A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the lower end of the monopole and the ground plane. One side of the antenna feedline is attached to the lower end of the monopole, the other side is attached to the ground plane, the Earth; this contrasts with a dipole antenna which consists of two identical rod conductors, with the signal from the transmitter applied between the two halves of the antenna. The monopole is a resonant antenna. Therefore, the length of the antenna is determined by the wavelength of the radio waves it is used with; the most common form is the quarter-wave monopole, in which the antenna is one quarter of the wavelength of the radio waves. The monopole antenna was invented in 1895 by radio pioneer Guglielmo Marconi.
Common types of monopole antenna are the whip, rubber ducky, random wire, inverted-L and T-antenna, inverted-F, mast radiator, ground plane antennas. The load impedance of the quarter-wave monopole is half that of the dipole antenna or 37.5+j21.25 ohms. The monopole antenna was invented in 1895 and patented 1896 by radio pioneer Guglielmo Marconi during his historic first experiments in radio communication, he began by using dipole antennas invented by Heinrich Hertz consisting of two identical horizontal wires ending in metal plates. He found by experiment that if instead of the dipole, one side of the transmitter and receiver was connected to a wire suspended overhead, the other side was connected to the Earth, he could transmit for longer distances. For this reason the monopole is called a Marconi antenna, although Alexander Popov independently invented it at about the same time. Like a dipole antenna, a monopole has an omnidirectional radiation pattern: it radiates with equal power in all azimuthal directions perpendicular to the antenna.
However, the radiated power varies with elevation angle, with the radiation dropping off to zero at the zenith of the antenna axis. It radiates vertically polarized radio waves. A monopole can be visualized as being formed by replacing the bottom half of a vertical dipole antenna with a conducting plane at right-angles to the remaining half. If the ground plane is large enough, the radio waves from the remaining upper half of the dipole reflected from the ground plane will seem to come from an image antenna forming the missing half of the dipole, which adds to the direct radiation to form a dipole radiation pattern. So the pattern of a monopole with a conducting, infinite ground plane is identical to the top half of a dipole pattern, with its maximum radiation in the horizontal direction, perpendicular to the antenna; because it radiates only into the space above the ground plane, or half the space of a dipole antenna, a monopole antenna will have a gain of twice the gain of a similar dipole antenna, a radiation resistance half that of a dipole.
Since a half-wave dipole has a gain of 2.19 dBi and a radiation resistance of 73 ohms, a quarter-wave monopole, the most common type, will have a gain of 2.19 + 3 = 5.19 dBi and a radiation resistance of about 36.8 ohms if it is mounted above a good ground plane. The general effect of electrically small ground planes, as well as imperfectly conducting earth grounds, is to tilt the direction of maximum radiation up to higher elevation angles; the ground plane used with a monopole may be the actual earth. This design is used for the mast radiator antennas employed in radio broadcasting at low frequencies, as well as other low frequency antennas such as the T-antenna and umbrella antenna. At VHF and UHF frequencies the size of the ground plane needed is smaller, so artificial ground planes are used to allow the antenna to be mounted above the ground. A common type of monopole antenna at these frequencies consists of a quarter-wave whip antenna with a ground plane consisting of several wires or rods radiating horizontally or diagonally from its base.
At gigahertz frequencies the metal surface of a car roof or airplane body makes a good ground plane, so car cell phone antennas consist of short whips mounted on the roof, aircraft communication antennas consist of a short conductor in an aerodynamic fairing projecting from the fuselage. The most common antenna used in mobile phones is the inverted-F antenna, a variant of the inverted-L monopole. Bending over the antenna saves space and keeps the it within the bounds of the mobile's case but the antenna has a low impedance. To improve the match the antenna is not fed from the end, rather some intermediate point, the end is grounded instead; the quarter-wave whip and rubber ducky antennas used with handheld radios such as walkie-talkies and cell phones are monopole antennas. These don't use a ground plane, the ground side of the transmitter is just connected to the ground connection on its circuit board; the hand and body of the person holding them may function as a rudimentary ground plane. Sometimes, monopole antennas are printed on a dielectric substrate to make it less fragile and they may be fabricated using the printed circuit board technologies.
Such antennas are
Low-noise block downconverter
A low-noise block downconverter is the receiving device mounted on satellite dishes used for satellite TV reception, which collects the radio waves from the dish and converts them to a signal, sent through a cable to the receiver inside the building. Called a low-noise block, low-noise converter, or low-noise downconverter, the device is sometimes inaccurately called a low-noise amplifier; the LNB is a combination of low-noise amplifier, frequency mixer, local oscillator and intermediate frequency amplifier. It serves as the RF front end of the satellite receiver, receiving the microwave signal from the satellite collected by the dish, amplifying it, downconverting the block of frequencies to a lower block of intermediate frequencies; this downconversion allows the signal to be carried to the indoor satellite TV receiver using cheap coaxial cable. The LNB is a small box suspended on one or more short booms, or feed arms, in front of the dish reflector, at its focus; the microwave signal from the dish is picked up by a feedhorn on the LNB and is fed to a section of waveguide.
One or more metal pins, or probes, protrude into the waveguide at right angles to the axis and act as antennas, feeding the signal to a printed circuit board inside the LNB's shielded box for processing. The lower frequency IF output signal emerges from a socket on the box to which the coaxial cable connects; the LNB gets its power from the receiver or set-top box, using the same coaxial cable that carries signals from the LNB to the receiver. This phantom power travels to the LNB. A corresponding component, called a block upconverter, is used at the satellite earth station dish to convert the band of television channels to the microwave uplink frequency; the signal received by the LNB is weak and it has to be amplified before downconversion. The low noise amplifier section of the LNB amplifies this weak signal while adding the minimum possible amount of noise to the signal; the low-noise quality of an LNB is expressed as the noise figure. This is the signal to noise ratio at the input divided by the signal to noise ratio at the output.
It is expressed as a decibels value. The ideal LNB a perfect amplifier, would have a noise figure of 0 dB and would not add any noise to the signal; every LNB introduces some noise but clever design techniques, expensive high performance low-noise components such as HEMTs and individual tweaking of the LNB after manufacture, can reduce some of the noise contributed by the LNB's components. Active cooling to low temperatures can help reduce noise too, is used in scientific research applications; every LNB off the production line has a different noise figure because of manufacturing tolerances. The noise figure quoted in the specifications, important for determining the LNB's suitability, is representative of neither that particular LNB nor the performance across the whole frequency range, since the noise figure most quoted is the typical figure averaged over the production batch. Satellites use comparatively high radio frequencies to transmit their TV signals; as microwave satellite signals do not pass through walls, roofs, or glass windows, it is preferable for satellite antennas to be mounted outdoors.
However, plastic glazing is transparent to microwaves and residential satellite dishes have been hidden indoors looking through acrylic or polycarbonate windows to preserve the external aesthetics of the home. The purpose of the LNB is to use the superheterodyne principle to take a block of high frequencies and convert them to similar signals carried at a much lower frequency; these lower frequencies travel through cables with much less attenuation, so there is much more signal left at the satellite receiver end of the cable. It is much easier and cheaper to design electronic circuits to operate at these lower frequencies, rather than the high frequencies of satellite transmission; the frequency conversion is performed by mixing a fixed frequency produced by a local oscillator inside the LNB with the incoming signal, to generate two signals equal to the sum of their frequencies and the difference. The frequency sum signal is filtered out and the frequency difference signal is amplified and sent down the cable to the receiver: C-band f IF = f LO − f recv Ku-band f IF = f recv − f LO where f is a frequency.
The local oscillator frequency determines what block of incoming frequencies is downconverted to the frequencies expected by the receiver. For example, to downconvert the incoming signals from Astra 1KR, which transmits in a frequency block of 10.70–11.70 GHz, to within a standard European receiver's IF tuning range of 950–2,150 MHz, a 9.75 GHz local oscillator frequency is used, producing a block of signals in the band 950–1,950 MHz. For the block of higher transmission frequencies used by Astra 2A and 2B, a different local oscillator frequency converts the block of incoming frequencies. A local oscillator frequency of 10.60 GHz is used to downconvert the block to 1,100–2,150 MHz, which
In radio and telecommunications a dipole antenna or doublet is the simplest and most used class of antenna. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole with a radiating structure supporting a line current so energized that the current has only one node at each end. A dipole antenna consists of two identical conductive elements such as metal wires or rods; the driving current from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the feedline to the transmitter or receiver is connected to one of the conductors; this contrasts with a monopole antenna, which consists of a single rod or conductor with one side of the feedline connected to it, the other side connected to some type of ground. A common example of a dipole is the "rabbit ears" television antenna found on broadcast television sets; the dipole is the simplest type of antenna from a theoretical point of view.
Most it consists of two conductors of equal length oriented end-to-end with the feedline connected between them. Dipoles are used as resonant antennas. If the feedpoint of such an antenna is shorted it will be able to resonate at a particular frequency, just like a guitar string, plucked. Using the antenna at around that frequency is advantageous in terms of feedpoint impedance, so its length is determined by the intended wavelength of operation; the most used is the center-fed half-wave dipole, just under a half-wavelength long. The radiation pattern of half-wave dipoles is maximum perpendicular to the conductor, falling to zero in the axial direction, thus implementing an omnidirectional antenna if installed vertically, or a weakly directional antenna if horizontal. Most antennas in use can be seen as based on the dipole. Although they may be used as standalone low-gain antennas, they are employed as driven elements in more complex antenna designs such as the Yagi antenna and driven arrays.
Dipole antennas are used to feed more elaborate directional antennas such as a horn antenna, parabolic reflector, or corner reflector. Engineers analyze vertical antennas on the basis of dipole antennas. German physicist Heinrich Hertz first demonstrated the existence of radio waves in 1887 using what we now know as a dipole antenna. On the other hand, Guglielmo Marconi empirically found that he could just ground the transmitter dispensing with one half of the antenna, thus realizing the vertical or monopole antenna. For the low frequencies Marconi employed to achieve long-distance communications, this form was more practical. In the early days of radio, the thus-named Marconi antenna and the doublet were seen as distinct inventions. Now, the "monopole" antenna is understood as a special case of a dipole which has a virtual element "underground". A short dipole is a dipole formed by two conductors with a total length L less than a half wavelength. Short dipoles are sometimes used in applications.
They can be analyzed using the results obtained below for the Hertzian dipole, a fictitious entity. Being shorter than a resonant antenna its feedpoint impedance includes a large capacitive reactance requiring a loading coil or other matching network in order to be practical as a transmitting antenna. To find the far-field electric and magnetic fields generated by a short dipole we use the result shown below for the Hertzian dipole at a distance r from the current and at an angle θ to the direction of the current, as being: H ϕ = i I h L k 4 π r e i sin E θ = ζ 0 H ϕ = i ζ 0 I h L k 4 π r e i sin where the radiator consists of a current of I h e
In radio communications, a radio receiver known as a receiver, wireless or radio is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna; the antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, recovers the desired information through demodulation; the information produced by the receiver may be in the form of sound, moving data. A radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. Radio receivers are widely used in modern technology, as components of communications, remote control, wireless networking systems. In consumer electronics, the terms radio and radio receiver are used for receivers designed to reproduce sound transmitted by radio broadcasting stations the first mass-market commercial radio application.
The most familiar form of radio receiver is a broadcast receiver just called a radio, which receives audio programs intended for public reception transmitted by local radio stations. The sound is reproduced either by a loudspeaker in the radio or an earphone which plugs into a jack on the radio; the radio requires electric power, provided either by batteries inside the radio or a power cord which plugs into an electric outlet. All radios have a volume control to adjust the loudness of the audio, some type of "tuning" control to select the radio station to be received. Modulation is the process of adding information to a radio carrier wave. Two types of modulation are used in analog radio broadcasting systems. In amplitude modulation the strength of the radio signal is varied by the audio signal. AM broadcasting is allowed in the AM broadcast bands which are between 148 and 283 kHz in the longwave range, between 526 and 1706 kHz in the medium frequency range of the radio spectrum. AM broadcasting is permitted in shortwave bands, between about 2.3 and 26 MHz, which are used for long distance international broadcasting.
In frequency modulation the frequency of the radio signal is varied by the audio signal. FM broadcasting is permitted in the FM broadcast bands between about 65 and 108 MHz in the high frequency range; the exact frequency ranges vary somewhat in different countries. FM stereo radio stations broadcast in stereophonic sound, transmitting two sound channels representing left and right microphones. A stereo receiver contains the additional circuits and parallel signal paths to reproduce the two separate channels. A monaural receiver, in contrast, only receives a single audio channel, a combination of the left and right channels. While AM stereo transmitters and receivers exist, they have not achieved the popularity of FM stereo. Most modern radios are "AM/FM" radios, are able to receive both AM and FM radio stations, have a switch to select which band to receive. Digital audio broadcasting is an advanced radio technology which debuted in some countries in 1998 that transmits audio from terrestrial radio stations as a digital signal rather than an analog signal as AM and FM do.
Its advantages are that DAB has the potential to provide higher quality sound than FM, has greater immunity to radio noise and interference, makes better use of scarce radio spectrum bandwidth, provides advanced user features such as electronic program guide, sports commentaries, image slideshows. Its disadvantage is that it is incompatible with previous radios so that a new DAB receiver must be purchased; as of 2017, 38 countries offer DAB, with 2,100 stations serving listening areas containing 420 million people. Most countries plan an eventual switchover from FM to DAB; the United States and Canada have chosen not to implement DAB. DAB radio stations work differently from AM or FM stations: a single DAB station transmits a wide 1,500 kHz bandwidth signal that carries from 9 to 12 channels from which the listener can choose. Broadcasters can transmit a channel at a range of different bit rates, so different channels can have different audio quality. In different countries DAB stations broadcast in either Band L band.
The signal strength of radio waves decreases the farther they travel from the transmitter, so a radio station can only be received within a limited range of its transmitter. The range depends on the power of the transmitter, the sensitivity of the receiver and internal noise, as well as any geographical obstructions such as hills between transmitter and receiver. AM broadcast band radio waves travel as ground waves which follow the contour of the Earth, so AM radio stations can be reliably received at hundreds of miles distance. Due to their higher frequency, FM band radio signals cannot travel far beyond the visual horizon; however FM radio has higher fidelity. So in many countries serious music is only broadcast by FM stations, AM stations specialize in radio news, talk radio, sports. Like FM, DAB signals travel by line of sight so reception distances are
In antenna engineering, side lobes or sidelobes are the lobes of the far field radiation pattern of an antenna or other radiation source, that are not the main lobe. The radiation pattern of most antennas shows a pattern of "lobes" at various angles, directions where the radiated signal strength reaches a maximum, separated by "nulls", angles at which the radiated signal strength falls to zero. In a directional antenna in which the objective is to emit the radio waves in one direction, the lobe in that direction is designed to have a larger field strength than the others; the other lobes are called "side lobes", represent unwanted radiation in undesired directions. A side lobe in the opposite direction from the main lobe is called the back lobe; the larger the antenna is compared to the radio wavelength, the more lobes its radiation pattern has. In transmitting antennas, excessive side lobe radiation wastes energy and may cause interference to other equipment. Classified information may be picked up by unintended receivers.
In receiving antennas, side lobes may pick up interfering signals, increase the noise level in the receiver. The power density in the side lobes is much less than that in the main beam, it is desirable to minimize the sidelobe level, measured in decibels relative to the peak of the main beam. The main lobe and side lobes occur for both conditions of transmit, for receive; the concepts of main and side lobes, radiation pattern, aperture shapes, aperture weighting, apply to optics and in acoustics fields such as loudspeaker and sonar design, as well as antenna design. For a rectangular aperture antenna having a uniform amplitude distribution, the first sidelobe is -13.26 dB relative to the peak of the main beam. For such antennas the radiation pattern has a canonical form of Radiation Pattern ∝ 20 log 10 Simple substitutions of various values of X into the canonical equation yield the following results: For a circular aperture antenna having a uniform amplitude distribution, the first sidelobe level is -17.57 dB relative to the peak of the main beam.
In this case, the radiation pattern has a canonical form of Radiation Pattern ∝ 10 log 10 where J 1 is the Bessel function of the first kind of order 1. Simple substitutions of various values of X into the canonical equation yield the following results: A uniform aperture distribution, as provided in the two examples above, gives the maximum possible directivity for a given aperture size, but it produces the maximum side lobe level. Side lobe levels can be reduced by tapering the edges of the aperture distribution at the expense of reduced directivity; the nulls between sidelobes occur when the radiation patterns passes through the origin in the complex plane. Hence, adjacent sidelobes are 180° out of phase to each other; because an antenna's far field radiation pattern is a Fourier Transform of its aperture distribution, most antennas will have sidelobes, unless the aperture distribution is a Gaussian, or if the antenna is so small, as to have no sidelobes in the visible space. Larger antennas have narrower main beams, as well as narrower sidelobes.
Hence, larger antennas have more sidelobes in the visible space. For discrete aperture antennas in which the element spacing is greater than a half wavelength, the spatial aliasing effect causes some sidelobes to become larger in amplitude, approaching the level of the main lobe. Grating lobes are a special case of a sidelobe. In such a case, the sidelobes should be considered all the lobes lying between the main lobe and the first grating lobe, or between grating lobes, it is conceptually useful to distinguish between sidelobes and grating lobes because grating lobes have larger amplitudes than most, if not all, of the other side lobes. The mathematics of grating lobes is the same as of X-ray diffraction. Sidelobes and Beamwidths - An Antenna Tutorial
Television New Zealand, more referred to as TVNZ, is a state-owned television network, broadcast throughout New Zealand and parts of the Pacific region. Although the network identifies as a national, part-public broadcaster, it is commercially funded. TVNZ was competition free until November 1989; this began the battle for ratings with the only real rival MediaWorks New Zealand, which operates channels Three, ThreeLife and The Edge TV. However, TVNZ still maintains a number of transmission advantages due to its long-standing relationship with the state-owned sister company Kordia. TVNZ operates playout services from its Auckland studio via Kordia's fibre and microwave network for TVNZ 1, TVNZ 2 and TVNZ Duke, with new media video services via the American-owned Brightcove, streamed on the Akamai RTMP/HLS DNS based caching network, its former channels include TVNZ Kidzone, TVNZ Heartland, TVNZ U, TVNZ 7, TVNZ 6, TVNZ Sport Extra. 90% of TVNZ's revenue is from commercial activity. The remainder of its funding comes from government funding agencies.
TVNZ was created in February 1980, through the merger of Television One and South Pacific Television. Until January 1989, it was paired with Radio New Zealand as the Broadcasting Corporation of New Zealand; the broadcaster was based in Television One's former headquarters at the Avalon television centre in Lower Hutt, however over the course of the 1980s, operations were moved to Auckland. In 1989, TVNZ moved to a new television centre in central Auckland. Broadcasting in New Zealand was deregulated in 1989; the Labour-led government under Helen Clark from 1999 to 2008 pursued a programme of public broadcasting reforms. New Zealand's wide-ranging adoption of neoliberal policies in the mid-1980s and 1990s had large sections of the state sector privatised; as a state owned enterprise, TVNZ enjoyed enormous commercial success and paid the Crown substantial dividends. However, the commercial success had been achieved through an unabashed pursuit of ratings through populist and tabloid content, prior to the 1999 election the National-led government was evidently positioning TVNZ for commercialisation Labour-led administrations since 1999 explicitly recognised the market failures of a wholly commercial broadcasting sector and re-emphasised television's cultural and democratic functions in their policy thinking.
The Clark government's highest profile broadcasting reform to date was the restructuring of TVNZ as a Crown entity in 2003. This introduced a dual remit whereby the broadcaster had to maintain its commercial performance while implementing a new public service Charter; the TVNZ Charter would require the negotiation and reconciliation of contradictory commercial and public service imperatives. The final version of the TVNZ Charter included a range of public service objectives and expectations. However, this dual remit precluded any transformation of TVNZ into fully-fledged public service broadcaster, TVNZ's efforts to balance its pursuit of commercial performance and Charter objectives were soon being criticised. Despite some investment in local content, including new documentaries and discussion programmes, the content on TV One and TV2 remained similar to the pre-charter schedules, with a continuing high proportion of light entertainment and reality-TV shows. TVNZ continues to pay dividends to the Crown.
However, from 2006 until 2009 TVNZ received $15.11 million each year from Government to assist it with fulfilling Charter obligations. There was much debate about the initial secrecy surrounding funding allocations and the programmes supported; the allocation of $5 million toward coverage of the 2008 Olympics, the rights for which are secured by a competitive tender between broadcasters, was the most controversial. In 2009 the Government gave control of that funding to funding agency NZ On Air. NZ On Air announced the creation of the contestable "Platinum Fund" in April 2009, setting aside the $15.11 million for high quality drama and other programme types. Following the election of a National Party-led government under John Key in 2008, the Charter was abolished in favour of a return to the 1990s model of a full commercial broadcaster. There is much debate on the future of TVNZ, which focuses on the nature of public service broadcasting and its commercial role. An example was in a memo called A More Public Broadcaster written by outgoing Chief Executive Ian Fraser to the board of TVNZ in October 2005, was obtained and released by Green MP Sue Kedgley.
The memo outlined three options. These were: TV One as a non-commercial network, like ABC in Australia, charged with delivering Charter values, merging with Radio New Zealand and Māori Television TV One a semi-commercial broadcaster with no more than six minutes of advertisements an hour like SBS in Australia TV One and TV2 remaining unchanged, but two new public service channels being broadcast via digital television. TV One and TV2 are now commercial with 15 – 20 minutes of ads per hour, plus ads overplayed over programs. On 15 February 2006, a group of 31 prominent New Zealanders signed an open letter, published as a full-page newspaper advertisement, calling for
In electronics and telecommunications, a transmitter or radio transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, applied to the antenna; when excited by this alternating current, the antenna radiates radio waves. Transmitters are necessary component parts of all electronic devices that communicate by radio, such as radio and television broadcasting stations, cell phones, walkie-talkies, wireless computer networks, Bluetooth enabled devices, garage door openers, two-way radios in aircraft, spacecraft, radar sets and navigational beacons; the term transmitter is limited to equipment that generates radio waves for communication purposes. Generators of radio waves for heating or industrial purposes, such as microwave ovens or diathermy equipment, are not called transmitters though they have similar circuits; the term is popularly used more to refer to a broadcast transmitter, a transmitter used in broadcasting, as in FM radio transmitter or television transmitter.
This usage includes both the transmitter proper, the antenna, the building it is housed in. A transmitter can be a separate piece of electronic equipment, or an electrical circuit within another electronic device. A transmitter and a receiver combined in one unit is called a transceiver; the term transmitter is abbreviated "XMTR" or "TX" in technical documents. The purpose of most transmitters is radio communication of information over a distance; the information is provided to the transmitter in the form of an electronic signal, such as an audio signal from a microphone, a video signal from a video camera, or in wireless networking devices, a digital signal from a computer. The transmitter combines the information signal to be carried with the radio frequency signal which generates the radio waves, called the carrier signal; this process is called modulation. The information can be added to the carrier in several different ways, in different types of transmitters. In an amplitude modulation transmitter, the information is added to the radio signal by varying its amplitude.
In a frequency modulation transmitter, it is added by varying the radio signal's frequency slightly. Many other types of modulation are used; the radio signal from the transmitter is applied to the antenna, which radiates the energy as radio waves. The antenna may be enclosed inside the case or attached to the outside of the transmitter, as in portable devices such as cell phones, walkie-talkies, garage door openers. In more powerful transmitters, the antenna may be located on top of a building or on a separate tower, connected to the transmitter by a feed line, a transmission line. Electromagnetic waves are radiated by electric charges undergoing acceleration. Radio waves, electromagnetic waves of radio frequency, are generated by time-varying electric currents, consisting of electrons flowing through a metal conductor called an antenna which are changing their velocity or direction and thus accelerating. An alternating current flowing back and forth in an antenna will create an oscillating magnetic field around the conductor.
The alternating voltage will charge the ends of the conductor alternately positive and negative, creating an oscillating electric field around the conductor. If the frequency of the oscillations is high enough, in the radio frequency range above about 20 kHz, the oscillating coupled electric and magnetic fields will radiate away from the antenna into space as an electromagnetic wave, a radio wave. A radio transmitter is an electronic circuit which transforms electric power from a power source into a radio frequency alternating current to apply to the antenna, the antenna radiates the energy from this current as radio waves; the transmitter impresses information such as an audio or video signal onto the radio frequency current to be carried by the radio waves. When they strike the antenna of a radio receiver, the waves excite similar radio frequency currents in it; the radio receiver extracts the information from the received waves. A practical radio transmitter consists of these parts: A power supply circuit to transform the input electrical power to the higher voltages needed to produce the required power output.
An electronic oscillator circuit to generate the radio frequency signal. This generates a sine wave of constant amplitude called the carrier wave, because it serves to "carry" the information through space. In most modern transmitters, this is a crystal oscillator in which the frequency is controlled by the vibrations of a quartz crystal; the frequency of the carrier wave is considered the frequency of the transmitter. A modulator circuit to add the information to be transmitted to the carrier wave produced by the oscillator; this is done by varying some aspect of the carrier wave. The information is provided to the transmitter either in the form of an audio signal, which represents sound, a video signal which represents moving images, or for data in the form of a binary digital signal which represents a sequence of bits, a bitstream. Different types of transmitters use different modulation methods to transmit information: In an AM transmitter the amplitude of the carrier wave is varied in proportion to the modulation signal.
In an FM transmitter the frequency of the carrier is varied by the modulation signal. In an FSK transmitter, which transmits digital data, the frequency of the carrier is shifted between two frequencies which represent the two binary digits, 0 and 1. Many oth