Question
What is the velocity factor of a transmission line?
Answer Options
- A) The ratio of its characteristic impedance to its termination impedance
- B) The ratio of its termination impedance to its characteristic impedance
- C) The velocity of a wave in the transmission line multiplied by the velocity of light in a vacuum
- D) The velocity of a wave in the transmission line divided by the velocity of light in a vacuum
Correct Answer: D
Explanation
The velocity factor (\text{VF}) of a transmission line is a crucial characteristic that determines the speed at which the \text{RF} signal travels through the line. The signal travels slower in any physical line than it does in a vacuum (the speed of light, c), due to the effect of the insulating material (dielectric) between the conductors.
The velocity factor (\text{VF}) is formally defined as the velocity of a wave in the transmission line divided by the velocity of light in a vacuum. Since the wave speed in the line is always less than c, the \text{VF} is always a number less than 1 (e.g., 0.66 for common polyethylene coax). It is essential for calculating the correct physical length of a transmission line when a specific electrical length is required.
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