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» Communicate technical information about how
some technological devices use the principles of
wave behavior and wave interactions with matter
to transmit and capture information and energy
˃ Science Practices
˃ Core Idea
˃ Cross-cutting concepts
» Lesson 1: EM modulation
˃ Activity: make a simple radio
˃ Activity: Build a cell phone detector
» Lesson 2: AM and FM
˃ Activity: Phet simulation
» Lesson 3: Communications closure
» Software Defined Radio Project
(Summative)
Encoding information in
radio waves
Wikipedia Commons
Take one nine volt battery, 1 nickle, and 1 AM radio.
Tune the radio to a region where you hear only static
Quickly Tap the nickel on the terminals of the battery
Record your observations in your notebook.
Take five minutes to investigate what factors might
influence the system.
» Reflection: Describe your observations and any
relationships between variables in your system.
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» How does your voice travel through space from your
cell phone to your friends cell phone?
» What are two ways in which radio waves can be
modified to carry information?
» How fast does your cell phone send information from
one phone, through the base stations, and to
another phone?
EM radiation is generated by
the acceleration of an
electron.
properties of electron
negative charge
almost negligible mass
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Wikipedia Commons
(NIST, 1994)
electron = elementary
negative charge
electric field
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Figure 1.
decreases with distance
according to an inversesquared law
exerts an electric force on
other charged particle
like charges = repulsion force
opposite = attractive force
note field lines are visual representation of field
direction (arrows) and strength (line density)
Figure 2.
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Magnetism in materials also produces fields that influence (applies a force to)
other magnetic materials
magnetic fields can influence electric charges
and likewise accelerating electric charges can create a magnetic field
An accelerating charged particle produces both an electric field and a magnetic
field that together form an electromagnetic disturbance that travels across
distances as a transverse wave.
Figure 1
» EM waves are
variations in the
electric field (E field)
and magnetic field
(B) over time.
» These waves travel
at a constant speed
» C = 3.00 x 109 m/s
An EM wave carries both energy and information from
one part of the universe to another.
Therefor EM radiation constitutes a transmission
channel!
remember Sugar, Soy, Glue; the rules of your
robot transmission channel?
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These are the ways that the properties of waves can be
affected by the radio source wether it be a natural
source, or a man-made
Amplitude (AM radio)
Frequency (or wavelength, FM radio)
Phase (RADAR)
Polarization (radio astronomy)
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Two types of polarization
» Linear polarization
˃ Either vertical or horizontal
» Circular polarization
In-phase
» Phase shifting cannot
be seen by the eye
» Detecting phase
changes
˃ Here the crest and trough are
colored to illustrate
+ in phase waves
+ out of phase waves
Wikipedia Commons
Out-of-phase
» How can the amplitude
of an EM wave be
changed?
˃ Amplitude corresponds to
intensity (# of photons)
˃ Amplitude modulation stores a
signal in a single frequency radio
wave (carrier wave)
Berserkerus/Wikimedia Commons
» Carrier wave
˃ One frequency that will be
added to by the signal
frequencies
» Signal Frequency
˃ Contains the information from
the source
Berserkerus/Wikimedia Commons
» Starter
» In a group of 2 or 3,
collect answers to the
following questions and
chose 1 member to be
a spokesperson.
1. Give some numbers of
radio stations in our
area.
2. What is your favorite
station and why.
3. What do the numbers
stand for?
4. Do you know where
the transmission
tower is?
» See “The History of Radio” for a full back ground on
the developments in the 19th and 20th century that
have led to our current technological
communication.
» Most common form of communication occurs on AM
and FM (Frequency Modulated) carrier waves.
» Lets look at how the information gets “encoded”
» How can the amplitude
of an EM wave be
changed?
˃ Amplitude corresponds to
intensity (# of photons)
˃ Amplitude modulation stores a
signal in a single frequency radio
wave (carrier wave)
» Carrier wave
˃ One frequency that will be
added to by the signal
frequencies
» Signal Frequency
˃ Contains the information from
the source
Note: Amplitude is not changing but frequency is
(Frequency Modulation)
Pros
» Very easy to detect with simple equipment (less
expensive)
» Narrow band width allows for more stations in a
geographical area without interference.
» Can be transmitted over long distances due to
ionospheric refraction.
» Can be very noisy
˃ Affected by electrical activity in the atmosphere
˃ Affected by other radio transmissions
» Fluctuates between day and night
˃ Night time allows for long range transmission
˃ HAM radio utilizes this
» Broadcast transmissions would interfere with each
other
» A carrier wave is generated at a source with a certain
power.
˃ This process required that a very consistent oscillation is generated in a
current.
˃ The frequency of the alternating current generated radio of the same
frequency.
» A Transmitting station adds a sound spectrum to the
carrier wave
» Less noise from atmosphere
˃ Clearer signal \
» Higher bandwidth
Cons
» More local, long distance transmission difficult
» Interference from objects like buildings, mountains,
etc…
» Wider bandwidth means less stations per geographic
area
» Watch this short video clip about how information is
encoded using frequency modulation
» https://www.youtube.com/watch?v=SmW4z76KgNQ
» FM transmitters are notoriously expensive to run
˃ They need complicated electronics to be able to encode the signal and carrier
waves
˃ They need lots of voltage in order to increase power to > 1000 watts
˃ Need to be cooled, maintained, and serviced
» While the switch to FM was advantageous for the
sound clarity and consistency, the expense
requirements changed the way broadcast
communication was done. …..Advertising time was
sold to cover the cost.
» Here
Voice contains a spectrum of
frequencies
The wiggling magnet finally
creates the compressional
waves that eventually reach our
ears.
Vibrations from the air
molecules strike a small
transducer (microphone)
Then the current is attached to a
speaker which ‘wiggles’ a
magnet at the same frequency
as the original sound.
The transducer changes the
energy from molecular air
motion to an electric current
The receiver then subtracts the
carrier to get the signal
The current varies in magnitude
on the same exact frequency as
the voice did also
We pick up that radio wave
when we tune a receiver to the
frequency of the carrier
That “signal” can be encoded
onto a radio “carrier” wave.
The radio carrier + signal is
amplified to > 1000 W
» So, now you know how
radio waves are
produced, but how do
they carry things like
voice, TV video, data
from spacecraft etc….?
» Interaction with material or fields (electric,
magnetic, or gravitational) as it travels.
» Human eyes are not
sensitive to
polarization
» Polarized sunglasses
work by excluding
reflected and scattered
light preferentially
since they tend to be
polarized in certain
directions
» Some animals (e.g. bees, ants, fish, octopuses, crickets) are
sensitive to polarized light and use it to navigate and enhance
their vision
Left Panel presents a normal image of a soft plastic CD case with no polarization information.
Right Panel shows the image with polarization information. The finger print on the CD becomes clearly visible.
(Nader Engheta, University of Pennsylvania)
» In radio astronomy the
degree of rotation of a
polarized signal
(Faraday rotation) gives
information about the
density of material
along the path of the
signal.
Satellite communication
frequently makes use of
polarization to send two
separate noninterfering signals at the
same frequency
» We have covered basics
of wave theory.
» The ways in which radio
waves can be modified
» From voice to radio and
back to voice.
1. "Astronomy: A Beginner's Guide to the Universe"
7th ed. Chaisson, E.; McMillan, S. Pearson Education
inc. 2013 p.503
2. "Mass of an Electron." Fundamental Physical
Constants. NIST, Oct. 1994. Web.
<http://physics.nist.gov/cuu/Constants/index.html>.
3. “Outer Space is not Empty: A Teaching Unit in
Astrochemistry”. RET 2004 Haystack Observatory
MIT. Wesley Johnson and Roy Riegel.
4. Course: ASTR 122: Birth, Life and Death of Stars
http://jersey.uoregon.edu/~imamura/122/astro.122.h
tml
5. http://www.pbs.org/wgbh/aso/tryit/radio/indext.html
6. http://galileo.phys.virginia.edu/classes/241L/emwaves
/emwaves.htm
7. http://www.astro.utu.fi/~cflynn/astroII/l4.html