IDEAL TRANSMISSION-LINE FUNDAMENTALS - Advanced Signal Integrity for High-Speed Digital Design - page 70

(a)
Electric field
Magnetic field
(b)
Figure 3-4 Electric and magnetic field patterns for homogeneous dielectrics in (a) a
stripline and (b) a microstrip.
e r = 1
e r > 1
Figure 3-5 How the electric field behaves at a dielectric boundary.
and tangential to the conductor surfaces. If we assume that the voltage is applied
with the positive value on the signal conductors as shown in Figure 3-4, we
can draw the electric and magnetic fields for various transmission-line structures
assuming a homogeneous dielectric.
When the dielectric is not homogeneous, as is almost always the case with
microstrip transmission lines, the field lines are distorted as they cross dielectric
boundaries. Figure 3-5 illustrates how the electric field lines are bent away from
the normal to the dielectric boundary when the relative dielectric permittivity on
the top half of the structures is smaller than that on the bottom .
To understand why the field lines are distorted, we must examine how elec-
trostatic fields behave at the boundaries between two dielectric regions. First,