Feed line

The Feed line in a radio transmission, reception or transceiver system is the physical cabling that carries the RF signal to and/or from the antenna. It is also called a transmission line. When operating properly, and under ideal conditions, the feed line should successfully carry all of the RF energy without any signal loss, and without radiating any energy or absorbing any energy. There are three common types of feed lines in use in modern wireless systems: the coaxial type, the twin-lead, and, at frequencies above 1 GHz, a waveguide is used.

The feed line is a very important part of the antenna system especially in certain conditions such as high frequency, weak signals or both, as they often go hand in hand. Feed line cabling has a specific impedance that must be matched with the transmitter/receiver/transceiver and the antenna to prevent signal loss. This matching is accomplished with an antenna tuning unit.

Twin Lead

Twin lead is also called parallel 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.

Coaxial Cable

Coaxial cable is a rounded cable with a center conductor and a braided or solid metallic shield, usually copper 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.

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.

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.

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