Antenna feed
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.
To carry the radio frequency (RF) current efficiently, the feedline is made of specialized cable called transmission line.
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.
It consists of two wire conductors running parallel to each other with a precisely constant spacing, molded in polyethylene insulating material in a flat ribbon-like cable.
Any unwanted external radio waves induce equal magnitude currents in the same direction (in phase) on both wires.
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 is probably the most widely used type of feedline, used for frequencies below the microwave (SHF) range.
In hard coax cable, used for high power transmitting applications like television transmitters, the shield is a rigid or flexible metal pipe containing a compressed gas such as nitrogen, and the internal conductor is held centered with periodic plastic spacers.
If the currents flowing on external surface are blocked, coax becomes unaffected by nearby metal objects and immune to interference.
Waveguide is used at microwave (SHF) frequencies, at which other types of feedline have excessive power losses.
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.
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.
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.
(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.)
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.
To emit a plane wave the individual antennas (elements) of a transmitting array must be fed current with a specific phase relationship.
