Question
Which of the following factors affect the feed point impedance of an antenna?
Answer Options
- A) Transmission line length
- B) Antenna height
- C) The settings of an antenna tuner at the transmitter
- D) The input power level
Correct Answer: B
Explanation
The feed point impedance (Z_A = R + jX) is the complex impedance presented by the antenna at the point where the transmission line connects. For a dipole, this is typically near 70 \Omega at resonance in free space. However, this impedance is highly susceptible to external influences, particularly proximity to the Earth.
The most significant external factor that affects the feed point impedance of an antenna is Antenna height (specifically, its height above ground). Changes in height alter the amount of power reflected back from the ground, which affects the reactive component (jX) and the resistive component (R) of the antenna’s impedance. The length of the transmission line, tuner settings, and power level do not affect the antenna’s inherent feed point impedance.
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