CHAPTER 2 - Conveying Power at Radio Frequency
104
2.11 Transmission lines – Design and Practical Realisation
A number of different geometries may be used to construct a transmission line. In this
section we will only be looking at two of the most common ones namely coaxial lines, mentioned in
section 2.3, and microstrip lines, which are very widely used in printed circuits.
2.11.1 Coaxial line
A coaxial line usually comes in the form of a coaxial cable with connectors at either side. A
typical cable structure is shown in Figure 2.11-1.
Figure 2.11-1 Coaxial Cable cutaway
The metallic shield is used as a ground reference while the signal is carried by the centre core.
Figure 2.11-2 Cross-section of a coaxial cable showing inner radius and outer radius
Figure 2.11-2 shows a cross-section of the coaxial cable and clearly indicates the dimensions
which are used to calculate the characteristic impedance of the line,
. The grey area between the
conductors is usually some type of dielectric but air would also work. The dielectric constant
of
the material interposed between the two conductors does influence the impedance of the line
therefore this quantity is also included in the calculation. This is shown by equation (2.11-1).
√
( )
Note that in many textbooks, and indeed in Microwave Office, diameters are used in place of radii
for this calculation. However it is apparent that either may be used without the value of
being
affected.
(2.11-1)
Conquer Radio Frequency
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