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Radio wave
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This article is about the radiation. For the generic oscillation, see Radio frequency. For the electronics, see Radio frequency engineering.
For other uses, see Radio Wave (disambiguation).
Animation of a half-wave dipole antenna radiating radio waves, showing the electric field lines. The antenna in the center is two vertical metal rods connected to a radio transmitter (not shown). The transmitter applies an alternating electric current to the rods, which charges them alternately positive (+) and negative (−). Loops of electric field leave the antenna and travel away at the speed of light; these are the radio waves. In this animation the action is shown slowed down enormously.
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as high as 300 GHz to as low as 3 kHz. At 300 GHz, the corresponding wavelength is 1 mm (0.039 in), and at 3 kHz is 100 km (62 mi). Like all other electromagnetic waves, they travel at the speed of light. Naturally occurring radio waves are generated by lightning, or by astronomical objects, the artificial ones are produced by transmitters and received by radio receivers.
Artificially generated radio waves are used for fixed and mobile radio communication, broadcasting, radar and other navigation systems, communications satellites, computer networks and many other applications. Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere; long waves can diffract around obstacles like mountains and follow the contour of the earth (ground waves), shorter waves can reflect off the ionosphere and return to earth beyond the horizon (skywaves), while much shorter wavelengths bend or diffract very little and travel on a line of sight, so their propagation distances are limited to the visual horizon.
To prevent interference between different users, the artificial generation and use of radio waves is strictly regulated by law, coordinated by an international body called the International Telecommunications Union (ITU), which defines radio waves as "electromagnetic waves of frequencies arbitrarily lower than 3 000 GHz, propagated in space without artificial guide".[1] The radio spectrum is divided into a number of radio bands on the basis of frequency, allocated to different uses.
Diagram of the electric fields (E) and magnetic fields (H) of radio waves emitted by a monopole radio transmitting antenna (small dark vertical line in the center). The E and H fields are perpendicular as implied by the phase diagram in the lower right.
Contents
[hide]
1 Discovery and exploitation
2 Propagation
3 Speed, wavelength, and frequency
4 Radio communication
5 Biological and environmental effects
6 See also
7 Notes
8 References
Discovery and exploitation[edit]
Main article: History of radio
Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including radio waves.
Radio waves were first predicted by mathematical work done in 1867 by Scottish mathematical physicist James Clerk Maxwell.[2] Maxwell noticed wavelike properties of light and similarities in electrical and magnetic observations. His mathematical theory, now called Maxwell's equations, described light waves and radio waves as waves of electromagnetism that travel in space, radiated by a charged particle as it undergoes acceleration. In 1887, Heinrich Hertz demonstrated the reality of Maxwell's electromagnetic waves by experimentally generating radio waves in his laboratory,[3] showing that they exhibited the same wave properties as light: standing waves, refraction, diffraction, and polarization. Radio waves, originally called "Hertzian waves",[4] were first used for communication in the mid 1890s by Guglielmo Marconi, who developed the first practical radio transmitters and receivers. The modern term "radio wave" replaced the original name "Hertzian wave" around 1912.
Propagation[edit]
Main article: Radio propagation
The study of radio propagation, how radio waves move in free space and over the surface of the Earth, is vitally important in the design of practical radio systems. Radio waves passing through different environments experience reflection, refraction, polarization, diffraction, and absorption. Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certain radio bands more useful for specific purposes than others. Practical radio systems mainly use three different techniques of radio propagation to communicate:[5]
Line of sight: This refers to radio waves that travel in a straight line from the transmitting antenna to the receiving antenna. It does not necessarily require a cleared sight path; at lower frequencies radio waves can pass through buildings, foliage and other obstructions. This is the only method of propagation possible at frequencies above 30 MHz. On the surface of the Earth, line of sight propagation is limited by the visual horizon to about 64 km (40 mi). This is the method used by cell phones, FM and television broadcasting and radar. By using dish antennas to transmit beams of microwaves, point-to-point microwave relay links transmit telephone and television signals over long distances up to the visual horizon. Ground stations can communicate with satellites and spacecraft billions of miles from Earth.
Indirect propagation: Radio waves can reach points beyond the line-of-sight by diffraction and reflection.[5] Diffraction allows a radio wave to bend around obstructions such as a building edge, a vehicle, or a turn in a hall. Radio waves also reflect from surfaces such as walls, floors, ceilings, vehicles and the ground. These propagation methods occur in short range radio communication systems such as cell phones, cordless phones, walkie-talkies, and wireless networks. A drawback of this mode is multipath propagation, in which radio waves travel from the transmitting to the receiving antenna via multiple paths. The waves interfere, often causing fading and other reception problems.
Ground waves: At lower frequencies below 2 MHz, in the medium wave and longwave bands, due to diffraction vertically polarized radio waves can bend over hills and mountains, and propagate beyond the horizon, traveling as surface waves which follow the contour of the Earth. This allows mediumwave and longwave broadcasting stations to have coverage areas beyond the horizon, out to hundreds of miles. As the frequency drops, the losses decrease and the achievable range increases. Military very low frequency (VLF) and extremely low frequency (ELF) communication systems can communicate over most of the Earth, and with submarines hundreds of feet underwater.
Skywaves: At medium wave and shortwave wavelengths, radio waves reflect off conductive layers of charged particles (ions) in a part of the atmosphere called the ionosphere. So radio waves directed at an angle into the sky can return to Earth beyond the horizon; this is called "skip" or "skywave" propagation. By using multiple skips communication at intercontinental distances can be achieved. Skywave propagation is variable and dependent on atmospheric conditions; it is most reliable at night and in the winter. Widely used during the first half of the 20th century, due to its unreliability skywave communication has mostly been abandoned. Remaining uses are by military over-the-horizon (OTH) radar systems, by some automated systems, by radio amateurs, and by shortwave broadcasting stations to broadcast to other countries.
Speed, wavelength, and frequency[edit]
Radio waves in vacuum travel at the speed of light.[6][7] When passing through a material medium, they are slowed according to that object's permeability and permittivity. Air is thin enough that in the Earth's atmosphere radio waves travel very close to the speed of light.
The wavelength is the distance from one peak of the wave's electric field (wave's peak/crest) to the next, and is inversely proportional to the frequency of the wave. The distance a radio wave travels in one second, in a vacuum, is 299,792,458 meters (983,571,056 ft) which is the wavelength of a 1 hertz radio signal. A 1 megahertz radio signal has a wavelength of 299.8 meters (984 ft).
Radio communication[edit]
Main article: Radio communication
In radio communication systems, information is carried across space using radio waves. At the sending end, the information to be sent, in the form of a time-varying electrical signal, is applied to a radio transmitter.[8] The information signal can be an audio signal representing sound from a microphone, a video signal representing moving images from a video camera, or a digital signal representing data from a computer. In the transmitter, an electronic oscillator generates an alternating current oscillating at a radio frequency, called the carrier because it serves to "carry" the information through the air. The information signal is used to modulate the carrier, altering some aspect of it, "piggybacking" the information on the carrier. The modulated carrier is amplified and applied to an antenna. The oscillating current pushes the electrons in the antenna back and forth, creating oscillating electric and magnetic fields, which radiate the energy away from the antenna as radio waves. The radio waves carry the information to the receiver location.
At the receiver, the oscillating electric and magnetic fields of the incoming radio wave push the electrons in the receiving antenna back and forth, creating a tiny oscillating voltage which is a weaker replica of the current in the transmitting antenna.[8] This voltage is applied to the radio receiver, which extracts the information signal. The receiver first uses a bandpass filter to separate the desired radio station's radio signal from all the other radio signals picked up by the antenna, then amplifies the signal so it is stronger, then finally extracts the information-bearing modulation signal in a demodulator. The recovered signal is sent to a loudspeaker or earphone to produce sound, or a television display screen to produce a visible image, or other devices. A digital data signal is applied to a computer or microprocessor, which interacts with a human user.
The radio waves from many transmitters pass through the air simultaneously without interfering with each other. They can be separated in the receiver because each transmitter's radio waves oscillate at a different rate, in other words each transmitter has a different frequency, measured in kilohertz (kHz), megahertz (MHz) or gigahertz (GHz). The bandpass filter in the receiver consists of a tuned circuit which acts like a resonator, similarly to a tuning fork.[8] It has a natural resonant frequency at which it oscillates. The resonant frequency is set equal to the frequency of the desired radio station. The oscillating radio signal from the desired station causes the tuned circuit to oscillate in sympathy, and it passes the signal on to the rest of the receiver. Radio signals at other frequencies are blocked by the tuned circuit and not passed on.
Biological and environmental effects[edit]
Radio waves are nonionizing radiation, which means they do not have enough energy to separate electrons from atoms or molecules, ionizing them, or break chemical bonds, causing chemical reactions or DNA damage. The main effect of absorption of radio waves by materials is to heat them, similarly to the infrared waves radiated by sources of heat such as a space heater or wood fire. The oscillating electric field of the wave causes polar molecules to vibrate back and forth, increasing the temperature; this is how a microwave oven cooks food. However, unlike infrared waves, which are mainly absorbed at the surface of objects and cause surface heating, radio waves are able to penetrate the surface and deposit their energy inside materials and biological tissues. The depth to which radio waves penetrate decreases with their frequency, and also depends on the material's resistivity and permittivity; it is given by a parameter called the skin depth of the material, which is the depth within which 63% of the energy is deposited. For example, the 2.45 GHz radio waves (microwaves) in a microwave oven penetrate most foods approximately 2.5 to 3.8 cm (1 to 1.5 inches). Radio waves have been applied to the body for 100 years in the medical therapy of diathermy for deep heating of body tissue, to promote increased blood flow and healing. More recently they have been used to create higher temperatures in hyperthermia treatment, to kill cancer cells. Looking into a source of radio waves at close range, such as the waveguide of a working radio transmitter, can cause damage to the lens of the eye by heating. A strong enough beam of radio waves can penetrate the eye and heat the lens enough to cause cataracts.[9][10][11][12][13]
Since the heating effect is in principle no different from other sources of heat, most research into possible health hazards of exposure to radio waves has focused on "nonthermal" effects; whether radio waves have any effect on tissues besides that caused by heating. Electromagnetic radiation has been classified by the International Agency for Research on Cancer (IARC) as "Possibly carcinogenic to humans".[14]
Radio waves can be shielded against by a conductive metal sheet or screen, an enclosure of sheet or screen is called a Faraday cage. A metal screen shields against radio waves as well as a solid sheet as long as the holes in the screen are smaller than about 1/20 of wavelength of the waves.[15]
See also[edit]
Look up radio wave in Wiktionary, the free dictionary.
Radio portal
Telecommunication portal
Radio astronomy
Television transmitter
Notes[edit]
Jump up ^ ITU Radio Regulations, Chapter I, Section I, General terms – Article 1.5, definition: radio waves or hertzian waves
Jump up ^ Harman, Peter Michael (1998). The natural philosophy of James Clerk Maxwell. Cambridge, England: Cambridge University Press. p. 6. ISBN 0-521-00585-X.
Jump up ^ "Heinrich Hertz: The Discovery of Radio Waves". Juliantrubin.com. Retrieved 2011-11-08.
Jump up ^ "22. Word Origins". earlyradiohistory.us.
^ Jump up to: a b Seybold, John S. (2005). Introduction to RF Propagation. John Wiley and Sons. pp. 3–10. ISBN 0471743682.
Jump up ^ "FREQUENCY & WAVELENGTH CALCULATOR". www.1728.org. Retrieved 15 January 2018.
Jump up ^ "National Radio Astronomy Observatory - National Radio Astronomy Observatory". National Radio Astronomy Observatory. Retrieved 15 January 2018.
^ Jump up to: a b c Brain, Marshall (2000-12-07). "How Radio Works". HowStuffWorks.com. Retrieved 2009-09-11.
Jump up ^ Kitchen, Ronald (2001). RF and Microwave Radiation Safety Handbook. Newnes. pp. 64–65. ISBN 0750643552.
Jump up ^ VanderVorst, André; Rosen, Arye; Kotsuka, Youji (2006). RF / Microwave Interaction with Biological Tissues. John Wiley and Sons. pp. 121–122. ISBN 0471752045.
Jump up ^ Graf, Rudolf F.; Sheets, William (2001). Build Your Own Low-power Transmitters: Projects for the Electronics Experimenter. Newnes. p. 234. ISBN 0750672447.
Jump up ^ Elder, Joe Allen; Cahill, Daniel F. (1984). Biological Effects of Radiofrequency Radiation. US Environmental Protection Agency. pp. 5.116–5.119.
Jump up ^ Hitchcock, R. Timothy; Patterson, Robert M. (1995). Radio-Frequency and ELF Electromagnetic Energies: A Handbook for Health Professionals. John Wiley and Sons. pp. 177–179. ISBN 0471284548.
Jump up ^ http://www.iarc.fr/en/media-centre/pr/2011/pdfs/pr208_E.pdf and http://monographs.iarc.fr/ENG/Classification/index.php
Jump up ^ Gerke, Daryl (2018). Electromagnetic Compatibility in Medical Equipment: A Guide for Designers and Installers. Routledge. p. 6.67. ISBN 1351453378.
References[edit]
James Clerk Maxwell, "A Dynamical Theory of the Electromagnetic Field", Philosophical Transactions of the Royal Society of London 155, 459–512 (1865).
Heinrich Hertz: "Electric waves; being researches on the propagation of electric action with finite velocity through space" (1893). Cornell University Library Historical Monographs Collection. Reprinted by Cornell University Library Digital Collections.
Karl Rawer: "Wave Propagation in the Ionosphere". Kluwer, Dordrecht 1993. ISBN 0-7923-0775-5
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Comes a Time
From Wikipedia, the free encyclopedia
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Comes a Time
Studio album by Neil Young and Crazy Horse
Released
October 21, 1978[1]
Recorded
November 28, 1975 – November 21, 1977
Studio
Triad Recording, Ft. Lauderdale, FL; Columbia Recording Studio, London; Wally Heider Recording Studio, San Francisco, CA; Woodland Sound Studios, Nashville; Sound Shop, Nashville, and Broken Arrow Ranch, Redwood City, CA
Genre
Country rock, folk, folk rock
Length
37:15
Label
Reprise
Producer
Neil Young, David Briggs, Ben Keith, Tim Mulligan
Neil Young and Crazy Horse chronology
Decade
(1977)Decade1977 Comes a Time
(1978) Rust Never Sleeps
(1979)Rust Never Sleeps1979
Professional ratings
Review scores
Source
Rating
Allmusic [2]
Robert Christgau A[3]
Rolling Stone (positive)[4]
Rolling Stone [5]
Comes a Time is the ninth album by Canadian singer-songwriter Neil Young, released in 1978. It is a return to the country/folk rock sound of Harvest (1972). "Comes a Time" is also the title song and a single release from the album.
Contents
[hide]
1 Production
2 Track listing
3 Personnel
4 Charts
4.1 Album
4.2 Singles
5 References
6 External links
Production[edit]
The album originally started out as a solo record, but when Young played it for Reprise executives they asked him if he would consider adding rhythm tracks to what he already had. Young agreed, and the end product was Comes a Time. Two songs ("Look Out For My Love" and "Lotta Love") were backed by Crazy Horse.
"Human Highway" was written several years prior to the album's release, and originally presented to Crosby, Stills, Nash & Young in 1974 for a proposed studio album by the group which never came to be.[citation needed] Much of the album features harmony vocals from Nicolette Larson. She also shares lead vocals with Young on "Motorcycle Mama".
For years, it was rumoured that Young had purchased some 200,000 copies of the album because he was unhappy with the sound, owing to damage that occurred to the master tape during shipment to the mixing facility. The version of the album most widely available today was personally remixed by Young from the safety copy of the original master. In a March 2014 interview with Rolling Stone, Young revealed that he, in fact, used the 200,000 LPs as shingles for a barn roof.[6]
Track listing[edit]
All songs written by Neil Young, except where noted.
Side one
No.
Title
Length
1. "Goin' Back" 4:43
2. "Comes a Time" 3:05
3. "Look Out for My Love" 4:06
4. "Lotta Love" 2:38
5. "Peace of Mind" 4:11
Side two
No.
Title
Writer(s)
Length
6. "Human Highway" 3:09
7. "Already One" 4:53
8. "Field of Opportunity" 3:08
9. "Motorcycle Mama" 3:08
10. "Four Strong Winds" Ian Tyson 4:07
Personnel[edit]
Neil Young – guitar, harmonica, vocals, production
Frank Sampedro – guitar, piano, vocals (on tracks 3 and 4)
Billy Talbot – bass, vocals (on tracks 3 and 4)
Ralph Molina – drums, vocals (on tracks 3 and 4)
Tim Mulligan – saxophone
Nicolette Larson – harmony vocals (except on tracks 3 and 4)
Ben Keith – steel guitar
Karl Himmel – drums
Tim Drummond – bass
Spooner Oldham – piano
Rufus Thibodeaux – fiddle
Joe Osborn – bass
Larrie Londin – drums
J. J. Cale – electric guitar
Farrell Morris – percussion
Rita Fey – autoharp
Bucky Barrett, Grant Boatwright, Johnny Christopher, Jerry Shook, Vic Jordan, Steve Gibson, Dale Sellers, Ray Edenton – acoustic guitars
Shelly Kurland, Stephanie Woolf, Marvin Chantry, Roy Christensen, Gary Vanosdale, Carl Gorodetzky, George Binkley, Steven Smith, Larry Harvin, Larry Lasson, Carol Walker, Rebecca Lynch, Virginia Christensen, Maryanna Harvin, George Kosmola, Martha McCrory, Chuck Cochran – strings
Technical
Ben Keith – production (except on tracks 3, 4 and 8)
Tim Mulligan – production (except on track 7)
David Briggs – production (on tracks 3 and 4)
Tim Mulligan, Michael Laskow, David McKinley, Danny Hilley, Mike Porter, Denny Purcell, Rich "Hoss" Adler, Ernie Winfrey, Gabby Garcia, Paul Kaminsky – engineering
Elliot Roberts – direction
Tom Wilkes – art direction
Coley Coleman – photography
Charts[edit]
Album[edit]
Year
Chart
Position
1978 Billboard Pop Albums 7[7]
Singles[edit]
Year
Single
Chart
Position
1978 "Four Strong Winds" Billboard Pop Singles 61[8]
References[edit]
Jump up ^ http://www.neilyoung.com/archives/discography/items/ny-cat.html
Jump up ^ Ruhlmann, William. Comes a Time at AllMusic. Retrieved 13 Nov 2009.
Jump up ^ Christgau, Robert. "Neil Young: Comes a Time > Consumer Guide Review". Robert Christgau. Retrieved 10 Mar 2006.
Jump up ^ Marcus, Greil (October 1978). "Neil Young Comes a Time > Album Review". Rolling Stone. Retrieved 24 September 2011. Posted at rollingstone.com January 2, 2002.
Jump up ^ Miles, Milo (January 23, 2003). "The Rolling Stone Hall of Fame: The Greatest Albums Ever Made: Neil Young Comes a Time > Album Review". Rolling Stone (914). p. 70. Archived from the original on 2007-10-30. Retrieved 12 January 2007. Posted at rollingstone.com December 30, 2002.
Jump up ^ "Neil Young on Pono, His New Album and Using LPs as Roof Shingles". Rolling Stone. Retrieved 9 April 2017.
Jump up ^ Comes a Time - Neil Young > Charts & Awards > Billboard Album at AllMusic. Retrieved 2 January 2005.
Jump up ^ Comes a Time - Neil Young > Charts & Awards > Billboard Singles at AllMusic. Retrieved 2 January 2005.
External links[edit]
Comes a Time at MySpace (streamed copy)
Samples from Amazon.com
[show]
v
t
e
Neil Young
Crazy Horse
The Stray Gators
Promise of the Real
Studio albums
Neil Young
Everybody Knows This Is Nowhere
After the Gold Rush
Harvest
On the Beach
Tonight's the Night
Zuma
American Stars 'n Bars
Comes a Time
Hawks & Doves
Re·ac·tor
Trans
Everybody's Rockin'
Old Ways
Landing on Water
This Note's for You
Freedom
Ragged Glory
Harvest Moon
Sleeps with Angels
Mirror Ball
Broken Arrow
Silver & Gold
Are You Passionate?
Greendale
Prairie Wind
Living with War
Chrome Dreams II
Fork in the Road
Le Noise
Americana
Psychedelic Pill
A Letter Home
Storytone
The Monsanto Years
Peace Trail
Hitchhiker
The Visitor
EPs
Eldorado
Live albums
Time Fades Away
Rust Never Sleeps
Live Rust
Life
Arc
Weld
Unplugged
Year of the Horse
Road Rock Vol. 1
Earth
Soundtracks
Journey Through the Past
Where the Buffalo Roam
Philadelphia
Dead Man
Compilations
Decade
Lucky Thirteen
Greatest Hits
Archives series
The Archives Vol. 1 1963–1972
Archives Performance
series
Live at the Fillmore East
Live at Massey Hall 1971
Sugar Mountain – Live at Canterbury House 1968
Live at the Riverboat 1969
Dreamin' Man Live '92
A Treasure
Live at the Cellar Door
Bluenote Café
Unreleased albums
Homegrown
Chrome Dreams
Songs
"Mr. Soul"
"Broken Arrow"
"Expecting to Fly"
"Sugar Mountain"
"The Loner"
"I've Been Waiting for You"
"Cinnamon Girl"
"Down by the River"
"Cowgirl in the Sand"
"Helpless"
"Ohio"
"Tell Me Why"
"After the Gold Rush"
"Only Love Can Break Your Heart"
"Southern Man"
"Don't Let It Bring You Down"
"When You Dance I Can Really Love"
"Bad Fog of Loneliness"
"A Man Needs a Maid"
"Heart of Gold"
"Are You Ready for the Country?"
"Old Man"
"The Needle and the Damage Done"
"Cortez the Killer"
"Will to Love"
"Like a Hurricane"
"Love Is a Rose"
"Lotta Love"
"Hey Hey, My My (Into the Black)"
"Pocahontas"
"Powderfinger"
"Sedan Delivery"
"Rockin' in the Free World"
"No More"
"Wrecking Ball"
"Downtown"
"Let's Impeach the President"
"Angry World"
"A Rock Star Bucks a Coffee Shop"
Films and Videos
Journey Through the Past
Human Highway
Neil Young in Berlin
Solo Trans
Year of the Horse
Greendale
Neil Young: Heart of Gold
CSNY/Déjà Vu
Neil Young Trunk Show
Neil Young Journeys
Books
Waging Heavy Peace: A Hippie Dream
Related
Discography
The Squires
The Mynah Birds
Buffalo Springfield
Crosby, Stills, Nash & Young
The Stills–Young Band
The Bridge: A Tribute to Neil Young
Myrmekiaphila neilyoungi
Pono
Old Black
Farm Aid
Bridge School Benefit
Pegi Young (former wife)
Carrie Snodgress
Category
Portal
[show]
v
t
e
Crazy Horse
Billy Talbot
Ralph Molina
Frank "Poncho" Sampedro
Danny Whitten
Jack Nitzsche
Nils Lofgren
George Whitsell
Greg LeRoy
John Blanton
Rick Curtis
Michael Curtis
Sonny Mone
Crazy Horse
Crazy Horse
Loose
At Crooked Lake
Crazy Moon
Left for Dead
Gone Dead Train: The Best of Crazy Horse 1971–1989
Scratchy: The Complete Reprise Recordings
Neil Young & Crazy Horse
Everybody Knows This Is Nowhere
After the Gold Rush
Tonight's the Night
Zuma
American Stars 'n Bars
Comes a Time
Rust Never Sleeps
Live Rust
Re·ac·tor
Trans
Life
Ragged Glory
Weld
Arc
Sleeps with Angels
Broken Arrow
Year of the Horse
Are You Passionate?
Greendale
Live at the Fillmore East
Americana
Psychedelic Pill
Related articles
Neil Young
The Rockets
Retrieved from "https://en.wikipedia.org/w/index.php?title=Comes_a_Time&oldid=818074285"
Categories:
1978 albums
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Albums produced by Neil Young
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