*Question G9D08:
How does a “screwdriver” mobile antenna adjust its feed point impedance?*  

*Correct Answer is  B: By varying the base loading inductance*  

### AI Response: (may contain errors)  

The correct answer to how a “screwdriver” mobile antenna adjusts its feed point impedance is:

**Correct Answer: B: By varying the base loading inductance**

### Explanation of the Correct Answer:
- A screwdriver antenna typically consists of a telescoping whip and a base that can adjust the loading coil's inductance. 
- By changing the position of the coil (which is often adjustable via a motor or manual mechanism), the inductance at the base of the antenna is varied. 
- This change in inductance affects the feed point impedance, allowing the antenna to be tuned for different frequencies.

### Why the Other Choices are Incorrect:
- **A: By varying its body capacitance**
  - While capacitance can affect impedance, a screwdriver antenna primarily adjusts its impedance through inductance, not capacitance. The body of the antenna does not typically have adjustable capacitance in this design.

- **C: By extending and retracting the whip**
  - Extending or retracting the whip changes the physical length of the antenna, which can affect resonance and tuning, but it does not directly adjust the feed point impedance in the same way that changing the inductance does.

- **D: By deploying a capacitance hat**
  - A capacitance hat can be used to increase the effective capacitance of an antenna, but it is not a feature of a screwdriver antenna. The screwdriver antenna relies on inductance changes rather than adding capacitance to adjust its impedance.

### Useful Formulas or Rules of Thumb:
- The impedance of an antenna can be affected by both inductance ($L$) and capacitance ($C$). The relationship can be described by the formula for the resonant frequency ($f_0$) of a loaded antenna:

$$
f_0 = \frac{1}{2\pi\sqrt{LC}}
$$

- For tuning antennas, remember that:
  - Increasing inductance lowers the resonant frequency.
  - Decreasing inductance raises the resonant frequency.

This understanding will help you grasp how a screwdriver antenna operates and why it adjusts its feed point impedance through inductance changes.