*Question E9D10:
How does radiation resistance of a base-fed whip antenna change below its resonant frequency?*  

*Correct Answer is  B: Radiation resistance decreases*  

### AI Response: (may contain errors)  

To understand how the radiation resistance of a base-fed whip antenna changes below its resonant frequency, let's break down the concepts involved.

### Correct Answer: Radiation resistance decreases

1. **Why the correct answer is correct:**
   - Below the resonant frequency, the antenna behaves more like a capacitive load. As the frequency decreases, the impedance of the antenna changes, leading to a decrease in radiation resistance. This is because the antenna is not efficiently radiating energy at frequencies below resonance, resulting in lower radiation resistance.

### Why the other choices are incorrect:

2. **Choice A: Radiation resistance increases**
   - This is incorrect because, as explained, the radiation resistance decreases below resonance due to the capacitive nature of the antenna at lower frequencies.

3. **Choice C: Radiation resistance becomes imaginary**
   - This is misleading. While the impedance of the antenna can become complex (having both real and imaginary components), the radiation resistance itself does not become imaginary. Instead, it decreases and remains a real value, although the overall impedance may have a reactive component.

4. **Choice D: Radiation resistance does not depend on frequency**
   - This is incorrect because radiation resistance is frequency-dependent. It varies with the frequency of operation, particularly around the resonant frequency of the antenna.

### Useful Formulas and Rules of Thumb:

- The radiation resistance ($R_r$) of a dipole antenna is typically around 73 ohms at resonance. Below resonance, it decreases.
- A rule of thumb is that as you move away from resonance (either above or below), the radiation resistance will change, and the antenna will become less efficient at radiating power.

In summary, below the resonant frequency, the radiation resistance of a base-fed whip antenna decreases due to its capacitive behavior, making it less effective at radiating energy.