Two of the four transmission lines treated in this chapter have few microwave applications because of excessive losses occasioned by radiation: their presence in a book devoted to microwave topics may therefore be questioned. However, they have been included because of the close 'family relationship' which exists between all four lines described, and also because the two 'interlopers' are direct ancestors of the microstrip transmission line, which is now of such great importance in microwave miniaturization and integrated circuit applications. Strictly speaking, the elliptic coaxial line should be included in this chapter, since it is the basic form from which the coaxial line and its relatives are derived. However, since its conductors are not circular in cross-section, and since it is of little practical importance, it has been included in Chapter 5, under the heading of 'lines of unusual cross-section.'

This chapter describes types of lines which may perhaps be regarded as 'hybrids' of those discussed in the two preceding chapters, since, of the two conductors which constitute each line, one is circular and the other rectangular in cross-section.

This chapter covers the following: lines with coupled conductors of circular cross-section;and lines with coupled conductors of rectangular cross-section.