2.3 Transmission Lines – an Introduction
41
Figure 2.3-7 Two-wire transmission line
For a parallel-wire line with air insulation, shown in Figure 2.3-7, the characteristic impedance may
be calculated as shown:
√
Where,
Z0
= Characteristic impedance of line
d = Distance between conductor centres
r = Conductor radius
= Relative permittivity of insulation between conductors
Figure 2.3-8 A coaxial transmission line
If the transmission line is coaxial
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in construction (Figure 2.3-8), the characteristic impedance
follows a different equation:
√
Where,
Z0
= Characteristic impedance of line
d1
= Inside diameter of outer conductor
d2
= Outside diameter of inner conductor
= Relative permittivity of insulation between conductors
In both equations, identical units of measurement must be used in both terms of the fraction.
If the insulating material is not air (or vacuum) both the characteristic impedance and the
propagation velocity will be affected. The ratio of a transmission line's true propagation velocity and
the speed of light in a vacuum is called the velocity factor of the line.
The velocity factor is purely a factor of the relative permittivity (or dielectric constant) of the
insulating material which is defined as the ratio of a material's electric field permittivity to that of a
pure vacuum.
14
Coaxial lines are explained further in section 2.11.1
(2.3-1)
(2.3-2)
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