Antenna feed

A radio transmitter or receiver is connected to an antenna which emits or receives the radio waves. The antenna feed system or antenna feed is the cable 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 of radio frequency, 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 current efficiently, the ''feedline'' connecting the transmitter or receiver to the antenna must be a special type of cable called transmission line. At microwave 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 feed antenna. 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 radios, the sleeve dipole antennas of wireless routers, and the PIFA antennas inside cellphones. In this case the feed system just consists of an impedance matching 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 and television stations. In this case the antenna is connected to the transmitter or receiver with a cable called a feedline. To carry the radio frequency (RF) current efficiently, the feedline is made of specialized cable called transmission line. The advantage of transmission line is that it has a uniform characteristic impedance to avoid abrupt impedance steps which cause the radio energy to be reflected back down the line. The main types of transmission line are parallel wire line (Twin lead), coaxial cable, and for microwaves waveguide.

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, and the antenna each has a specific characteristic impedance, which is the ratio of voltage 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 waves" (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 ohms, 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 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: 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.

Since in an impedance matched transmitter, the transmitter's source resistance is equal to the antenna load resistance, and both are in series in 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 in the transmitter's output stage resistance.

In radio receivers an impedance mismatch with the antenna causes a similar reduction in the signal energy from the antenna reaching the receiver. However at lower frequencies below 40 MHz this is not such a problem, because the thermal noise floor in receivers is far below atmospheric noise, so the weak signal from the antenna can simply be amplified in the receiver to compensate for power loss from any mismatch, without contaminating it with noise.

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 antennas and parallel wire lines, or unbalanced, in which one side of the line is connected to ground, for example monopole antennas and coaxial cable. To connect balanced and unbalanced components, a two port device called a balun is used. A balun is a matching network, usually a transformer, that couples 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, currents will occur on the outside of the coaxial cable shield, causing 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 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 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 antennas 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, so it can be carried into the building using a cheaper coaxial cable feedline.

Radar 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
* polarizers 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 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

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