Information in Radio Waves

Transcription

1 Using History of Radio presentation: Slide 1: The History of Radio and Radio Astronomy - Introduce the purpose of the overall lesson, which is to give perspective on where all their household technology (which almost all rely on radio waves in one way or another) stems from. Slide 2: The Beginning... - Give students a few moments to answer these questions. Have them do it in their heads or write them down. Discuss as a class. Slide 3: The Beginning... (2) - Maxwell combined 4 very well known mathematical equations together that became part of his theory of electromagnetism. His theory and equations showed how light can exist outside of the visible spectrum. - Hertz was the first person to demonstrate in a lab that radio waves could be produced as well as detected therefore proving the existence of electromagnetic waves as proposed by Maxwell. Slide 4: First Uses - Present this slide speaking in only beeps. Either that, or mention this was first piece of technology that was capable of do wireless communication (machine to machine). Mention that the early radio devices used what was called a spark-gap transmitter. Used pulse modulation (signal was either on or off). Slide 5: Radio s Evolution - Highlight that this wasn t possible with the basic technology of the wireless telegraph and only became possible when the transmitters became more efficient and receivers more sensitive. - Fessenden used a high frequency alternator-transmitter which was able to produce a continuous sine wave capable of being modulated in amplitude up and down to carry sound information. Slide 6: Radio s Evolution (2) - There is a significant time gap between the previous slide and this one. Ask your students why they think this is. The answer being that during World War 1, all radio patents were seized by the military (the navy) to prevent other nations spies getting access to the technology. All developments and improvements during the war were based on AM and were done by the Navy. - Soon after the war, the government released control of all radio patents and the company RCA was founded with the purpose of redistributing those patents. - AM radio lingered a while, delayed possibly from the seizures of patents during the war. Radio became mainstream in this time with many AM radio stations popping up. AM had its obvious limitations and amatuer operators started to fiddle with encoding sound through varying the frequency instead of the amplitude. This was officially achieved by Edwin Armstrong in 1933.

2 - Interesting story: Orson Welles broadcast in Ask students if they have heard of it. While the overall level of panic is debated, one humorous true story to come out of it: Many listeners sued the network for "mental anguish" and "personal injury". All suits were dismissed, except for a claim for a pair of black men's shoes (size 9B) by a Massachusetts man, who spent his shoe money to escape the Martians. Welles insisted the man be paid. Press. Schroeder, Andreas (2005). Scams!. True stories from the Edge (2nd ed.). Annick p. 43. ISBN X Slide 7: Things Take Off... - Most or all of your students will be too young to remember TV before cable and satellite. Inform them that even today, they can get channels on their TVs via an antenna. Slide 8:Interlude - A nice diagram that details Radio from its invention to modern digital contributions/events Slide 9: Radio Astronomy - Note that all radio astronomy advances were being made simultaneously with the previous radio history, beginning with the same two people: Maxwell and Hertz. - Edison and Lodge were not the only two scientists of the time to attempt detection of cosmic waves, but were two of the most notable to do so: Edison wrote a proposed apparatus for detecting radio waves from the sun. No evidence that he actually build and tested the device but even so it would not have been successful (overall insensitive and only sensitive to very long wavelengths which are scattered by the ionosphere). Lodge was also unsuccessful, building an apparatus that was again too insensitive. His device was sensitive to centimeter wavelengths (which can penetrate the ionosphere) but was hampered by the extensive interference in his area. - Others of note: Johannes Wilsing ( ) and Julius Scheiner ( ) Charles Nordman Max Planck (very important, his theory of black body radiation curves may have been the key to further attempts of detecting solar radio waves) Oliver Heaviside - theorized the existence of an ionized layer of the atmosphere Slide 10: Karl Jansky - Karl Jansky, working with a degree in physics and working for Bell Telephone Laboratories out of New Jersey was given the task of investigating sources of static that would be possible

3 interference for voice transmissions. - Designed and build an antenna sensitive to the frequency 20.5 MHz (~14.5 m). Was mounted and able to rotate in any direction. Slide 11: Karl Jansky cont. - After months of observational data, Jansky identified three sources of static: 1. Nearby thunderstorms 2. Distant Thunderstorms 3. faint steady hiss of unknown origin - Jansky spent over a year studying the unknown hiss and noted that it rose and fell ~once a day. Thought at first that it was the sun, but after a while the brightest point moved away from the Sun. Slide 12: Karl Jansky cont. - The rise and fall of the signal followed a sidereal day, therefore wasn t coming from the Sun. Turned out to be from the center of the Galaxy. - Widely publicized, appearing in the New York Times May 5th, Wanted to further investigate the radio emissions from the Milky Way and proposed a large dish antenna (100 feet) for this purpose but Bell Labs had the answer they wanted and didn t care for further investigation. Jansky never worked in astronomy again. Slide 13: Grote Reber - Grote Reber wasn t the only scientist who sought to duplicate Jansky s findings but the timing was bad for any research being done as it was the great depression. - Reber was affected by the depression as well, not being able to get a job at an observatory like he wanted but decided to do the research on his own in his backyard. The picture in this slide is how the telescope he made looked in 1970 (painted red, white and blue). Slide 14: Grote Reber cont. - Build a dish in his backyard of Wheaton, Illinois at his own expense. Was 31.4 ft in diameter and of a parabolic dish design meaning it reflects wavelengths of all sizes to a single focus. - Tried three different receivers before confirming Jansky s discovery in Reber used his telescope to survey the sky and then plotted his data as a contour map, detailing the brightness of the radiation. - Reber s work in astronomy laid the groundwork for research after World War 2. Slide 15: Important Times in Radio Astronomy - After years and multitudes of attempts by many scientists, radio waves from the Sun are finally detected by James Stanley Hey

4 - Jan Oort and Hendrick C. van de Hulst predict the existence of the 21-cm line. H.C. van de Hulst should get all the credit though as it was his theory that was simply assigned and supported by Oort. - Edward Mills Purcell and Harold Irving Ewen went on to confirm the existence of the 21-cm line. They built a horn antennae that they hung out the windows of their lab at Harvard. Picture of this slide is the horn antenna outside of their window. - Utilized a frequency switching technique that effectively cancelled out noise sources, and successfully detected the hydrogen line in 1951 Slide 16: Other important times - During WW2, interferometry was discovered while using radar. Soon after, the technique s usefulness was recognized for radio astronomy as the resolution of a telescope relies on the wavelength divided by the size of the instrument s aperture. Instead of making the dish bigger, multiple dishes could be used at once thereby increasing the aperture by the distance between the two foci (baseline) , VLBI is born (very long baseline interferometry) is born. Radio information is recorded in two or more very distant locations and then correlated using very accurate clocks. Allows for very high angular resolution. To this day, VLBI is one of the most prominent tools used by radio astronomers. U.S. were the first to achieve fringes. The canadians soon after. Both used different techniques, the Canadians analog recording and the U.S. digital. VLBI becomes more and more useful as technology developments in data storage allow for more and more data to be recorded at ever increasing rates. Slide 17: More Dates - Arecibo is the largest single dish radio antenna in the world with an aperture of 1000 ft. - Dish is fully stationary while the focus can be moved using the cable system that holds it up. Slide 18: More Dates (2) - Irwin Shapiro, in 1966 and 1967, conducted his famous 4th test of general relativity at the Haystack Observatory in Westford, MA. - His test theorized and confirmed that radar pulses that are bounced off the surfaces of mercury and venus, when favorably aligned, should take ~200 microseconds longer to return due to the gravitational pull of the Sun. Animation: Slide 19: The VLA - The VLA is a large radio telescope array consisting of 27 radio antennae that are in a Y- shaped configuration in New Mexico. The entire array has a total of 4 different configurations allowing for greater and greater distances between the antennae, the maximum creating a single dish equivalent to 22 miles in size.

5 Been featured in many films including: Contact in 1997 Terminator Salvation in 2009 Transformers: Dark Side of the Moon (incorrectly shown to exist before it was actually constructed) Slide 20-25: Local Efforts - Introduces and gives links that show the developments made by MIT in radio technologies. - MITs efforts and research at the time can be used to explain and show students how research is often done. The military finds an issue with technology and the limitations of it, research money is doled out to fix the problem. As more things pop up, more money is given out and more research is done. - slide 25 gives a good list of topics for students who want to know more in the subject of radio astronomy.