*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)  

When dealing with antennas, radiation resistance is an important concept. It represents the part of an antenna's impedance that is responsible for radiating energy into space, as opposed to dissipating it as heat.

### Explanation:

1. **Correct Answer: Radiation resistance decreases (B)**

   - **Why it is correct:**  
     Below its resonant frequency, a base-fed whip antenna behaves more like a capacitive reactance. As the frequency decreases further from resonance, the radiation resistance also decreases. This is because the antenna is less efficient at radiating energy when it is not at its resonant frequency.

2. **Why the other choices are incorrect:**

   - **A: Radiation resistance increases**  
     This is incorrect because, as mentioned, the radiation resistance actually decreases below the resonant frequency, not increases.

   - **C: Radiation resistance becomes imaginary**  
     Radiation resistance is a real part of the antenna's impedance. While the overall impedance of the antenna can have an imaginary component (reactance), the radiation resistance itself remains a real value.

   - **D: Radiation resistance does not depend on frequency**  
     This is incorrect because radiation resistance does indeed depend on frequency. It varies with frequency, especially as you move away from the resonant frequency of the antenna.

### Useful Formula:

- The total impedance of an antenna can be expressed as:  
  $$ Z = R_r + R_l + jX $$
  where:
  - $R_r$ = radiation resistance
  - $R_l$ = loss resistance
  - $jX$ = reactance (imaginary part)

- As frequency changes, the reactance $jX$ changes, and this affects the overall impedance, including the radiation resistance $R_r$.

Understanding how radiation resistance changes with frequency helps in designing and tuning antennas for optimal performance.