Twin-lead

A ribbon cable, also called two-wire or twin lead, is a two-pole electrical cable which is used in the field of high frequency technology for symmetric transmission of high frequency signals. Common use of this cable is the electrical connection of symmetric antenna in the frequency range of a few kHz to a few 100 MHz.

Sometimes the ribbon cable is incorrectly referred to as " twin lead ".

The two electrical conductors are located at this line in a characteristic spacing from one another which is determined, inter alia, the characteristic impedance of the cable, and are held by spacers and a plastic jacket of the cable along this characteristic spacing. The band line represents one of several types of so-called Lecher line

In contrast to the balanced cables used in the field of data transmission, the distances between the conductors are made of tighter tolerances, there is no twist in front, and to achieve a high characteristic impedance, the conductors are arranged at a greater spatial distance.

General

Middle of the 20th century, ribbon cables are used in tube receivers and the first television receiver to connect the receiving apparatus to the antenna, since it the symmetric reception antenna can be connected without any impedance matching and attenuation of the received signal is less than in the current normal coaxial cables. The disadvantage of the ribbon cable is because the RF energy is transmitted in the unprotected area between the two conductors, a higher sensitivity to interference and only a small screening attenuation - more spatially almost parallel routing ribbon cables have strong and often undesired couplings on. For cable routing, a ribbon cable, for example, make sure there are no metal objects next to the ribbon cable are.

Done to implement the antenna signals to the receivers had become customary for later coaxial cable and the coaxial connectors using so-called balun.

Other applications are in the field of amateur radio stations for connecting balanced antennas. It can also be just two conductors, which are fixed at regular intervals by spacers run the ribbon conductor. The primary dielectric then represents the air between the conductors

Ribbon cables can be directly connected, among others, the following antennas:

  • Dipole antennas, especially λ/2-Dipole with symmetrical feeding as the folded dipole
  • Yagi -Uda antenna
  • Butterfly antenna
  • Rhombusantenne

Parameter

The characteristic impedance of a ribbon cable as shown in the sketch opposite is:

Of the relative permittivity, which is a material constant of the insulation material used between the conductors.

Typical impedances of ribbon cables are 600 Ω, 450 Ω, 300 Ω and 75 Ω. In particular, the ribbon cable 300 Ω was in early radio and the first television receiver, a typical value, that can be easily implemented in newer receivers coaxial antenna terminal through a balun impedance matching of a factor of 4 in the usual coaxial cables 75 Ω. For the greater achievements of transmission facilities, the characteristic impedance is chosen at 185 Ω, since there the losses are minimal.

For a given insulation material result in the band line following optimal impedances:

  • Minimum attenuation at a / d ≈ 2.276 with Z = 175.6 Ω
  • Largest electric strength at a / d ≈ 2.932 with Z = 208.6 Ω
  • Maximum power transfer at a / d ≈ 2.146 with Z = 167.7 Ω
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