Transcript Electromagnetic Propagation

Electromagnetic Wave Propagation
High School Science Demonstration
Melanie Leong
June 25, 2003
Early Radio to Radio Astronomy
A Brief History of Radio & A Quick Introduction to Radio Astronomy
Onizuka Visitor Center Presentation
Melanie Leong
July 5, 2003
Agenda
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Brief History of Radio
Explanation of Electromagnetic Waves and Frequency
Electromagnetic Wave Propagation - Spark Gap Generator
Quick Introduction to Radio Astronomy
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.
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
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 Fessenden’s
voice and “O Holy Night” playing on their morse code receivers.
– 1906-1912, Radio Broadcast Development
History of Radio (p4)
• 1924, Spark transmission was phased out
• 1870’s-1920’s, 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 Jansky’s 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
Grote Reber’s Radio Sky Maps
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 can’t see or feel them, but they are around us.
Explanation of Frequency
• Everything you see, and can’t 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.
Diagram of Waves & Frequency
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.
Diagram of Demonstration
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ignition coil
key
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+
radio
battery
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.
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.
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
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
CSO Atmospheric Transmission
350 µm Survey of Orion KL
Sagittarius A - Galactic Center
Arches Cluster in Sagittarius - Optical
Arches Cluster in Sagittarius - Radio
Sagittarius Arches in Optical & Radio
Sgr Arches in Optical & X-Ray/IR
Sagittarius - Arches Cluster
Optical
X-Ray/Infrared
Radio
QuickTime™ and a Y UV 420 codec decompressor are needed to see this picture.
Summary
• There’s 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.