E9D10: Radiation resistance: How radiation resistance of a base-fed

Q: How does radiation resistance of a base-fed whip antenna change below its resonant frequency?

The correct answer is B: Radiation resistance decreases. The correct answer is B: Radiation resistance decreases. As a base-fed whip antenna operates below its resonant frequency, the radiation resistance decreases. Below resonance, the antenna is shorter than optimal, reducing its radiation resistance. Radiation resistance is the resistance that represents power radiated as RF. For a given antenna type, radiation resistance depends on the antenna's electrical length. As you go below resonance (shorter antenna), the radiation resistance decreases. This is because a shorter antenna is less efficient at radiating - it has less effective length to create radiation. The radiation resistance of a short vertical is much lower than a full-size quarter-wave vertical. This is why electrically short antennas often have low radiation resistance and need good ground systems to minimize other losses.

Updated: Dec 9, 2025 | Source: 2024-2028 Question Pool | Topic: E9D

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How does radiation resistance of a base-fed whip antenna change below its resonant frequency?

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Deep Dive: E9D10

The correct answer is B: Radiation resistance decreases. As a base-fed whip antenna operates below its resonant frequency, the radiation resistance decreases. Below resonance, the antenna is shorter than optimal, reducing its radiation resistance. Radiation resistance is the resistance that represents power radiated as RF. For a given antenna type, radiation resistance depends on the antenna's electrical length. As you go below resonance (shorter antenna), the radiation resistance decreases. This is because a shorter antenna is less efficient at radiating - it has less effective length to create radiation. The radiation resistance of a short vertical is much lower than a full-size quarter-wave vertical. This is why electrically short antennas often have low radiation resistance and need good ground systems to minimize other losses.

Why Other Answers Are Wrong

Option A: Incorrect. Radiation resistance doesn't increase below resonance - it decreases. Shorter antennas have lower radiation resistance. Option C: Incorrect. Radiation resistance doesn't stay the same. It changes with frequency/electrical length. Option D: Incorrect. Radiation resistance doesn't become purely reactive. It's still resistive, just lower in value. The antenna also has reactive components, but radiation resistance itself is still resistive (just smaller).

Exam Tip

Below resonance = Decreased radiation resistance. Remember: As a base-fed whip operates below its resonant frequency, radiation resistance decreases because the shorter antenna is less efficient at radiating.

Memory Aid

**B**elow **R**esonance = **D**ecreased **R**adiation **R**esistance (think 'BR = DRR')

Real-World Example

You have a base-fed whip that's resonant at 14 MHz (full quarter-wave). The radiation resistance is about 36 ohms. You operate it at 7 MHz (below resonance, electrically shorter). The radiation resistance is now much lower (maybe 10-15 ohms) because the shorter antenna is less efficient at radiating. This is why short antennas need loading coils and good ground systems.

Source & Coverage

Question Pool: 2024-2028 Question Pool

Subelement: E9D

Reference: FCC Part 97.3

Key Concepts

Radiation resistance Below resonance Base-fed whip Antenna length

Verified Content

Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E9D topic.