lecture_24 - C o m m u n i c a t i o n b y R a d i o...

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Unformatted text preview: C o m m u n i c a t i o n b y R a d i o Advantages: Speed: velocity of light exceeds physical transportation speeds Cost is small compared to space voyages or probes Commonly used bands in the radio spectrum. What determines the choice of communication frequency? 1. Economy: cost per photon proportional to energy and frequency 2. Freedom from interference: sum of galactic background, cosmic background quantum limit; atmospheric absorption eliminated from space-based radios 3. Cosmic guideposts: e.g., cosmic waterhole between H spin-flip (1420 MHz) and OH lines (16001700 Mhz) Lattimer, AST 248, Lecture 24 p.1/16 Lattimer, AST 248, Lecture 24 p.2/16 R a d i o T e l e s c o p e s Grote Reber, first parabolic dish Wheaton, IL 1937 Green Bank Telescope Largest fully steerable dish Lattimer, AST 248, Lecture 24 p.3/16 R a d i o T e l e s c o p e s Arecibo Radio Telescope Largest single-aperture telescope VLA - Very Large Array Socorro, New Mexico Lattimer, AST 248, Lecture 24 p.4/16 C o m m u n i c a t i o n b y R a d i o Sources of interference. Total is the sum. Lattimer, AST 248, Lecture 24 p.5/16 C o m m u n i c a t i o n b y R a d i o Signal and Noise, Bandpass Need to select particular frequency (channel center) and bandpass (channel width). An optimum receiver has bandpass equal to signals. Too large bandpass admits extraneous noise. Too small bandpass excludes some signal. Narrower signal of given power has higher signal-to-noise. Number of channels = Frequency- range Bandpass . The cosmic waterhole alone has frequency range 1721 MHz - 1420 MHz = 301 MHz. With 1 MHz bandpass, number of channels = 301. With 1 Hz bandpass, number of channels = 301 million. Minimum bandpass width of about 0.1 Hz set by electron density in interstellar medium. In window from 1000 MHz to 100,000 MHz, there are 1 million million channels of .1 Hz width. Also must consider frequency drift caused by Doppler shifts due to relative velocities of home and alien planets. Lattimer, AST 248, Lecture 24 p.6/16 H i s t o r y o f S E T I Early pioneers were Guglielmo Marconi (1874 - 1937) and Nikola Tesla (1856 - 1943), Marconi made radio practical and sent first transatlantic radio communications. Tesla invented fluorescent lights and developed AC as an energy delivery system. Both felt they were receiving radio signals from Martians. Giuseppe Cocconi and Philip Morrison began modern SETI around 1959, and developed the idea of the cosmic waterhole hypothesis, that civilizations would use the hydrgen spin-flip frequency of 1420 MHz as a guidepost for their own signals. Separately, Frank Drake began using the Green Bank Radio Observatory in 1960 to search for radio signals from Ceti and Eridanii, two solar-like stars about 12 light-years away (Project Ozma)....
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lecture_24 - C o m m u n i c a t i o n b y R a d i o...

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