1.2. Transmission Line Characteristics

In practice, the most common segments of the regular transmission lines of a certain length. If the length of a regular transmission line significantly exceeds the length of the wavelength line L , then such a line is called long.

It is known from electrodynamics that the transmission line can be characterized by its running parameters:  is the characteristic impedance; R 1 - linear resistance,

Ohm / m; G 1 —three conductance, 1 Ohm; L 1 — pursuit inductance, GN / m; C 1 - running capacity, f / m. The linear resistance R 1 and conductivity G 1 depend on the conductivity of the material of the wires and the quality of the dielectric surrounding these wires, respectively. The lower the heat loss in the metal of the wires and in the dielectric, the smaller, respectively, R 1 and G 1. The frightening inductance L 1 and capacitance C 1 are determined by the shape and size of the cross section of the wires, as well as the distance between them.

The propagation coefficient of the wave in the line является is generally a complex value and can be represented by the following expression

 R 1  jL 1 G 1  jC 1  jk, where  is the damping coefficient of the influence wave; k — coefficient of phase.

Transmission Line Requirements:

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minimal energy loss during propagation;

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the transmission line must not radiate;

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the transmission line must be matched to the input and load.

The characteristic impedance of a two-wire transmission line is determined by the formula

276  d 

lg,  r  r  

where  r is the relative dielectric constant of the dielectric; r is the radius of the conductors; d is the distance between the conductors.

The characteristic impedance of the coaxial transmission line is determined by the formula

138  r 



lg   1  , r  r 2 

where r 1 is the radius of the outer conductor (braid); r 2 is the radius of the inner conductor (central core).

See also

• 1.1 Introduction. Classification of transmission lines. Primary requirements.