ABSTRACT
The physical realization of a coplanar waveguide* is indicated in Figure 10.1.1. This t.l. was first studied by C.P. Wen.1 It is important to note that if the word “waveguide” appears in this transmission line name, in this case, there is no similarity to the well-known “waveguides” discussed in Appendix A2. “CPW” is a full planar t.l., since in contrast to the microstrip case, here there is no bottom ground conductor.** From this point of view, the “CPW” is very similar to the slot line studied in Chapter 9. As shown in Figure 10.1.1, this t.l. is composed of a central conductor of width “w,” separated from two lateral conductors by a distance “s” called the “slot.” All the conductors of thickness “t,” are placed on a dielectric slab of height “h” and dielectric and magnetic constant “er” and “µr.” The extension “w1” and “w2” of the two lateral conductors is supposed to be infinite, but in practice is many times the length of the signal wavelength.***
According to the discussion in Appendix A2, the “CPW” has a zero cut-off frequency, but its low order propagation mode is indicated with “qTEM”**** because it is not a real “TEM.” However, the error we make in evaluating the fundamental propagation mode as a pure “TEM” is negligible for frequencies up to some tens of GHz.2,3,4 After this limit, dispersion arises and the propagation mode tends to be nearly a “TE,”***** with the magnetic field elliptically polarized along longitudinal planes. From this point of view, there is a big difference between microstrips and striplines studied in Chapters 2 and 3, while there is some similarity with the slot line propagation mode since this t.l. does not support a real “TEM” mode. Due to the elliptical magnetic field polarization, the “CPW” is a t.l. well suited to have energy exchange with ferrimagnetic materials, as we will show later.
