materials voided the signal more or less,
reflecting only part of it.
Next we tried to move the devices, putting them at different
angles and closer or further from the reflecting material. Results
were similar to the previous experiment – the further the distance
the weaker the signal. What about slant, just as I stated before,
keeping devices at the approximately same angles made more current
received by the receiver.
Advanced reflection
After experimenting with basic reflection rules, our curiosity
brought us to create more complex setting and include more than one
reflecting material:
We tried to bounce the signal multiple times before it reached
the Receiver and our results varied a lot. Depending on the path we
built we got stronger or weaker signal. In some cases we could not
get signal at all. Generally the more reflectors we used the weaker
the strength of the signal was. Some particular settings were making
the signal stronger though; these were settings in which reflecting
materials bound the signal inside the area between transmitter and
receiver letting only small parts of it escape.
- Optical fiber An optical fiber became an alternative to
copper cable and wireless networks. It is a glass or
plastic wire that carries light instead of current.
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Optical fiber is able to carry much more data (it has a wider
bandwidth) than any other kind of media (wireless or copper cable
for instance). It is also much faster and more efficient than other
forms of communication, as well as it allows transmission over
longer distances. Fibers are immune to data loss, noise and
electromagnetic interference.
Optical fibers use a specific rule of reflection called ‘total
internal reflection’, which means that the light wave is bound
within the core of the fiber, which acts as a waveguide for the
light it carries. It is built from two different kinds of glass,
having two different refraction indexes (refraction index of
cladding layer must be lower than the refraction index of core
material). The larger the index of refraction, the more slowly light
travels in that medium. The output is
that the light wave is reflecting from
the ‘cladding’ layer glass and thus moving further through the
fiber. The angle at which the light wave is inserted is critical, as
not everyone will work and make the wave reflect. See the diagram
and image below:
There are two general modes optical fibers work on:
-
Single Mode Fibers (SMF) – working only on one mode; used for
links longer than 200 meters
Multi-Mode Fibers (MMF) – supporting several modes at the same
time; usually used for short-distance communication
Optical fiber has many applications, most of which are
connected with networks and communication. It is definitely a
revolution among media world, and it will probably take place of
copper cables in the nearest future. Optical fiber is able to
deliver information over very long distances with repeaters only
every 50-80 km. Erbium-doped fiber allows the signal to regenerate
without using any electrical power, which means it is far more
efficient and effective. Another big advantage of optical fiber is
its bandwidth, capable of caring more data at the same time,
measured not in Mb/s but Gb/s! The latest rumors say that speeds of
1Tb/s are being developed! Fiber is also immune to electrical
interference, which prevents cross-talk between signals in different
cables and pickup of environmental noise.
Other uses of optical fiber include
medicine, illumination applications (like
this Frisbee illumination on the picture
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beside), decorations and art. In some buildings, optical fibers are
used to route sunlight from the roof to other parts of the building.
A lot of signs or images use optical fiber for illumination effects
either to make it more visible or just because of its original
effect, or often both.
Fibers have many uses in remote sensing. In some applications,
the sensor is itself an optical fiber. In other cases, fiber is used
to connect a non-fiber optic sensor to a measurement system.
Depending on the application, fiber may be used because of its small
size, or the fact that no electrical power is needed at the remote
location, or because many sensors can be multiplexed along the
length of a fiber by using different wavelengths of light for each
sensor, or by sensing the time delay as light passes along the fiber
through each sensor.
The last experiment on reflection we did was a very specific
setting of transmitter and receiver put beside each other. Opposite
them we put a reflecting material, so that overall setup looked
exactly like that one below:
Again we measured strength of the signal depending on different
materials and came out with very similar results and conclusions as
with the basic reflection instance. That settings reminded us about
a specific device, called radar.
- Radar Radar is an acronym for Radio
Detection and Ranging. The term became
used as a new word and entered English
language with time, and is no longer
written using capital letters.
Radar
is
electromagnetic
range, altitude,
both moving and
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a
device
that
uses
waves to identify the
direction, or speed of
fixed objects such as
aircraft, ships, motor vehicles, weather formations or terrain. It
transmits the signal, which is being reflected by the target and
pick up the reflected signal by the receiver, which usually located
very close to the transmitter (as on the diagram above). The signal
that returns is often quite weak, but this does not make a big
problem as the signal can be amplified. This makes radar a very
useful tool, even at long distances.
Radar devices use different frequencies, ranging from 3 MHz to
300 GHz, depending on the particular function the device is used to.
For example, air traffic control stations use radars that work on 12 GHz frequencies.
Radar is used in many contexts, including meteorological
detection, measuring ocean surface waves, air traffic control,
police detection of speeding traffic, and by the military.
Further explorations
Below there are a few additional topics and examples concerning
electromagnetic waves.
- Electromagnetic spectrum Waves in the electromagnetic spectrum vary in size from very
long radio waves the size of buildings, to very short gamma-rays
smaller than the size of the nucleus of an atom.
The electromagnetic spectrum should be familiar to most of us.
The microwave we use to heat our food and the cell phones all use
waves that are part of the Electromagnetic Spectrum. The light that
our eyes can see is also part of it. This visible part of the
electromagnetic spectrum consists of the colours that we see - from
reds and oranges, through blues and purples. Each of these colours
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actually corresponds to a different wavelength of light.
- Modulation Modulation is the process of varying a periodic waveform, like
a tone, in order to use that signal to convey, in a similar fashion
as a musician may modulate the tone from a musical instrument by
varying its volume, timing and pitch. Usually a high-frequency
sinusoid waveform is used to carry the signal. The three key
parameters of a sine wave are its amplitude ("volume"), its phase
("timing") and its frequency ("pitch"), all of which can be modified
in accordance with a low frequency information signal to obtain the
modulated signal. Modulation is used to make the signal stronger and
carry it over long distances, as well as regenerate it.
A device that performs modulation is called a modulator, and
one that does the inverse task is called a demodulator. Very often a
device that can do both jobs is used and it has a well known name of
a modem (modulator-demodulator).
- Maxwell’s equations Maxwell's equations are a set of four partial differential
equations that describe the properties of the electric and magnetic
fields and relate them to their sources, charge density and current
density. These equations prove that light is an electromagnetic
wave. Individually, the equations are known as Gauss' law, Gauss'
law for magnetism, Faraday's law of induction, and Ampere’s law with
Maxwell's correction:
xH=
0
E/
t+j
xE=-
0
H/
t
.
H=0
.
E=
/
0
, where H is the magnetic field (A/m), E is the electric field
-12
(V/m), j is the vector current density (A/m2),
0 = 8.8542 x 10
-7
F/m is the permittivity of free space,
x 10
H/m is the
0 = 4
permeability
(C/m3).
of
free
space,
and
is
the
scalar
charge
density
Gauss' law describes how electric charge can create and alter
electric fields. In particular, electric fields tend to point away
from positive charges, and towards negative charges. Gauss' law is
the primary explanation of why opposite charges attract, and like
repel: The charges create certain electric fields, which other
charges then respond to via an electric force.
Gauss' law for magnetism states that magnetism is unlike
electricity in that there are not distinct "north pole" and "south
Page 13
pole" particles (such particles, which exist in theory only, would
be called magnetic monopoles) that attract and repel the way
positive and negative charges do. Instead, north poles and south
poles necessarily come as pairs (magnetic dipoles). In particular,
unlike the electric field which tends to point away from positive
charges and towards negative charges, magnetic field lines always
come in loops, for example pointing away from the north pole outside
of a bar magnet but towards it inside the magnet.
Faraday's law of induction describes how a changing magnetic
field can create an electric field. This is, for example, the
operating principle behind many electric generators: Mechanical
force (such as the force of water falling through a hydroelectric
dam) spins a huge magnet, and the changing magnetic field creates an
electric field which drives electricity through the power grid.
Ampere’s law with Maxwell's correction states that magnetic
fields can be generated in two ways: By electrical current (this was
the original "Ampere’s law") and by changing electric fields (this
was Maxwell's correction, also called the displacement current
term).
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References
• http://science.hq.nasa.gov
• http://en.wikipedia.org
• Physics – Hans C. Ohanian
• AIT Telecommunication Fundamentals – Dr. Robert Stewart
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