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
The DRDO Airborne Early Warning and Control System is a project of India's Defence Research and Development Organisation to develop an airborne early warning and control system for the Indian Air Force. It is referred to as DRDO NETRA AEW&CS system. In 2003, the Indian Air Force and Defence Research and Development Organisation carried out a joint study of the system-level requirements and feasibility of development for an Airborne Early Warning and Control system; the government approved the project for the development of the AEWAC system by DRDO. Primary responsibility for the project was with DRDO's Bengaluru-based Centre for Airborne Systems, which led the design, system integration and testing of the system. LRDE was responsible for the design of the radar array; the Defence Electronics Application Laboratory, based in Dehradun, was responsible for the Data Link and Communication Systems for AEW&CS. The DRDO AEWACS programme aims to deliver three radar-equipped surveillance aircraft to the Indian Air Force.
The aircraft platform selected was the Embraer ERJ 145. Three ERJ 145 were procured from Embraer at a cost of US $300 Million, including the contracted modifications to the airframe; the project goal was to deploy these AEW&C aircraft by 2013. India's sole previous effort to develop an AEWAC system was the Airborne Surveillance Platform, but the programme, codenamed Airavat, was ended after the only test-bed crashed; the AEW&C project aimed to supplement the larger and more capable EL/W-2090 AWACS acquired by the IAF from Israel. Three EL/W-2090 systems have been ordered, with follow-on orders of 3 more expected in 2010. Apart from providing the IAF with a cheaper and hence, more flexible AEW&C platform as a backup to its more capable EL/W-2090 class systems, the DRDO AEW&C project aimed to develop the domestic ability to design and operationalise airborne surveillance platforms; the delivery of six additional systems ordered in October 2010 is to begin from 2015. In June 2010, it was reported that the Indian Air Force is said to be looking at acquiring up to 20 additional systems, in addition to the existing systems on order.
The responsibility between various DRDO laboratories is split as follows: LRDE - Primary radar DEAL - Communication Systems and Data Link DARE - Self Protection suite, Electronic Support Measurement EW DLRL - Communication Support Measures CABS - IFF & Overall Programme Management and development of the data handling system, mission computers et al. Various Indian private sector firms are involved in the programme. National Aerospace Laboratories contributed to the aerodynamic studies of the antenna array, flight modelling of the entire AEWACS platform. Two radiating planar arrays assembled back-to-back and mounted on top of the fuselage in an active antenna array unit will provide 240° coverage like Erieye; the AAAU is configured to compactly house 10 × 2 antenna array panels, 160 transmit receive 10 × 2 antenna array panels, 160 transmit receive multi-modules dividers, beam forming units, beam control units, power supply units and related electronic devices including cables and connectors.
This has been achieved through an innovative and iterative process to arrive at the AAAU with minimal dimensions and optimum mass properties. A unique feature of this Indian TRMM design is that eight trans-receive modules are combined compactly to form a single TRMM, thus facilitating high density installation of 160 of them in the AAAU to power the surveillance radar. Additionally, the aircraft has other mission capabilities like identification friend or foe and communication support measures, C-band line-of-sight and Ku-band SATCOM datalinks, etc. similar to those on the AWACS and CAEW systems. The important modes of operation of the primary radar system are the surface surveillance and the air surveillance; the sensor has the abilities to search, track-while-scan, priority tracking, high performance tracking, etc. In priority tracking, the targets will be placed in full track mode if these cross the primary surveillance area. In high performance tracking, additional measurements will be made to improve the tracking accuracy.
Utilising active aperture technology, the radar provides a fast-beam agile system that can operate in several modes concurrently. Inter-operability with AWACS, other AEW&C aircraft and ground-exploitation stations is ensured using the data-links with voice and data channels; the aircraft cabin houses five operator work stations to adequately meet requirements of the operational mission tasks. An air-to-air refuelling system enables extended operations at times of need; the endurance of the platform aircraft is about nine hours with one air-to-air refuelling. The AEWACS aircraft will have an active electronically scanned array primary radar with IFF; the system will have ESM and CSM ability. Datalinks to network the AEWACS with fighters, ground-based control systems will be provided, as will be the SATCOM; the aircraft will have a comprehensive self-defence suite. The avionics suite will be linked via a datahandling system, controlled by Mission computers. DRDO's public overview of the AEWACS aircraft stated: The Radar will have an extended range mode against fighter aircraft, will consist of two back to back AESA arrays, with an additional dedicated IFF array.
The ESM system will be able to track sources with a directional accuracy of 2 deg. RMS and a frequency accuracy of 1 MHz; the ESM system will have complete 360 degree coverage in azimuth and have a database of up to 3000 emitters against which threats will be scanned. Communication Support Measure system will analyse and record intercepted commu
Freezing rain is the name given to rain maintained at temperatures below freezing by the ambient air mass that causes freezing on contact with surfaces. Unlike a mixture of rain and snow, ice pellets, or hail, freezing rain is made of liquid droplets; the raindrops become supercooled while passing through a sub-freezing layer of air hundreds of meters above the ground, freeze upon impact with any surface they encounter, including the ground, electrical wires and automobiles. The resulting ice, called glaze ice, can accumulate to a thickness of several centimeters and cover all exposed surfaces; the METAR code for freezing rain is FZRA. A storm that produces a significant thickness of glaze ice from freezing rain is referred to as an ice storm. Although these storms are not violent, freezing rain is notorious for causing travel problems on roadways, breaking tree limbs, downing power lines from the weight of accumulating ice. Downed power lines cause power outages in affected areas while accumulated ice can pose significant overhead hazards.
It is known for being dangerous to aircraft since the ice can effectively'remould' the shape of the airfoil and flight control surfaces. Freezing rain is associated with the approach of a warm front, when subfreezing air is trapped in the lowest levels of the atmosphere while warm air advects in aloft; this happens, for instance, when a low pressure system moves from the Mississippi River Valley toward the Appalachian Mountains and the Saint Lawrence River Valley of North America during the cold season, with a strong high pressure system sitting further east. This setup is known as cold-air damming, is characterized by cold and dry air at the surface within the region of high pressure; the warm air from the Gulf of Mexico is the fuel for freezing precipitation. Freezing rain develops when falling snow encounters a layer of warm air aloft around the 800 mbar level, causing the snow to melt and become rain; as the rain continues to fall, it passes through a layer of subfreezing air just above the surface and cools to a temperature below freezing.
If this layer of subfreezing air is sufficiently deep, the raindrops may have time to freeze into ice pellets before reaching the ground. However, if the subfreezing layer of air at the surface is shallow, the rain drops falling through it will not have time to freeze and they will hit the ground as supercooled rain; when these supercooled drops make contact with the ground, power lines, tree branches, aircraft, or anything else below 0 °C, a portion of the drops freezes, forming a thin film of ice, hence freezing rain. The specific physical process by which this occurs is called nucleation. Surface observations by manned or automatic stations are the only direct confirmation of freezing rain. One can never see directly freezing rain or snow on weather radars, Doppler or conventional. However, it is possible to estimate the area covered by freezing rain with radars indirectly; the intensity of the radar echoes is proportional to the form of its diameter. In fact, rain has much stronger reflective power than snow but its diameter is much smaller.
So the reflectivity of rain coming from melted snow is only higher. However, in the layer where the snow is melting, the wet flakes still have a large diameter and are coated with water so the returns to the radar is much stronger; the presence of this brightband indicates. This could be producing rain on the ground or the possibility of freezing rain if the temperature is below freezing; this artifact can be located, with a cross-section through radar data. The height and slope of the brightband will give clues to the extent of the region where melting occurs, it is possible to associate this clue with surface observations and numerical models prediction to produce output such as the ones seen on television weather programs that divide radar echoes into rain and snow precipitations. Freezing rain causes major power outages by forming glaze ice; when the freezing rain or drizzle is light and not prolonged, the ice formed is thin and causes only minor damage. When large quantities accumulate, however, it is one of the most dangerous types of winter hazard.
When the ice layer exceeds 6.4 mm, tree limbs with branches coated in ice can break off under the enormous weight and fall onto power lines. Windy conditions and lightning, when present, will exacerbate the damage. Power lines coated with ice become heavy, causing support poles and lines to break; the ice that forms on roadways makes vehicle travel dangerous. Unlike snow, wet ice provides no traction, vehicles will slide on gentle slopes; because freezing rain does not hit the ground as an ice pellet but still as a rain droplet, it conforms to the shape of the ground, or object such as a tree branch or car. This makes one thick layer of ice called "glaze". Freezing rain and glaze ice on a large scale is called an ice storm. Effects on plants can be severe. Trees may snap as they are fragile during winter weather. Pine trees are victims of ice storms as their needles will catch the ice, but not be able to support the weight. In February 1994, a severe ice storm caused over $1 billion in damage in the Southern United States in Mississippi, Tennessee and Western North Carolina the
A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder. Communications satellites are used for television, radio and military applications. There are 2,134 communications satellites in Earth’s orbit, used by both private and government organizations. Many are in geostationary orbit 22,200 miles above the equator, so that the satellite appears stationary at the same point in the sky, so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it; the high frequency radio waves used for telecommunications links travel by line of sight and so are obstructed by the curve of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between separated geographical points. Communications satellites use a wide range of microwave frequencies. To avoid signal interference, international organizations have regulations for which frequency ranges or "bands" certain organizations are allowed to use.
This allocation of bands minimizes the risk of signal interference. The concept of the geostationary communications satellite was first proposed by Arthur C. Clarke, along with Vahid K. Sanadi building on work by Konstantin Tsiolkovsky. In October 1945 Clarke published an article titled "Extraterrestrial Relays" in the British magazine Wireless World; the article described the fundamentals behind the deployment of artificial satellites in geostationary orbits for the purpose of relaying radio signals. Thus, Arthur C. Clarke is quoted as being the inventor of the communications satellite and the term'Clarke Belt' employed as a description of the orbit. Decades a project named Communication Moon Relay was a telecommunication project carried out by the United States Navy, its objective was to develop a secure and reliable method of wireless communication by using the Moon as a passive reflector and a natural communications satellite. The first artificial Earth satellite was Sputnik 1. Put into orbit by the Soviet Union on October 4, 1957, it was equipped with an on-board radio-transmitter that worked on two frequencies: 20.005 and 40.002 MHz.
Sputnik 1 was launched as a major step in the exploration of rocket development. However, it was not placed in orbit for the purpose of sending data from one point on earth to another; the first satellite to relay communications was an intended lunar probe. Though the spacecraft only made it about halfway to the moon, it flew high enough to carry out the proof of concept relay of telemetry across the world, first from Cape Canaveral to Manchester, England; the first satellite purpose-built to relay communications was NASA's Project SCORE in 1958, which used a tape recorder to store and forward voice messages. It was used to send a Christmas greeting to the world from U. S. President Dwight D. Eisenhower. Courier 1B, built by Philco, launched in 1960, was the world's first active repeater satellite; the first artificial satellite used to further advances in global communications was a balloon named Echo 1. Echo 1 was the world's first artificial communications satellite capable of relaying signals to other points on Earth.
It soared 1,600 kilometres above the planet after its Aug. 12, 1960 launch, yet relied on humanity's oldest flight technology — ballooning. Launched by NASA, Echo 1 was a 30-metre aluminized PET film balloon that served as a passive reflector for radio communications; the world's first inflatable satellite — or "satelloon", as they were informally known — helped lay the foundation of today's satellite communications. The idea behind a communications satellite is simple: Send data up into space and beam it back down to another spot on the globe. Echo 1 accomplished this by serving as an enormous mirror, 10 stories tall, that could be used to reflect communications signals. There are two major classes of communications satellites and active. Passive satellites only reflect the signal coming from the source, toward the direction of the receiver. With passive satellites, the reflected signal is not amplified at the satellite, only a small amount of the transmitted energy reaches the receiver. Since the satellite is so far above Earth, the radio signal is attenuated due to free-space path loss, so the signal received on Earth is very weak.
Active satellites, on the other hand, amplify the received signal before retransmitting it to the receiver on the ground. Passive satellites are little used now. Telstar was the second direct relay communications satellite. Belonging to AT&T as part of a multi-national agreement between AT&T, Bell Telephone Laboratories, NASA, the British General Post Office, the French National PTT to develop satellite communications, it was launched by NASA from Cape Canaveral on July 10, 1962, in the first sponsored space launch. Relay 1 was launched on December 13, 1962, it became the first satellite to transmit across the Pacific Ocean on November 22, 1963. An immediate antecedent of the geostationary satellites was the Hughes Aircraft Company's Syncom 2, launched on July 26, 1963. Syncom 2 was the first communications satellite in a geosynchronous orbit, it revolved around the earth once per day at constant speed, but because it still had north-south motion, special equipment was needed to track it. Its successor, Syncom 3 was the first geostationary communications satellite.
Syncom 3 obt
Alternating current is an electric current which periodically reverses direction, in contrast to direct current which flows only in one direction. Alternating current is the form in which electric power is delivered to businesses and residences, it is the form of electrical energy that consumers use when they plug kitchen appliances, televisions and electric lamps into a wall socket. A common source of DC power is a battery cell in a flashlight; the abbreviations AC and DC are used to mean alternating and direct, as when they modify current or voltage. The usual waveform of alternating current in most electric power circuits is a sine wave, whose positive half-period corresponds with positive direction of the current and vice versa. In certain applications, different waveforms are used, such as square waves. Audio and radio signals carried on electrical wires are examples of alternating current; these types of alternating current carry information such as sound or images sometimes carried by modulation of an AC carrier signal.
These currents alternate at higher frequencies than those used in power transmission. Electrical energy is distributed as alternating current because AC voltage may be increased or decreased with a transformer; this allows the power to be transmitted through power lines efficiently at high voltage, which reduces the energy lost as heat due to resistance of the wire, transformed to a lower, voltage for use. Use of a higher voltage leads to more efficient transmission of power; the power losses in the wire are a product of the square of the current and the resistance of the wire, described by the formula: P w = I 2 R. This means that when transmitting a fixed power on a given wire, if the current is halved, the power loss due to the wire's resistance will be reduced to one quarter; the power transmitted is equal to the product of the voltage. Power is transmitted at hundreds of kilovolts, transformed to 100 V – 240 V for domestic use. High voltages have disadvantages, such as the increased insulation required, increased difficulty in their safe handling.
In a power plant, energy is generated at a convenient voltage for the design of a generator, stepped up to a high voltage for transmission. Near the loads, the transmission voltage is stepped down to the voltages used by equipment. Consumer voltages vary somewhat depending on the country and size of load, but motors and lighting are built to use up to a few hundred volts between phases; the voltage delivered to equipment such as lighting and motor loads is standardized, with an allowable range of voltage over which equipment is expected to operate. Standard power utilization voltages and percentage tolerance vary in the different mains power systems found in the world. High-voltage direct-current electric power transmission systems have become more viable as technology has provided efficient means of changing the voltage of DC power. Transmission with high voltage direct current was not feasible in the early days of electric power transmission, as there was no economically viable way to step down the voltage of DC for end user applications such as lighting incandescent bulbs.
Three-phase electrical generation is common. The simplest way is to use three separate coils in the generator stator, physically offset by an angle of 120° to each other. Three current waveforms are produced that are equal in magnitude and 120° out of phase to each other. If coils are added opposite to these, they generate the same phases with reverse polarity and so can be wired together. In practice, higher "pole orders" are used. For example, a 12-pole machine would have 36 coils; the advantage is. For example, a 2-pole machine running at 3600 rpm and a 12-pole machine running at 600 rpm produce the same frequency. If the load on a three-phase system is balanced among the phases, no current flows through the neutral point. In the worst-case unbalanced load, the neutral current will not exceed the highest of the phase currents. Non-linear loads may require an oversized neutral bus and neutral conductor in the upstream distribution panel to handle harmonics. Harmonics can cause neutral conductor current levels to exceed that of all phase conductors.
For three-phase at utilization voltages a four-wire system is used. When stepping down three-phase, a transformer with a Delta primary and a Star secondary is used so there is no need for a neutral on the supply side. For smaller customers only a single phase and neutral, or two phases and neutral, are taken to the property. For larger installations all three phases and neutral are taken to the main distribution panel. From the three-phase main panel, both single and three-phase circuits may lead off. Three-wire single-phase systems, with a single center-tapped transformer giving two live conductors, is a common distribution scheme for res
A tuner is a subsystem that receives radio frequency transmissions like radio broadcasts and converts the selected carrier frequency and its associated bandwidth into a fixed frequency, suitable for further processing because a lower frequency is used on the output. Broadcast FM/AM transmissions feed this intermediate frequency directly into a demodulator that convert the radio signal into audio-frequency signals that can be fed into an amplifier to drive a loudspeaker. More complex transmissions like PAL/NTSC, DAB, DVB-T/DVB-S/DVB-C etc. use a wider frequency bandwidth with several subcarriers. These are transmitted inside the receiver as an intermediate frequency; the next step is either to process subcarriers like real radio transmissions or to sample the whole bandwidth with A/D at a rate faster than the Nyquist rate, at least twice the IF frequency. A tuner can refer to a radio receiver or standalone audio component that are part of an audio system, to be connected to a separate amplifier.
The verb tuning in radio contexts means adjusting the radio receiver to receive the desired radio signal carrier frequency that a particular radio station uses. The simplest tuner consists of an inductor and capacitor connected in parallel, where the capacitor or inductor is made to be variable; this creates a resonant circuit. Combined with a detector known as a demodulator, it becomes the simplest radio receiver called a crystal set. Older models would realize manual tuning by means of mechanically operated ganged variable capacitors. Several sections would be provided on a tuning capacitor, to tune several stages of the receiver in tandem, or to allow switching between different frequency bands. A method used a potentiometer supplying a variable voltage to varactor diodes in the local oscillator and tank circuits of front end tuner, for electronic tuning. Modern radio tuners use a superheterodyne receiver with tuning selected by adjustment of the frequency of a local oscillator; this system shifts the radio frequency of interest to a fixed frequency so that it can be tuned with fixed-frequency band-pass filter.
Still phase locked loop methods were used, with microprocessor control. In a self-contained radio receiver for audio, the signal from the detector after the tuner is run through a volume control and to an amplifier stage; the amplifier feeds either headphones. In a tuner component of an audio system, the output of the detector is connected to a separate external system of amplifiers and speakers; the broadcast audio FM band is around 100 times higher in frequency than the AM band and provides enough space for a bandwidth of 50 kHz. This bandwidth is sufficient to transmit both stereo channels with the full hearing range. Sometimes, additional subcarriers are used for unrelated data transmissions; the left and right audio signals must be combined into a single signal, applied to the modulation input of the transmitter. FM stereo allows left and right channels to be transmitted; the availability of FM stereo, a quieter VHF broadcast band, better fidelity led to the specialization of FM broadcasting in music, tending to leave AM broadcasting with spoken-word material.
Standalone audio stereo FM tuners are sought after for audiophile and TV/FM DX applications those produced in the 1970s and early 1980s, when performance and manufacturing standards were among the highest. In many instances the tuner may be modified to improve performance. A growing hobby trend is the electronics specialists that buy and restore these vintage FM or AM/FM audio tuners; the restoration begins with replacing the electrolytic capacitors that may age over time. The tuner is outfitted with better sounding upgraded parts. Prices have increased relative to the increasing demand for the older audio tuners; those with the most value are the best sounding, most rare, the best DX capable and the known build quality of the component, as it left the factory. Most of the early tuner models were manufactured to receive only the AM broadcast band; as FM became more popular, the limitations of AM became more apparent, FM became the primary listening focus for stereo and music broadcasting. Few companies manufacture dedicated FM or AM/FM tuners now, as these bands are most included in a low cost chip for A/V systems, more as an afterthought, rather than designed for the critical FM listener.
In Europe, where a second AM broadcast band is used for longwave broadcasting, tuners may be fitted with both the standard medium wave and the additional longwave band. However, radios with only medium wave are common in countries where there are no longwave broadcasters. Radios are sold with only FM and longwave, but no medium wave band; some tuners may be equipped with one or more short wave bands. A television tuner converts a radio frequency analog television or digital television transmission into audio and video signals which can be further processed to produce sound and a picture. Different tuners are used for different television standards such as PAL, NTSC, ATSC, SECAM, DVB-C, DVB-T, DVB-T2, ISDB, T-DMB, open cable. An example frequency range is 48.25 MHz - 855.25 MHz, with a tuning frequency step size of 31.25, 50 or 62.5 kHz. Modern solid-state internal TV-tuner modules weigh arou