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RF and Microwave Engineering: Fundamentals of Wireless Communications by Frank Gustrau

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4.8 Two-Wire Line

A two-wire transmission line is a symmetrical (balanced) line with two conductors of equal size and shape (see Figure 4.25a). The cylindrical conductors have a diameter d and are separated by distance D (centre-to-centre). Such a transmission line supports a TEM-wave. Signal and return conductors carry opposite currents I+ and I and are encircled by closed magnetic field lines images/c04_I0092.gif (see Figure 4.25b). Between the two conductors we have open electric field lines images/c04_I0093.gif due to different electric potentials on the conductors.

Figure 4.25 Geometry of a two-wire line and electric and magnetic field distribution of a TEM-wave.

4.25

In a practical design the conductors are surrounded by dielectric isolating material in order to ensure a constant cross-section along the line. In this case the fundamental mode changes from TEM to quasi-TEM. We will neglect the influence of dielectric material in our discussion. Ready-to-use formulas for the characteristic impedance of two-wire lines surrounded by dielectric material can be found in the literature [7]. Furthermore, EM simulation software can provide data for such transmission line geometries.

Modifications of the basic two-wire line concept ...

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