Question
What is the purpose of making a Yagi’s parasitic elements either longer or shorter than resonance?
Answer Options
- A) Wind torque cancellation
- B) Mechanical balance
- C) Control of phase shift
- D) Minimize losses
Correct Answer: C
Explanation
A Yagi antenna’s directors and reflectors are not connected to the feed line (they are parasitic) but still carry \text{RF} current induced by the driven element’s field. To function correctly, they must absorb energy and re-radiate it with a specific phase relationship relative to the driven element.
This required phase relationship is achieved by tuning the element’s length: making the element slightly longer than resonance causes it to exhibit inductive reactance (acting as a reflector) which lags the phase; making it slightly shorter than resonance causes it to exhibit capacitive reactance (acting as a director) which leads the phase. Therefore, the purpose is to control of phase shift to ensure the energy combines constructively in the forward direction.
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