TABLE OF CONTENTS The signal and ground currents of multiconductor transmission lines such as the coaxial line each flow on a separate conductor. The coaxial line is an enclosed transmission line: the outer conductor completely confines the electromagnetic field and permits no interaction with sources outside. Because of this property, coaxial transmission line finds use in interconnecting microwave modules and systems where high isolation is needed. However, the weight, size, and cost of coaxial line make it undesirable for use within individual microwave modules and at lower frequencies. Figure 3.15 shows three commonly used open conduc tor, wire transmission lines. Like coaxial line, the dominant mode of each is the TEM. Unlike the coaxial line, the electromagnetic field of an open conductor transmission line, although concentrated near the conductors, decays gradually away from the wires, finally reaching zero at an infinite distance. The signal and ground conductor diameters and their separation determine the characteristic impedance of these transmission lines. For the two-wire transmission line in Figure 3.15(a), we have [6]
where 2 s is the spacing between the centers of the wires, d is their diameter, and ? r is the dielectric constant of the surrounding medium. For the three-wire line of Figure 3.15(b), the characteristic impedance is given by [6]
Figure 3.15(c) shows a single...
