EMC Question of the Week: December 29, 2025
A plane wave propagates along a straight parallel-wire transmission until it comes to the end of the line. At that point, the transmission line ends and the signal sees nothing but free space. If the characteristic impedance of the transmission line is 377 Ω. What is the reflection coefficient at the end of the line?
- 1
- 0
- -1
- j1
Answer
The best answer is “a.” The reflection coefficient at an open circuit termination is one. All of the power in the plane wave is reflected back into the transmission line. And, although there is no such thing as a perfect open-circuit termination, even if we account for the capacitance associated with the fringing electric field at the end of the wires, the reflection coefficient will be very nearly equal to 1.
The characteristic impedance of a transmission line should never be confused with the intrinsic impedance of a dielectric. The characteristic impedance of a transmission line is the ratio of the voltage to the current in wave propagating along the line. The intrinsic impedance of a dielectric is the ratio of the magnitudes of the electric and magnetic fields in a plane wave propagating in that dielectric. A transmission line with a characteristic impedance of 377 Ω does not inherently radiate any more or less than transmission lines with any other characteristic impedance.
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