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with their colonies and naval fleets during world war i the development of continuous wave radio transmitters rectifying electrolytic and crystal radio receiver detectors enabled amplitude modulation am radiotelephony to be achieved by reginald fessenden and others allowing audio to be transmitted on 2 november 1920 the first commercial radio broadcast was transmitted by westinghouse electric and manufacturing company in pittsburgh under the call sign kdka the broadcast featured live coverage of the 1920 united states presidential election 8 principles edit radio waves are radiated by electric charges undergoing acceleration 21 22 they are generated artificially by time varying electric currents consisting of electrons flowing back and forth in a metal conductor called an antenna 23 24 as they travel farther from the transmitting antenna radio waves spread out so their signal strength intensity in watts per square meter decreases see inverse square law so radio transmissions can only be received within a limited range of the transmitter the distance depending on the transmitter power the antenna radiation pattern receiver sensitivity background noise level and presence of obstructions between transmitter and receiver an omnidirectional antenna transmits or receives radio waves in all directions while a directional antenna transmits radio waves in a beam in a particular direction or receives waves from only one direction 25 26 27 28 radio waves travel at the speed of light in vacuum 29 and at slightly lower velocity in air 30 the other types of electromagnetic waves besides radio waves infrared visible light ultraviolet x rays and gamma rays can also carry information and be used for communication the wide use of radio waves for telecommunication is mainly due to their desirable propagation properties stemming from their longer wavelength 24 radio waves have the ability to pass through the atmosphere in any weather foliage and at longer wavelengths through most building materials by diffraction longer wavelengths can bend around obstructions and unlike other electromagnetic waves they tend to be scattered rather than absorbed by objects larger than their wavelength communication systems edit radio communication information such as sound is converted by a transducer such as a microphone to an electrical signal which modulates a radio wave produced by the transmitter a receiver intercepts the radio wave and extracts the information bearing modulation signal which is converted back to a human usable form with another transducer such as a loudspeaker comparison of am and fm modulated radio waves in radio communication systems information is carried across space using radio waves at the sending end the information to be sent is converted by some type of transducer to a time varying electrical signal called the modulation signal 24 31 the modulation signal may be an audio signal representing sound from a microphone a video signal representing moving images from a video camera or a digital signal consisting of a sequence of bits representing binary data from a computer the modulation signal is applied to a radio transmitter in the transmitter an electronic oscillator generates an alternating current oscillating at a radio frequency called the carrier wave because it serves to generate the radio waves that carry the information through the air the modulation signal is used to modulate the carrier varying some aspect of the carrier wave impressing the information in the modulation signal onto the carrier different radio systems use different modulation methods 32 amplitude modulation am in an am transmitter the amplitude strength of the radio carrier wave is varied by the modulation signal 32 3 frequency modulation fm in an fm transmitter the frequency of the radio carrier wave is varied by the modulation signal 32 33 frequency shift keying fsk used in wireless digital devices to transmit digital signals the frequency of the carrier wave is shifted between frequencies 32 58 orthogonal frequency division multiplexing ofdm a family of digital modulation methods widely used in high bandwidth systems such as wi fi networks cellphones digital television broadcasting and digital audio broadcasting dab to transmit digital data using a minimum of radio spectrum bandwidth it has higher spectral efficiency and more resistance to fading than am or fm in ofdm multiple radio carrier waves closely spaced in frequency are transmitted within the radio channel with each carrier modulated with bits from the incoming bitstream so multiple bits are being sent simultaneously in parallel at the receiver the carriers are demodulated and the bits are combined in the proper order into one bitstream 33 many other types of modulation are also used in some types the carrier wave is suppressed and only one or both modulation sidebands are transmitted 34 the modulated carrier is amplified in the transmitter and applied to a transmitting antenna which radiates the energy as radio waves the radio waves carry the information to the receiver location 35 at the receiver the radio wave induces a tiny oscillating voltage in the receiving antenna a weaker replica of the current in the transmitting antenna 24 31 this voltage is applied to the radio receiver which amplifies the weak radio signal so it is stronger then demodulates it extracting the original modulation signal from the modulated carrier wave the modulation signal is converted by a transducer back to a human usable form an audio signal is converted to sound waves by a loudspeaker or earphones a video signal is converted to images by a display while a digital signal is applied to a computer or microprocessor which interacts with human users 32 the radio waves from many transmitters pass through the air simultaneously without interfering with each other because each transmitter s radio waves oscillate at a different frequency measured in hertz hz kilohertz khz megahertz mhz or gigahertz ghz the receiving antenna typically picks up the radio signals of many transmitters the receiver uses tuned circuits to select the desired radio signal out of all the signals picked up by the antenna and reject the others a tuned circuit acts like a resonator similar to a tuning fork 31 it has a natural resonant frequency at which it oscillates the resonant frequency of the receiver s tuned circuit is adjusted by the user to the frequency of the desired radio station this is called tuning 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 36 bandwidth edit frequency spectrum of a typical modulated am or fm radio signal it consists of a component c at the carrier wave frequency f c displaystyle f_ c with the modulated information contained in two narrow bands of frequencies called sidebands sb just above and below the carrier frequency the bandwidth bw is the amount of spectrum occupied by the sidebands a modulated radio wave carrying an information signal occupies a range of frequencies the information in a radio signal is usually concentrated in narrow frequency bands called sidebands sb just above and below the carrier frequency the width in hertz of the frequency range that the radio signal occupies the highest frequency minus the lowest frequency is called its bandwidth bw 32 37 for any given signal to noise ratio a given bandwidth can carry the same amount of information regardless of where in the radio frequency spectrum it is located bandwidth is a measure of information carrying capacity the bandwidth required by a radio transmission depends on the data rate of the information being sent and the spectral efficiency of the modulation method used how much data it can transmit in each unit of bandwidth different types of information signals carried by radio have different data rates for example a television signal has a greater data rate than an audio signal 32 38 the radio spectrum the total range of radio frequencies that can be used for communication in a given area is a limited resource 37 3 each radio transmission occupies a portion of the total spectrum available radio spectrum is regarded as an economic good which has a monetary cost and is in increasing demand in some parts of the radio spectrum the right to use a frequency band or even a single radio channel is bought and sold for millions of dollars so there is an incentive to employ technology to minimize the spectrum used by radio services 38 a slow transition from analog to digital radio transmission technologies began in the late 1990s 39 40 part of the reason for this is that digital modulation can transmit more information in a given bandwidth than analog modulation the modulation itself is more efficient and loss compression further improves efficiency digital modulation also has greater noise immunity than analog associated digital signal processors have more power and flexibility than analog circuits and a wide variety of information can be transmitted using the same digital modulation 32 because it is a fixed resource which is in demand by an increasing number of users the radio spectrum has become increasingly congested in recent decades and the need to use it more effectively is driving many additional radio innovations such as trunked radio systems spread spectrum ultra wideband transmission frequency reuse dynamic spectrum management frequency pooling and cognitive radio 38 itu frequency bands edit the itu arbitrarily divides the radio spectrum into 12 bands each beginning at a wavelength which is a power of ten 10 n metres with corresponding frequency of 3 times a power of ten and each covering a decade of frequency or wavelength 3 41 each of these bands has a traditional name 42 band name abbreviation frequency wavelength extremely low frequency elf 3 30 hz 100 000 10 000 km super low frequency slf 30 300 hz 10 000 1 000 km ultra low frequency ulf 300 3 000 hz 1 000 100 km very low frequency vlf 3 30 khz 100 10 km low frequency lf 30 300 khz 10 1 km medium frequency mf 300 3 000 khz 1 000 100 m band name abbreviation frequency wavelength high frequency hf 3 30 mhz 100 10 m very high frequency vhf 30 300 mhz 10 1 m ultra high frequency uhf 300 3 000 mhz 100 10 cm super high frequency shf 3 30 ghz 10 1 cm extremely high frequency ehf 30 300 ghz 10 1 mm tremendously high frequency thf 300 3 000 ghz 0 3 3 0 thz 1 0 0 1 mm it can be seen that the bandwidth the absolute range of frequencies contained in each band is not equal but increases exponentially as the frequency increases each band contains ten times the bandwidth of the preceding band 43 though not defined by the itu 42 the term tremendously low frequency tlf has been used for wavelengths from 1 3 hz 300 000 100 000 km 44 regulation edit further information radio regulation the airwaves are a resource shared by many users two radio transmitters in the same area that attempt to transmit on the same frequency will interfere with each other causing garbled reception often to the extent that neither transmission may be received clearly 37 interference with radio transmissions can not only have a high economic cost but it can also be life threatening for example in the case of interference with emergency communications or air traffic control 45 46 to prevent interference between different users the emission of radio waves is strictly regulated by national laws and coordinated by an international body the international telecommunication union itu which allocates bands in the radio spectrum for different uses 37 3 radio transmitters must be licensed by governments under a variety of license classes depending on use and are restricted to certain frequencies and power levels in some classes such as radio and television broadcasting stations the transmitter is given a unique identifier consisting of a string of letters and numbers called a call sign which must be used in all transmissions 47 in order to adjust maintain or internally repair radio transmitters individuals must hold a government license such as the general radiotelephone operator license in the us obtained by taking a test demonstrating adequate technical and legal knowledge of safe radio operation 48 exceptions to the above rules allow the unlicensed operation by the public of low power short range transmitters in consumer products such as cell phones cordless phones wireless devices walkie talkies citizens band radios wireless microphones garage door openers and baby monitors in the us these fall under part 15 of the federal communications commission fcc regulations many of these devices use the ism bands a series of frequency bands throughout the radio spectrum reserved for unlicensed use although they can be operated without a license like all radio equipment these devices generally must be type approved before the sale 49 radio jamming is the deliberate radiation of radio signals designed to interfere with the reception of other radio signals jamming devices are called signal suppressors or interference generators or just jammers 9 during wartime militaries use jamming to interfere with enemies tactical radio communication since radio waves can pass beyond national borders some totalitarian countries that practice censorship use jamming to prevent their citizens from listening to broadcasts from radio stations in other countries jamming is usually accomplished by a powerful transmitter that generates noise on the same frequency as the target transmitter 10 11 us federal law prohibits the nonmilitary operation or sale of any type of jamming devices including ones that interfere with gps cellular wi fi and police radars 15 applications edit main article applications of radio see also radio spectrum applications and radio receiver applications radio has many practical applications which include broadcasting voice communication data communication radar radiolocation and remote control see also edit radio portal amateur radio portal electromagnetic radiation and health internet radio list of radios list of specific models of radios outline of radio radio quiet zone references edit radio oxford living dictionaries oxford university press 2019 archived from the original on 24 march 2019 retrieved 26 february 2019 definition of radio encyclopedia pcmagazine website ziff davis 2018 archived from the original on 24 march 2019 retrieved 26 february 2019 1 2 3 4 ellingson steven w 2016 radio systems engineering cambridge university press pp 1 4 isbn 978 1316785164 1 2 3 4 125 years discovery of electromagnetic waves karlsruhe institute of technology 16 may 2022 archived from the original on 14 july 2022 retrieved 14 july 2022 1 2 bondyopadhyay prebir k 1995 guglielmo marconi the father of long distance radio communication an engineer s tribute ...