Until recently, three principal classes had been known in the electrical
cir- cuitry. They were as follows: 1) The lumped-constant circuit, which
should be called a zero-dimensional circuit, in the sense that the
circuit elements are much smaller in size as compared with the
wavelength in all three spatial directions. 2) The distributed-constant
circuit, which should be called a one-dimensional circuit, in the sense
that the circuit elements are much smaller than the wavelength in two
directions but comparable to the wavelength in one di- rection. 3) The
waveguide circuit, which should be called a three-dimensional circuit,
in the sense that the circuit elements are comparable to the wavelength
in all three directions. The principal subject of this book is the
analysis and design (synthesis) theories for another circuit class which
appeared in the late 1960s and became common in the 1970s. This new
circuit class is 4) the planar circuit, which should be called a
two-dimensional circuit, in the sense that the circuit elements are much
smaller in size as compared with the wavelength in one direction, but
comparable to the wavelength in the other two directions.