EMC Question of the Week: September 17, 2017

The ratio of the electric field strength to the magnetic field strength for a wave propagating in RG58 coaxial cable (Z₀ = 50 Ω, ε_r =2.3) is,

1. 50 Ω
2. 250 Ω
3. 377 Ω
4. > 10 MΩ

Answer

The correct answer is "b". The ratio of the electric field strength to the magnetic field strength for a wave propagating in a transmission line is equal to the intrinsic impedance of the dielectric. In this example, the dielectric has a relative permittivity equal to 2.3. Therefore, the intrinsic impedance is $$\eta =\sqrt{\frac{\mu }{\epsilon }}=\sqrt{\frac{{\mu }_0}{{\epsilon }_0{\epsilon }_r}}\approx \frac{377\Omega }{\sqrt{2.3}}\approx 250\mathrm{\Omega .}$$

Note that the "intrinsic impedance" of the dielectric is not the same thing as the "characteristic impedance" of the transmission line. Characteristic impedance is the ration of the voltage to the current in a wave propagating on the transmission line (in this example, 50 Ω).

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