Question
How does radiation resistance of a base-fed whip antenna change below its resonant frequency?
Answer Options
- A) Radiation resistance increases
- B) Radiation resistance decreases
- C) Radiation resistance becomes imaginary
- D) Radiation resistance does not depend on frequency
Correct Answer: B
Explanation
Radiation resistance (R_{Rad}) is the equivalent resistance that converts input power into useful, radiated energy. For any antenna, R_{Rad} is highly dependent on its physical length relative to the signal wavelength. An antenna that is very short relative to its operating wavelength is an inefficient radiator.
If a base-fed whip antenna is operated below its resonant frequency (meaning it is electrically short), the current magnitude decreases along the antenna and the antenna becomes highly capacitive. As the length decreases (or frequency decreases further below resonance), the radiation resistance decreases significantly, drastically lowering the antenna’s overall radiation efficiency.
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