Electromagnetic Wave Propagation

High School Science Demonstration

Melanie Leong

June 25, 2003

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

Onizuka Visitor Center Presentation

Melanie Leong

July 5, 2003

Agenda

• 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

•

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

•

key

battery

+-

ignition coil

radio

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 aYUV420 codec decompressorare 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.