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Radio Transmitter

Radio transmitter

A transmitter (sometimes abbreviated XMTR) is an electronic device which with the aid of an antenna propagates an electromagnetic signal such as radio, television, or other telecommunications. A transmitter usually has a power supply, an oscillator, a modulator, and amplifiers for audio (AF), intermediate frequency (IF) and radio frequency (RF). The modulator is the device which piggybacks (or modulates) the signal information onto the carrier frequency, which is then broadcast. Sometimes a device (for example, a cell phone) contains both a transmitter and a radio receiver, with the combined unit referred to as a transceiver. More generally and in communications and information processing, a "transmitter" is any object (source) which sends information to an observer (receiver). For example, in industrial process control a "transmitter" is any device which converts measurements from a sensor into a signal to be received, usually sent via wires, by some display or control device located a distance away. Some "transmitters" use 4-20 mA current loop or digital methods for transmission of measurements. Some such transmitters even send process signals as 3-15 psi varying pneumatic pressure. When used in this more general sense, vocal cords may also be considered an example of a "transmitter".

History

In the early days of radio engineering, radio frequency energy was generated using arcs or mechanical alternators (of which a rare example survives at the SAQ transmitter in Grimeton, Sweden). In the 1920s electronic transmitters, based on vacuum tubes, began to be used.

Electromagnetic concepts

In principle any conductor (wire) carrying an alternating current will radiate a radio signal. Thus a basic transmitter is just an oscillator connected directly to a wire antenna. Since transmitters require excellent frequency stability, there are usually several amplifier stages between oscillator and antenna. The intermediate amplifier stages prevent changes in the antenna circuit from affecting the frequency of the oscillator. Often the transmitter frequency is not the frequency produced by the oscillator, but one of its harmonics. This is generated from the oscillator's output by a non-linear device (e.g. a diode or an overdriven amplifier), then filtered with combinations of inductors and capacitors, and then amplified. Special standard frequency transmitters use frequency synthesis referenced to a very stable atomic clock. Since this procedure, which gives the most precise carrier frequencies, is very complex, it is not used in most transmitters. Typically a quartz crystal is used as a frequency reference, which provides adequate stability for nearly all purposes. Historically mechanically-tuned variable-frequency oscillators were used, and are still found in classic amateur radio and antique equipment. During the generation and amplification, harmonics are created. These usually have to be filtered out by resonant circuits before reaching the antenna. Vacuum tubes are still occasionally used as amplifier elements in high-power stages, for more than a few kilowatts of radio-frequency power. At high transmitting powers these tubes are water-cooled. For microwave transmitters, special semiconductor components or vacuum tubes (such as the klystron, cavity magnetron or TWT) are needed, because signals of these frequencies and power levels cannot be processed with normal semiconductors. The information to be transmitted is then added by modulation of the frequency, amplitude or phase of the carrier.

Cooling of final stages

Low-power transmitters do not require special cooling equipment. For medium-power transmitters, up to a few hundred watts, air cooling with fans is used. At power levels over a few kilowatts, the output stage is liquid cooled. Since the coolant directly touches the high-voltage anodes of the tubes, only distilled, deionised water or a special dielectric coolant can be used in the cooling circuit. This high-purity coolant is in turn cooled by a heat exchanger, where the second circuit can use water of ordinary quality because it is not in contact with energized parts. Very-high-power tubes of small physical size may use evaporative cooling by water in contact with the anode. The production of steam allows a high heat flow in a small space.

Protection equipment

The high voltage used in high power transmitters (up to 20 kV) require extensive protection equipment. Also, transmitters are exposed to damage from lightning. Transmitters may be damaged if operated without an antenna, so protection circuits must detect the loss of the antenna and switch off the transmitter immediately. Tube-based transmitters must have power applied in the proper sequence, with the filament voltage applied before the anode voltage, otherwise the tubes can be damaged. The output stage must be monitored for standing waves, which indicate that generated power is not being radiated but instead is being reflected back into the transmitter. Lightning protection is required between the transmitter and antenna. This consists of spark gaps and gas-filled surge arresters to limit the voltage that appears on the transmitter terminals. The control instrument that measures the voltage standing-wave ratio switches the transmitter off briefly if a higher voltage standing-wave ratio is detected after a lightning strike, as the reflections are probably due to lightning damage. If this does not succeed after several attempts, the antenna is likely damaged and the transmitter will remain switched off. In some transmitting plants UV detectors are fitted in critical places, to switch off the transmitter if an arc is detected. With water-cooled output stages the electrical conductivity of the water must be supervised carefully. If it exceeds a certain value, suitable countermeasures (replacement with highly pure water or switching off the transmitter) must be taken. Further, the operating voltages, modulation factor, frequency and other transmitter parameters are monitored for protection and diagnostic purposes. The parameters may be displayed locally or at a remote control room.

Building

A transmitter site will have a control building to shelter the transmitter components and control devices. This is usually a purely functional building, which may contain apparatus for both radio and television transmitters. To reduce transmission line loss the transmitter building is usually immediately adjacent to the antenna for VHF and UHF sites, but for lower frequencies it may be desirable to have a distance of a few score or several hundred metres between the building and the antenna. Some transmitting towers have enclosures built into the tower to house radio relay link transmitters or other, relatively low-power transmitters.

Legal and Regulatory Aspects

Since radio waves go over borders, international agreements control radio transmissions. In European countries like Germany often the national Post Office is the regulating authority. In the United States broadcast and industrial transmitters are regulated by the FCC. In Canada technical aspects of broadcast and radio transmitters are controlled by Industry Canada, but broadcast content is regulated separately by the CRTC.

Planning

As in any costly undertaking, the planning of a high power transmitter site requires great care. This begins with the location. A minimum distance, which depends on the transmitter frequency, transmitter power, and the design of the transmitting antennas, is required to protect people from the radio frequency energy. Transmitters for long and medium wave require good grounding and soil of high electrical conductivity. Locations at the sea or in river valleys are ideal, but the flood danger must be considered. Transmitters for UHF are best on high mountains to improve the range (see radio propagation). The antenna pattern must be considered because it is costly to change the pattern of a long-wave or medium-wave antenna. Transmitting antennas for long and medium wave are usually implemented as a mast radiator. Similar antennas with smaller dimensions are used also for short wave transmitters, if these send in the round spray enterprise. For arranging radiation at free standing steel towers fastened planar arrays are used. Radio towers for UHF and TV transmitter can be implemented in principle as grounded constructions. Towers may be steel lattice masts or reinforced concrete towers with antennas mounted at the top. Some transmitting towers for UHF have high-altitude operating rooms and/or facilities such as restaurants and observation platforms, which are accessible by elevator. Such towers are usually called TV tower. For microwaves one uses frequently parabolic antennas. These can be set up for applications of radio relay links on transmitting towers for UKW to special platforms. For the program passing on of television satellites and the funkkontakt to space vehicles large parabolic antennas with diameters of 3 to 100 meters of diameters are necessary. These plants, which can be used if necessary also as radio telescope, are established on free standing constructions, whereby there are also numerous special designs, like the radio telescope in Arecibo.

Transmitters in Culture

Some cities in Europe, like Muehlacker, Ismaning, Langenberg, Kalundborg, Hoerby and Allouis became famous as sites of powerful transmitters. Some transmitting towers like the radio tower Berlin or the TV tower Stuttgart became landmarks of cities. Many transmitting plants have very high radio towers, which are masterpieces of engineering.

Records

- Tallest radio mast
- 1974-1991:Konstantynow for 2000kilowatt longwave transmitter, 646.38 metres
- 1963-1974 and since 1991, KVLY Tower
- Highest power
- Longwave, transmitter Taldom, 2500 kW
- Medium wave, transmitter Bolshakovo, 2500 kW
- Highest transmission sites (Europe)
- UKW Pic du Aigu bei Chamonix
- MW Pic Blanc in Andorra

Broadcasting

In broadcasting, the part which contains the oscillator, modulator, and sometimes audio processor, is called the exciter. Confusingly, the high-power amplifier which the exciter then feeds into is often called the "transmitter" by broadcast engineers. The final output is given as transmitter power output (TPO), although this is not what most stations are rated by. Effective radiated power (ERP) is used when calculating station coverage, even for most non-broadcast stations. It is the TPO, minus any attenuation or radiated loss in the line to the antenna, multiplied by the gain (magnification) which the antenna provides toward the horizon. This is important, because the electric utility bill for the transmitter would be enormous otherwise, as would the cost of a transmitter. For most large stations in the VHF- and UHF-range, the transmitter power is no more than 20% of the ERP. For VLF, LF, MF and SW the ERP is not determined separately. In most cases the transmission power found in lists of transmitters is the value for the output of the transmitter. This is only correct for omnidirectional aerials with a length of a quarter wavelength or shorter. For other aerial types there are gain factors, which can reach values until 50 for shortwave directional beams in the direction of maximum beam intensity. Since some authors take account of gain factors of aerials of transmitters for frequencies below 30 MHz and others not, there are often discrepancies of the values of transmitted powers.

External links

- [http://hawkins.pair.com/radio.html Jim Hawkins' Radio and Broadcast Technology Page]
- [http://www.wcov.com/technical/transmitter/transmitter.html WCOV-TV's Transmitter Technical Website] Category:Radio Category:Telecommunications equipment Category:Radar ja:送信機

Electronics

The field of electronics is the study and use of systems that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves and semiconductors. The design and construction of electronic circuits to solve practical problems is part of the fields of electronic engineering, and the hardware design side of computer engineering. The study of new semiconductor devices and their technology is sometimes considered as a branch of physics.

Electronic devices today

Electronic devices are used to perform a wide variety of tasks. The main uses of electronic circuits are the controlling, processing and distribution of information, and the conversion and distribution of electric power. Both of these uses involve the creation or detection of electromagnetic fields and electric currents. While electrical energy had been used for some time to transmit data over telegraphs and telephones, the development of electronics truly began in earnest with the advent of radio.

CAD/CAM of electronic circuits

Today's electronics engineers enjoy the ability to design circuits using premanufactured building blocks such as power supplies, resistors, capacitors, semiconductors (such as transistors), and integrated circuits. Electronic design automation software programs include schematic capture programs such as ORCAD , used to make circuit diagrams and printed circuit board layouts.

Electronic systems

One way of looking at an electronic system is to divide it into the following parts: # Inputs – Electronic or mechanical sensors (or transducers), which take signals (in the form of temperature, pressure, etc.) from the physical world and convert them into current/voltage signals. # Signal processing circuits – These consist of electronic components connected together to manipulate, interpret and transform the signals. # Outputs – Actuators or other devices (also transducers) that transform current/voltage signals back into useful physical form. One example is a television set. Its input is a broadcast signal received by an antenna or fed in through a cable. Signal processing circuits inside the television extract the brightness, colour and sound information from this signal. The output devices are a cathode ray tube that converts electronic signals into a visible image on a screen and magnet driven audio speakers.

Electronic test equipment

- Ammeter, e.g. Galvanometer (Measure current)
- Ohmmeter, e.g. Wheatstone bridge (Measure resistance)
- Voltmeter (Measures voltage)
- Multimeter (Measures all of the above)
- Oscilloscope (Measures all of the above, except Ohm, as they change over time)
- Logic analyzer (Tests digital circuits)
- Spectrum analyzer (SA) (Measures spectral energy of signals)
- Vector signal analyzer (VSA) (Like the SA but it can also perform many more useful digital demodulation functions)
- Electrometer (Measures charge)
- Frequency counter (Measures frequency)
- Time-domain reflectometer for testing integrity of long cables

Electronic components

- Electronic components
- Electronic Devices and Circuits

Analog circuits

Most analog electronic appliances, such as radio receivers, are constructed from arrays of a few types of circuits.
- Analog computer
- Analog multipliers
- electronic amplifiers
- electronic filters
- electronic oscillators
- Phase-locked loops
- electronic mixers
- Power conversion
- Electronic Power Supply
- impedance matchers
- operational amplifiers
- comparators

Digital circuits

Computers, electronic clocks, and programmable logic controllers (used to control industrial processes) are constructed of digital circuits. Digital Signal Processors are another example. Building-blocks:
- logic gates
- flip-flops
- counters
- registers
- multiplexers
- Schmitt triggers Highly integrated devices:
- microprocessors
- microcontrollers
- DSP
- Field Programmable Gate Array

Mixed-signal circuits

Mixed-signal circuits, also known as hybrid circuits, are becoming increasingly common. Mixed circuits contain both analog and digital components. analog to digital converters and digital to analog converters are the primary examples. Other examples are transmission gates and buffers.

Heat dissipation

Heat generated by electronic circuitry must be dissipated to improve reliability. Techniques for heat dissipation can include heatsinks and fans for air cooling, and other forms of computer cooling such as liquid cooling for computers .

Noise

Associated with all electronic circuits is noise. Types of noise include
- Shot noise in resistors.
- Johnson-Nyquist noise (Thermal noise) in resistors.
- White noise
- 1/f noise (pink noise, or flicker noise)
- Gaussian noise

Electronics theory

- Mathematical methods in electronics
- Digital circuits
- Analog electronics

External links

Tutorials and projects

- [http://www.electronicsinfoline.com/ Electronics Infoline] Directory for electronics projects.
- [http://www.opamp-electronics.com/tutorials/index.htm Basic Electronic Tutorials On DC, AC, Semiconductor and Digital Theory] Extensive free tutorial material and store.
- [http://www.electronics-tutorials.com/ Electronics tutorials] Modest site, mostly focused on radio electronics, awkward layout.
- Williamson Labs' [http://www.williamson-labs.com/ Electronics tutorial]
- Ian Purdie's [http://my.integritynet.com.au/purdic Electronics tutorial]s
- Iguana Labs' [http://www.iguanalabs.com/maintut.htm Electronics Tutorials and Kits]
- [http://www.electronicdefinitions.com Electronic Meanings and Acronyms]
- [http://www.ibiblio.org/obp/electricCircuits/ Lessons in Electric Circuits] – A free series of textbooks on the subjects of electricity and electronics.
- [http://www.radio-electronics.com/ Radio-Electronics.Com] Free information and resources covering radio and electronics
- [http://www.electronicschat.org/echatwiki/ A hobbyist wiki]
- [http://www.falstad.com/circuit/ Circuit simulator with voltage and current visualization]
- [http://allaboutcircuits.com A comprehensive guide to making integrated circuits]
- [http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html HyperPhysics]
- [http://www.talkingelectronics.com/te_interactive_index.html "Talking Electronics"] Great for amateurs, commercial kits.
- [http://electronics.esolberg.com/ Electronic parts library]
- [http://www.work-readyelectronics.org Work Ready Electronics] Free instructional online course materials for Community College Electronics Instructors and Students.

Some other good sites

- [http://endtas.com/robot/ Endtas robotics community website with lots of free robotic projects. Do it yourself]
- [http://www.ieee.org/ IEEE]
- [http://www.spectrum.ieee.org/ IEEE spectrum]
- [http://www.elexp.com/links.htm Electronix Express]
- [http://www.electronicspoint.com/ Electronics Discussions] Web access to electronics related newsgroups. Category:Electronics Category:Electronic engineering ko:전자공학 ms:Elektronik ja:電子工学 simple:Electronics th:อิเล็กทรอนิกส์

Device

A device can be taken to mean:
- electrical device designed to carry power, but not use it
- instrument
- small appliance such as a TV set
- machine or functional part of a machine
- component of a computer such as a printer or network card
- device file or special file, an interface for a device driver that appears in a file system as if it were an ordinary file
- information appliance such as a cell phone or PDA
- coat of arms
- member of the pop band devices
- an "idea" or "mechanism" that somebody 'thought up'
- an invention
- the word device is known as a primary word, meaning that it is so basic it is hard to define; this explains the multitude of definitions and examples for the word device.

Antenna (electronics)

] Most simply, an antenna (U.S.) or aerial (UK) is an electronic component designed to transmit or receive radio waves. The words "antenna" and "aerial" are used throughout this article with precisely the same meaning. More specifically, an antenna is an arrangement of conductors designed to radiate (transmit) an electromagnetic field in response to an applied alternating electromotive force (EMF) and the associated alternating electric current. Alternatively, if an antenna is placed into an electromagnetic field, that field will induce an alternating current upon the antenna, and EMF between its terminals. See radio frequency induction.

Overview

There are two fundamental types of antennas. The first type is omni and the second type is directional. Omni type of antennas function in all possible directions whereas directional type of antennas work only in a single direction,i.e, "Line of Sight(LOS)". The first type couples to the electric field of an electromagnetic wave, and usually consists of a length of wire in which an electric charge moves back and forth (electric dipole). The second type couples to the magnetic field of an electromagnetic wave, and is usually a coil or loop of wire (magnetic dipole). By adding additional conducting rods or coils (called elements) and varying their length, spacing, and orientation, an antenna with specific desired properties can be created, such as a Yagi-Uda Antenna (often abbreviated to "Yagi"). Typically, antennas are designed to operate in a relatively narrow frequency range. The design criteria for receiving and transmitting antennas differ slightly, but generally an antenna can receive and transmit equally as well. This property is called reciprocity. The vast majority of antennas are simple vertical rods a quarter of a wavelength long. Such antennas are simple in construction, usually inexpensive, and both radiate in and receive from all horizontal directions (omnidirectional). One limitation of this antenna is that it does not radiate or receive in the direction in which the rod points. This region is called the antenna blind cone or null. Antennas have practical use for the transmission and reception of radio frequency signals (radio, TV, etc.), which can travel over great distances at the speed of light, and pass through nonconducting walls (although often there is a variable signal reduction depending on the type of wall, and natural rock can be very defective to radio signals).

Antenna effectiveness

Antennas may be omni and directional. There are several critical parameters that affect an antenna's performance and can be adjusted during the design process. These are resonant frequency, impedance, gain, aperture or radiation pattern, polarization, efficiency and bandwidth. Transmit antennas may also have a maximum power rating, and receive antennas differ in their noise rejection properties.

Resonant frequency

The resonant frequency is related to the electrical length of the antenna. This is usually the physical length of the wire multiplied by the ratio of the speed of wave propagation in the wire. Typically an antenna is tuned for a specific frequency, and is effective for a range of frequencies usually centered on that resonant frequency. However, the other properties of the antenna (especially radiation pattern and impedance) change with frequency, so the antenna's resonant frequency may merely be close to the center frequency of these other more important properties. Antennas can be made resonant on harmonic frequencies and with lengths that are fractions of the target frequency. Some antenna designs have multiple resonant frequencies, and some are relatively effective over a very broad range of frequencies. The most commonly known type of wide band aerial is the logarithmic or log aerial but its gain is usually much lower than that of a specific or narrower band aerial.

Impedance

Impedance is similar to refractive index in optics. As the electric wave travels through the different parts of the antenna system (radio, feed line, antenna, free space) it may encounter differences in impedance. At each interface, some fraction of the wave's energy will reflect back to the source, forming a standing wave in the feed line. The ratio of maximum power to minimum power in the wave can be measured and is called the standing wave ratio (SWR). A SWR of 1:1 is ideal. A SWR of 1.5:1 is considered to be marginally acceptable in low power applications where power loss is more critical, although an SWR as high as 6:1 may still be usable with the right equipment. Minimizing impedance differences at each interface will reduce SWR and maximize power transfer through each part of the antenna system. Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line and radio by adjusting the impedance of the feed line, using the feed line as an impedance transformer. More commonly, the impedance is adjusted at the load (see below) with an antenna tuner, a balun, a matching transformer, matching networks composed of inductors and capacitors, or matching sections such as the gamma match.

Gain

capacitor An antenna has gain if it radiates more strongly in one direction than in another. Gain is measured by comparing an antenna to a model antenna, typically the isotropic antenna which radiates equally in all directions. Often a dipole is also used as a practical reference as the isotropic source cannot be realised in practice, but it has 2.1 dB gain over an isotropic source. Most practical antennas radiate more than the isotropic antenna in some directions and less in others. Gain is inherently directional; the gain of an antenna is usually measured in the direction which it radiates best. Gain is one-dimensional. Gain does not mean that the antenna radiates more power than is fed to it, merely that it distributes the power more strongly in some directions than in others. Aperture, and radiation pattern are closely related to gain. Aperture is the shape of the "beam" cross section in the direction of highest gain, and is two-dimensional. (Sometimes aperture is expressed as the radius of the circle that approximates this cross section or the angle of the cone.) Radiation pattern is the three-dimensional plot of the gain, but usually only the two-dimensional horizontal and vertical cross sections of the radiation pattern are considered. Antennas with high gain typically show side lobes in the radiation pattern. Side lobes are peaks in gain other than the main lobe (the "beam"). Side lobes detract from the antenna quality whenever the system is being used to determine the direction of a signal, as in radar systems.

Efficiency

Efficiency is the ratio of power actually radiated to the power put into the antenna terminals. A dummy load may have a SWR of 1:1 but an efficiency of 0, as it absorbs all power and radiates none, showing that SWR alone is not an effective measure of an antenna's efficiency. Radiation in an antenna is caused by radiation resistance which can only be measured as part of total resistance including loss resistance.

Bandwidth

The bandwidth of an antenna is the range of frequencies over which it is effective, usually centered around the resonant frequency. The bandwidth of an antenna may be increased by several techniques, including using thicker wires, replacing wires with cages to simulate a thicker wire, tapering antenna components (like in a feed horn), and combining multiple antennas into a single assembly and allowing the natural impedance to select the correct antenna. Small antennas are usually preferred for convenience, but there is a fundamental limit relating bandwidth, size and efficiency. Of the parameters above, SWR is most easily measured. Impedance can be measured with specialized equipment, as it relates to the complex SWR. Measuring radiation pattern requires a sophisticated setup including significant clear space (enough to get into the antenna's far field) or an anechoic chamber designed for antenna measurements, careful study of experiment geometry, and specialised measurement equipment such as robots that rotate the antenna during the measurements. Bandwidth depends on the overall effectiveness of the antenna, so all of these parameters must be understood to understand bandwidth. However, typically bandwidth is measured by only looking at SWR, i.e., by finding the frequency range over which the SWR is less than a given value. Bandwidth over which an antenna exhibits a particular radiation pattern might also be considered.

Polarization

The polarization of an antenna or orientation of the radio wave is determined by the electric field or E-plane. The ionosphere changes the polarization of signals unpredictably, so for signals which will be reflected by the ionosphere, polarization is not crucial. However, for line-of-sight communications, it can make a tremendous difference in signal quality to have the transmitter and receiver using the same polarization. Polarizations commonly considered are linear, such as vertical and horizontal, and circular, which is divided into right-hand and left-hand circular.

Transmission and receiving

All of these parameters are expressed in terms of a transmission antenna, but are identically applicable to a receiving antenna, due to reciprocity. Impedance, however, is not applied in an obvious way; for impedance, the impedance at the load (where the power is consumed) is most critical. For a transmitting antenna, this is the antenna itself. For a receiving antenna, this is at the (radio) receiver rather than at the antenna. Antennas used for transmission have a maximum power rating, beyond which heating, arcing or sparking may occur in the components, which may cause them to be damaged or destroyed. Raising this maximum power rating usually requires larger and heavier components, which may require larger and heavier supporting structures. Of course, this is only a concern for transmitting antennas; the power received by an antenna rarely exceeds the microwatt range. If an antenna is to be used for reception at very low frequencies (below about ten megahertz), its noise rejection capabilities become important. At such frequencies, signals are reflected very effectively by the ionosphere; however, at these frequencies there are many forms of natural radio noise, including the noise produced by lightning. Successfully rejecting these forms of noise is an important antenna feature. For example, a small coil of wire with many turns is more able to reject such noise than a vertical antenna. However, the vertical will radiate much more effectively on transmit, where extraneous signals are not a concern.

Theoretical antenna types

- A dielectric resonator is a variation on the conventional antenna in which an insulator with a large dielectric constant is used to modify the electromagnetic field. It is claimed that the dielectric contains the antenna's near field and therefore prevents it from interfering with other nearby antennas or circuits, making it suitable for miniature equipment such as mobile phones.
- A feedhorn is an antenna system that handles the incoming waveform from the dish to the focal point. It usually comprises of a series of rings with decreasing radius in order to drive the signal to the polarizer.
- An isotropic radiator is an antenna that radiates equally in all directions. It is considered to be a point in space with no dimensions and no mass. Most antennas' gains are measured with reference to an isotropic radiator, and are rated in dBi (decibels with respect to an isotropic radiator). This antenna type is purely theoretical and is not achievable in real life.

Practical antenna models

There are many variations of antennas, but here are a few common models. More can be found in :Category:Radio frequency antenna types.
- The dipole antenna is simply two wires pointed in opposite directions arranged either horizontally or vertically, with one end of each wire connected to the radio and the other end hanging free in space. Variations of the dipole include the folded dipole and the whip antenna which is really just half of a dipole using a ground plane as the image of the second half. The dipole antenna is usually a multiple of a half wavelength long. For this reason, the dipole antenna is sometimes referred to as the half-wave antennna. Generally, the dipole is considered to be omnidirectional in the plane perpendicular to the axis of the antenna, but it has deep nulls in the directions of the axis. The popular J-pole antenna is a variation of the half dipole with a built in quarter wave transmission line impedance matching section.
- The yagi-Uda antenna is a directional variation of the dipole with parasitic elements added with functionality similar to adding a reflector and lenses (directors) to focus a filament lightbulb.
- The groundplane antenna takes the form of a driven vertical element 1/4 wave long in the center of a grounded plane 1/2 wave in diameter. The end of the vertical element nearest the ground plane is connected to the radio, and the far end is in hanging in free space. The ground plane can take the form of the natural Earth surface, or a network of wires and ground rods, or a solid metal sheet, or four wires arranged as two crossed dipoles and centrally connected to ground.
- The (large) loop antenna is similar to a dipole, except that the ends of the dipole are connected to form a circle, triangle (delta loop antenna) or square. Typically a loop is a multiple of a half or full wavelength in circumference. A circular loop gets higher gain (about 10%) than the other forms of large loop antenna, as gain of this antenna is directly proportional to the area enclosed by the loop, but circles can be hard to support in a flexible wire, making squares and triangles much more popular. Large loop antennas are more immune to localized noise partly due to lack of a need for a groundplane. The large loop has its strongest signal in the plane of the loop, and nulls in the axis perpendicular to the plane of the loop.
- The small loop antenna, also called the magnetic loop antenna is a loop of wire (in other words, both ends of the wire connect to the radio) less than a wavelength in circumference. Typically, the circumference is less than 1/10 for a receiving loop, and less than 1/4 for a transmitting loop. Unlike nearly all other antennas in this list, this antenna detects the magnetic component of the electromagnetic wave. As such, it is less sensitive to near field electric noise when properly shielded. The receiving aperture can be greatly increased by bringing the loop into resonance with a tuning capacitor. Due to the small size of the loop, the radiation pattern is 90 degrees from that of the large loop. The radiation pattern is perpendicular to the plane of the loop, with sharp nulls in the plane of the loop.
- The electrically short antenna is an open-end wire far less than 1/4 wavelength in length - in other words only one end of the antenna is connected to the radio, and the other end is hanging free in space. Unlike nearly all other antennas in this list, this antenna detects only the electric field of the wave instead of the electromagnetic field - think of the free end of the wire as measuring the voltage of that point in space, as opposed to measuring both the voltage and the magnetic field. Its receiving aperture can be greatly increased by increasing the voltage; by adding an inductor or resonator tuned to resonance with the signals of interest. Electrically short antennas are typically used where operating wavelength is large and space is limited, e.g. for mobile transceivers operating at long wavelengths.
- The microstrip antenna consists of a patch of metalization on a ground plane. These are low profile, light weight antennas, most suitable for aerospace and mobile applications. Because of their low power handling capability, these antennas can be used in low-power transmitting and receiving applications. Microstrip antennas are the most commonly used antennas in mobile communications, satellite links, W-LAN and so on because circuit functions can be directly integrated to the microstrip antenna to form compact tranceivers and spatial power combiners.
- The quad antenna is an array of square loops that vary in size. The quad is related to the loop in exactly the same way the yagi is related to the dipole. Typically, the quad needs fewer elements to get the same gain as a yagi. Variations of the quad include the delta loop antenna which uses a triangle instead of a square, requiring fewer supports for large wavelength antennas.
- The random wire antenna is simply a very long (greater than one wavelength) wire with one end connected to the radio and the other in free space, arranged in any way most convenient for the space available. Folding will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more than the folding typically hurts.) Typically, a random wire antenna will also require an antenna tuner, as it might have a random impedance that varies nonlinearly with frequency.
- The Beverage antenna is a form of directional long-wire antenna which uses a resistive termination at one end and feed from the other.
- The helical antenna is a directional antenna suited for receiving signals that are either circular polarized or randomly polarized. These are usually used with satellites, and are frequently used for the driven element on a dish.
- The Phased array antenna is a group of independently fed active elements in which the relative phases of the respective signals feeding the elements are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. In plain language, this is a directional antenna that can be aimed without moving any parts.
- Synthetic aperture radar uses a series of observations separated in time and space to simulate a very large antenna. More generally, interferometry allows the combining of signals from several radio receivers or a single moving receiver.
- A trailing wire antenna is used by submarines when submerged. These antennas are designed to pick up transmissions in the low frequency (LF) and very low frequency (VLF) ranges.
- An evolved antenna refers to an antenna fully or substantially designed using a computer algorithm based on Darwinian evolution.

External links

- [http://hamradio.co.in/tcvr/antena.php Antenna] Antena for Ham / Amateur Radio
- [http://www.maxstream.net/helpdesk/article-27 dBi vs. dBd] How to measure antenna gain
- [http://www.radio-electronics.com/info/antennas/index.php Radio-Electronics.Com] Further information regarding antennas
- [http://www.dxzone.com/catalog/Antennas/ Antenna Plans] Over 400 amateur radio antenna plans and documents from [http://www.dxzone.com dxzone.com]
- [http://www.vias.org/simulations/simusoft_twoaerials.html Learning by Simulations] Interactive simulation of two coupled antennas
- [http://www.n0hr.com/total NØHR.com Best Ham Radio Links] Ham radio antenna sites sorted by band, design, and homebrew vs. commercial antenna products.
- Category:Amateur radio Category:Electrical components Category:Radio electronics ms:Antena ja:空中線

Electromagnetic radiation

Electromagnetic radiation is a propagating wave in space with electric and magnetic components. These components oscillate at right angles to each other and to the direction of propagation. The term electromagnetic radiation is also used as a synonym for electromagnetic waves in general, even if they are not radiating or travelling in free space. This sense includes, for example, light travelling through an optical fiber, or electrical energy travelling within a coaxial cable. Electromagnetic (EM) radiation carries energy and momentum which may be imparted when it interacts with matter.

Physics

Theory

Electromagnetic waves of much lower frequency than visible light were predicted by Maxwell's equations and subsequently discovered by Heinrich Hertz. Maxwell derived a wave form of the electric and magnetic equations which made explicit the wave nature of the electric and magnetic fields. These equations displayed the symmetry of the fields. According to the theory, a time-varying electric field generates a magnetic field and vice versa. Thus, an oscillating electric field creates an oscillating magnetic field, which in turn creates an oscillating electric field, and so on. By this means an EM wave is produced which propagates through space.

Properties

Electric and magnetic fields exhibit the property of superposition. This means that the field due to a particular particle or time-varying electric or magnetic field adds to the fields due to other causes. (As magnetic and electric fields are vector fields, this is the vector addition of all the individual electric and magnetic field vectors.) As a result, EM radiation is influenced by various phenomena such as refraction and diffraction. For example, a travelling EM wave incident on a particular arrangement of atoms induces oscillation in the atoms and thus causes them to emit their own EM waves (called wavelets). These emissions interfere with the impinging wave and alter its form. In refraction, a wave moving from one medium to another of a different density changes its speed and direction when it enters the new medium. The ratio of the refractive indices of the media determines the extent of refraction. Refraction is the mechanism by which light disperses into a spectrum when it is shone through a prism. The physics of electromagnetic radiation is electrodynamics, a subfield of electromagnetism. EM radiation exhibits both wave properties and particle properties at the same time (see wave-particle duality). These characteristics are mutually exclusive and appear separately in different circumstances: the wave characteristics appear when EM radation is measured over relatively larger timescales and over larger distances, and the particle characteristics are evident when measuring smaller distances and timescales. EM radiation's behaviours as a wave and as a stream of particles have been confirmed by a large number of experiments.

Wave model

An important aspect of the wave nature of light is frequency. The frequency of a wave is its rate of oscillation and is measured in hertz, the SI unit of frequency, equal to one oscillation per second. Light usually comprises a spectrum of frequencies which sum to form the resultant wave. In addition, frequency affects properties like refraction, in which different frequencies undergo a different level of refraction. A wave has troughs and crests. The wavelength is the distance from crest to crest. Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom. Frequency has an inverse relationship to the concept of wavelength. When waves travel from one medium to another, their frequency remains exactly the same - only their wavelength and/or speed changes. Waves can also be described by their radiant energy. Interference is the superposition of two or more waves resulting in a new wave pattern. The way that these coincide causes different types of interference.

Particle model

In the particle model of EM radiation, EM radiation is quantized as particles called photons. Quantisation of light represents the discrete packets of energy which constitute the radiation. The frequency of the radiation determines the magnitude of the energy of the particles. Moreover, these particles are emitted and absorbed by charged particles, so photons act as transporters of energy. A photon absorbed by an atom excites an electron and elevates it to a higher energy level. If the energy is great enough, the electron is liberated from the atom in a process called ionization. Conversely, an electron which descends to a lower energy level in an atom emits a photon of light equal to the energy difference. The energy levels of electrons in atoms are discrete. Therefore, each element has its own characteristic frequencies. Together these effects explain the absorption spectra of light. The dark bands in the spectrum are due to the atoms in the intervening medium which absorb different frequencies of the light. The composition of the medium through which the light travels determines the nature of the absorption spectrum. For instance, in a distant star, dark bands in the light it emits are due to the atoms in the atmosphere of the star. These bands correspond to the allowed energy levels in the atoms. A similar phenomenon occurs for emission. As the electrons descend to lower energy levels, a spectrum which represents the jumps between the energy levels of the electrons is exhibited. This is manifested in the emission spectrum of nebulae.

Speed of propagation

Any electric charge which accelerates, or any changing magnetic field, produces electromagnetic radiation. Electromagnetic information about the charge travels at the speed of light. Accurate treatment thus incorporates a concept known as retarded time (as opposed to advanced time, which is unphysical in light of causality), which adds to the expressions for the electrodynamic electric field and magnetic field. These extra terms are responsible for electromagnetic radiation. When any wire (or other conducting object such as an antenna) conducts alternating current, electromagnetic radiation is propagated at the same frequency as the electric current. Depending on the circumstances, it may behave as a wave or as particles. As a wave, it is characterized by a velocity (the speed of light), wavelength, and frequency. When considered as particles, they are known as photons, and each has an energy related to the frequency of the wave given by Planck's relation E = hν, where E is the energy of the photon, h = 6.626 × 10^-34 J·s is Planck's constant, and ν is the frequency of the wave. One rule is always obeyed regardless of the circumstances. EM radiation in a vacuum always travels at the speed of light, relative to the observer, regardless of the observer's velocity. (This observation led to Albert Einstein's development of the theory of special relativity.)

Electromagnetic spectrum

Generally, EM radiation is classified by wavelength into electrical energy, radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays. The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. More in-depth information about the physical properties of objects, gases, or even stars can be obtained from this type of device. It is widely used in astrophysics. For example, many hydrogen atoms emit radio waves which have a wavelength of 21.12 cm.

Light

EM radiation with a wavelength between 400 nm and 700 nm is detected by the human eye and perceived as visible light. If radiation having a frequency in the visible region of the EM spectrum shines on an object, say, a bowl of fruit, this results in our visual perception of identifying information from the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this not-entirely-understood "psychophysical phenomenon," most humans perceive a bowl of fruit. In the vast majority of cases, however, the information carried by light is not directly apprehensible by human senses. Natural sources produce EM radiation across the spectrum; so, too, can human technology manipulate a broad range of wavelengths. Optical fiber transmits light which, although not suitable for direct viewing, can carry data. Those data can be translated into sound or an image. The coded form of such data is similar to that used with radio waves.

Radio waves

Radio waves carry information by varying amplitude and by varying frequency within a frequency band. When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the electrons of the conducting material. This effect (the skin effect) is used in antennas. EM radiation may also cause certain molecules to absorb energy and thus to heat up; this is exploited in microwave ovens.

Derivation

Electromagnetic waves as a general phenomenon were predicted by the classical laws of electricity and magnetism, known as Maxwell's equations. If you inspect Maxwell's equations without sources (charges or currents) then you will find that, along with the possibility of nothing happening, the theory will also admit nontrivial solutions of changing electric and magnetic fields. (For symbol definitions see magnetic field.) :

\nabla \cdot \mathbf = 0

\nabla \times \mathbf = -\frac \mathbf

\nabla \cdot \mathbf = 0

\nabla \times \mathbf = \mu_0 \epsilon_0 \frac \mathbf

\mathbf=\mathbf=\mathbf

is a solution, but there might be other solutions as well. Let us employ a useful identity from vector calculus. :

\nabla \times \left( \nabla \times \mathbf \right) = \nabla \left( \nabla \cdot \mathbf \right) - \nabla^2 \mathbf

Where

\mathbf

can be any vector function. Taking the curl of the curl equations and applying the identity, we get the following. :

\nabla^2 \mathbf = \mu_0 \epsilon_0 \frac \mathbf

\nabla^2 \mathbf = \mu_0 \epsilon_0 \frac \mathbf

These types of equations are identified as linear wave equations with wave speed

\frac

. Amazingly, this speed happens to be exactly the speed of light! Maxwell's equations have unified the permittivity of free space

\epsilon_0

, the permeability of free space

\mu_0

, and the speed of light itself:

c = \frac

. Before this derivation it was not known that there was such a strong relationship between light and electricity and magnetism. But these are only two equations and we started with four, so there is still more information pertaining to these waves hidden within Maxwell's equations. Let's consider a generic vector wave for the electric field. :

\mathbf = \mathbf_0 f\left( \hat \cdot \mathbf - c t \right)

Here

\mathbf_0

is the constant amplitude,

f

is any second differentiable function,

\hat

is a unit vector in the direction of propagation, and

\hat

is a position vector. We observe that

f\left( \hat \cdot \mathbf - c t \right)

is a generic solution to the wave equation. In other words :

\nabla^2 f\left( \hat \cdot \mathbf - c t \right) = \frac \frac f\left( \hat \cdot \mathbf - c t \right)

, for a generic wave traveling in the

\hat

direction. The proof of this is trivial. This form will satisfy the wave equation, but will it satisfy all of Maxwell's equations, and with what corresponding magnetic field? :

\nabla \cdot \mathbf = \hat \cdot \mathbf_0 f'\left( \hat \cdot \mathbf - c t \right) = 0

\mathbf \cdot \hat = 0

The first of Maxell's equations implies that electric field is orthogonal to the direction the wave propagates. :

\nabla \times \mathbf = \hat \times \mathbf_0 f'\left( \hat \cdot \mathbf - c t \right) = -\frac \mathbf

\mathbf = \frac \hat \times \mathbf

The second of Maxwell's equations yields the magnetic field. The remaining equations will be satisfied by this choice of

\mathbf,\mathbf

. Not only are the electric and magnetic field waves traveling at the speed of light, but they have a special restricted orientation and proportional magnitudes,

\mathbf_0 = c \mathbf_0

. The electric field, magnetic field, and direction of wave propagation are all orthogonal and the wave propagates in the same direction as

\mathbf \times \mathbf

. Visualizing yourself as an electromagnetic wave traveling forward, the electric field might be oscillating up and down, while the magnetic field oscillates right and left; but you can rotate this picture around with the electric field oscillating right and left and the magnetic field oscillating down and up. This is a different solution that is traveling in the same direction. This arbitrariness in the orientation, with respect to propagation direction, is known as polarization.

References

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-
-

External links

; General
- [http://www.sengpielaudio.com/calculator-wavelength.htm Conversion of frequency to wavelength and back - electromagnetic, radio and sound waves]
- [http://www.scienceofspectroscopy.info The Science of Spectroscopy - a learning tool for spectroscopy] ; Patents
- Greenleaf Whittier Pickard - - Intelligence intercommunication by magnetic wave component ko:전자기파 ja:電磁波

Signal

Signaling or signal may mean:

Scientific concepts

- A signal (information theory) is a flow of information;
- A signal (computing) is an event (a message, a data structure) transmitted between computational processes;
- Signals and slots is a software pattern implementation closely related to the Observer pattern.
- Signaling (economics) is a subtle means of conveying information;
- Signaling (telecommunication) is a part of some communication protocols;
- Signals (biology) are electrochemical activity in an organism;

Proper names

- Signal (band), a Bulgarian rock band;
- Signal (magazine)
- Signal paper, a discontinued brand of carbonless copy paper;
- Signal programming language;
- Signal (toothpaste);
- Signal 1 and Signal 2 radio stations;
- Signal (subscription service), service of Data Broadcasting Corporation that provided real-time stock quotes;

Other

- A traffic signal or railway signal for the control of traffic.
- In courting or mating rituals of animals, signals are instinctively sent to communicate readiness or other relevant information.
- In a partnership card game, a player's choice of card to play at a particular time, which gives information to a partner (see Signal (bridge)).
- In baseball and other sports, strategy is communicated between players and coaches through hand signals and other types of signals.
- Distress signal
- Smoke signal
- Message ja:信号

Radio

Radio is the wireless transmission of signals, by modulation of electromagnetic waves with frequencies below those of light.

Radio waves

Radio waves are a form of electromagnetic radiation, created whenever a charged object (e.g. an electron) accelerates with a frequency that lies in the radio frequency (RF) portion of the electromagnetic spectrum. In radio, this acceleration is caused by an alternating current in an antenna. Radio frequencies occupy the range from a few tens of hertz to a few hundred gigahertz. Other types of electromagnetic radiation, with frequencies above the RF range are infrared, visible light, ultraviolet, X-rays and gamma rays. Since the energy of an individual photon of radio frequency is too low to remove an electron from an atom, radio waves are classified as non-ionizing radiation.

non-ionizing radiation
Radio transmission diagram and electromagnetic waves.

Electromagnetic radiation travels (propagates) by means of oscillating electromagnetic fields that pass through the air and the vacuum of space equally well, and does not require a medium of transport (such as the aether). When radio waves pass an electrical conductor, the oscillating electric or magnetic field (depending on the shape of the conductor) induces an alternating current and voltage in the conductor. This can be transformed into audio or other signals that carry information. Although the word 'radio' is used to describe this phenomenon, the transmissions which we know as television, radio, radar, and cell phone are all classed as radio frequency emissions.

History and invention

The identity of the original inventor of radio, at the time called wireless telegraphy, is contentious. The controversy over who invented the radio, with the benefit of hindsight, can be broken down as follows: :Q1: Who invented 'wireless transmission of data' (spark-gap radio)? :A1: Alexander Popov, Guglielmo Marconi, Nikola Tesla (possibly in that order). :Q2: Who invented amplitude-modulated (AM) radio, so that more than one station can send signals (as opposed to spark-gap radio, where one transmitter covers the entire bandwidth of the spectrum)? :A2: Reginald Fessenden _{[http://www.invent.org/hall_of_fame/59.html]} and Lee de Forest. :Q3: Who invented frequency-modulated (FM) radio, so that an audio signal can avoid "static," that is, interference from electrical equipment and atmospherics? :A3: Edwin H. Armstrong and Lee de Forest. Early radios ran the entire power of the transmitter through a carbon microphone. While some early radios used some type of amplification through electric current or battery, through the mid 1920s the most common type of receiver was the crystal set. In the 1920s, amplifying vacuum tubes revolutionized both radio receivers and transmitters.

Discovery and development

The theoretical basis of the propagation of electromagnetic waves was first described in 1873 by James Clerk Maxwell in his paper to the Royal Society A dynamical theory of the electromagnetic field, which followed his work between 1861 and 1865. In 1878 David E. Hughes was the first to transmit and receive radio waves when he noticed that his induction balance caused noise in the receiver of his homemade telephone. He demonstrated his discovery to the Royal Society in 1880 but was told it was merely induction. It was Heinrich Rudolf Hertz who, between 1886 and 1888, first validated Maxwell's theory through experiment, demonstrating that radio radiation had all the properties of waves (now called Hertzian waves), and discovering that the electromagnetic equations could be reformulated into a partial differential equation called the wave equation. William Henry Ward was issued on April 30, 1872. Mahlon Loomis was issued on July 30, 1872. Landell de Moura, a Brazilian priest and scientist, conducted experiments after 1893 (but at least by 1894). He did not publicize his achievement until 1900. Claims have been made that Nathan Stubblefield invented radio before either Tesla or Marconi, but his device seems to have worked by induction transmission rather than radio transmission.

Wireless age

In 1893 in St. Louis, Missouri, Tesla made devices for his experiments with the electricity. Addressing the Franklin Institute in Philadelphia and the National Electric Light Association, he described and demonstrated in detail the principles of their work. [http://www.ieee-virtual-museum.org/collection/people.php?taid=&id=1234597&lid=1] They contained all the elements that were later incorporated into radio systems before the development of the vacuum tube. He initially experimented with magnetic receivers, unlike the coherers used by Marconi and other early experimenters. [http://www.teslasociety.com/teslarec.pdf]. Tesla is usually considered the first to apply the mechanism of electrical conduction to wireless practices. On 19 August 1894, British physicist Sir Oliver Lodge demonstrated the reception of Morse code signalling using radio waves using a detecting device called a coherer, a tube filled with iron filings which had been invented by Temistocle Calzecchi-Onesti at Fermo in Italy in 1884. Edouard Branly of France and Popov of Russia later produced improved versions of the coherer. Alexander Popov, who was the first to develop a practical communication system based on the coherer, is usually considered to have been the inventor of radio. In 1894 he built a coherer and presented it to the Russian Physical and Chemical Society on May 7 1895 [http://www.ieee.org/organizations/history_center/milestones_photos/popov.html]. In March 1896, he effected transmission of radio waves between different campus buildings in Saint Petersburg, but didn't care to apply for a patent. The Indian physicist, Jagdish Chandra Bose, during the years 1894-1900, performed pioneering research on radio waves and created waves as short as 5 mm. [http://www.ieee-virtual-museum.org/collection/people.php?taid=&id=1234735&lid=1] In November 1894 J.C. Bose ignited gunpowder and rang a bell at a distance using electromagnetic waves, confirming that communication signals can be sent without using wires. But he was not interested in patenting his work too. In 1896 Marconi was awarded what is sometimes recognised as the world's first patent for radio with British Patent 12039, Improvements in transmitting electrical impulses and signals and in apparatus there-for. In 1897 he established the world's first radio station on the Isle of Wight, England. The same year in the U.S., some key developments in radio's early history were created and patented by Tesla. The U.S. Patent Office reversed its decision in 1904, awarding Marconi a patent for the invention of radio, possibly influenced by Marconi's financial backers in the States, who included Thomas Edison and Andrew Carnegie. Some believe this was made for financial reasons, allowing the U.S. government to avoid having to pay the royalties that were being claimed by Tesla for use of his patents. In 1909, Marconi, with Karl Ferdinand Braun, was also awarded the Nobel Prize in Physics for "contributions to the development of wireless telegraphy". However, Tesla's patent (number 645576) was reinstated in 1943 by the U.S. Supreme Court, shortly after his death. This decision was based on the fact that prior art existed before the establishment of Marconi's patent. Some believe the decision was also made for financial reasons, to allow the U.S. government to avoid having to pay damages that were being claimed by the Marconi Company for use of its patents during World War I.

"Wireless" factories and vacuum tubes

Marconi opened the world's first "wireless" factory in Hall Street, Chelmsford, England in 1898, employing around 50 people. Around 1900, Tesla opened the Wardenclyffe Tower facility and advertised services. By 1903, the tower structure neared completion. Various theories exist on how Tesla intended to achieve the goals of this wireless system (reportedly, a 200 kW system). Tesla claimed that Wardenclyffe, as part of a World System of transmitters, would have allowed secure multichannel transceiving of information, universal navigation, time synchronization, and a global location system. The next great invention was the vacuum tube detector, invented by a team of Westinghouse engineers. On Christmas Eve, 1906, Reginald Fessenden (using his heterodyne principle) transmitted the first radio audio broadcast in history from Brant Rock, Massachusetts. Ships at sea heard a broadcast that included Fessenden playing O Holy Night on the violin and reading a passage from the Bible. The world's first radio news program was broadcast August 31, 1920 by station 8MK in Detroit, Michigan. The world's first regular wireless broadcasts for entertainment commenced in 1922 from the Marconi Research Centre at Writtle near Chelmsford, England.

20th century

Developments in the early 20th century (1900-1959):
- Aircraft used commercial AM radio stations for navigation. This continued through the early 1960s when VOR systems finally became widespread (though AM stations are still marked on U.S. aviation charts).
- In the early 1930s, single sideband and frequency modulation were invented by amateur radio operators. By the end of the decade, they were established commercial modes.
- Radio was used to transmit pictures visible as television as early as the 1920s. Standard analog transmissions started in North America and Europe in the 1940s.
- In 1954, Regency introduced a pocket transistor radio, the TR-1, powered by a "standard 22.5V Battery". Developments in the latter half of the 20th century (1960-1999):
- In 1960, Sony introduced their first transistorized radio, small enough to fit in a vest pocket, and able to be powered by a small battery. It was durable, because there were no tubes to burn out. Over the next twenty years, transistors displaced tubes almost completely except for very high power, or very high frequency, uses.
- In 1963 color television was commercially transmitted, and the first (radio) communication satellite, TELSTAR, was launched.
- In the late 1960s, the U.S. long-distance telephone network began to convert to a digital network, employing digital radios for many of its links.
- In the 1970s, LORAN became the premier radio navigation system. Soon, the U.S. Navy experimented with satellite navigation, culminating in the invention and launch of the GPS constellation in 1987.
- In the early 1990s, amateur radio experimenters began to use personal computers with audio cards to process radio signals. In 1994, the U.S. Army and DARPA launched an aggressive, successful project to construct a software radio that could become a different radio on the fly by changing software.
- Digital transmissions began to be applied to broadcasting in the late 1990s.

Uses of radio

software radio software radio Many of radio's early uses were maritime, for sending telegraphic messages using Morse code between ships and land. One of the earliest users included the Japanese Navy scouting the Russian fleet during the Battle of Tsushima in 1905. One of the most memorable uses of marine telegraphy was during the sinking of the RMS Titanic in 1912, including communications between operators on the sinking ship and nearby vessels, and communications to shore stations listing the survivors. Radio was used to pass on orders and communications between armies and navies on both sides in World War I; Germany used radio communications for diplomatic messages once its submarine cables were cut by the British. The United States passed on President Woodrow Wilson's Fourteen Points to Germany via radio during the war. Broadcasting began to become feasible in the 1920s, with the widespread introduction of radio receivers, particularly in Europe and the United States. Besides broadcasting, point-to-point broadcasting, including telephone messages and relays of radio programs, became widespread in the 1920s and 1930s. Another use of radio in the pre-war years was the development of detecting and locating aircraft and ships by the use of radar (RAdio Detecting And Ranging). Today, radio takes many forms, including wireless networks, mobile communications of all types, as well as radio broadcasting. Read more about radio's history. Before the advent of television, commercial radio broadcasts included not only news and music, but dramas, comedies, variety shows, and many other forms of entertainment. Radio was unique among dramatic presentation that it used only sound. For more, see radio programming. There are a number of uses of radio:

Audio

- AM broadcast radio sends music and voice in the Medium Frequency (MF—0.300 MHz to 3 MHz) radio spectrum. AM radio uses amplitude modulation, in which louder sounds at the microphone causes wider fluctuations in the transmitter power while the transmitter frequency remains unchanged. Transmissions are affected by static because lightning and other sources of radio add their radio waves to the ones from the transmitter.
- FM broadcast radio sends music and voice, with higher fidelity than AM radio. In frequency modulation, louder sounds at the microphone cause the transmitter frequency to fluctuate farther, the transmitter power stays constant. FM is transmitted in the Very High Frequency (VHF—30 MHz to 300 MHz) radio spectrum. FM requires more radio frequency space than AM and there are more frequencies available at higher frequencies, so there can be more stations, each sending more information. Another effect is that shorter VHF radio waves act more like light, travelling in straight lines, hence the reception range is generally limited to about 50-100 miles. During unusual upper atmospheric conditions, FM signals are occasionally reflected back towards the Earth by the ionosphere, resulting in Long distance FM reception. FM receivers are subject to the capture effect, which causes the radio to only receive the strongest signal when multiple signals appear on the same frequency. FM receivers are relatively immune to lightning and spark interference.
- FM Subcarrier services are secondary signals transmitted "piggyback" along with the main program. Special receivers are required to utilize these services. Analog channels may contain alternative programming, such as reading services for the blind, background music or stereo sound signals. In some extremely crowded metropolitan areas, the subchannel program might be an alternate foreign language radio program for various ethnic groups. Subcarriers can also transmit digital data, such as station identification, the current song's name, web addresses, or stock quotes. In some countries, FM radios automatically retune themselves to the same channel in a different district by using sub-bands.
- Aviation voice radios use VHF AM. AM is used so that multiple stations on the same channel can be received. (Use of FM would result in stronger stations blocking out reception of weaker stations due to FM's capture effect). Aircraft fly high enough that their transmitters can be received hundreds of miles (kilometres) away, even though they are using VHF.
- Marine voice radios can use AM in the shortwave High Frequency (HF—3 MHz to 30 MHz) radio spectrum for very long ranges or narrowband FM in the VHF spectrum for much shorter ranges.
- Government, police, fire and commercial voice services use narrowband FM on special frequencies. Fidelity is sacrificed to use a smaller range of radio frequencies, usually five kilohertz of deviation (5 thousand cycles per second), rather than the 75 used by FM broadcasts and 25 used by TV sound.
- Civil and military HF (high frequency) voice services use shortwave radio to contact ships at sea, aircraft and isolated settlements. Most use single sideband voice (SSB), which uses less bandwidth than AM. SSB sounds like ducks quacking on an AM radio. Viewed as a graph of frequency versus power, an AM signal shows power where the frequencies of the voice add and subtract with the main radio frequency. SSB cuts the bandwidth in half by suppressing the carrier and (usually) lower sideband. This also makes the transmitter about three times more powerful, because it doesn't need to transmit the unused carrier and sideband.
- TETRA, Terrestrial Trunked Radio is a digital cell phone system for military, police and ambulances.
- Commercial services such as XM and Sirius offer digital Satellite radio.

Telephony

- Cell phones transmit to a local cell transmitter/receiver site, which connects to the public service telephone network through an optic fiber or microwave radio. When the phone leaves the cell radio's area, the central computer switches the phone to a new cell. Cell phones originally used FM, but now most use various digital encodings.
- Satellite phones come in two types: INMARSAT and Iridium. Both types provide world-wide coverage. INMARSAT uses geosynchronous satellites, with aimed high-gain antennas on the vehicles. Iridium provides cell phones, except the cells are satellites in orbit.

Video

- Television sends the picture as AM, and the sound as FM, on the same radio signal.
- Digital television encodes three bits as eight strengths of AM signal. The bits are sent out-of-order to reduce the effect of bursts of radio noise. A Reed-Solomon error correction code lets the receiver detect and correct errors in the data. Although any data could be sent, the standard is to use MPEG-2 for video, and five CD-quality (44.1 kHz) audio channels (center, left, right, left-back and right back). With all this, it takes only half the bandwidth of an analog TV signal because the video data is compressed.

Navigation

- All satellite navigation systems use satellites with precision clocks. The satellite transmits its position, and the time of the transmission. The receiver listens to four satellites, and can figure its position as being on a line that is tangent to a spherical shell around each satellite, determined by the time-of-flight of the radio signals from the satellite. A computer in the receiver does the math.
- Loran systems also used time-of-flight radio signals, but from radio stations on the ground.
- VOR systems (used by aircraft), have a antenna array that transmits two signals simultaneously. A directional signal rotates like a lighthouse at a fixed rate. When the directional signal is facing north, an omnidirectional signal pulses. By measuring the difference in phase of these two signals, an aircraft can determine its bearing from the station. An aircraft can get readings from two VORs, and locate its position at the intersection of the two beams.
- Radio direction-finding is the oldest form of radio navigation. Before 1960 navigators used movable loop antennas to locate commercial AM stations near cities. In some cases they used marine radiolocation beacons, which share a range of frequencies just above AM radio with amateur radio operators.

Radar

- Radar detects things at a distance by bouncing radio waves off them. The delay caused by the echo measures the distance. The direction of the beam determines the direction of the reflection. The polarization and frequency of the return can sense the type of surface.
- Navigational radars scan a wide area two to four times per minute. They use very short waves that reflect from earth and stone. They are common on commercial ships and long-distance commercial aircraft
- General purpose radars generally use navigational radar frequencies, but modulate and polarize the pulse so the receiver can determine the type of surface of the reflector. The best general-purpose radars distinguish the rain of heavy storms, as well as land and vehicles. Some can superimpose sonar data and map data from GPS position.
- Search radars scan a wide area with pulses of short radio waves. They usually scan the area two to four times a minute. Sometimes search radars use the doppler effect to separate moving vehicles from clutter.
- Targeting radars use the same principle as search radar but scan a much smaller area far more often, usually several times a second or more.
- Weather radars resemble search radars, but use radio waves with circular polarization and a wavelength to reflect from water droplets. Some weather radar use the doppler to measure wind speeds.

Emergency services

- emergency position-indicating rescue beacons (EPIRBs), emergency locating transmitters or personal locator beacons are small radio transmitters that satellites can use to locate a person or vehicle needing rescue. Their purpose is to help rescue people in the first day, when survival is most likely. There are several types, with widely-varying performance.

Data (digital radio)

- The oldest form of digital broadcast was spark gap telegraphy, used by pioneers such as Marconi. By pressing the key, the operator could send messages in Morse code by energizing a rotating commutating spark gap. The rotating commutator produced a tone in the receiver, where a simple spark gap would produce a hiss, indistinguishable from static. Spark gap transmitters are now illegal, because their transmissions span several hundred megahertz. This is very wasteful of both radio frequencies and power.
- The next advance was continuous wave telegraphy, or CW, in which a pure radio frequency, produced by a vacuum tube electronic oscillator was switched on and off by a key. A receiver with a local oscillator would "heterodyne" with the pure radio frequency, creating a whistle-like audio tone. CW uses less than 100Hz of bandwidth. CW is still used, these days primarily by amateur radio operators (hams). Strictly, on-off keying of a carrier should be known as "Interrupted Continuous Wave" or ICW.
- Radio teletypes usually operate on short-wave (HF) and are much loved by the military because they create written information without a skilled operator. They send a bit as one of two tones. Groups of five or seven bits become a character printed by a teletype. From about 1925 to 1975, radio teletype was how most commercial messages were sent to less developed countries. These are still used by the military and weather services.
- Aircraft use a 1200 Baud radioteletype service over VHF to send their ID, altitude and position, and get gate and connecting-flight data.
- Microwave dishes on satellites, telephone exchanges and TV stations usually use quadrature amplitude modulation (QAM). QAM sends data by changing both the phase and the amplitude of the radio signal. Engineers like QAM because it packs the most bits into a radio signal. Usually the bits are sent in "frames" that repeat. A special bit pattern is used to locate the beginning of a frame.
- Systems that need reliability, or that share their frequency with other services may use "corrected orthogonal frequency-division multiplexing" or COFDM. COFDM breaks a digital signal into as many as several hundred slower subchannels. The digital signal is often sent as QAM on the subchannels. Modern COFDM systems use a small computer to make and decode the signal with digital signal processing, which is more flexible and far less expensive than older systems that implemented separate electronic channels. COFDM resists fading and ghosting because the narrow-channel QAM signals can be sent slowly. An adaptive system, or one that sends error-correction codes can also resist interference, because most interference can affect only a few of the QAM channels. COFDM is used for WiFi, some cell phones, Digital Radio Mondiale, Eureka 147, and many other local area network, digital TV and radio standards.
- Most new radio systems are digital, see also:Digital TV, Satellite Radio, Digital Audio Broadcasting.

Heating

Radio-frequency energy generated for heating of objects is generally not intended to radiate outside of the generating equipment, to prevent interferance with other radio signals.
- Microwave ovens use intense radio waves to heat food. (Note: It is a common misconception that the radio waves are tuned to the resonant frequency of water molecules. The microwave frequencies used are actually about a factor of 10 below the resonant frequency.)
- Diathermy equipment is used in surgery for sealing of blood vessels.
- Induction furnaces are used for melting metal for casting.

Mechanical Force

- Tractor beams: Radio waves exert small electrostatic and magnetic forces. These are enough to perform station-keeping in microgravity environments.
- Conceptually, Spacecraft propulsion: Radiation pressure from intense radio waves has been proposed as a propulsion method for an interstellar probe called Starwisp. Since the waves are long, the probe could be a very light-weight metal mesh, and thus achieve higher accelerations than if it were a solar sail.

Other

solar sail
- Amateur radio is a hobby where enthusiasts who purchase or build their own equipment and use radio for their own enjoyment. They may also provide an emergency and public-service radio service. This has been of great use, saving lives in many instances. Radio amateurs are able to use frequencies in a large number of narrow bands throughout the radio spectrum. Radio amateurs use all forms of encoding, including obsolete and experimental ones. Several forms of radio were pioneered by radio amateurs and later became commercially important, including FM, single-sideband AM, digital packet radio and satellite repeaters.
- Personal radio services such as Citizens' Band Radio, Family Radio Service, Multi-Use Radio Service and others exist in North America to provide simple, (usually) short range communication for individuals and small groups, without the overhead of licensing. Similar services exist in other parts of the world.
- Wireless energy transfer: A number of schemes have been proposed that transmit power using microwaves, and the technique has been demonstrated. (See Microwave power transmission). These schemes include, for example, solar power stations in orbit beaming energy down to terrestrial users.
- Radio remote control: Use of radio waves to transmit control data to a remote object as in some early forms of guided missile, some early TV remotes and a range of model boats, cars and aeroplanes. Large industrial remote-controlled equipment such as cranes and switching locomotives now usually use digital radio techniques to ensure safety and reliability.

Radio transmitter

History

Electromagnetic concepts

Cooling of final stages

Protection equipment

Building

Legal and Regulatory Aspects

Planning

Transmitters in Culture

Records

Broadcasting

See also

External links

Electronics

Electronic devices today

CAD/CAM of electronic circuits

Electronic systems

Electronic test equipment

Electronic components

Analog circuits

Digital circuits

Mixed-signal circuits

Heat dissipation

Noise

Electronics theory

See also

External links

Tutorials and projects

Some other good sites

Device

Antenna (electronics)

Overview

Antenna effectiveness

Resonant frequency

Impedance

Gain

Efficiency

Bandwidth

Polarization

Transmission and receiving

Theoretical antenna types

Practical antenna models

See also

External links

Electromagnetic radiation

Physics

Theory

Properties

Wave model

Particle model

Speed of propagation

Electromagnetic spectrum

Light

Radio waves

Derivation

See also

References

External links

Signal

Scientific concepts

Proper names

Other

Radio

Radio waves

History and invention

Discovery and development

Wireless age

"Wireless" factories and vacuum tubes

20th century

Uses of radio

Audio

Telephony

Video

Navigation

Radar

Emergency services

Data (digital radio)

Heating

Mechanical Force

Other