Feed line
Encyclopedia
In a radio antenna
, the feed line is the cable or other transmission line
that connects the antenna with the radio transmitter
or receiver. In a transmitter, it feeds the radio frequency
(RF) current
from the transmitter to the antenna, where it is radiated as radio wave
s. In a receiver it transfers the RF signal picked up by the antenna to the receiver. In order to carry RF current efficiently, feed lines are made of specialized types of cable called transmission line
. The most widely used types of feed line are coaxial cable
, twin-lead
, ladder line, and at microwave
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
. 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 equal it can cause a condition called standing wave
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.
wire
. It is two conductors
molded
into an insulating
material and running alongside one another in a flat and parallel ribbon-like wire. The distance between the two wires is small relative to the wavelength
of the RF signal carried on the wire. Furthermore, the RF current
in one wire is equal in magnitude
and opposite in direction
to the RF current on the other wire (it is inverted). Thus, if both wires radiate
energy equally, the radiated energies will cancel each other out and there will be near zero radiation at any distance from the wire. Twin lead is also immune to external noise or RF energies. Any unwanted external noise or unwanted RF energy induced on the wire from external energy sources will be induced in both wires at the same time and equally in magnitude and direction. At the end of the transmission line the inverted signal wire is restored to normal (non-inverted now) and added back to the original non-inverted signal wire by the receiving circuitry. Any noise will now be equal in magnitude and opposite in direction and cancel itself out.
Twin lead is considered a balanced transmission line.
or aluminum. The center conductor is separated from the outer shield by a dielectric
, which is usually foam, air or a compressed gas such as nitrogen
. The shield is covered with an outer cable sheath.
Coaxial cable is considered an unbalanced line
.
A waveguide is considered an unbalanced transmission line
.
Larger lists are available in other articles, references, and directly from manufacturers.
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
, the feed line is the cable or other transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
that connects the antenna with the radio transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
or receiver. In a transmitter, it feeds the radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
from the transmitter to the antenna, where it is radiated as radio wave
Radio Wave
Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...
s. In a receiver it transfers the RF signal picked up by the antenna to the receiver. In order to carry RF current efficiently, feed lines are made of specialized types of cable called transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
. The most widely used types of feed line are coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
, twin-lead
Twin-lead
Twin-lead cable is a two-conductor ribbon cable used as a transmission line to carry radio frequency signals.- Characteristics and uses:...
, ladder line, and at microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
frequencies, waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
.
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
Characteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...
. This must be matched to the impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
of the antenna and the transmitter, to transfer power efficiently to the antenna. If these impedances are not equal it can cause a condition called standing wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...
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
Antenna tuner
An antenna tuner, transmatch or antenna tuning unit is a device connected between a radio transmitter or receiver and its antenna to improve the efficiency of the power transfer between them by matching the impedance of the equipment to the antenna...
in the transmitter.
Twin Lead
Twin lead is also called parallelParallel (geometry)
Parallelism is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. The assumed existence and properties of parallel lines are the basis of Euclid's parallel postulate. Two lines in a plane that do not...
wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...
. It is two conductors
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
molded
Molding (process)
Molding or moulding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern....
into an insulating
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...
material and running alongside one another in a flat and parallel ribbon-like wire. The distance between the two wires is small relative to the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the RF signal carried on the wire. Furthermore, the RF current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
in one wire is equal in magnitude
Magnitude (mathematics)
The magnitude of an object in mathematics is its size: a property by which it can be compared as larger or smaller than other objects of the same kind; in technical terms, an ordering of the class of objects to which it belongs....
and opposite in direction
Direction (geometry, geography)
Direction is the information contained in the relative position of one point with respect to another point without the distance information. Directions may be either relative to some indicated reference , or absolute according to some previously agreed upon frame of reference Direction is the...
to the RF current on the other wire (it is inverted). Thus, if both wires radiate
Radiate
Radiate is a verb. See radiation.Radiate may also refer to an antic Roman bronze coin....
energy equally, the radiated energies will cancel each other out and there will be near zero radiation at any distance from the wire. Twin lead is also immune to external noise or RF energies. Any unwanted external noise or unwanted RF energy induced on the wire from external energy sources will be induced in both wires at the same time and equally in magnitude and direction. At the end of the transmission line the inverted signal wire is restored to normal (non-inverted now) and added back to the original non-inverted signal wire by the receiving circuitry. Any noise will now be equal in magnitude and opposite in direction and cancel itself out.
Twin lead is considered a balanced transmission line.
Coaxial Cable
Coaxial cable is a rounded cable with a center conductor and a braided or solid metallic shield, usually copperCopper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
or aluminum. The center conductor is separated from the outer shield by a dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
, which is usually foam, air or a compressed gas such as nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
. The shield is covered with an outer cable sheath.
Coaxial cable is considered an unbalanced line
Unbalanced line
In Electrical engineering, an unbalanced line is a transmission line, usually coaxial cable, whose conductors have unequal impedances with respect to ground; as opposed to a balanced line.Microstrip and single-wire lines are also unbalanced lines....
.
Wave Guide
A waveguide is a hollow metallic conductor with a circular or square cross-section. The RF signal travels along the inside of the tube, guided by its size and shape, similar to the way sound travels in a tube. The metallic construction of the wave guide keeps it from radiating energy outwards and also prevents unwanted energy from entering the waveguide.A waveguide is considered an unbalanced transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
.
Feed line characteristics
This is a comparison of a few common feed line characteristics.Larger lists are available in other articles, references, and directly from manufacturers.
| type | impedance Characteristic impedance The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm... |
velocity factor |
|---|---|---|
| twin-lead Twin-lead Twin-lead cable is a two-conductor ribbon cable used as a transmission line to carry radio frequency signals.- Characteristics and uses:... |
300 | 82% |
| ladder line | 450, 600 | 95% |
| coax | 50, 75 | 66% |