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A twin-lead is a cable with two wires that are arranged symmetrically parallel to each other; it is commonly used as a receiver antenna cable that carries very low power in a balanced configuration. It looks like a two-conductor ribbon cable with a larger, fixed spacing between the centers of the two wires. The size of the wire and the spacing in between determine radio frequency transmission line impedance. This cable is still very much used due to the simplicity in terminating the cable that is usually connected to a balanced antenna on the other end. The termination at the antenna side is also a simple screw type or is molded permanently to the antenna.
The twin-lead signal cables are balanced-type cables wherein neither of the wires is connected to common or ground. A drawback is the lack of shielding in the cable since there are two parallel cables with signal. Unlike the twin-lead, the unbalanced coaxial cable has a shield that surrounds the inner wire that carries the signal. The advantages of this cable are the cost per unit length and the simplicity of termination.
Twin-lead cables are similar to ribbon cables, although ribbon cables have wires that are very close to the other wire. Ribbon cables are frequently used in digital buses that carry voltages between 0 and 5 volts (V) or between 0 and 3 V depending on the bus driver’s logic family. Certain digital buses use differential-type outputs and inputs that use twisted-pairs that allow faster bit rates and better immunity to external electromagnetic interference.
Early twin-lead cables looked like a ladder in which the rung-like materials are insulator spacers that held the two wires in place. In transmitter applications, the separation between the two wires of this cable is increased to allow for higher voltages and, therefore, higher power to be sent to the antenna. Most radio transmitters use a single-ended output radio frequency power amplifier that uses an unbalanced to balanced transformer to drive the transmission line. The antenna that connects to a transmission line is usually a loop, a dipole, or a variation of a loop and dipole antenna.
For broadcast receiver systems, the twin-lead antenna works very well even when the receiver is far from the broadcasting station. The capacitance between two wires of the parallel-running wires of the twin-lead helps to attenuate any signal that may reach the receiver by getting induced directly into the twin-lead. The result is usually a longitudinal current that cancels out since the receiver is extracting the balanced output.