Feed line
The cable connecting antenna to transmitter or receiver.
In a radio system, the feed line (also called a feedline or feeder) is the cable or other transmission line that links the antenna to the radio transmitter or receiver. When transmitting, the feed line carries radio frequency (RF) current from the transmitter to the antenna, where the antenna converts that current into radio waves. When receiving, the feed line transfers the tiny RF voltage induced in the antenna by incoming radio waves to the receiver for processing.
To transfer RF current efficiently, feed lines must be a specialized type of cable known as transmission line. The most common types are coaxial cable, twin-lead, ladder line, and, at microwave frequencies, waveguide—a hollow metal pipe that guides radio waves.
The feed line is especially critical in transmitting setups. Each type of transmission line has a specific characteristic impedance, which must be matched to the impedance of both the antenna and the transmitter for efficient power transfer. If these impedances are not matched, a condition called standing waves can develop on the feed line, causing RF energy to reflect back toward the transmitter. This wastes power and can overheat the transmitter. To correct this, an antenna tuner (or matching network) is used at the transmitter, and sometimes a matching network is placed at the antenna. The degree of mismatch is measured with an SWR meter, which indicates the standing wave ratio on the line.
Twin-lead is a type of feed line consisting of two parallel wire conductors held at a constant spacing by a flat, ribbon-like polyethylene insulator. It is used to connect FM radios and television receivers to their antennas (though coaxial cable has largely replaced it for TV), and as a feed line for low-power transmitters like those in amateur radio. The distance between the two wires is small relative to the wavelength of the RF signal. The RF current in one wire is equal in magnitude but opposite in direction to that in the other wire, so far from the line, the radio waves radiated by each wire cancel each other out. For the same reason, twin-lead is largely immune to radio noise and interference, as long as both wires remain equally distant from large metal objects or other parallel wires.
- Types
- coaxial cable, twin-lead, ladder line, waveguide
- Characteristic impedance
- each type has a specific characteristic impedance
Lore & Background
The feed line is a critical component, particularly with a transmitting antenna, that must be adjusted to work correctly with the antenna and transmitter. Each type of transmission line has a specific characteristic impedance, which must be matched to the impedance of the antenna and the transmitter to transfer power efficiently. If these impedances are not matched it can cause a condition called standing waves 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.
The most widely used types of feed line are coaxial cable, twin-lead, ladder line, and at microwave frequencies, waveguide. Coaxial cable consists of a wire center conductor and a braided or solid metallic shield conductor, usually copper or aluminum, separated by a dielectric. Twin-lead consists of two wire conductors running parallel with constant spacing, molded in polyethylene. Waveguide is a hollow metallic conductor or pipe used at microwave frequencies where other types have excessive power losses.
Reader's Guide
The feed line is a fundamental component in any radio system where the antenna is located separately from the transmitter or receiver, such as broadcast television antennas, satellite dishes, cell tower sector antennas, rotating radar antennas, and radio and television station antenna towers. Its significance lies in its ability to carry radio frequency current efficiently between the radio and the antenna, using specialized transmission line types that maintain a uniform characteristic impedance to avoid reflections. The choice of feed line type depends on frequency and application: coaxial cable is widely used for frequencies below microwave, twin-lead is used for FM radios and television receivers as well as low power amateur radio transmitters, and waveguide is employed at microwave frequencies. The feed line's legacy includes the development of impedance matching techniques and instruments like the SWR meter to ensure efficient power transfer and prevent transmitter damage from reflected energy.
Did You Know?
- Twin-lead is largely immune to radio noise and RFI because unwanted external waves induce equal currents in the same direction on both wires, which cancel at a balanced receiver input.
- Coaxial cable's shield conductor isolates the cable's interior currents from external electromagnetic fields, making it unaffected by nearby metal objects if external surface currents are blocked.
- Waveguide runs are often pressurized with nitrogen gas to keep moisture out.
- The degree of mismatch between feedline and antenna is measured by an SWR meter, which measures the standing wave ratio.
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