RF and Microwave Engineering: Fundamentals of Wireless Communications

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 (see Figure 4.25b). Between the two conductors we have open electric field lines 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.

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 ...