Question
Which of the following has the biggest effect on the velocity factor of a transmission line?
Answer Options
- A) The characteristic impedance
- B) The transmission line length
- C) The insulating dielectric material
- D) The center conductor resistivity
Correct Answer: C
Explanation
The speed at which a radio wave travels through a transmission line (its velocity factor, VF) is governed by the line’s electrical properties. The velocity factor is mathematically related to the line’s dielectric constant (\epsilon_r), which is the measure of the material’s ability to store electric field energy.
Since the velocity of propagation depends on the electric field, the material that separates the conductors has the biggest effect on the VF. Therefore, the insulating dielectric material has the biggest effect on the velocity factor of a transmission line, with air or foam dielectrics resulting in high VF (close to 1.0), and solid plastic dielectrics resulting in low VF (around 0.66).
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