Transmitter
 | This article includes a list of general references, but it lacks sufficient corresponding inline citations. (October 2022) |
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.In
electronic device which produces radio waves with an antenna with the purpose of signal transmission up to a radio receiver. The transmitter itself generates a radio frequency alternating current, which is 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
communication purposes; or radiolocation, such as radar and navigational transmitters. Generators of radio waves for heating or industrial purposes, such as microwave ovens or diathermy
equipment, are not usually called transmitters, even though they often have similar circuits.
The term is popularly used more specifically to refer to a broadcast transmitter, a transmitter used in broadcasting, as in FM radio transmitter or television transmitter. This usage typically includes both the transmitter proper, the antenna, and often the building it is housed in.
Description
electromagnetic waves (radio waves
) to transport information (in this case sound) over a distance.A transmitter can be a separate piece of electronic equipment, or an
carrier signal. It combines the carrier with the modulation signal, a process called modulation. The information can be added to the carrier in several different ways, in different types of transmitters. In an amplitude modulation (AM) transmitter, the information is added to the radio signal by varying its amplitude. In a frequency modulation (FM) transmitter, it is added by varying the radio signal's frequency
slightly. Many other types of modulation are also used.
The radio signal from the transmitter is applied to the
feed line, that is a transmission line
.
Radio transmitters
two way radio
transmitting on 27 MHz with a power of 4 W, that can be operated without a licenseConsumer products that contain transmitters
laptop computer and home wireless router (background) which connects it to the Internet, creating a home Wi-Fi network. Both have Wi-Fi modems, automated microwave transmitters and receivers operating on 2.4 GHz which exchange data packets with the internet service provider (ISP).
Operation
radio waves, showing the electric field lines. The antenna in the center is two vertical metal rods, with an alternating current applied at its center from a radio transmitter (not shown). The voltage charges the two sides of the antenna alternately positive (+) and negative (−). Loops of electric field (black lines) leave the antenna and travel away at the speed of light
; these are the radio waves. This animation shows the action slowed enormouslyElectromagnetic waves are radiated by electric charges when they are accelerated.[1][2] 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 and thus accelerating.[3][2] An alternating current flowing back and forth in an antenna will create an oscillating magnetic field around the conductor. The alternating voltage will also 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
video signal into the radio frequency current to be carried by the radio waves. When they strike the antenna of a radio receiver
, the waves excite similar (but less powerful) radio frequency currents in it. The radio receiver extracts the information from the received waves.
Components
A practical radio transmitter mainly consists of the following parts:
- In high power transmitters, a power supply circuit to transform the input electrical power to the higher voltages needed to produce the required power output.
- An quartz crystal. The frequencyof the carrier wave is considered the frequency of the transmitter.
- A bits, a bitstream. Different types of transmitters use different modulationmethods to transmit information:
- In an AM (amplitude modulation) transmitter the amplitude (strength) of the carrier wave is varied in proportion to the modulation signal.
- In an FM (frequency modulation) transmitter the frequency of the carrier is varied by the modulation signal.
- In an FSK (binary digits, 0 and 1.
- OFDM (bitsare being sent simultaneously, in parallel. At the receiver the carriers are demodulated and the bits are combined in the proper order into one bitstream.
- Many other types of modulation are also used. In large transmitters the oscillator and modulator together are often referred to as the exciter.
- A radio frequency (RF) amplifier to increase the power of the signal, to increase the range of the radio waves.
- An impedance matching (antenna tuner) circuit to transform the output impedance of the transmitter to match the impedance of the antenna (or the transmission line to the antenna), to transfer power efficiently to the antenna. If these impedances are not equal, it causes a condition called standing waves, in which the power is reflected back from the antenna toward the transmitter, wasting power and sometimes overheating the transmitter.
In higher frequency transmitters, in the UHF and microwave range, free running oscillators are unstable at the output frequency. Older designs used an oscillator at a lower frequency, which was multiplied by frequency multipliers to get a signal at the desired frequency. Modern designs more commonly use an oscillator at the operating frequency which is stabilized by phase locking to a very stable lower frequency reference, usually a crystal oscillator.
Regulation
Two radio transmitters in the same area that attempt to transmit on the same frequency will interfere with each other, causing garbled reception, so neither transmission may be received clearly.
Interference with radio transmissions can not only have a large economic cost, it can be life-threatening (for example, in the case of interference with emergency communications or air traffic control
).
For this reason, in most countries, use of transmitters is strictly controlled by law. Transmitters must be licensed by governments, under a variety of license classes depending on use such as
marine radio, Airband, Amateur and are restricted to certain frequencies and power levels. A body called the International Telecommunication Union (ITU) allocates the frequency bands in the radio spectrum to various classes of users. In some classes, each transmitter is given a unique call sign consisting of a string of letters and numbers which must be used as an identifier in transmissions. The operator of the transmitter usually must hold a government license, such as a general radiotelephone operator license
, which is obtained by passing a test demonstrating adequate technical and legal knowledge of safe radio operation.
Exceptions to the above regulations allow the unlicensed use of low-power short-range transmitters in consumer products such as
Part 15 of the Federal Communications Commission (FCC) regulations. Although they can be operated without a license, these devices still generally must be type-approved
before sale.
History
The first primitive radio transmitters (called
radio noise
which interfered with other transmitters. Damped wave emissions were banned by international law in 1934.
Two short-lived competing transmitter technologies came into use after the turn of the century, which were the first
Poulsen arc) in 1904 and the Alexanderson alternator
around 1910, which were used into the 1920s.
All these early technologies were replaced by
traveling wave tube
.
The invention of the
wireless modems) in portable devices operate automatically, in the background, to exchange data with wireless networks
.
The need to conserve bandwidth in the increasingly congested
noise immunity, and greater flexibility and processing power of digital signal processing integrated circuits
.
-
radiotelegraphycommunication in 1895–1897 -
High power spark gapradiotelegraphytransmitter in Australia around 1910. -
1 MW US NavyPoulsen arctransmitter which generated continuous waves using an electric arc in a magnetic field, a technology used for a brief period from 1903 until vacuum tubes took over in the 20s -
An Alexanderson alternator, a huge rotating machine used as a radio transmitter at very low frequency from about 1910 until World War 2 -
-
One of the BBC's first broadcast transmitters, early 1920s, London. The 4 triode tubes, connected in parallel to form an oscillator, each produced around 4 kilowatts with 12 thousand volts on their anodes. -
Armstrong's first experimental FM broadcast transmitter W2XDG, in the Empire State Building, New York City, used for secret tests 1934–1935. It transmitted on 41 MHz at a power of 2 kW. -
Transmitter assembly of a 20 kW, 9.375 GHzwaveguidewhich conducts them to the dish antenna.
See also
- List of transmission sites
- List of radios – List of specific models of radios
- Radio transmitter design
- Repeater
- Transmitter station
- Transposer
- Television transmitter
References
- ISBN 978-0495386933.
- ^ ISBN 978-1316785164.
- ISBN 9781118585733.
- ^ Brain, Marshall (2000-12-07). "How Radio Works". HowStuffWorks.com. Retrieved 2009-09-11.
External links
Look up transmitter in Wiktionary, the free dictionary.