I was learning about crosstalk in PCB from the High-speed signal propagation book by Howard Johnson.
It says mutual inductance coupling is high in microstrip line compared to mutual capacitive coupling.
May I know why it is like that
Isn't that normal ? Distributed Inductance along Line Length and Magnetic Coupling due to this reason is dominate. Capacitive Coupling occurs at fringe sides of the Line and it's ( Electrical Coupling ) relatively low compare to Magnetic Coupling.
Stripline impedance is vertical to ground plane with nH/mm and pF/mm such that a w:h ratio in FR4 of 2:1 is about 50 Ohms from Zo=sqrt(L/C) There is lateral flux coupling from coplanar tracks by M being a fraction of L and C due to the thin edge crosstalk which is about 50% of the voltage for a 0.1ns risetime (near 4GHz) when the geometry for l:w:h = 10:1:1.
Directional couplers vary in design but depend on the controlled power ratio of the coupling and directional properties.