The transmission lines in the previous sections consist of one or two conductors. Rectangular and circular waveguides have one conductor and support TE modes as fundamental wave modes. Coaxial line, microstrip, strip line, coplanar and two-wire lines are two conductor lines that have a signal trace and a return trace and support TEM or quasi-TEM waves. Coaxial line, microstrip, strip line and coplanar line are unbalanced, where the signal and return trace are of different size or shape. The two-wire line is a balanced line, where the two conductors are of equal geometry.
We will now look at symmetrical three-conductor lines where we have two signal traces that are of equal geometry and a third conductor representing a common ground. As examples we look at three-conductor lines that are derived from a two-wire line and microstrip line.
A two-wire line is an open line that is susceptible to coupling from external electromagnetic fields. Furthermore, the electromagnetic fields from the waves on the transmission line reach out into the space around the two-wire line (see Figure 4.25b). We can improve the situation by surrounding the two-wire line by a cylindrical shielding enclosure (shielded two-wire line). Now the field of the wave on the line is limited to the interior of the cylindrical enclosure (see Figure 4.27a). Now we have a three-conductor line with two signal traces and a common ground.