Editing Radio wave (section)

== Radio communication ==
{{main|Radio}}
In [[radio communication]] systems, information is transported 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]].<ref name="HowStuffWorks">{{cite web |last=Brain |first=M. |date=7 Dec 2000 |title=How Radio Works |url=http://electronics.howstuffworks.com/radio8.htm |url-status=live |archive-url=https://web.archive.org/web/20091002050151/http://electronics.howstuffworks.com/radio8.htm |archive-date=2 October 2009 |access-date=11 Sep 2009 |website=HowStuffWorks.com}}</ref> The information, called the [[modulation|modulation 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 wave]]'' because it creates the radio waves that "carry" the information through the air. The information signal is used to [[modulation|modulate]] the carrier, altering some aspect of it, encoding the information on the carrier. The modulated carrier is amplified and applied to an [[antenna (radio)|antenna]]. The oscillating current pushes the [[electron]]s in the antenna back and forth, creating oscillating [[electric field|electric]] and [[magnetic field]]s, 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.<ref name="HowStuffWorks" /> 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 [[amplifier|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 device|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 one or more [[tuned circuit]]s which act like a [[resonator]], similarly to a [[tuning fork]].<ref name="HowStuffWorks">{{cite web |last=Brain |first=M. |date=7 Dec 2000 |title=How Radio Works |url=http://electronics.howstuffworks.com/radio8.htm |url-status=live |archive-url=https://web.archive.org/web/20091002050151/http://electronics.howstuffworks.com/radio8.htm |archive-date=2 October 2009 |access-date=11 Sep 2009 |website=HowStuffWorks.com}}</ref>  The tuned circuit 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.