Feed Line - Twin Lead

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.