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A radio
transmitter In electronics and telecommunications, a radio transmitter or just transmitter (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna (radio), antenna with the purpose of sig ...
or receiver is connected to an antenna which emits or receives the
radio wave Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths g ...
s. The antenna feed system or antenna feed is the
cable Cable may refer to: Mechanical * Nautical cable, an assembly of three or more ropes woven against the weave of the ropes, rendering it virtually waterproof * Wire rope, a type of rope that consists of several strands of metal wire laid into a hel ...
or conductor, and other associated equipment, which connects the transmitter or receiver with the antenna and makes the two devices compatible. In a radio transmitter, the transmitter generates an
alternating current Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Alternating current is the form in w ...
of
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...
, and the feed system feeds the current to the antenna, which converts the power in the current to radio waves. In a radio receiver, the incoming radio waves excite tiny alternating currents in the antenna, and the feed system delivers this current to the receiver, which processes the signal. To transfer
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...
current efficiently, the '' feedline'' connecting the transmitter or receiver to the antenna must be a special type of cable called
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
. At
microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
frequencies, waveguide is often used, which is a hollow metal pipe carrying radio waves. In a parabolic (dish) antenna the feed is usually also defined to include the ''feed antenna ( feed horn)'' which emits or receives the radio waves. Particularly in transmitters, the feed system is a critical component which impedance matches the antenna, feedline, and transmitter. To accomplish this, the feed system may also include circuits called '' antenna tuning units'' or '' matching networks'' between the antenna and feedline and the feedline and transmitter. On an antenna the ''feed point'' is the point on the driven antenna element at which the feedline is connected.


Components

In a transmitter, the antenna feed is considered to be all components between the transmitter's final amplifier and the antenna's feedpoint. In a receiver, it is all components between the antenna and the receiver's input terminals. In some cases such as parabolic dishes it is also defined to include the feed antenna or feed horn. In some radios the antenna is attached directly to the transmitter or receiver, such as the whip antennas mounted on walkie talkies and portable
FM radio FM broadcasting is a method of radio broadcasting that uses frequency modulation (FM) of the radio broadcast carrier wave. Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to transmit high fidelity, high-f ...
s, the sleeve
dipole antenna In radio and telecommunications a dipole antenna or doublet is one of the two simplest and most widely used antenna types, types of antenna; the other is the monopole antenna, monopole. The dipole is any one of a class of antennas producin ...
s of wireless routers, and the PIFA antennas inside cellphones. In this case the feed system just consists of an
impedance matching In electrical engineering, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or ...
circuit (if needed) between the antenna and transmitter or receiver, which matches the impedance of the antenna to the radio. In other cases the antenna is located separately from the transmitter or receiver, such as broadcast television antennas and satellite dishes mounted on the roofs of residences, the sector antenna on cell towers of cellular base stations, the rotating radar antennas at airports, and the antenna towers of
radio Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3  hertz (Hz) and 300  gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...
and
television station A television station is a set of equipment managed by a business, organisation or other entity such as an amateur television (ATV) operator, that transmits video content and audio content via radio waves directly from a transmitter on the earth's s ...
s. In this case the antenna is connected to the transmitter or receiver with a cable called a feedline. To carry the
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...
(RF) current efficiently, the feedline is made of specialized cable called
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
. The advantage of transmission line is that it has a uniform
characteristic impedance The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a wave travelling in one direction along the line in the absence of reflections in th ...
to avoid abrupt impedance steps which cause the radio energy to be reflected backward down the line. The main types of transmission line are parallel wire line ( Twin lead),
coaxial cable Coaxial cable, or coax (pronounced ), is a type of electrical cable consisting of an inner Electrical conductor, conductor surrounded by a concentric conducting Electromagnetic shielding, shield, with the two separated by a dielectric (Insulat ...
, and for microwaves waveguide.


Feed line

In a radio antenna, the feed line (feedline), or feeder, is the cable or other
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
that connects the antenna with the radio
transmitter In electronics and telecommunications, a radio transmitter or just transmitter (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna (radio), antenna with the purpose of sig ...
or receiver. In a transmitting antenna, it feeds the
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...
(RF) current from the transmitter to the antenna, where the energy in the current is radiated as
radio wave Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths g ...
s. In a receiving antenna it transfers the tiny RF voltage induced in the antenna by the radio wave to the receiver. In order to carry
RF current Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the up ...
efficiently, feed lines are made of specialized types of cable called
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
. The most widely used types of feed line are
coaxial cable Coaxial cable, or coax (pronounced ), is a type of electrical cable consisting of an inner Electrical conductor, conductor surrounded by a concentric conducting Electromagnetic shielding, shield, with the two separated by a dielectric (Insulat ...
, twin-lead, ladder line, and at
microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
frequencies, waveguide. Particularly with a transmitting antenna, the feed line is a critical component that must be adjusted to work correctly with the antenna and transmitter. Each type of transmission line has a specific
characteristic impedance The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a wave travelling in one direction along the line in the absence of reflections in th ...
. This must be matched to the impedance of the antenna and the transmitter, to transfer power efficiently to the antenna. If these impedances are not matched it can cause a condition called
standing wave In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect t ...
s on the feed line, in which the RF energy is reflected back toward the transmitter, wasting energy and possibly overheating the transmitter. This adjustment is done with a device called an antenna tuner in the transmitter, and sometimes a matching network at the antenna. The degree of mismatch between the feedline and the antenna is measured by an instrument called an SWR meter (standing wave ratio meter), which measures the '' standing wave ratio'' (SWR) on the line. :


Twin-lead

Twin lead is used to connect
FM radio FM broadcasting is a method of radio broadcasting that uses frequency modulation (FM) of the radio broadcast carrier wave. Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to transmit high fidelity, high-f ...
s and television receivers with their antennas, although it has been largely replaced in the latter application by coaxial cable, and as a feedline for low power transmitters such as
amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency radio spectrum, spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emer ...
transmitters. It consists of two wire conductors running parallel to each other with a precisely constant spacing, molded in
polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bott ...
insulating material in a flat ribbon-like cable. The distance between the two wires is small relative to the
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
of the RF signal carried on the wire. The RF current in one wire is equal in magnitude and opposite in direction to the RF current on the other wire. Thus, far from the line, the radio waves radiated by one wire will be opposite in phase and will cancel the waves radiated by the other wire. For the same reason, twin lead is also largely immune to
radio noise In radio reception, radio noise (commonly referred to as radio static) is unwanted random radio frequency electrical signals, fluctuating voltages, always present in a radio receiver in addition to the desired radio signal. Radio noise is a comb ...
and radio frequency interference (RFI), as long as both wires are kept equally far from any large metal objects or other parallel wires. Any unwanted external radio waves induce equal magnitude currents in the same direction (in phase) on both wires. Since as long as the receiver input is balanced it only responds to differential (opposite) currents, the noise currents are cancelled out. Twin lead is commonly called a type of " balanced line", however, this needs to be moderated with common sense: All types of cabling, either parallel wire or coaxial, are able to carry balanced current, and all can carry unbalanced current, which will radiate. For that reason every type of feedline requires some attention to make it "balanced", and can become "unbalanced" if neglected; all should be fed with balanced current and connected through current-type baluns (or "line isolators") at a few points along the line, to remove the noise brought in as unbalanced current.


Coaxial cable

Coaxial cable is probably the most widely used type of feedline, used for frequencies below the
microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
( SHF) range. It consists of a wire center conductor and a braided or solid metallic "shield" conductor, usually
copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...
or
aluminum Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
surrounding it. The center conductor is separated from the outer shield by a
dielectric In electromagnetism, a dielectric (or dielectric medium) is an Insulator (electricity), electrical insulator that can be Polarisability, polarised by an applied electric field. When a dielectric material is placed in an electric field, electric ...
, usually plastic foam, to keep the separation between the two conductors precisely constant. The shield is covered with an outer plastic insulation jacket. In ''hard coax'' cable, used for high power transmitting applications like
television transmitter A television transmitter is a transmitter that is used for terrestrial television, terrestrial (over-the-air) television broadcasting. It is an electronic device that radiates radio waves that carry a video signal representing moving images, alon ...
s, the shield is a rigid or flexible metal pipe containing a compressed gas such as
nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
, and the internal conductor is held centered with periodic plastic spacers. Coax is called " unbalanced line", since the shield conductor is usually connected to electrical ground, however the currents that flow along the center conductor are balanced by opposite currents that skim along the interior surface of the shield; only the current flowing on the exterior surface of the coaxial shield is actually unbalanced. If that current can be blocked, then the coax becomes a "balanced line". Coaxial cable's great advantage is that the enclosing shield conductor isolates the cable's interior currents from external electromagnetic fields. If the currents flowing on external surface are blocked, coax becomes unaffected by nearby metal objects and immune to interference.


Waveguide

Waveguide is used at
microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
( SHF) frequencies, at which other types of feedline have excessive power losses. A waveguide is a hollow metallic conductor or pipe. It can have a circular or square cross-section. Waveguide runs are often pressurized with nitrogen gas to keep moisture out. The RF signal travels through the pipe similarly to the way sound travels in a tube. The metal walls keep it from radiating energy outwards and also prevent interference from entering the waveguide. Because of the cost and maintenance waveguide entails, microwave antennas often have the output stage of the transmitter or the RF front end of the receiver located at the antenna, and the signal is fed to or from the rest of the transmitter or receiver at a lower frequency, using coaxial cable. A waveguide is considered an unbalanced
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
.


Impedance matching

Particularly with a transmitting antenna, the antenna feed is a critical component that must be adjusted to function compatibly with the antenna and transmitter. The transmitter output terminals, the
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
, and the antenna each has a specific
characteristic impedance The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a wave travelling in one direction along the line in the absence of reflections in th ...
, which is the ratio of
voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
to current at the terminals of the device. To transfer maximum power between the transmitter and antenna the transmitter and feedline must be impedance matched to the antenna. This means the transmitter and antenna must have the same resistance and equal but opposite reactance. The feedline must also be impedance matched to the transmitter. If this condition is met, the antenna will absorb all the power supplied by the feedline. If the impedances at either end of the line do not match, it will cause a condition called “
standing wave In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect t ...
s" (high VSWR) on the feedline, in which some of the RF power is not radiated by the antenna but is reflected back toward the transmitter, wasting energy and possibly overheating the transmitter. Most transmitters have a standard output impedance of 50 
ohm Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm. Ohm or OHM may also refer to: People * Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm'' * Germán Ohm (born 1936), Mexican boxer * Jörg Ohm (1 ...
s, designed to feed 50 ohm coaxial cable. The transmitter is matched to the feedline by a device called an '' antenna tuner'', ''antenna tuning unit'', or '' matching network'', which may be a circuit in the transmitter, or a separate piece of equipment connected between the transmitter and feedline. There may be another matching network between the antenna and feedline, to match the feedline to the antenna. In consumer wireless devices that operate at fixed frequencies the matching network is not adjustable and is enclosed in the device's case. In large transmitters like broadcasting stations and transmitters that may operate on different frequencies like shortwave stations, the antenna tuner is adjustable. Changes in the transmitter frequency or adjustments to the transmitter output stage or antenna typically change the impedance, so after any work is done on the transmitter or antenna the SWR must be checked and the matching network adjusted. To adjust the matching network the simplest instrument to measure the degree of mismatch between the feedline and the antenna is called an SWR meter (standing wave ratio meter), which reports the '' standing wave ratio'' (SWR) on the line: The ratio of the adjacent maximum and minimum voltage or current on the line. A ratio of 1:1 indicates an impedance match, meaning that the load is completely resistive so all of the power is absorbed and none is reflected. A higher ratio indicates a mismatch and reflected power. The matching network is adjusted until the SWR is below an acceptable limit. Other more advanced instruments are impedance bridges and antenna analyzers. Since in an impedance matched transmitter, the transmitter's source resistance is equal to the feedline impedance and the antenna load resistance, and both are in series on the feedline and consume equal power, the maximum power that can be delivered to the antenna is 50% of the transmitter's output power; the other 50% is dissipated as heat by the resistance in the transmitter's output stage. (The matched feedline does dissipate a small amount of power through a small resistance, but the majority of its apparent resistive impedance is merely the voltage required to overcome the inductive and capacitive reactances of the feedline, which in and of themselves cause no loss.) In radio receivers an impedance mismatch with the antenna causes a similar reduction in the signal energy from the antenna that reaches the receiver, which is also a maximum of 50% of the power of the intercepted signal, and power delivered to the receiver is far less when the line is mismatched and SWR is high. However, loss at frequencies below 10~20 MHz is not much of a problem, because the
thermal noise A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
floor in receivers is far below
atmospheric noise Atmospheric noise is radio noise, or "static", caused by natural atmospheric processes, primarily lightning discharges in thunderstorms. On a worldwide scale, there are about 40 lightning flashes per second, or lightning discharges per day ...
already embedded in the signal, so a weak signal from the antenna can simply be amplified in the receiver to compensate for power lost from any mismatch, without noticeably contaminating it with noise. At frequencies above 20 MHz atmospheric noise radiates freely into space, and so is low enough in received signals that it approaches the level of the receiver's own internally generated noise; at those VHF and UHF frequencies, amplification degrades the signal to noise ratio, and receive signal impedance matching is an important concern for receiving faint signals.


Balanced and unbalanced feeds

Transmission lines and their attached components can be classified as either balanced, in which both sides of the line have the same impedance to ground, for example
dipole antenna In radio and telecommunications a dipole antenna or doublet is one of the two simplest and most widely used antenna types, types of antenna; the other is the monopole antenna, monopole. The dipole is any one of a class of antennas producin ...
s and parallel wire lines, or unbalanced, in which one side of the line is connected to ground, for example monopole antennas and
coaxial cable Coaxial cable, or coax (pronounced ), is a type of electrical cable consisting of an inner Electrical conductor, conductor surrounded by a concentric conducting Electromagnetic shielding, shield, with the two separated by a dielectric (Insulat ...
. To connect balanced and unbalanced components, a two port device called a balun is used. A balun is a
transformer In electrical engineering, a transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any coil of the transformer produces ...
that couples between balanced and unbalanced transmission line components. For example, to feed a dipole antenna from an unbalanced feedline like coaxial cable, the feedline is connected to the antenna through a balun. Without the balun, the unbalanced part of the current will flow on the outside of the coaxial cable shield, causing the outer surface of the shield to act as an antenna.


Other feed components

More complicated feeds may have other components besides the feedline and matching networks: A receiving antenna with a long feedline may have an
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It is a two-port electronic circuit that uses electric power from a power su ...
at the antenna, called a low-noise amplifier (LNA) which increases the power of the weak radio signals to compensate for attenuation in the feedline. At
microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
frequencies ordinary types of transmission line have excessive power losses, so for low losses microwaves must be carried by waveguide, a hollow metal pipe which conducts the radio waves. Due to the high cost and maintenance requirements, long waveguide runs are avoided, and the
parabolic antenna A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or p ...
s used at microwave frequencies often have the RF front end of the receiver, or parts of the transmitter, located at the antenna. For example, in satellite dishes the feedhorn on the dish which collects the microwaves is attached to a circuit called a low-noise block downconverter (LNB or LNC), which converts the high microwave frequency to a lower
intermediate frequency In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier wave is shifted as an intermediate step in Transmission (telecommunications), transmission or reception. The intermediate frequency is ...
, so it can be carried into the building using a cheaper coaxial cable feedline.
Radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
and satellite communications antennas may handle radio waves of multiple frequencies and polarizations, and may be used as both transmitting and receiving antennas, so the feed system carries radio signals traveling in both directions. Therefore, these antennas often have more complicated feeds that include specialized components like * directional couplers, which couple out radio waves moving in one direction but not the other, to separate the received signal from the transmitted signal *
polarizer A polarizer or polariser is an optical filter that lets light waves of a specific polarization (waves), polarization pass through while attenuation, blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed ...
s which pass radio waves of one polarization * orthomode transducers which combine or separate radio signals of different polarizations * diplexers which combine or separate two different frequencies * phase shifters, which alter the phase of the radio waves *waveguide switches * waveguide rotary joints An '' array antenna'' or ''antenna array'' consists of multiple antennas which are connected to a single transmitter or receiver which work together to emit or receive the radio waves. The feed systems of array antennas are understandably more complex than single antennas. The feed network must divide the transmitter power evenly between the antennas. To emit a plane wave the individual antennas ''(elements)'' of a transmitting array must be fed current with a specific phase relationship. Similarly with receiving arrays the currents from each element may need to be phase shifted so that they combine in phase in the receiver. This may require phase shifting networks at each element. In
phased array In antenna (radio), antenna theory, a phased array usually means an electronically scanned array, a computer-controlled Antenna array, array of antennas which creates a radio beam, beam of radio waves that can be electronically steered to point ...
antennas, a type of array antenna in which the beam can be steered electronically to different directions, each antenna element is fed current through a programmable phase shifter, which are controlled by a computer.


See also

* Standing wave ratio * Antenna tuner


References

{{DEFAULTSORT:Antenna Feed Feed