EMC Question of the Week: June 30, 2025
For a signal propagating in a parallel-wire transmission line, which of the following must remain constant in order to avoid reflections?
- the ratio of the voltage to the current
- the ratio of the electric- to magnetic-field strengths
- the diameter of the wires
- the direction of propagation
Answer
The best answer is “a.” Reflections occur when there is a change in the characteristic impedance. This is the ratio of the voltage to the current in a forward propagating wave. All of the other parameters can change, but there won't be a reflection unless the characteristic impedance also changes.
The magnitude of the ratio of the electric- to magnetic-field strengths in the TEM wave propagating down the transmission line is equal to the intrinsic impedance of the dielectric. It's possible to change the intrinsic impedance of the dielectric without changing the characteristic impedance of the transmission line.
One example of this is feeding a TEM cell with an RG-58 cable. The TEM cell has a 50-Ω characteristic impedance and an air dielectric. The RG-58 coaxial cable has a 50-Ω characteristic impedance and a dielectric that has a relative permittivity of approximately 2.3. A lot of things change at the cable-cell interface including the size and shape of the conductors and the velocity of propagation. However, there is no reflection as long as the 50-Ω characteristic impedance is maintained.
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