## EMC Question of the Week: April 29, 2024

A stripline trace in a circuit board is 0.2 mm wide and 0.25 mm from the planes above and below it. The dielectric is FR-4 (ε_{r} = 4.3). What is the ratio of the electric field strength to the magnetic field strength associated with a signal propagating along the trace?

- about 50 Ω
- about 180 Ω
- about 377 Ω
- depends on trace thickness

## Answer

The best answer is “b.” Striplines support one of mode of propagation, the TEM mode. This is plane wave propagation. The ratio of the electric field strength to the magnetic field strength is the wave impedance. For plane wave propagation, the wave impedance is equal to the intrinsic impedance of the medium. In this case, the intrinsic impedance is approximately, $\eta =\raisebox{1ex}{$377\text{\hspace{0.33em}}\Omega $}\!\left/ \!\raisebox{-1ex}{$\sqrt{4.3}$}\right.\approx 180\text{\hspace{0.33em}}\mathrm{\Omega}$.

Note that the characteristic impedance of a transmission line is not the same thing as the intrinsic impedance of the dielectric. The characteristic impedance is the ratio of the voltage to the current in a forward propagating wave. Unlike the wave impedance, the characteristic impedance is a function of the trace width, height, and thickness.

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