Electromagnetic Wave Propagation

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Electromagnetic Wave Propagation

• High School Science Demonstration
• Melanie Leong
• June 25, 2003

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Early Radio to Radio AstronomyA Brief History of
Radio A Quick Introduction to Radio Astronomy

• Onizuka Visitor Center Presentation
• Melanie Leong
• July 5, 2003

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Agenda

• Brief History of Radio
• Explanation of Electromagnetic Waves and
  Frequency
• Electromagnetic Wave Propagation - Spark Gap
  Generator
• Quick Introduction to Radio Astronomy

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A Brief History of Radio

• Radio is a new subject in terms of science
  astronomy
• We receive transmitted radio waves for enjoyment,
  entertainment, and information
• 100 years ago Radio, as we know it, did not
  exist.
• 1884, James Clerk Maxwell
• Calculated the speed electromagnetic waves travel
  is approximately the speed of light.
• Visible light forms only a small part of the
  spectrum of electromagnetic waves.

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History of Radio (p2)

• 1888, Heinrich Hertz
• Proved that electricity could be transformed into
  electromagnetic waves.
• These waves travel at the speed of light.
• 1896, Guglielmo Marconi
• Built a wireless telegraph, a spark gap
  transmitter receiver
• On December 12, 1901, accomplished the Atlantic
  Leap from Poldhu, Cornwall, England to Signal
  Hill, Newfoundland

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History of Radio (p3)

• 1900, Reginald Fessenden
• Continuous Waves necessary to transmit speech and
  music
• December 23, 1900, First wireless voice message
  to colleague
• Hello Brant.
• One. Two. Three. Four.
• Is it snowing where you are Mr. Theissen?
• If it is, telegraph back and let me know.
• Transmit and receive stations were only 1 mile
  apart, but this heralded the beginning of radio
  telephony.
• On Christmas Day 1906, Ships 100 miles away heard
  Fessendens voice and O Holy Night playing on
  their morse code receivers.
• 1906-1912, Radio Broadcast Development

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History of Radio (p4)

• 1924, Spark transmission was phased out
• 1870s-1920s, Telephone developed and in service
• 1932, Karl Jansky
• While doing research for Bell Labs for
  transatlantic radio service, he observed radio
  waves coming from the center of the Milky Way
  Galaxy.
• Not allowed to investigate more . . .
• 1941, Grote Reber
• In 1933, Read about Janskys work, published in
  the NY Times
• By 1939, Built the first radio telescope in his
  backyard
• By 1941, Measured and recorded the first radio
  sky map

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Grote Rebers Radio Sky Maps
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What are Electromagnetic Waves?

• An electromagnetic wave is an energy wave
  produced from an electrical discharge.
• Electromagnetic waves have rise and fall cycles.
• The number of rise and fall cycles per second is
  its frequency.
• We cant see or feel them, but they are around us.

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Explanation of Frequency

• Everything you see, and cant see, resonate at a
  specific frequency.
• Frequencies are sinusoidal waves.
• Speed of Light Frequency x Wavelength
• Example The wavelength of a signal resonating
  at 3kHz is
• 3 x 108 m/s 100 kilometers or 62 miles!
• 3 x 103 Hz
• Lower frequencies have longer wavelengths. This
  characteristic allows these frequencies to be
  used for Morse code and amateur radio.

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Diagram of Waves Frequency
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Frequency Bands

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Simple Demo

• Reviewing what we know
• Electromagnetic waves propagate through space.
• If there is an excitation - electrical discharge,
  what do you predict will happen?
• Electromagnetic waves will propagate from its
  source to throughout the room.
• That means a detector placed anywhere in this
  room will indicate that the waves have propagated.

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Diagram of Demonstration

ignition coil
-
key

radio
battery
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Field Coverage

• This spark gap generator transmits broadband
  emissions - It covers a large band of
  frequencies.
• With a radio tuned to an AM or Shortwave station,
  noise will be heard when the spark gap
  transmitter is energized. (530kHz to 4200kHz)
• Tune to another AM/Shortwave station, you should
  get the same electrical discharge noise.
• If the signal is regulated, or tuned, to one
  band of frequencies, communication can be gained.

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Field Coverage (p2)

• If there is a powerful discharge and you walked
  outside and down the street. Will the noise
  still be detected? Yes!
• Which is why full time activation is prohibited
  by the FCC.

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EM Wave Summary

• Electromagnetic Wavelength - Distance of One
  Cycle (peak to peak)
• Electromagnetic Frequency - Number of Cycles in
  One Second
• Speed of Light Wavelength x Frequency
• Electromagnetic Waves propagate through space
  from an electrical discharge
• Electromagnetic Wave Propagation Uses -
  Communications, Astronomy, and much more

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What is Radio Astronomy?

• It is the study of electromagnetic emissions from
  celestial objects in the radio spectrum band.
• Atoms and molecules in space emit their own
  unique electromagnetic waves.
• Radio telescopes can see cold objects, they do
  not emit light
• Able to detect radio sources behind interstellar
  clouds hidden from optical viewing.
• Can detect distant galaxies at the edge of the
  Universe

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CSO Atmospheric Transmission
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350 µm Survey of Orion KL
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Sagittarius A - Galactic Center
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Arches Cluster in Sagittarius - Optical
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Arches Cluster in Sagittarius - Radio
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Sagittarius Arches in Optical Radio
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Sgr Arches in Optical X-Ray/IR
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Sagittarius - Arches Cluster
Optical
X-Ray/Infrared
Radio
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Summary

• Theres much more out there than visible light.
• What is seen in radio waves is very different
  than optical.
• Electromagnetic waves detected by Radio
  Telescopes tell us more about what else is going
  on out there.
• All spectrum bands are used to observe, measure,
  and interpret what is going on out in the
  Universe.
• Many different ground based and space based
  telescopes are made to accomplish this.