EMC Question of the Week: September 3, 2017

The propagation velocity of an electromagnetic wave propagating in a transmission line with an air dielectric is,

1. 3 x 10⁸ meters/sec
2. 377 Ω
3. dependent on the amplitude of the field strength
4. dependent of the characteristic impedance of the transmission line

Answer

The correct answer is "a". The velocity of propagation in a transmission line is a function of the dielectric properties. In air or free space, this velocity is the speed of light or 3 x 10⁸ meters/sec.

377 Ω is the intrinsic impedance of free space. The velocity of propagation is independent of the source amplitude or the geometry of the conductors in the transmission line. The characteristic impedance of the transmission line is also a function of the dielectric properties; but for a given dielectric such as air, the velocity of propagation is independent of the characteristic impedance. 

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